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The effect of flight efficiency on gap‐crossing ability in Amazonian forest birds
Abstract
The ability to move across the landscape is a fundamental property of species that can determine their chances of persistence in fragmented landscapes and in rapidly changing environments. Despite its importance, empirical evidence showing the effect of movement capacity on patterns of movement across fragmented landscapes is limited. In this study, we examine the role of flight efficiency on the likelihood of crossing a man‐made habitat gap. We used data from the Biological Dynamics of Forest Fragments Projects on recaptures of banded birds in an Amazonian forest bisected by a road. For a total of 45 species, we estimated flight efficiency using the hand‐wing index (a proxy for the wing's aspect ratio) and used it as a predictor of the probability of road crossing in phylogenetic binomial regression models. We found that flight efficiency was a strong predictor of road‐crossing probability: species with high hand‐wing indices crossed the road more frequently than those with low hand‐wing indices. In contrast, other characteristics such as body mass, diet, flocking behavior, and foraging stratum did not show significant associations with road‐crossing probability. Our results suggest that proxies of flight efficiency such as the hand‐wing index can be powerful tools for predicting the vulnerability of bird species to forest fragmentation.
Abstract in Spanish is available with online material
... Frugivore responses to the matrix are driven by response traits that determine sensitivity to disturbance (24). For example, wing morphology is related to flight efficiency and predicts the ability of bird species to cross deforested areas (25). Generally, response traits are not randomly distributed but clustered in certain phylogenetic lineages or taxonomic groups of frugivores (26). ...
... These behaviors determine within-habitat and cross-habitat dispersal: Seeds dispersed in the matrix can arrive from both the forest and the fine-grained vegetation of the matrix (15,36), and the same is expected to happen in the forest (i.e., some seed arrival from the matrix; 15). Importantly, frugivore behavior can be explained by differences in habitat specificity and mobility (25,37), which in turn are related to specific response traits (24). Indeed, frugivores were larger and had higher flight efficiency in the matrix than in the forest. ...
... Body mass is typically both a response and an effect trait because it is related to susceptibility to defaunation and capacity to disperse large seeds over long distances (e.g., ref. 53). The HWI is a measure of wing pointedness, a proxy for wing aspect ratio and flight efficiency in birds and is positively related to flight strength and ability to cross open habitats (25,54). We obtained species-level mean body mass and mean HWI of birds from the AVONET database (55). ...
Seed dispersal by frugivores is a fundamental function for plant community dynamics in fragmented landscapes, where forest remnants are typically embedded in a matrix of anthropogenic habitats. Frugivores can mediate both connectivity among forest remnants and plant colonization of the matrix. However, it remains poorly understood how frugivore communities change from forest to matrix due to the loss or replacement of species with traits that are less advantageous in open habitats and whether such changes ultimately influence the composition and traits of dispersed plants via species interactions. Here, we close this gap by using a unique dataset of seed-dispersal networks that were sampled in forest patches and adjacent matrix habitats of seven fragmented landscapes across Europe. We found a similar diversity of frugivores, plants, and interactions contributing to seed dispersal in forest and matrix, but a high turnover (replacement) in all these components. The turnover of dispersed seeds was smaller than that of frugivore communities because different frugivore species provided complementary seed dispersal in forest and matrix. Importantly, the turnover involved functional changes toward larger and more mobile frugivores in the matrix, which dispersed taller, larger-seeded plants with later fruiting periods. Our study provides a trait-based understanding of frugivore-mediated seed dispersal through fragmented landscapes, uncovering nonrandom shifts that can have cascading consequences for the composition of regenerating plant communities. Our findings also highlight the importance of forest remnants and frugivore faunas for ecosystem resilience, demonstrating a high potential for passive forest restoration of unmanaged lands in the matrix.
... In this study, Poisson regression models identified the diversity of diet as a significant predictor of venom's functional activity, demonstrating the value of such models in making ecological predictions. Similarly, in a study on Amazonian forest birds, count data played a crucial role in unraveling the relationship between body mass, flight efficiency, diet, and road-crossing frequency [8]. Here, binomial regression models provided valuable insights into the predictors of road-crossing, which serves as a proxy for the bird's ability to cross habitat gaps-an essential survival skill in the rapidly changing Amazonian landscape. ...
... Each species is assumed to have the same rate σ i = σ, for i = v, u, z, y, x and possess independent identical Wiener processes W i t = W t , for i = v, u, z, y, x. For the count trait evolution shown in the lower-left panel of Figure 1, The tips values at t = 570 denoted as (x, y, z, u, v) are assumed to have values Y = (2,8,12,5,16). Note that one can also consider generating the sample through a tree traversal [28] where starting with the root node with a given value then each successive internal node (the circled points in the figure) is simulated using the status of the starting node plus or minus a Poisson random variable with the rate equal to the branch length multiplied by the status of the nodes where the plus or minus is determined by a Bernoulli trial with value 1 or −1 with probability drawn from a uniform distribution. ...
... The conceptual regression curves shown in the upper-right panel of Figure 1 using two types of trees and a toy dataset with trait values Y = (2, 8,12,5,16) for dependent count variable, and x = (23.4, 26.7, 24.5, 30.6, 32.5) for quantitative covariate trait variable are shown in Figure 2. ...
Regression models are extensively used to explore the relationship between a dependent variable and its covariates. These models work well when the dependent variable is categorical and the data are supposedly independent, as is the case with generalized linear models (GLMs). However, trait data from related species do not operate under these conditions due to their shared common ancestry, leading to dependence that can be illustrated through a phylogenetic tree. In response to the analytical challenges of count-dependent variables in phylogenetically related species, we have developed a novel phylogenetic negative binomial regression model that allows for overdispersion, a limitation present in the phylogenetic Poisson regression model in the literature. This model overcomes limitations of conventional GLMs, which overlook the inherent dependence arising from shared lineage. Instead, our proposed model acknowledges this factor and uses the generalized estimating equation (GEE) framework for precise parameter estimation. The effectiveness of the proposed model was corroborated by a rigorous simulation study, which, despite the need for careful convergence monitoring, demonstrated its reasonable efficacy. The empirical application of the model to lizard egg-laying count and mammalian litter size data further highlighted its practical relevance. In particular, our results identified negative correlations between increases in egg mass, litter size, ovulation rate, and gestation length with respective yearly counts, while a positive correlation was observed with species lifespan. This study underscores the importance of our proposed model in providing nuanced and accurate analyses of count-dependent variables in related species, highlighting the often overlooked impact of shared ancestry. The model represents a critical advance in research methodologies, opening new avenues for interpretation of related species data in the field.
... We quantify the effect of fragmentation on these populations on the basis of their aversion to forest edges, accounting for continuous gradients in tree cover 28 . We then use Bayesian phylogenetic mixed-effect models to assess whether fragmentation sensitivity is best predicted by historical disturbance or hand-wing index (HWI)-a metric of wing shape that predicts dispersal distance 21 and gap-crossing ability in forest birds 29,30 . We use negative (inverse) hand-wing index score (nHWI) to represent dispersal limitation, because this helps to clarify the mechanistic link with fragmentation sensitivity (see Methods). ...
... This within-guild variation can be explained by differences in dispersal ability among members of the same trophic group. For example, terrestrial and understorey insectivores are gene rally less dispersive and more sensitive to habitat fragmentation than canopy or aerial insectivores 14,42 , suggesting that flight efficiency and gap-crossing ability outweigh diet as the key factor determining responses to fragmentation in tropical forests 29,30 . ...
... The effect of dispersal limitation on fragmentation sensitivity makes sense in light of evidence from observational 24,49 and experimental studies 29,30 indicating that forest bird species with reduced dispersal capacity are much less inclined to cross gaps of inhospitable habitat. For a substantial proportion of tropical forest species, constrained gap-crossing ability reduces population connectivity in forested landscapes 24 and constrains recolonization of isolated habitat Populations were classified as fragmentation sensitive if they were identified as 'Forest-core' by BIOFRAG. ...
Species sensitivity to forest fragmentation varies latitudinally, peaking in the tropics. A prominent explanation for this pattern is that historical landscape disturbance at higher latitudes has removed fragmentation-sensitive species or promoted the evolution of more resilient survivors. However, it is unclear whether this so-called extinction filter is the dominant driver of geographic variation in fragmentation sensitivity, particularly because climatic factors may also cause latitudinal gradients in dispersal ability, a key trait mediating sensitivity to habitat fragmentation. Here we combine field survey data with a morphological proxy for avian dispersal ability (hand-wing index) to assess responses to forest fragmentation in 1,034 bird species worldwide. We find that fragmentation sensitivity is strongly predicted by dispersal limitation and that other factors—latitude, body mass and historical disturbance events—have relatively limited explanatory power after accounting for species differences in dispersal. We also show that variation in dispersal ability is only weakly predicted by historical disturbance and more strongly associated with intra-annual temperature fluctuations (seasonality). Our results suggest that climatic factors play a dominant role in driving global variation in the impacts of forest fragmentation, emphasizing the need for more nuanced environmental policies that take into account local context and associated species traits.
... Dispersal ability can also define which species colonize islands (MacArthur and Wilson, 1967;Lees and Gilroy, 2014) or which species are able to persist in fragmented landscapes (Burkey, 1989;Moore et al., 2008;Lees and Peres, 2009). Likewise, population connectivity in fragmented landscapes depends on species dispersal ability, which are mostly unreported for most tropical bird species (Hartfelder et al., 2020;Tourinho et al., 2022;Claramunt et al., 2022). ...
... On the other hand, recent studies have shown that flight capabilities and long-distance flight efficiency likely represent a key aspect for avian dispersal and gap-crossing tendencies (Moore et al., 2008;Ibarra-Macias et al., 2011;Claramunt et al., 2012;Claramunt, 2021;Claramunt et al., 2022). Therefore, flight speed could also represent an important variable, as faster birds may be more prone to successfully cross a riverine barrier than slower birds. ...
... These results are consistent with theoretical expectations of increased long-distance flight efficiency with higher aspect ratio wings (Norberg, 1990;Pennycuick, 2008) and adds to the mounting evidence of the usefulness of the hand-wing index as a proxy for avian dispersal ability (Claramunt et al., 2012(Claramunt et al., , 2022Claramunt and Wright, 2017;Sheard et al., 2020;Claramunt, 2021). On the other hand, we found no effect of body mass on dispersal capacity within our sample. ...
Amazonian rivers represent known barriers for avian dispersal, reducing gene flow and enhancing differentiation. Despite the importance of rivers in the avian evolutionary process, we have made only minor advances in understanding the limitations imposed by rivers on flying birds. To fill that gap, we conducted dispersal-challenge experiments over water, assessing the flying capabilities of 84 tropical bird species of 22 different avian families. We mist-netted and released 484 birds from a stationary boat on the Rio Branco, northern Amazonia, at increasing distances from the shore, including 249 individuals at 100; 219 at 200; 8 at 300; and 5 at 400 m. A successful trial was represented by a bird reaching the riverbank, whereas a failure would refer to birds not reaching the shore and landing on the water, when they were rescued by our team. Our main goal was to understand if the outcome in the experiments could be predicted by (i) phylogenetic constraints, (ii) morphology (body mass and wing shape), (iii) flight speed, (iv) ecological preferences (stratum, habitat, and river-island specialization), and (v) psychological reluctance to fly. Nearly two thirds of the individuals (332) were successful in reaching the riverbank, whereas 152 failed. We found significant differences among lineages. Whereas seven avian families succeeded in all of the trials, two families (antbirds and wrens) were particularly bad dispersers (<40% success). The hand-wing index (HWI) was the single most powerful predictor of trial success. Flying speed was also a significant predictor of success. Overall, ecological attributes had a low explanatory power. Only forest stratum preference had a significant, although weak, effect on dispersal ability: canopy- and ground-dwellers performed better than understory birds. However, we found no effect of habitat preference or river-island specialization on dispersal ability. Our speed estimates for 64 bird species are among the first produced for the tropics and suggest slower flying speeds than those reported from temperate migratory birds. Although birds showed behavioral differences when presented with the opportunity to fly away from the boat, we found no evidence that their reluctance to fly could predict the outcome in the experiments. This represents the first experimental study evaluating the riverine effect through dispersal ability of Amazonian birds, providing important insights to better understand dispersal limitations provided by riverine barriers.
... This observation is further supported by research on 2D habitat configuration that shows greater predation rates from raptors and other predators in more heterogenous habitats compared to more homogenous ones (Preston 1990, Batary andBaldi 2004). Moreover, species with larger HWI tend to have a greater gapcrossing ability (Claramunt et al. 2022), perhaps allowing them to easily maneuver and persist in patchy habitats (but see Jones et al. 2023). Indeed, larger HWI has been found to be more prevalent in areas with increased disturbance (Claramunt et al. 2022, Naka et al. 2022, Weeks et al. 2023. ...
... Moreover, species with larger HWI tend to have a greater gapcrossing ability (Claramunt et al. 2022), perhaps allowing them to easily maneuver and persist in patchy habitats (but see Jones et al. 2023). Indeed, larger HWI has been found to be more prevalent in areas with increased disturbance (Claramunt et al. 2022, Naka et al. 2022, Weeks et al. 2023. However, an alternative explanation for the increase in HWI could be an increase in the presence of migratory insectivorous species, which are known to increase along forest edges (Terraube et al. 2016). ...
Understanding how 3D habitat structure drives biodiversity patterns is key to predicting how habitat alteration and loss will affect species and community-level patterns in the future. To date, few studies have contrasted the effects of three-dimensional (3D) habitat composition with those of 3D habitat configuration on biodiversity, with existing investigations often limited to measures of taxonomic diversity (i.e., species richness). Here, we examined the influence of Light Detecting and Ranging (LiDAR)-derived 3D habitat structure–both its composition and configuration–on multiple facets of bird diversity. Specifically, we used data from the National Ecological Observatory Network (NEON) to test the associations between eleven measures of 3D habitat structure and avian species richness, functional and trait diversity, and phylogenetic diversity. We found that 3D habitat structure was the most consistent predictor of avian functional and trait diversity, with little to no effect on species richness or phylogenetic diversity. Functional diversity and individual trait characteristics were strongly associated with both 3D habitat composition and configuration, but the magnitude and the direction of the effects varied across the canopy, subcanopy, midstory, and understory vertical strata. Our findings suggest that 3D habitat structure influences avian diversity through its effects on traits. By examining the effects of multiple aspects of habitat structure on multiple facets of avian diversity, we provide a broader framework for future investigations on habitat structure.
Posted on 24 May 2024 | The copyright holder is the author/funder. All rights reserved. No reuse without permission. | https://doi.org/10.22541/au.171654107.71445289/v1 | This is a preprint and has not been peer-reviewed. Data may be preliminary.
... However, the importance of dispersal ability likely varies with latitude and may not be a critical constraint for temperate bird species which tend to have broader environmental tolerances [57,73] and often engage in movements such as latitudinal migration, altitudinal migration, and nomadic behaviour [203]. In contrast, tropical bird species tend to have much lower dispersal capability [203], and several studies have shown the limited ability of rain-forest birds to cross even small gaps between forests [204][205][206][207]. Thus, dispersal ability is probably more important in limiting the shift rates of tropical birds, as supported by a meta-analysis of documented elevational shifts [71]. ...
... Across this gradient, lower elevational limits contracted significantly over time, while upper elevational limits were slow to shift, generally moving only within continuous blocks of forest. These results indicate that the isolation of forest blocks limits upslope colonisation of understory birds [204,207]. However, this study was restricted to fragmented forest and did not include a control system of continuous forest. ...
Mountains hold much of the world's taxonomic diversity, but global climate change threatens this diversity by altering the distributions of montane species. While numerous studies have documented upslope shifts in elevational ranges, these patterns are highly variable across geographic regions and taxa. This variation in how species' range shifts are manifesting along elevational gradients likely reflects the diversity of mechanisms that determines eleva-tional ranges and modulates movements, and stands in contrast to latitudinal gradients, where range shifts show less variability and appear more predictable. Here, we review observed elevational range shifts in a single taxonomic group-birds-a group that has received substantial research attention and thus provides a useful context for exploring variability in range shifts while controlling for the mechanisms that drive range shifts across broader taxonomic groups. We then explore the abiotic and biotic factors that are known to define elevational ranges, as well as the constraints that may prevent birds from shifting. Across the literature, temperature is generally invoked as the prime driver of range shifts while the role of precipitation is more neglected. However, temperature is less likely to act directly on elevational ranges, instead mediating biotic factors such as habitat and food availability, predator activity, and parasite prevalence, which could in turn modulate range shifts. Dispersal ability places an intrinsic constraint on elevational range shifts, exacerbated by habitat fragmentation. While current research provides strong evidence for the importance of various drivers of elevational ranges and shifts, testing the relative importance of these factors and achieving a more holistic view of elevational gradients will require integration of expanding datasets, novel technologies, and innovative techniques.
... Rivers have been shown as effective barriers to dispersal in tropical birds (Haffer, 1997;Moncrieff et al., 2024;Naka et al., 2012;Naka et al., 2022) and other taxa, e.g., frogs (Fouquet et al., 2012), primates (Fordham et al., 2020), and butterflies (Rosser et al., 2021). However, the extent to which a river, or any geographic barrier, serves to reduce gene flow likely depends on the dispersal mode or capability of the species of interest (Claramunt et al., 2012;Claramunt et al., 2022;Naka et al., 2022;Nazareno et al., 2021). The goldencollared manakin is capable of flying substantial distances over open water (Moore et al., 2008), and we observed radiotagged females dispersing beyond a kilometer from the lek where they were tagged. ...
Hybrid zones are dynamic systems where natural selection, sexual selection, and other evolutionary forces can act on reshuffled combinations of distinct genomes. The movement of hybrid zones, individual traits, or both are of particular interest for understanding the interplay between selective processes. In a hybrid zone involving two lek-breeding birds, secondary sexual plumage traits of Manacus vitellinus, including bright yellow collar and olive belly color, have introgressed asymmetrically ~50 km across the genomic center of the zone into populations more genetically similar to Manacus candei. Males with yellow collars are preferred by females and are more aggressive than parental M. candei, suggesting that sexual selection was responsible for the introgression of male traits. We assessed the spatial and temporal dynamics of this hybrid zone using historical (1989 - 1994) and contemporary (2017 - 2020) transect samples to survey both morphological and genetic variation. Genome-wide SNP data and several male phenotypic traits show that the genomic center of the zone has remained spatially stable, whereas the olive belly color of male M. vitellinus has continued to introgress over this time period. Our data suggest that sexual selection can continue to shape phenotypes dynamically, independent of a stable genomic transition between species.
... Recently, it has been suggested that birds with low hand-wing indexes, as a proxy of flight efficiency, tend to disperse less frequently across humanmade gaps. This characteristic makes some bird species more vulnerable to the effects of forest fragmentation in the Amazonia, such as Microcerculus bambla, a phylogenetically close species of T. sinaloa (Claramunt et al. 2022). Although estimates of the hand-wing index for T. sinaloa are not available, their wings are round and short, adapted for flying through intricate microhabitats with climbing vines (Malpica et al. 2022). ...
Context
Tropical dry forests (TDFs) are one of the richest and also one of the most threatened ecosystems in the world due to anthropization. In Mexico, only a minimal proportion of TDF is conserved in protected areas, typically surrounded by human-modified landscapes. Habitat modification can impact gene flow, affecting the populations’ genetic structure, and ultimately, the long-term persistence of natural populations.
Objectives
We examined the influence of landscape features on the genetic connectivity of Thryophilus sinaloa, a common and highly territorial TDF-associated bird species. We conducted our study in a Mexican landscape along the Pacific coast characterized by a protected area surrounded by a heterogenous human-modified landscape.
Methods
We genotyped 90 individuals from 20 localities at 24,549 SNPs derived from 3RADseq and de novo assembling techniques to examine the relationship between population genetic patterns and landscape features using a resistance surface optimization framework.
Results
Populations were genetically structured in two groups across the landscape. An open-areas resistance surface, along with geographic distance, reduced genetic connectivity. This finding suggests that protected areas are partially isolated from TDF fragments and other non-protected areas.
Conclusions
Our research highlights the impact of TDF landscape modification by human-induced activities on the genetic connectivity of a common bird. Our study reveals that the only TDF reserve in the region is mostly disconnected from other remnants of non-protected areas of continuous TDF. The increasing deterioration of the habitat could eventually cause a decrease in genetic diversity and effective population size. Moreover, genetic differentiation could be enhanced as habitat patches are more isolated, increasing the likelihood of local extinctions.
... Distance from water sources and human activity intensity were not identified as primary factors influencing nest site selection by magpies and grey magpies in this study. This finding can be attributed to the adaptability and flying abilities of these bird species, their larger activity range (Claramunt et al., 2022), the proximity of Xuanwu Lake (which offers better and easily accessible water supply) to the campus, and their overall strong resource access capabilities and adaptability to urban habitats. ...
Introduction
Understanding the birds’ breeding strategies in urban habitats is vital for ensuring their continued existence. Therefore, more research must be conducted on bird breeding and urban adaptation strategies in urban green spaces. This study aimed to address this gap by investigating the influence of landscape factors on the selection of bird nest sites.
Methods
Data on the presence and absence of magpie (Pica pica) and gray magpie (Cyanopica cyana) nests were collected through field surveys conducted in the campus of Nanjing Forestry University during the 2023 breeding season. Generalized additive models (GAMs) incorporating landscape variables were employed to assess the effects of these predictors on nest occurrence. The model with the lowest Akaike’s information criterion value was selected among the candidate GAMs.
Results
Below is a summary of the main results. Nest tree height (TH), distance from the central lawn (D), and tree coverage (TC) within the sampled area were identified as the primary landscape factors influencing nest site choice. Conversely, factors such as the shortest distance to the water source, herb coverage, shrub coverage, percentage of buildings, and percentage of hard pavement did not significantly impact on nest site selection. Furthermore, the nesting potential of magpies and grey magpies initially increased with tree height, reaching a maximum at ca. TH=25 meters after which it began to decline. The nesting occurrence rate showed an initial decrease tendency with increasing distance from the central lawn, reaching a minimum at D=400 meters, and then increased with further distance. Additionally, nesting potential decreased initially with increasing of TC in the range of 0–20%, fluctuated evenly between 20–60% TC, and decreased rapidly when TC exceeded 60%.
Discussion
This study provides valuable insights into the selection of nest sites by birds in urban habitats, specifically with respect to landscape factors. The understanding of the impact of urban green spaces on urban birds and the underlying mechanisms of their behavior contributes to the conservation of wild birds and promotes the harmonious development of urban areas.
... This positive feedback loop is mediated through relatively poor dispersal capacity in these birds, the cost (tradeoff) of the low metabolic rate plus wings and tails specialized for foraging efficiency at the cost of flight efficiency. Poor dispersal capacity in these tropical birds is a severe handicap in humanfragmented tropical landscapes, contributing to widespread declines in tropical insectivorous birds (Sherry, 2021;Claramunt et al., 2022). The rampant habitat loss of tropical forests inevitably fragments them; and agricultural pesticides and other human activities further degrade what habitats remain . ...
Ecological niches are pivotal in addressing questions of species richness gradients like the Latitudinal Diversity Gradient (LDG). The Hutchinsonian niche hypervolume model and derivatives are some of the most proven tools. Accordingly, species occupy mathematically convenient spaces in relation to functional, especially trophic, relationships, as well as the physical environment. In one application, the number of species in a community is a function of average niche sizes, overlaps, and total niche volume. Alternatively, the number of coexisting species derives from invasibility criteria in relation to species-interaction modules. The daunting complexity of tropical communities begs the question of how well these ecologically inspired paradigms accommodate present knowledge of species interactions and functional relationships. Recent studies of hyperdiverse tropical insectivorous bird species suggests reevaluating the applicability of such concepts. Here I review Neotropical, arthropod-feeding bird species interactions needed to explain these species’ trophic relationships, including their diets, feeding substrates, and behavioral and morphological traits relevant to resource acquisition. Important emergent generalizations include extraordinary specializations on both prey resource locations (substrates) and behaviors, rather than on particular resources per se, and a preponderance of adaptations to exploit the anti-predator traits of prey, traits evolved in response to other predators. These specializations and implicit arms races necessitate evolutionary approaches to niches necessary to understand the relevant natural history and ecology, how these species compete interspecifically, and even how these predator species interact with prey via evolutionary enhancements. These findings, compared and contrasted with prevailing concepts and findings, suggest expanding niche concepts to accommodate both the large temporal and regional geographic scales to understand the accumulated species richness of the mainland Neotropics. These trophic specializations also highlight why many of these birds are so sensitive to human disturbances, especially habitat loss, fragmentation, and degradation.
... Rivers have been shown as effective barriers to dispersal in tropical birds (Haffer, 1997;Naka, Bechtoldt, Henriques, & Brumfield, 2012;Naka, Costa, Lima, & Claramunt, 2022) and other taxa, e.g., frogs (Fouquet et al. 2012), primates (Fordham, Shanee, & Peck, 2020), butterflies (Rosser, Shirai, Dasmahapatra, Mallet, & Freitas, 2021). However, the extent that a river, or any geographic barrier, serves to reduce gene flow likely depends on the dispersal mode or capability of the species of interest (Claramunt, Derryberry, Remsen, & Brumfield, 2012;Claramunt, Hong, & Bravo, 2022;Naka et al., 2022;Nazareno, Knowles, Dick, & Lohmann, 2021). Additionally, dispersal is highly predictive of hybrid zone widths (McEntee, Burleigh, & Singhal, 2020). ...
A bstract
Hybrid zones are dynamic systems where natural selection, sexual selection, and other evolutionary forces can act on reshuffled combinations of distinct genomes. The movement of hybrid zones, individual traits, or both are of particular interest for understanding the interplay between selective processes. In a hybrid zone involving two lek-breeding birds, secondary sexual plumage traits of Manacus vitellinus , including bright yellow collar and olive belly color, have introgressed asymmetrically ∼50 km across the genomic center of the zone into populations more genetically similar to Manacus candei . Males with yellow collars are preferred by females and are more aggressive than parental M. candei , suggesting that sexual selection was responsible for the introgression of male traits. We assessed the spatial and temporal dynamics of this hybrid zone using historical (1989 - 1994) and contemporary (2017 - 2020) transect samples to survey both morphological and genetic variation. Genome-wide SNP data and several male phenotypic traits show that the genomic center of the zone has remained spatially stable, whereas the olive belly color of male M. vitellinus has continued to introgress over this time period. Our data suggest that sexual selection can continue to shape phenotypes dynamically, independent of a stable genomic transition between species.
... Additionally, experimental dispersal challenges over water are now available for some Neotropical birds (Moore et al., 2008;Naka et al., 2022). Claramunt et al. (2022) found that wing morphology was a significant predictor of overwater flight ability and Weeks et al. (2022) found a significant association between flight ability and natal dispersal distance in a phylogenetically diverse set of 114 bird species, supporting wing measurements from research specimens as a proxy for dispersal. ...
Understanding the factors that govern variation in genetic structure across species is key to the study of speciation and population genetics. Genetic structure has been linked to several aspects of life history, such as foraging strategy, habitat association, migration distance, and dispersal ability, all of which might influence dispersal and gene flow. Comparative studies of population genetic data from species with differing life histories provide opportunities to tease apart the role of dispersal in shaping gene flow and population genetic structure. Here, we examine population genetic data from sets of bird species specialized on a series of Amazonian habitat types hypothesized to filter for species with dramatically different dispersal abilities: stable upland forest, dynamic floodplain forest, and highly dynamic riverine islands. Using genome-wide markers, we show that habitat type has a significant effect on population genetic structure, with species in upland forest, floodplain forest, and riverine islands exhibiting progressively lower levels of structure. Although morphological traits used as proxies for individual-level dispersal ability did not explain this pattern, population genetic measures of gene flow are elevated in species from more dynamic riverine habitats. Our results suggest that the habitat in which a species occurs drives the degree of population genetic structuring via its impact on long-term fluctuations in levels of gene flow, with species in highly dynamic habitats having particularly elevated gene flow. These differences in genetic variation across taxa specialized in distinct habitats may lead to disparate responses to environmental change or habitat-specific diversification dynamics over evolutionary time scales.
Two recent studies come to different yet complementary conclusions about the factors — species traits, climate conditions and past disturbances — that determine the responses of bird species to forest loss and fragmentation.
Dispersal is a fundamental process in evolution and ecology. Due to the predominant role of flight in bird movement, their dispersal capabilities can be estimated from their flight morphology. Most predictors of flight efficiency require an estimate of the total wing area, but the existing methods for estimating wing area are multi‐stepped and prone to compounding error. Here, we validated a new method for estimating the total wing area that requires only the measurement of the wingspan plus two measurements from the folded wings of study skin specimens: wing length and wing width. We demonstrate that the new folded‐wing method estimates total wing area with high precision across a variety of avian groups and wing shapes. In addition, the new method performs as well as the old method when used to estimate natal dispersal distances of North American birds. The folded‐wing method will allow for estimates of the total wing to be readily obtained from thousands of specimens in ornithological collections, thus providing critical information for studies of flight and dispersal in birds.
Bird–plant seed‐dispersal networks are structural components of ecosystems. The role of bird species in seed‐dispersal networks (from less [peripheral] to more connected [central]), determines the interaction patterns and their ecosystem services. These roles may be driven by morphological and functional traits as well as evolutionary, geographical and environmental properties acting at different spatial extents.
It is still unknown if such drivers are equally important in determining species centrality at different network levels, from individual local networks to the global meta‐network representing interactions across all local networks.
Using 308 networks covering five continents and 11 biogeographical regions, we show that at the global meta‐network level species' range size was the most important driver of species centrality, with more central species having larger range sizes, which would facilitate the interaction with a higher number of plants and thus the maintenance of seed‐dispersal interactions.
At the local network level, body mass was the only driver with a significant effect, implying that local factors related to resource availability are more important at this level of network organisation than those related to broad spatial factors such as range sizes. This could also be related to the mismatch between species‐level traits, which do not consider intraspecific variation, and the local networks that can depend on such variation.
Taken together, our results show that the drivers determining species centrality are relative to the levels of network organisation, suggesting that prediction of species functional roles in seed‐dispersal interactions requires combined local and global approaches.
As human density increases, biodiversity must increasingly co-exist with urbanization or face local extinction. Tolerance of urban areas has been linked to numerous functional traits, yet few globally consistent patterns have emerged to explain variation in urban tolerance, which stymies attempts at a generalizable predictive framework. Here, we calculate an Urban Association Index (UAI) for 3,768 bird species in 137 cities across all permanently inhabited continents. We then assess how this UAI varies as a function of ten species-specific traits and further test whether the strength of trait relationships vary as a function of three city-specific variables. Of the ten species traits, nine were significantly associated with urban tolerance. Urban-associated species tend to be smaller, less territorial, have greater dispersal ability, broader dietary and habitat niches, larger clutch sizes, greater longevity, and lower elevational limits. Only bill shape showed no global association with urban tolerance. Additionally, the strength of several trait relationships varied across cities as a function of latitude and/or human population density. For example, the associations of body mass and diet breadth were more pronounced at higher latitudes, while the associations of territoriality and longevity were reduced in cities with higher population density. Thus, the importance of trait filters in birds varies predictably across cities, indicating biogeographic variation in selection for urban tolerance that could explain prior challenges in the search for global patterns. A globally informed framework that predicts urban tolerance will be integral to conservation as increasing proportions of the world's biodiversity are impacted by urbanization.
Natal dispersal—the movement from birth site to first breeding site—determines demographic and population genetic dynamics and has important consequences for ecological and evolutionary processes. Recent work suggested that one of the main factors determining natal dispersal distances is the cost of locomotion. We evaluated this hypothesis using band recovery data to estimate natal dispersal distances for 50 North American bird species. We then analyzed the relationships between dispersal distances and a suite of morphological and ecological predictors, including proxies for the cost of locomotion (flight efficiency), using phylogenetic regression models. We found that flight efficiency, population size, and habitat influence natal dispersal distances. We discuss how the effects of population size and habitat can also be related to mobility and locomotion. Our findings are consistent with a predominant effect of adaptations for mobility on dispersal distances. Dispersal is fundamental to many processes in ecology and evolution. We evaluated the hypothesis that the cost of movement is most influential in dispersal process in North American birds. We found that flight efficiency, population size, and habitat influence natal dispersal distance, although the individual effects are difficult to disentangle.
Distribution data from the North American Breeding Bird Survey shows an increasing mismatch between species distribution and climate.
The factors responsible for variation in dispersal distances across species remain poorly understood. Previous comparative studies found differing results and equivocal support for theoretical predictions. Here I re‐examine factors that influence natal dispersal distances in British birds while taking into account the cost of transport as estimated from proxies of long‐distance flight efficiency. First, I show that flight efficiency, as estimated by the hand‐wing index, the aspect ratio, or the lift‐to‐drag ratio, is a strong predictor of dispersal distances among resident species. Most migratory species showed a similar pattern, but a group of species with relatively low aerodynamic efficiency showed longer‐than‐expected dispersal distances, making the overall trend independent of flight efficiency. Ecological, behavioral, and life history factors had a small or nil influence on dispersal distances, with most of their influence likely mediated by adaptations for the use of space reflected in flight efficiency. This suggests that dispersal distances in birds are not determined by adaptive strategies for dispersal per se, but are predominantly influenced by the energetic cost of movement.
Brazil has a monitoring system to track annual forest conversion in the Amazon and most recently to monitor the Cerrado biome. However, there is still a gap of annual land use and land cover (LULC) information in all Brazilian biomes in the country. Existing countrywide efforts to map land use and land cover lack regularly updates and high spatial resolution time-series data to better understand historical land use and land cover dynamics, and the subsequent impacts in the country biomes. In this study, we described a novel approach and the results achieved by a multi-disciplinary network called MapBiomas to reconstruct annual land use and land cover information between 1985 and 2017 for Brazil, based on random forest applied to Landsat archive using Google Earth Engine. We mapped five major classes: forest, non-forest natural formation, farming, non-vegetated areas, and water. These classes were broken into two sub-classification levels leading to the most comprehensive and detailed mapping for the country at a 30 m pixel resolution. The average overall accuracy of the land use and land cover time-series, based on a stratified random sample of 75,000 pixel locations, was 89% ranging from 73 to 95% in the biomes. The 33 years of LULC change data series revealed that Brazil lost 71 Mha of natural vegetation, mostly to cattle ranching and agriculture activities. Pasture expanded by 46% from 1985 to 2017, and agriculture by 172%, mostly replacing old pasture fields. We also identified that 86 Mha of the converted native vegetation was undergoing some level of regrowth. Several applications of the MapBiomas dataset are underway, suggesting that reconstructing historical land use and land cover change maps is useful for advancing the science and to guide social, economic and environmental policy decision-making processes in Brazil.
An organism’s ability to disperse influences many fundamental processes, from speciation and geographical range expansion to community assembly. However, the patterns and underlying drivers of variation in dispersal across species remain unclear, partly because standardised estimates of dispersal ability are rarely available. Here we present a global dataset of avian hand-wing index (HWI), an estimate of wing shape widely adopted as a proxy for dispersal ability in birds. We show that HWI is correlated with geography and ecology across 10,338 (>99%) species, increasing at higher latitudes and with migration, and decreasing with territoriality. After controlling for these effects, the strongest predictor of HWI is temperature variability (seasonality), with secondary effects of diet and habitat type. Finally, we also show that HWI is a strong predictor of geographical range size. Our analyses reveal a prominent latitudinal gradient in HWI shaped by a combination of environmental and behavioural factors, and also provide a global index of avian dispersal ability for use in community ecology, macroecology, and macroevolution. In birds, the hand-wing index is a morphological trait that can be used as a proxy for flight efficiency. Here the authors examine variation of hand-wing index in over 10,000 bird species, finding that it is higher in migratory and non-territorial species, and lower in the tropics.
In a rapidly changing world, it is important to understand how environmental modifications by humans affect species behavior. This is not a simple task, since we need to deal with a multitude of species and the different external contexts that affect their behavior. Here, we investigate how interpatch short‐distance movements of 73 common forest bird species can be predicted by forest cover and forest isolation. We modeled bird movement as a function of environmental covariates, species traits – body mass and feeding habit – and phylogenetic relationships using Joint Species Movement Models. We used field data collected in forest edges and open pastures of six 600 × 600 m plots in the Atlantic Forest biodiversity hotspot. We found that birds fly larger distances and visit more forest patches and remnant trees with decreasing forest cover. Increasing landscape isolation results in larger flight distances, and it increases the use of trees as stepping‐stones for most species. Our results show that birds can adjust their behavior as a response to spatial modification in resource distribution and landscape connectivity. These adjusted behaviors can potentially contribute to ecosystem responses to habitat modification.
Background:
The drivers of space use patterns of multi-species groups have been poorly studied, although mixed-species avian flocks are common throughout the world. In a mixed-species flock, multiple species move together and maintain proximity. The different species may or may not have conflicting preferences of space use. We hypothesized that the space use patterns of the flock are driven by a single species.
Methods:
We investigated the behavioral drivers of space use patterns of mixed-species flocks in Amazonian Peru by mapping 95% fixed-kernel home ranges of three flocks, which then we divided into high-use (inner 55% kernel utilization distribution) and low-use areas (lying outside the high-use area). We quantified the foraging and anti-predator behavior of individual birds in the flocks. We tested whether foraging and anti-predator behavior of different species were different in high use and low use areas of the flock.
Results:
We collected 455 spatial points and 329 foraging and anti-predator behavior observations on three flocks. The single best model for explaining the space use patterns of the flocks contained only vegetation density that surrounded Dusky-throated Antshrikes.
Conclusion:
The results are consistent with the hypothesis that a single species in mixed-species flocks has a disproportionately large influence on space use patterns. The surrounding vegetation density of the Dusky-throated Antshrike was the only driver of space use patterns of flocks supported by our data. The results may apply to flocks pantropically, many of which are led by species that behave similarly to the Dusky-throated Antshrike, e.g. Asian flocks led by drongos (Dicrurus spp.).
From butterflies to elephants, the rapidly developing science of movement ecology is providing increasingly detailed spatio-temporal data on a wide array of mobile animals. Thus, this discipline also holds great promise for improving the conservation of wildlife. To measure progress toward this promise, we investigated the degree to which movement ecology research is connected to conservation goals as well as the proportion of studies that were incorporated into federal and international status assessments for mobile species at risk. We examined 13,349 “movement ecology” papers published between 1990 and 2014 and found that explicit connections to conservation and management were made in 35% (n = 4, 672) of these papers, with the number of connections increasing over time. We then measured the uptake of movement ecology research into species status assessment and recovery plans (n = 72 documents) produced by three different governance agencies for 12 endangered mobile species. We found that on average 60% of available movement ecology research was used in the status assessment process, demonstrating that when movement ecology research is available, it is generally being utilized in conservation planning. However, for 25% of these species, there was little movement research available to be used, highlighting that knowledge gaps remain for some at-risk species despite the general growth of movement ecology research. We outline opportunities for movement ecology to promote more effective conservation of taxa that move.
Secondary forest habitats are increasingly recognized for their potential to conserve biodiversity in the tropics. However, the development of faunal assemblages in secondary forest systems varies according to habitat quality and species‐specific traits. In this study, we predicted that the recovery of bird assemblages is dependent on secondary forest age and level of isolation, the forest stratum examined, and the species’ traits of feeding guild and body mass. This study was undertaken in secondary forests in central Panama; spanning a chronosequence of 60‐, 90‐, and 120‐year‐old forests, and in neighboring old‐growth forest. To give equal attention to all forest strata, we employed a novel method that paired simultaneous surveys in canopy and understory. This survey method provides a more nuanced picture than ground‐based studies, which are biased toward understory assemblages. Bird reassembly varied according to both habitat age and isolation, although it was challenging to separate these effects, as the older sites were also more isolated than the younger sites. In combination, habitat age and isolation impacted understory birds more than canopy‐dwelling birds. Proportions of dietary guilds did not vary with habitat age, but were significantly different between strata. Body mass distributions were similar across forest ages for small‐bodied birds, but older forest supported more large‐bodied birds, probably due to control of poaching at these sites. Canopy assemblages were characterized by higher species richness, and greater variation in both dietary breadth and body mass, relative to understory assemblages. The results highlight that secondary forests may offer critical refugia for many bird species, particularly specialist canopy‐dwellers. However, understory bird species may be less able to adapt to novel and isolated habitats and should be the focus of conservation efforts encouraging bird colonization of secondary forests.
An intact forest landscape (IFL) is a seamless mosaic of forest and naturally treeless ecosystems with no remotely detected signs of human activity and a minimum area of 500 km². IFLs are critical for stabilizing terrestrial carbon storage, harboring biodiversity, regulating hydrological regimes, and providing other ecosystem functions. Although the remaining IFLs comprise only 20% of tropical forest area, they account for 40% of the total aboveground tropical forest carbon. We show that global IFL extent has been reduced by 7.2% since the year 2000. An increasing rate of global IFL area reduction was found, largely driven by the tripling of IFL tropical forest loss in 2011–2013 compared to that in 2001–2003. Industrial logging, agricultural expansion, fire, and mining/resource extraction were the primary causes of IFL area reduction. Protected areas (International Union for Conservation of Nature categories I to III) were found to have a positive effect in slowing the reduction of IFL area from timber harvesting but were less effective in limiting agricultural expansion. The certification of logging concessions under responsible management had a negligible impact on slowing IFL fragmentation in the Congo Basin. Fragmentation of IFLs by logging and establishment of roads and other infrastructure initiates a cascade of changes that lead to landscape transformation and loss of conservation values. Given that only 12% of the global IFL area is protected, our results illustrate the need for planning and investment in carbon sequestration and biodiversity conservation efforts that target the most valuable remaining forests, as identified using the IFL approach.
Migratory flight performance has direct or carry-over effects on fitness. Therefore, selection is expected to act on minimizing the costs of migratory flight, which increases with the distance covered. Aerodynamic theory predicts how morphological adaptations improve flight performance. These predictions have rarely been tested in comparative analyses that account for scaling and phylogenetic effects. We amassed a unique dataset of 149 European bird species and 10 morphological traits. Mass-adjusted aspect ratio increased, while mass-adjusted heart weight and wing loading decreased with increasing migration distance. These results were robust to whether the analyses were based on the entire species pool or limited to passerines or migrants. Our findings indicate that selection due to migration acts on wing traits that reduce the energetic cost of transportation to increase the flight range. Consequently, the demands for high ‘exercise organ’ performance might be low, and hence such energetically expensive tissues are not associated (pectoral muscle) or are inversely associated (heart) with long migration distance.
Dispersal knowledge is essential for conservation management, and demand is growing. But are we accumulating dispersal knowledge at a pace that can meet the demand? To answer this question we tested for changes in dispersal data collection and use over time. Our systematic review of 655 conservation-related publications compared five topics: climate change, habitat restoration, population viability analysis, land planning (systematic conservation planning) and invasive species. We analysed temporal changes in the: (i) questions asked by dispersal-related research; (ii) methods used to study dispersal; (iii) the quality of dispersal data; (iv) extent that dispersal knowledge is lacking, and; (v) likely consequences of limited dispersal knowledge. Research questions have changed little over time; the same problems examined in the 1990s are still being addressed. The most common methods used to study dispersal were occupancy data, expert opinion and modelling, which often provided indirect, low quality information about dispersal. Although use of genetics for estimating dispersal has increased, new ecological and genetic methods for measuring dispersal are not yet widely adopted. Almost half of the papers identified knowledge gaps related to dispersal. Limited dispersal knowledge often made it impossible to discover ecological processes or compromised conservation outcomes. The quality of dispersal data used in climate change research has increased since the 1990s. In comparison, restoration ecology inadequately addresses large-scale process, whilst the gap between knowledge accumulation and growth in applications may be increasing in land planning. To overcome apparent stagnation in collection and use of dispersal knowledge, researchers need to: (i) improve the quality of available data using new approaches; (ii) understand the complementarities of different methods and; (iii) define the value of different kinds of dispersal information for supporting management decisions. Ambitious, multi-disciplinary research programs studying many species are critical for advancing dispersal research.
Integrated, efficient, and global prioritization approaches are necessary to manage the ongoing loss of species and their associated function. "Evolutionary distinctness" measures a species' contribution to the total evolutionary history of its clade and is expected to capture uniquely divergent genomes and functions. Here we demonstrate how such a metric identifies species and regions of particular value for safeguarding evolutionary diversity.
Among the world's 9,993 recognized bird species, evolutionary distinctness is very heterogeneously distributed on the phylogenetic tree and varies little with range size or threat level. Species representing the most evolutionary history over the smallest area (those with greatest "evolutionary distinctness rarity") as well as some of the most imperiled distinct species are often concentrated outside the species-rich regions and countries, suggesting they may not be well captured by current conservation planning. We perform global cross-species and spatial analyses and generate minimum conservation sets to assess the benefits of the presented species-level metrics. We find that prioritizing imperiled species by their evolutionary distinctness and geographic rarity is a surprisingly effective and spatially economical way to maintain the total evolutionary information encompassing the world's birds. We identify potential conservation gaps in relation to the existing reserve network that in particular highlight islands as effective priority areas.
The presented distinctness metrics are effective yet easily communicable and versatile tools to assist objective global conservation decision making. Given that most species will remain ecologically understudied, combining growing phylogenetic and spatial data may be an efficient way to retain vital aspects of biodiversity.
Seasonal movements are common in Neotropical forest birds. Species that engage in the largest movements, altitudinal and intratropical migrants, are predominantly frugivores or nectarivores of canopy/edge or dry habitats. Migration by such species may be necessitated by the high spatial and temporal variation in their resource base. We hypothesize that seasonal movements within the tropics predisposed these birds to migration out of the tropics. Indeed, most Nearctic passerine migrants are drawn from Neotropical taxa that exhibit altitudinal or intratropical migration. Conversely, taxa comprising the most sedentary group of Neotropical residents (understory insectivores) are poorly represented among Nearctic migrants. Further evidence of an evolutionary link between seasonal movements within the tropics and long-distance migration is provided by similarities in habitat use and diet between overwintering Nearctic migrants and closely related tropical residents. Both groups tend to be frugivorous/nectarivorous, inhabit "open" habitat, and engage in seasonal movements. The only difference in their movements is one of scale. These results help explain the behavior and ecology of Nearctic birds overwintering in the Neotropics.
This article considers the statistical issues relevant to the comparative method in evolutionary biology. A generalized Linear model (GLM) is presented for the analysis of comparative data, which can be used to address questions regarding the relationship between traits or between traits and environments, the rate of phenotypic evolution, the degree of phylogenetic effect, and the ancestral state of a character. Our approach thus emphasizes the similarity among evolutionary questions asked in comparative studies. We then discuss ways of specifying the sources of error involved in a comparative study (e.g., measurement error, error due to evolution along a phylogeny, error due to misspecification of a phylogeny) and show how the impact of these sources of error can be taken into account in a comparative analysis. In contrast to most existing phylogenetic comparative methods, our procedure offers substantial flexibility in the choice of microevolutionary assumptions underlying the statistical analysis, allowing researchers to choose assumptions that are most appropriate for their particular set of data and evolutionary question. In developing the approach, we also propose novel ways of incorporating within-species variation and/or measurement error into phylogenetic analyses, of estimating ancestral states, and of considering both continuous (quantitative) and categorical (qualitative or ''state'') characters in the same analysis.
Conservation of Tropical Birds has been written by four conservation biologists whose expertise spans all the tropical regions of the world. It is the first book to cover all the major issues in tropical bird conservation. Current problems faced by tropical bird conservationists are summarised and potential solutions outlined based on the results of case studies. Birds are key indicators of ecosystem health, and such a well-studied group of organisms, that they provide an excellent lens through which to examine global conservation problems caused by phenomena such as climate change, declines in ecosystem services, habitat loss, fires, overexploitation, and invasive species. Therefore, the book also provides an engaging synopsis of the general issues in conservation and the problems faced by other wildlife. This book serves as an important resource and companion to all people interested in observing and conserving birds in the tropics and elsewhere. © 2011 Navjot S. Sodhi, Cagan H. Sekercioglu, Jos Barlow, Scott K. Robinson.
Tropical forests are being lost at an alarming rate. Studies from various tropical locations report losses of forest birds as possibly direct or indirect results of deforestation. Although it may take a century for all the sensitive species to be ex-tirpated from a site following habitat loss, species with larger or heavier bodies and those foraging on insects, fruits, or both are particularly extinction prone. Larger-or heavier-bodied species may occur at low densities, increasing their vulnerability to habitat alterations. Insectivores are vulnerable for reasons such as the loss of preferred microhabitats, poor dispersal abilities, and/or ground nesting habits that make them susceptible to predation. The lack of year-round availability of fruits may make survival in deforested or fragmented areas difficult for frugivores. Extirpation of large preda-tors, superior competitors, pollinators, and seed dispersers may have repercussions for tropical ecosystem functioning. Large tropical reserves that adequately protect existing forest avifauna are needed. Sound ecological knowledge of tropical forest avifauna for biodiversity-friendly forest management practices is also needed but sorely lacking.
Aim The aim of this study is to answer the questions: (1) do small organisms disperse farther than large, or vice versa; and (2) does the observed pattern differ for passive and active dispersers? These questions are central to several themes in biogeography (including microbial biogeography), macroecology, metacommunity ecology and conservation biology.
Location The meta-analysis was conducted using published data collected worldwide.
Methods We collected and analysed 795 data values in the peer-reviewed literature for direct observations of both maximal dispersal distance and mass of the dispersing organisms (e.g. seeds, not trees). Analysed taxa ranged in size from bacteria to whales. We applied macroecology analyses based on null models (using Monte Carlo randomizations) to test patterns relative to specific hypotheses.
Results Collected dispersal distance and mass data spanned 9 and 21 orders of magnitude, respectively. Active dispersers dispersed significantly farther (P < 0.001) and were significantly greater in mass (P < 0.001) than passive dispersers. Overall, size matters: larger active dispersers attained greater maximum observed dispersal distances than smaller active dispersers. In contrast, passive-disperser distances were random with respect to propagule mass, but not uniformly random, in part due to sparse data available for tiny propagules.
Conclusions Size is important to maximal dispersal distance for active dispersers, but not for passive dispersers. Claims that microbes disperse widely cannot be tested by current data based on direct observations of dispersal: indirect approaches will need to be applied. Distance–mass relationships should contribute to a resolution of neutral and niche-based metacommunity theories by helping scale expectations for dispersal limitation. Also, distance–mass relationships should inform analyses of latitudinal species richness and conservation biology topics such as fragmentation, umbrella species and taxonomic homogenization.
Translocation experiments showed that a woodcreeper bird species is able to move between isolated forest fragments, but this ability is limited by increasing interpatch distances. Larger distances (> 100 m) were overcome by using small stepping-stones (isolated trees), which enhance connectivity and are useful for the species conservation in fragmented landscapes.
Experimentos de translocação demonstraram que uma espécie de arapaçu é capaz de mover-se entre fragmentos florestais, mas essa habilidade é limitada por seu isolamento. Distâncias maiores que 100 m foram vencidas com o uso de árvores isoladas, as quais aumentam a conectividade e são úteis para a conservação dessa espécie em paisagens fragmentadas.
In fragmented landscapes, species persistence within isolated habitat patches is governed by a myriad of species life-history, habitat patch and landscape characteristics. We investigated the inter-specific variation in non-forest gap-crossing abilities of an entire tropical forest-dependent avifauna. We then related this measure of dispersal ability to species life-history characteristics and occupancy data from 31 variable-sized forest patches sampled within the same fragmented forest landscape. A total of 5436 gap-crossing movements of 231 forest-dependent bird species were observed across ten linear forest gaps of varying widths, adjacent to large areas of undisturbed forest. Species persistence in isolated fragments was strongly linked to gap-crossing ability. The most capable gap-crossers were medium to large-bodied species in the large insectivore, frugivore and granivore guilds, matching the most prevalent subset of species in small forest patches. However, some competent gap-crossing species failed to occur in small patches, and minimum forest-patch area requirements were more important in determining patch occupancy for these species. Narrow forest gaps (4–70 m) created by roads and power-lines may become territory boundaries, thereby eliminating home-range gap-crossing movements for many forest species, but permit rarer dispersal events. Wider gaps (>70 m) may inhibit gap-crossing behaviour for all but the most vagile species. Although patch size and quality may be the most important factors in structuring species assemblages in forest fragments, our results show that the degree of patch isolation and permeability of the surrounding matrix also explain which species can persist in forest isolates. Reducing the number and width of forest-dividing gaps; maintaining and/or creating forest corridors and increasing matrix permeability through the creation and maintenance of ‘stepping-stone’ structures will maximise the species retention in fragmented tropical forest landscapes.
The fact that species vary in their vulnerability to extinction is well documented, but the reasons for these differences remain poorly understood. Why should some species/families/guilds decline rapidly with increasing anthropogenic disturbance, while others either tolerate or proliferate in disturbed habitats? We investigated the bird species composition in 31 primary forest patches of varying size in a region of the Amazonian ‘Arc of Deforestation’ and assessed which species life-history traits predisposed individual species to extinction. Medium-sized non-flocking canopy frugivores/ominvores of low primary forest dependence were least likely to go extinct in small patches, while small-bodied flock-following primary-forest-dependent terrestrial insectivores were most fragmentation sensitive. We found highly idiosyncratic relationships between the minimum size of forest patches occupied by different species and their territory size requirements estimated based on other Amazonian studies. This suggests that avian assemblages in forest fragments primarily comprise species that either have good dispersal abilities or are highly tolerant to the non-forest matrix, rather than those whose minimum spatial requirements can be met by the size of available forest fragments.
The effects of habitat gaps on breeding success and parental daily energy expenditure (DEE) were investigated in great tits
(Parus major) and blue tits (Cyanistes caeruleus) in urban parkland (Cardiff, UK) compared with birds in deciduous woodland (eastern England, UK). Tree canopy height, the
percentage of gap in the canopy and the percentage of oak (in the wood only) within a 30m radius of nest boxes were obtained
from airborne remote-sensed data. Breeding success was monitored and parental DEE (great tits: both habitats; blue tits: park
only) was measured using doubly labelled water in birds feeding young. In the park, mean (±SD) tree height (7.5±4.7m) was
less than in the wood (10.6±4.5m), but the incidence of gaps (32.7±22.6%) was greater (9.2±14.7%). Great tits and blue
tits both reared fewer young in the park and chick body mass was also reduced in park-reared great tits. Park great tits had
a higher DEE (86.3±12.3kJday−1) than those in the wood (78.0±11.7kJday−1) and, because of smaller brood sizes, worked about 64% harder for each chick reared. Tits in the park with more than about
35% gap around their boxes had higher DEEs than the average for the habitat. In the wood, great tits with less oak around
their boxes worked harder than average. Thus structural gaps, and functional gaps generated by variation in the quality of
foraging habitat, increased the costs of rearing young.
The current worldwide concern about tropical deforestation raises questions about the sustainability of avian populations in isolated forest fragments. One of the most important issues concerns the sizes of forest fragments necessary to maintain populations and the genetic variation within them. We address this by: (1) using mtDNA sequence variation to infer aspects of the population structure of four species of understory birds from four sites in southern Costa Rican rainforest; and (2) determining whether forest fragmentation that has occurred in the last 50years has had an effect on the amount of within-population variation for the species in question. High levels of between-population differentiation (D
xy
) were found over a relatively small geographic scale (Henicorhina leucosticta), bicolored antbird (Gymnopithys leucaspis), and gray-headed tanager (Eucometis penicillata), suggesting that these species are highly sedentary and exhibit strong female philopatry. No mtDNA variation was found in Plain Antvireo (Dysithamnus mentalis). In all three of the polymorphic species there was a significant decrease in mtDNA nucleotide diversity in populations isolated by forest fragmentation as compared to populations in contiguous primary forest. Even in relatively large (250–1000ha) forest reserves, sedentary avian species have lost roughly half (range 43–85) of the nucleotide diversity in mtDNA over a relatively short period of time. Our results indicate that sedentary avian species in forest fragments isolated by clearing have undergone severe reductions in effective population size due to population bottlenecks perpetuated by prolonged isolation and potential edge effects.
Birds living in fragmented habitat may occupy territories comprising more than one patch. This paper uses a theoretical model to investigate the costs (in terms of time and energy) of crossing gaps between patches for birds feeding young in the nest, using the great tit (Parus major) as an example. When the proportion of foraging trips involving gap-crossing was small (25%), gaps of about 300–550m (depending on body mass and flight speed) could be crossed without exceeding likely maximum sustainable daily energy expenditure (DEEmax). However, a penalty of time lost in crossing gaps of about one hour was incurred. For more gap-crossing (due to larger brood size and/or a greater proportion of gap-crossing trips), distances that could be crossed decreased rapidly to about 50–100m and time lost increased to more than six hours. Crossing gaps at maximum range speed, rather than at the slower minimum power speed, reduced flight times by 42% and slightly reduced overall daily energy expenditure because the higher flight costs per minute were more than off-set by the shorter flight times. Smaller body mass (17g versus 19g) was advantageous for gap-crossing, the distances which could be crossed without exceeding DEEmax being almost doubled for the smaller mass. The influence of changes in wing morphology, fat load and prey load size on the energetics of gap-crossing were also considered. Although the model was constructed for a woodland bird, problems of time and energy expenditure associated with gap-crossing will affect many species which exploit patchy resources, especially when the spacing of the patches increases, for example due to habitat loss and modification. In landscapes where semi-natural habitat is highly fragmented and most surviving patches are small (e.g., many farming landscapes) the costs of multiple patch use may represent another mechanism by which habitat fragmentation reduces the reproductive potential of the inhabitants of habitat patches which are of acceptable or even good quality, but are small.
Dispersal can stimulate speciation by facilitating geographical expansion across barriers or inhibit speciation by maintaining gene flow among populations. Therefore, the relationship between dispersal ability and speciation rates can be positive or negative. Furthermore, an 'intermediate dispersal' model that combines positive and negative effects predicts a unimodal relationship between dispersal and diversification. Because both dispersal ability and speciation rates are difficult to quantify, empirical evidence for the relationship between dispersal and diversification remains scarce. Using a surrogate for flight performance and a species-level DNA-based phylogeny of a large South American bird radiation (the Furnariidae), we found that lineages with higher dispersal ability experienced lower speciation rates. We propose that the degree of fragmentation or permeability of the geographical setting together with the intermediate dispersal model are crucial in reconciling previous, often contradictory findings regarding the relationship between dispersal and diversification.
1. Informative Bayesian priors can improve the precision of estimates in ecological studies or estimate parameters for which little or no information is available. While Bayesian analyses are becoming more popular in ecology, the use of strongly informative priors remains rare, perhaps because examples of informative priors are not readily available in the published literature.
2. Dispersal distance is an important ecological parameter, but is difficult to measure and estimates are scarce. General models that provide informative prior estimates of dispersal distances will therefore be valuable.
3. Using a world-wide data set on birds, we develop a predictive model of median natal dispersal distance that includes body mass, wingspan, sex and feeding guild. This model predicts median dispersal distance well when using the fitted data and an independent test data set, explaining up to 53% of the variation.
4. Using this model, we predict a priori estimates of median dispersal distance for 57 woodland-dependent bird species in northern Victoria, Australia. These estimates are then used to investigate the relationship between dispersal ability and vulnerability to landscape-scale changes in habitat cover and fragmentation.
5. We find evidence that woodland bird species with poor predicted dispersal ability are more vulnerable to habitat fragmentation than those species with longer predicted dispersal distances, thus improving the understanding of this important phenomenon.
6. The value of constructing informative priors from existing information is also demonstrated. When used as informative priors for four example species, predicted dispersal distances reduced the 95% credible intervals of posterior estimates of dispersal distance by 8–19%. Further, should we have wished to collect information on avian dispersal distances and relate it to species’ responses to habitat loss and fragmentation, data from 221 individuals across 57 species would have been required to obtain estimates with the same precision as those provided by the general model.
phangorn is a package for phylogenetic reconstruction and analysis in the R language. Previously it was only possible to estimate phylogenetic trees with distance methods in R. phangorn, now offers the possibility of reconstructing phylogenies with distance based methods, maximum parsimony or maximum likelihood (ML) and performing Hadamard conjugation. Extending the general ML framework, this package provides the possibility of estimating mixture and partition models. Furthermore, phangorn offers several functions for comparing trees, phylogenetic models or splits, simulating character data and performing congruence analyses.
Availability: phangorn can be obtained through the CRAN homepage http://cran.r-project.org/web/packages/phangorn/index.html. phangorn is licensed under GPL 2.
Contact: klaus.kschliep@snv.jussieu.fr
Supplementary information: Supplementary data are available at Bioinformatics online.
Some forest-dwelling tropical bird species are unable or unwilling to fly even a few hundred meters across habitat discontinuities, restricting connectivity of isolated populations in fragmented landscapes. Experimental evidence for flight limitation has reliably predicted occurrence across archipelagoes of habitat fragments varying in their distance from potential source populations. Mechanistic explanations for large differences in flight capacity of tropical birds, even over remarkably short distances, have not been tested. We evaluated myoglobin concentration in pectoralis muscles and hearts of eight Neotropical species for which experimental evidence revealing a wide range of flight abilities exists. We found a strong positive relationship between myoglobin concentration in the pectoralis muscles, but not hearts, and average overwater flight distance during dispersal-challenge experiments. The approximately 2.5-fold difference in pectoralis myoglobin concentration is directly associated with flight capacity and allows predictions of species or species groups most likely to be impeded by habitat discontinuities and therefore at conservation risk in fragmented landscapes.
Significance
Connectivity, or the degree to which individuals can move across landscapes, is essential for species persistence and the maintenance of biodiversity. While connectivity is increasingly understood for some individual species and landscapes, understanding and predicting connectivity for entire communities remains elusive despite its importance for biodiversity theory and conservation practice. We address this long-standing problem by providing a framework based on allometric scaling, the relationship between morphology and organism traits. We apply the framework to a diverse community of birds using field experiments and observed data on dispersal and species distribution. Based on our framework, we are able to explain substantial variation in the observed bird community structure, highlighting how species traits related to movement can predict biodiversity across landscapes.
Some theories predict habitat specialists should be less dispersive and migratory than generalists, while other theories predict the opposite. We evaluated the cross‐species relationship between the degree of habitat specialization and dispersal and migration status in 101 bird species breeding in North America and the United Kingdom, using empirical estimates of the degree of habitat specialization from breeding bird surveys and mean dispersal distance estimates from large‐scale mark‐recapture studies. We found that habitat specialists dispersed farther than habitat generalists, and full migrants had more specialized habitat than partial migrants or resident species. To our knowledge this is the first large‐scale, multi‐species study to demonstrate a positive relationship between the degree of habitat specialization and dispersal, and it is opposite to the pattern found for invertebrates. This finding is particularly interesting because it suggests that trade‐offs between the degree of habitat specialization and dispersal ability are not conserved across taxonomic groups. This cautions against extrapolation of trait co‐occurrence from one species group to another. In particular, it suggests that efforts aimed at conserving the most habitat‐specialist temperate‐breeding birds will not lead to conservation of the most dispersal limited species. This article is protected by copyright. All rights reserved.
Introduction.- First Steps in R for Phylogeneticists.- Phylogenetic Data in R.- Plotting Phylogenies.- Phylogeny Estimation.- Analysis of Macroevolution with Phylogenies.- Developing and Implementing Phylogenetic Methods in R.
A growing body of literature seeks to explain variation in range shifts using species' ecological and life history traits, with expectations that shifts should be greater in species with greater dispersal ability, reproductive potential, and ecological generalization. Despite strong theoretical support for species' traits as predictors of range shifts, empirical evidence from contemporary range shift studies remains limited in extent and consensus. We conducted the first comprehensive review of species' traits as predictors of range shifts, collecting results from 51 studies across multiple taxa encompassing over 11,000 species' responses for 54 assemblages of taxonomically-related species occurring together in space. We used studies of assemblages that directly compared geographic distributions sampled in the 20(th) century prior to climate change with resurveys of distributions after contemporary climate change, and then tested whether species traits accounted for heterogeneity in range shifts. We performed a formal meta-analysis on study-level effects of body size, fecundity, diet breadth, habitat breadth, and historic range limit as predictors of range shifts for a subset of 21 studies of 26 assemblages with sufficient data. Range shifts were consistent with predictions based on habitat breadth and historic range limit. However, body size, fecundity, and diet breadth showed no significant effect on range shifts across studies, and multiple studies reported significant relationships that contradicted predictions. Current understanding of species' traits as predictors of range shifts is limited, and standardized study is needed for traits to be valid indicators of vulnerability in assessments of climate change impacts. This article is protected by copyright. All rights reserved.
Roads through tropical forest create linear disturbances that have unknown consequences for forest birds. We studied how a narrow, rarely used road through otherwise undisturbed Amazonian forest affected the movements and area requirements of understory birds that form mixed-species flocks. Differences in road maintenance led to two distinct treatments along the same road. Trees along the "closed" road formed a partial canopy connecting the two sides of the road, although the roadway itself was kept open. The "open" road was regularly maintained, making a complete opening 10-30 m wide. We followed 15 flocks, 5 each in interior forest, along the open road, and along the closed road. These flocks were led by Thamnomanes antshrikes, and each flock had a discreet, permanent territory. Flock territory size (mean = 8.5 ba) did not differ among the three locations. The open road formed the territorial boundary for all five flocks, although birds moved within a few meters of the edge of the road. Tbe closed road was less of a barrier: 2 of 5 flocks used both sides of the closed road. Playback experiments showed that flocks readily crossed the closed road to approach agonistic vocalizations. Along the open road, even though birds responded to playback by becoming agitated and moving to the extreme edge of the roadside vegetation, they were less likely to cross the road and did so only after a longer duration of playback. Our results suggest that flocks respond to a road as they would to a long linear gap. They use the vegetation along the edges of the road, but because they are unwilling to cross the open area, it becomes a flock territory boundary. Similarly, as in forest gaps, successional change along the closed road produced suitable habitat for flocks. Although this suggests that roads are a trivial problem, we caution that this result applies only to narrow roads that are not accompanied by deforestation or other disturbance.
We sampled understory insectivorous birds in Amazonian forest fragments from before isolation through 9 yr after isolation. We accumulated 3658 mist net captures of 84 insectivorous species in five 1-ha fragments and four 10-ha fragments. Abundance and species richness declined dramatically after isolation, even though fragments were separated from continuous forest by only 70-650 m. Three species of obligate army ant followers disappeared within the first 2 yr after isolation. Mixed-species flocks containing 13 commonly netted species disintegrated within 2-3 yr after isolation, although three species that dropped out of flocks persisted in fragments. Among insectivores not associated with flocks or army ants, only two species of edge specialists were unaffected by fragmentation. Overall, loss of forest insectivores was not compensated for by an increase in nonforest or previously uncommon species. Secondary vegetation surrounding fragments strongly affected use of fragments after isolation. Fragments surrounded by Vismia, the dominant regrowth where felled forest was burned and temporarily used as cattle pasture, remained depauperate. In contrast, many species returned to fragments by moving through regenerating forest dominated by Cecropia, which occurred in areas where the felled forest was not burned. Both 1- and 10-ha fragments surrounded by Cecropia were used by ant followers by 5 yr after isolation. Mixed-species flocks reassembled in 10-ha fragments surrounded by Cecropia by 7-9 yr after isolation, and augmented their group territories by foraging in secondary forest outside fragments. Solitary species were more variable in their responses, although several species returned to 10-ha fragments surrounded by Cecropia. Terrestrial insectivores, such as Sclerurus leafscrapers and various antbirds, did not return to any fragments, and appear to be the group most vulnerable to fragmentation. Ordination of the insectivore community showed that 1-ha fragments diverged from their pre-isolation communities more than did 10-ha fragments. Communities in 10-ha fragments surrounded by Cecropia were more closely associated with pre-isolation communities than those in fragments surrounded by Vismia. Over time, communities in 10-ha fragments surrounded by Cecropia became more like pre-isolation communities, although communities in other fragments generally continued to diverge.
Interpreting the Logistic Regression ModelInference for Logistic RegressionLogistic Regression with Categorical PredictorsMultiple Logistic RegressionSummarizing Effects in Logistic RegressionProblems
Dispersal is fundamental in determining biodiversity responses to rapid climate change, but recently acquired ecological and evolutionary knowledge is seldom accounted for in either predictive methods or conservation planning. We emphasise the accumulating evidence for direct and indirect impacts of climate change on dispersal. Additionally, evolutionary theory predicts increases in dispersal at expanding range margins, and this has been observed in a number of species. This multitude of ecological and evolutionary processes is likely to lead to complex responses of dispersal to climate change. As a result, improvement of models of species’ range changes will require greater realism in the representation of dispersal. Placing dispersal at the heart of our thinking will facilitate development of conservation strategies that are resilient to climate change, including landscape management and assisted colonisation.
Synthesis
This article seeks synthesis across the fields of dispersal ecology and evolution, species distribution modelling and conservation biology. Increasing effort focuses on understanding how dispersal influences species' responses to climate change. Importantly, though perhaps not broadly widely‐recognised, species' dispersal characteristics are themselves likely to alter during rapid climate change. We compile evidence for direct and indirect influences that climate change may have on dispersal, some ecological and others evolutionary. We emphasise the need for predictive modelling to account for this dispersal realism and highlight the need for conservation to make better use of our existing knowledge related to dispersal.
I censused forest canopy birds from two emergent trees in lowland wet forest in Costa Rica from April 1985 to May 1986. Composition of the canopy avifauna did not differ overall between census sites. I recorded 89 species and 2,944 individuals during 49 censuses. Forest canopy was dominated by frugivores, especially large-bodied (> 100 g) frugivores, and parrots. Furthermore, in contrast to the avifauna of forest canopies in Panama and Peru, I found that the canopy avifauna was primarily composed of forest species, rather than scrub species. Most species occurred in intra- or interspecific flocks. I found that the abundance of small frugivores and small insectivores was seasonally variable. Extent of seasonal variation in fruit crop sizes of Dipteryx panamensis may have contributed to the annual variation observed in psittacids. Avifauna of the forest canopy, with few exceptions, was distinct from the understory avifauna; few of the common,understory species were recorded in the canopy. Further, in contrast to the canopy avifauna, the understory avifauna was dominated, in terms of species number, by insectivores. Key words: Canopy; Costa Rica; frugivores; rain forest; seasonality; tropical.
Land cover and land use surrounding fragmented habitat can greatly impact species persistence by altering resource availability, edge effects, or the movement of individuals throughout a landscape. Despite the potential importance of the landscape matrix, ecologists still have limited understanding of the relative effects of different types of land cover and land uses on species patterns and processes in natural systems. Here we investigated whether Neotropical resident bird communities in limestone forest patches differed if they were embedded in three different human-dominated matrix types (agriculture, peri-urban development, and bauxite mining) relative to sites in continuous forest in central Jamaica. We found that species richness, community composition, and abundances were matrix-dependent, with agricultural landscapes supporting greater avian diversity and more intact community assemblages than either peri-urban or bauxite landscapes. Abundance of almost 70% of species differed in forest embedded in the different landscape matrix types. Traits related to resource use best predicted species responses, including diet guild, nest height, habitat association, and foraging strata. Insectivores, frugivores, canopy nesters, understory and canopy foragers, and forest-restricted species rarely observed in matrix habitats had lower abundances in forest fragments embedded in human-dominated matrix types than in continuous forest. In contrast, nectarivores, omnivores, granivores, ground and multi-strata nesters, ground foragers, and species regularly in matrix habitats were least sensitive to forest fragmentation. Results suggest that structure, composition, and land use disturbance regimes in matrix areas impact overall habitat quality in landscapes by potentially mediating resource availability inside as well as outside forest habitat. This study reinforces the importance of differentiating among land cover and land uses in fragmentation research and lends support to the hypothesis that resource availability may be a primary factor driving Neotropical bird responses to fragmentation.
Previous studies that tracked the movements of single bird species within human- modified landscapes have shown that the ability of forest birds to move across matrix habitat differs among species. Functional guild specificity as well as land- scape characteristics have been shown to influence bird movements, entailing different movement behaviour of birds within a community. Studies investigating how both these factors influence the movements of entire bird assemblages across fragmented landscapes are essential but have rarely been conducted. In this study, we determined how species’ traits and different forest matrices influence bird movements among nine forest patches in a highly fragmented South African landscape. We combined 90 h of bird observations with capture–mark–recaptures (104 754 mist-net hours) to distinguish between movements among patches (all birds that conduct long-distance movements across the landscape) and move- ments within patches (all resident birds that conduct only short-distance move- ments within a fragment). Overall, we detected a high bird movement activity across the fragmented landscape. Dietary specialization, habitat affinity and body mass strongly shaped the relative distribution of bird species across the nine fragments with frugivorous birds, forest specialists and large-bodied species showing the highest movement abilities. In contrast, resident insectivores and forest generalists tended to move only within particular forest fragments. Our results suggest that remnant forest fragments may represent valuable stepping stones as well as permanent habitat for local bird assemblages. We emphasize that beside the conservation of natural forests, the maintenance of nearby, structurally rich forest fragments is pivotal in maintaining regional forest bird assemblages in human-modified landscapes.
We examined temporal variation in abundance of understory birds and fruiting plants in young (5-7 yr) and old (25-35 yr) successional habitats and in intact, lowland rain forest at Estacion Biologica La $elva, Costa Rica, between January 1985 and May 1986. Fruit abundance varied seasonally in each habitat but was consistently greatest in the youngest site. Frugivores and nectarivores accounted for four (forest and older successional) or five (younger successional) of the five most frequently captured bird species in each habitat. Capture rates of arboreal frugivores and arboreal frugivore-insectivores were greatest in the youngest site and were not different between older habitats. Temporal variation in capture rates of frugivores resulted from habitat shifts by resident individuals and from arrival and departure of altitudinal and latitudinal migrants. Capture rates of frugivores correlated with fruit abundance in forest and the older successional habitat but not in the youngest site. Capture rates of nectarivores and insectivores varied over time and among habitats, but rates showed no correlation with capture rates of frugivores. The lack of positive correlations in seasonal capture rates among trophic groups and the correlation between frugivores and fruit abundance support the view that temporal and spatial variation in bird abundance in tropical bird communities is at least partially in response to variation in resource abundance. Received 3 November 1989, accepted 28 July 1990.
Roads through tropical forest create linear disturbances that have unknown consequences for forest birds. We studied how a narrow, rarely used road through otherwise undisturbed Amazonian forest affected the movements and area requirements of understory birds that form mixed-species flocks. Differences in road maintenance led to two distinct treatments along the same road. Trees along the “closed” road formed a partial canopy connecting the two sides of the road, although the roadway itself was kept open. The “open” road was regularly maintained, making a complete opening 10–30 m wide.We followed 15 flocks, 5 each in interior forest, along the open road, and along the closed road. These flocks were led by Thamnomanes antshrikes, and each flock had a discreet, permanent territory. Flock territory size (mean = 8.5 ha) did not differ among the three locations. The open road formed the territorial boundary for all five flocks, although birds moved within a few meters of the edge of the road. The closed road was less of a barrier: 2 of 5 flocks used both sides of the closed road. Playback experiments showed that flocks readily crossed the closed road to approach agonistic vocalizations. Along the open road, even though birds responded to playback by becoming agitated and moving to the extreme edge of the roadside vegetation, they were less likely to cross the road and did so only after a longer duration of playback. Our results suggest that flocks respond to a road as they would to a long linear gap. They use the vegetation along the edges of the road, but because they are unwilling to cross the open area, it becomes a flock territory boundary. Similarly, as in forest gaps, successional change along the closed road produced suitable habitat for flocks. Although this suggests that roads are a trivial problem, we caution that this result applies only to narrow roads that are not accompanied by deforestation or other disturbance.
The potential negative effects of forest fragmentation on animal movement and dispersal, and its consequences for population persistence, require an understanding of how modified landscapes affect movement decisions of forest species. We used a dispersal challenge experiment, whereby we released individuals of six bird species in a cattle pasture at different distances from forest edge (0, 50, 100 and 150 m), in a fragmented tropical landscape in Mexico to investigate the gap-crossing abilities and movement behavior of six species of tropical forest birds. Gaps as narrow as 50 m affected movement behavior of tropical forest birds. A sharp change in movement behavior in gaps 100 m and larger suggested the presence of a threshold distance beyond which birds are less likely to attempt and successfully navigate during trans-gap flights. Bird responses varied with degree of forest dependence: three forest-restricted species showed greater latency to cross gaps, independent of gap width, as compared with forest-unrestricted species. Gap width had a stronger effect on the orientation and destination of forest-restricted species than that of forest-unrestricted species. The concordance of our results with those found in species-distribution and radio-tracking studies indicates that dispersal challenge experiments provide reliable predictive information about response of forest birds to gaps between isolated forest fragments. In the landscape we studied, reducing gaps to no more than 50 m and providing corridors or shade trees between fragments should facilitate movements of forest birds.
Natal dispersal is a process that is critical in the spatial dynamics of populations, including population spread recolonization, and gene flow. It is a central focus of conservation issues for many vertebrate species. Using data for 77 bird and 68 mammal species we tested whether median and maximum natal dispersal distances were correlated with body mass, diet type, social system, taxonomic family, and migratory status. Body mass and diet type were found to predict both median and maximum natal dispersal distances in mammals: large species dispersed farther than small ones, and carnivorous species dispersed farther than herbivores and omnivores. Similar relationships occurred for carnivorous bird species, but not for herbivorous or omnivorous ones. Natal dispersal distances in birds or mammals were not significantly related to broad categories of social systems. Only in birds were factors such as taxonomic relatedness and migratory status correlated with natal dispersal, and then only for maximum distances. Summary properties of dispersal processes appeared to be derived from interactions among behavioral and morphological characteristics of species and from their linkages to the dynamics of resource availability in landscapes. In all the species we examined, most dispersers moved relatively short distances, and long-distance dispersal was uncommon. On the basis of these findings, we fit an empirical model based on the negative exponential distribution for calculating minimum probabilities that animals disperse particular distances from their natal areas. This model, coupled with knowledge of a species' body mass and diet type, can be used to conservatively predict dispersal distances for different species and examine possible consequences of large-scale habitat alterations on connectedness between populations. Taken together, our results can provide managers with the means to identify species vulnerable to landscape-level habitat changes such as forest fragmentation. In addition, our dispersal models can be used to predict which species in a community are likely to be the most vulnerable to loss of connectedness and allow managers to test the merits of alternative habitat conservation plans.