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A Rapid, Strong, and Convergent Genetic Response to Urban Habitat Fragmentation in Four Divergent and Widespread Vertebrates
PLOS ONE
Abstract and Figures
Urbanization is a major cause of habitat fragmentation worldwide. Ecological and conservation theory predicts many potential impacts of habitat fragmentation on natural populations, including genetic impacts. Habitat fragmentation by urbanization causes populations of animals and plants to be isolated in patches of suitable habitat that are surrounded by non-native vegetation or severely altered vegetation, asphalt, concrete, and human structures. This can lead to genetic divergence between patches and in turn to decreased genetic diversity within patches through genetic drift and inbreeding.
We examined population genetic patterns using microsatellites in four common vertebrate species, three lizards and one bird, in highly fragmented urban southern California. Despite significant phylogenetic, ecological, and mobility differences between these species, all four showed similar and significant reductions in gene flow over relatively short geographic and temporal scales. For all four species, the greatest genetic divergence was found where development was oldest and most intensive. All four animals also showed significant reduction in gene flow associated with intervening roads and freeways, the degree of patch isolation, and the time since isolation.
Despite wide acceptance of the idea in principle, evidence of significant population genetic changes associated with fragmentation at small spatial and temporal scales has been rare, even in smaller terrestrial vertebrates, and especially for birds. Given the striking pattern of similar and rapid effects across four common and widespread species, including a volant bird, intense urbanization may represent the most severe form of fragmentation, with minimal effective movement through the urban matrix.
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Supplementary resources (6)
... Similarly, the relationship between amphibian genetic diversity or differentiation and the degree of urbanization of several North American cities was not significant in the study by Schmidt and Garroway (2021). On the contrary, Delaney et al. (2010) showed that urbanization decreased genetic diversity and increased genetic differentiation in three lizard species and one bird species, mainly due to higher road density in urban areas. Likewise, Stillfried et al. (2017) showed that urban populations of wild boars exhibited lower genetic diversity levels than suburban ones in Berlin. ...
... For instance, the very subtle differences in genetic diversity simulated under the scenario 1 recall the empirical results of Khimoun et al. (2020). In contrast, the sharper differences simulated under the scenario 3 are akin to the significant genetic contrasts between urban and non-urban settings observed by Delaney et al. (2010) in several species. Although our results do not provide an explanation for these specific empirical findings, they show that differences in dispersal patterns could be sufficient to generate similar genetic patterns. ...
... Besides, genetic differentiation levels measured between these two types of habitats were intermediate, as compared with the high and low levels measured within UGS and forest patches, respectively. Similar genetic responses to urbanization have already been empirically observed in several species, from birds and reptiles (Delaney et al., 2010), to rodents (DeMarco et al., 2021;Gortat et al., 2015) and larger mammals (Stillfried et al., 2017;Wandeler et al., 2003). In our simulations, they mainly stem from the fact that the contribution of forest areas to the overall amount of reachable habitat was much larger than that of UGS in most cities. ...
... To increase protected area coverage and connectivity, which allows for larger effective population sizes, there have been strong drives to increase the number of transfrontier parks and their coverage (Van Aarde & Jackson 2007;Hayward & Kerley 2009;Winterbach et al. 2013). However, human activity, a strong limiting factor for population connectivity (Delaney et al. 2010;Crooks et al. 2011;Creel et al. 2019;Brennan et al. 2020;Thatte et al. 2020), within transfrontier conservation areas have hindered these efforts (Hilty et al. 2020). Indeed, human activity is resulting in increased population isolation (Cushman et al. 2016;Creel et al. 2019;Brennan et al. 2020;Dures et al. 2021) and genetic structuring among subpopulations of carnivores (Tensen et al. 2016;Creel et al. 2019;Dures et al. 2021), within and between protected areas. ...
... Coupled with protected area size and the degree of isolation (Hanski & Gilpin 1991), anthropogenic pressure is becoming increasingly important in determining the degree of connectivity between protected areas (Delaney et al. 2010;Crooks et al. 2011;Creel et al. 2019;Brennan et al. 2020;Thatte et al. 2020). For example, human activity within the Kavango-Zambezi Transboundary Conservation Area limits large mammal population connectivity between the protected area (Cushman et al. 2016;Creel et al. 2019;Brennan et al. 2020;Dures et al. 2021), facilitating genetic differentiation among populations (Creel et al. 2019;Dures et al. 2021). ...
... The geographic location for closely-related individuals of lions and leopards provides evidence for connectivity within the GLTCA. Connectivity between protected areas is influenced by protected area size and the degree of isolation (Hanski & Gilpin 1991), human activity (Delaney et al. 2010;Crooks et al. 2011;Creel et al. 2019;Brennan et al. 2020;Thatte et al. 2020), and the ecological traits of the disperser in question (Creel et al. 2019;Dures et al. 2020). Despite the relatively large area and close proximity of LNP with BNP (< 15 km, well within the normal dispersal range for both lions and leopards; Fattebert et al. 2013;Dolrenry et al. 2014), I did not record first-degree lion relatives shared across LNP and BNP, unlike leopards. ...
Terrestrial carnivore population declines are driven by habitat loss and fragmentation, prey-depletion, persecution, and retaliatory killings. Population strongholds now centre on protected areas, that face increasing human pressure, resulting in population isolation, declining prey populations, and livestock intrusion. I therefore aimed to investigate dispersal and connectivity, and diets of lions (Panthera leo) and leopards (P. pardus) in response to human-use and wildlife density gradients in the Greater Limpopo Transfrontier Conservation Area (GLTCA). Firstly, I investigated dispersal and connectivity for these carnivores across the GLTCA, using single nucleotide polymorphisms. I present evidence that in the prey-depleted Mozambique portion of the GLTCA, lion and leopard dispersal distances are higher compared with the prey-abundant Kruger National Park (KNP). I also provide the first evidence for long-range dispersal in female lions. Despite evidence for connectivity occurring across the GLTCA, I recorded population structuring across the region for both carnivores, likely due to habitat fragmentation by human activities in the Mozambique portion of the GLTCA. I then assessed carnivore diet responses to prey depletion and livestock availability by comparing lion and leopard diets in the prey-abundant/livestock-absent KNP (South Africa), with the prey-depleted/livestock-abundant Limpopo National Park (LNP, Mozambique), using scat analyses. Lions and leopards downshifted their prey size selection in LNP relative to KNP. Despite both carnivores expanding their dietary niche breadths in LNP relative to KNP, diet overlap did not differ between sites. This suggests that even when prey is depleted, lions and leopards can partition food resources, which likely limits competition. Despite cattle (Bos taurus) being the most abundant ungulate in LNP, lions and leopards strongly avoided cattle, supporting the notion that carnivores can perceive the risk of hunting livestock and modulate their foraging behaviour to reduce human-carnivore conflict. Should my findings reflect general patterns in carnivore dispersal and diet responses to human-use and wildlife gradients, then carnivore conservation initiates across their range should focus on prey population rehabilitation, improved livestock husbandry practices, the establishment of effective dispersal corridors and improved human-tolerance towards large carnivores.
... In addition to landscape modification, human activity on the landscape (e.g., recreation, hunting and gathering) can also affect wildlife (Suraci et al. 2019) and it is much more difficult to measure. However, it is important to quantify the effects of human activity on wildlife because they can vary parks and preserves with high levels of human activity could be ecological traps (Schlaepfer et al. 2002) if animal populations are declining due to human activity, but individuals are not able to move to alternate habitat because of the impermeability of the urban matrix (Delaney et al. 2010). ...
... The negative effects of recreation compound the numerous conservation challenges in fragmented landscapes, such as lack of connectivity and the resulting loss of genetic diversity in isolated populations (Aguilar et al. 2008; it is a relative generalist, the common side-blotched lizard is rarely found in highly altered greenspace such as mowed lawns and landscaped urban parks, and has limited dispersal ability (Doughty and Sinervo 1994). Its inability to move between isolated habitat fragments can result in substantial genetic isolation among populations (Delaney et al. 2010). ...
... We observed a slight trend toward a negative relationship between occupancy and human activity, but with considerable uncertainty. A lack of flexibility in habitat use may prevent the orange-throated whiptail from moving away from human activity if there is little suitable habitat nearby (Gill et al. 2001), as it is small and has limited dispersal ability (Delaney et al. 2010). Previous studies have shown that prey abundance, specifically abundance of Crematogaster ants, is an important predictor of orange-throated whiptail abundance (Ver Hoef et al. 2001). ...
The world is urbanizing rapidly, resulting in increasing rates of habitat loss and fragmentation. Protected areas are commonly established to restrict development and conserve native ecological communities. Yet urban protected areas often receive high levels of recreational activity, which can reduce their conservation effectiveness because of disturbance to animals. Recreation has negative consequences for many animal species, but its effects on reptiles are largely unknown. We evaluated the effects of non-consumptive recreation on reptiles within urban protected areas in a fragmented landscape in coastal southern California, USA. We surveyed lizards and snakes along a gradient of recreation intensity and modeled species richness, community composition, and occupancy in relation to human activity along with other variables known to affect reptile distributions. We observed a decline in lizard species richness in association with human activity. Richness of habitat specialists was not related to recreation, but smaller-bodied lizards and lizards with narrower active temperature ranges were less common at sites with high human activity. Human activity was associated with a decline in occupancy of the common side-blotched lizard (Uta stansburiana), no meaningful relationship with occupancy of the orange-throated whiptail (Aspidoscelis hyperythra) and a positive relationship with western fence lizard (Sceloporus occidentalis) occupancy and/or detection probability. Our study demonstrates that increasing rates of recreation activity can reduce the ability of urban protected areas to conserve diverse reptile assemblages.
... the face of environmental change (Lewis 2006;Delaney et al. 2010;Callens et al. 2011;Frankham et al. 2017). ...
Understanding genetic structure and diversity among remnant populations of rare species can inform conservation and recovery actions. We used a population genetic framework to spatially delineate gene pools and estimate gene flow and effective population sizes for the endangered California Freshwater Shrimp Syncaris pacifica. Tissues of 101 individuals were collected from 11 sites in 5 watersheds, using non-lethal tissue sampling. Single Nucleotide Polymorphism markers were developed de novo using ddRAD-seq methods, resulting in 433 unlinked loci scored with high confidence and low missing data. We found evidence for strong genetic structure across the species range. Two hierarchical levels of significant differentiation were observed: (i) five clusters (regional gene pools, FST = 0.38–0.75) isolated by low gene flow were associated with watershed limits and (ii) modest local structure among tributaries within a watershed that are not connected through direct downstream flow (local gene pools, FST = 0.06–0.10). Sampling sites connected with direct upstream-to-downstream water flow were not differentiated. Our analyses suggest that regional watersheds are isolated from one another, with very limited (possibly no) gene flow over recent generations. This isolation is paired with small effective population sizes across regional gene pools (Ne = 62.4–147.1). Genetic diversity was variable across sites and watersheds (He = 0.09–0.22). Those with the highest diversity may have been refugia and are now potential sources of genetic diversity for other populations. These findings highlight which portions of the species range may be most vulnerable to future habitat fragmentation and provide management consideration for maintaining local effective population sizes and genetic connectivity.
... These processes may decrease genetic diversity within subpopulations and increase genetic differentiation between them (Johnson & Munshi-South, 2017). These fine-scale genetic predictions have been recorded for some species (e.g., Delaney et al., 2010), but are not universal and should be considered in the context of urban and natural landscape features that restrict or facilitate gene flow in a particular system . ...
Urbanization leads to complex environmental changes and poses multiple challenges to organisms. Amphibians are highly susceptible to the effects of urbanization, with land use conversion, habitat destruction, and degradation ranked as the most significant threats. Consequently, amphibians are declining in urban areas, in both population numbers and abundance, however, the effect of urbanization on population genetic parameters remains unclear. Here, we studied the genomic response to urbanization in two widespread European species, the common toad Bufo bufo (26 localities, 480 individuals), and the smooth newt Lissotriton vulgaris (30 localities, 516 individuals) in three geographic regions: southern and northern Poland and southern Norway. We assessed genome‐wide SNP variation using RADseq (ca. 42 and 552 thousand SNPs in toads and newts, respectively) and adaptively relevant major histocompatibility complex (MHC) class I and II genes. The results linked most of the genetic differentiation in both marker types to regional (latitudinal) effects, which also correspond to historical biogeography. Further, we did not find any association between genetic differentiation and level of urbanization at local scales for either species. However, urban smooth newts, but not toads, have lower levels of within‐population genome‐wide diversity, suggesting higher susceptibility to the negative effects of urbanization. A decreasing level of genetic diversity linked to increasing urbanization was also found for MHC II in smooth newts, while the relationship between MHC class I diversity and urbanization differed between geographic regions. We did not find any effects of urbanization on MHC diversity in the toad populations. Although two genetic environment association analyses of genome‐wide data, LFMM and BayPass, revealed numerous (219 in B. bufo and 7040 in L. vulgaris) SNPs statistically associated with urbanization, we found a marked lack of repeatability between geographic regions, suggesting a complex and multifaceted response to natural selection elicited by life in the city.
... However, as of 2021, 56% of the human population was estimated to live in urban settings (United Nations, 2022), and projections indicate 68% of the global population is expected to live in urban areas by 2050 (United Nations Department of Economic and Social Affairs, 2018). Urban development and its associated fragmentation can limit gene flow (Delaney et al., 2010) and species richness (Theodorou et al., 2020), and can provide non-native species with opportunities to colonize disturbed landscapes and outcompete native species (Cadotte et al., 2017;Dearborn & Kark, 2010). These problems are exacerbated as cities are typically located in biodiverse places (Kowarik, 2011). ...
The threat to biodiversity posed by urban expansion is well researched and supported. Since the late 1990s, the field of urban ecology has been expanding along with the developed landscapes it studies. Past reviews have shown unequal publication rates in urban ecology literature for taxonomic groups and research locations. Herein, we explore differences in the publication rate of urban studies by vertebrate groups, but also expand on previous investigations by broadening the scope of the literature searched, exploring trends in subtopics within the urban wildlife literature, identifying geographic patterns of such publications, and comparing the rate at which non‐native and threatened and endangered species are studied in urban settings. We used linear and segmented regression to assess publication rates and Fisher's exact tests for comparisons between groups. All vertebrate groups show an increasing proportion of urban studies through time, with urban avian studies accelerating most sharply and herpetofauna appearing to be understudied. Non‐native mammals are more studied than non‐native birds, and threatened and endangered herpetofauna and mammals are more likely to be studied than threatened and endangered birds in urban areas. The plurality of urban wildlife studies are found in North America, while there is a dearth of studies from Africa, Asia, and South America. Our results can help inform decisions of urban ecologists on how to better fill in knowledge gaps and bring a greater degree of equity into the field.
... This finding aligns with a large body of literature showing that roads impede dispersal for a variety of taxa, often leading to reduced population sizes and genetic drift (Holderegger & Di Giulio, 2010). Reptile species are subject to direct mortality on roads (Andrews et al., 2015), and roads act as a barrier to gene flow for small-bodied, dispersal-limited reptiles (Delaney et al., 2010). ...
Aim
The development of natural habitats into urban land uses has greatly accelerated in the recent past due to human activities. This habitat development disrupts species' natural dispersal processes and can lead to both direct and indirect impacts on dispersal. Whether human activities result in restricted or facilitated dispersal may depend on a species' development tolerance; however, this premise has not been tested. We examined the impact of urbanization and road networks on the dispersal of three lizard species in the context of their development tolerance.
Location
Curaçao.
Methods
To quantify species' development tolerance, we modelled three lizard species abundances at sites based on surrounding landscape development. Using microsatellite genotypes, we conducted individual‐based resistance surface analyses and modelled the effect of habitat development on genetic admixture to assess indirect dispersal restriction and facilitation. We explored direct facilitation of dispersal using network analysis of mitochondrial haplotypes.
Results
Phyllodactylus martini, a native gecko species, was the least tolerant of development and experienced indirect dispersal restriction due to roads, according to resistance surface analyses. Anolis lineatus, a native anole species, exhibited a neutral relationship with development. Resistance surfaces and Structure analyses showed that A. lineatus faced indirect dispersal restrictions from roads and developed areas, while mitochondrial haplotype networks suggested they benefited from occasional human‐facilitated long‐distance dispersal events. Hemidactylus mabouia, an introduced gecko species, was the most tolerant of development, and experienced no dispersal restriction, but mitochondrial haplotypes suggest direct long‐distance dispersal facilitation.
Main Conclusions
Our findings highlight development tolerance as a key predictor of dispersal impact for these species and future work should test whether these patterns are upheld in other systems. Understanding how human activities affect species' dispersal will aid in managing introduced species while promoting connectivity for native species navigating dispersal challenges in dynamic landscapes.
... Resource shifts caused by climate change have forced species ranges to shift and contract at unprecedented rates, exacerbating human-wildlife conflict (4)(5)(6)(7). Human development and land use further degrades and fragments existing habitat, impacting critical processes such as migration, dispersal, and gene flow (8). As these pressures force humans and wildlife into even closer proximity, there is an increase in shared landscapes and subsequently human-wildlife interactions (9,10). ...
Human–wildlife conflict is an important factor in the modern biodiversity crisis and has negative effects on both humans and wildlife (such as property destruction, injury, or death) that can impede conservation efforts for threatened species. Effectively addressing conflict requires an understanding of where it is likely to occur, particularly as climate change shifts wildlife ranges and human activities globally. Here, we examine how projected shifts in cropland density, human population density, and climatic suitability—three key drivers of human–elephant conflict—will shift conflict pressures for endangered Asian and African elephants to inform conflict management in a changing climate. We find that conflict risk (cropland density and/or human population density moving into the 90th percentile based on current-day values) increases in 2050, with a larger increase under the high-emissions “regional rivalry” SSP3 - RCP 7.0 scenario than the low-emissions “sustainability” SSP1 - RCP 2.6 scenario. We also find a net decrease in climatic suitability for both species along their extended range boundaries, with decreasing suitability most often overlapping increasing conflict risk when both suitability and conflict risk are changing. Our findings suggest that as climate changes, the risk of conflict with Asian and African elephants may shift and increase and managers should proactively mitigate that conflict to preserve these charismatic animals.
... Fragmented landscapes can result in decreased habitat connectivity, wherein plant and animal populations experience decreased dispersal among patches, diminished reproduction within patches, and smaller population sizes through time (Fletcher et al. 2018). Decreased connectivity and population size can have myriad genetic consequences for populations (Templeton et al. 1990), including restrictions or cessation of gene flow (Delaney et al. 2010) and decreased genetic diversity (Schlaepfer et al. 2018;Lino et al. 2019;González et al. 2020), loss of diversity via genetic drift (Holderegger and Di Giulio 2010), increased inbreeding depression, reduced adaptive potential, and increased frequency of deleterious alleles (Keyghobadi 2007). Through an interplay of these mechanisms, habitat fragmentation contributes to significant negative demographic effects on populations that can drive both population extirpation and ultimately even species extinctions. ...
In the North American longleaf pine (Pinus palustris) ecosystem, the Gopher Tortoise (Gopherus polyphemus) is a keystone species that has declined significantly over the last century. Habitat degradation and fragmentation may have caused G. polyphemus to become separated into small, isolated local populations that suffer from decreased genetic diversity or inbreeding depression. Here we use genome-scale methods to sequence thousands of loci for 336 G. polyphemus individuals from 11 sites across southern Alabama to estimate population genetic structure and levels of genetic diversity. We found a pattern of isolation by distance among samples, where geographic distance predicted genetic difference. Principal components and structure analyses supported the existence of three weak genetic populations comprising individuals from (1) Fred T. Stimpson State Game Sanctuary and Perdido Wildlife Management Area, (2) Conecuh National Forest and Solon Dixon Forestry Education Center, and (3) Geneva State Forest Wildlife Management Area. We did not observe strong variation in genetic diversity or effective population size metrics among sampling locations or genetic populations identified by population structure analyses. Our results suggest that G. polyphemus historically operated on larger geographic scales than those considered by contemporary mark-recapture studies. Absence of variation in population genetic metrics suggests that either effects of fragmentation have not manifested themselves, or that the effects are similar across all locations. Given the common use of translocations in Gopher Tortoise management, we provide a framework for tortoise translocations based on our genomic data.
... Moreover, the presence of energy infrastructure incited mass long-term avoidance by mule deer Odocoileus hemionus in Wyoming, USA (Sawyer et al. 2017), whereas high traffic volume on roads reduced the likelihood of ungulate crossing into new habitats in Sweden (Olsson et al. 2008). In a fragmented landscape, habitat patches become smaller and more isolated (Lees andPeres, 2009, Ceia-Hasse et al. 2018), leading to a lack of gene flow between patches (Delaney et al. 2010, Fenderson et al. 2014) and subsequently becoming more likely to suffer local extinction from genetic isolation (Corlatti et al. 2009, Koumoundouros et al. 2009). ...
Linear infrastructure represent a barrier to movement for many species, reducing the connectivity of the landscapes in which they reside. Of all linear infrastructure, roads and fences are two of the most ubiquitous, and are understood to reduce landscape connectivity for wildlife. However, what is often neglected consideration is a holistic approach of modelling the effects of multiple types of linear infrastructure simultaneously. Few studies have examined this, typically assessing the impacts of a singular kind of infrastructure on landscape connectivity. Therefore, the aim of this study is to address the relative importance of considering multiple kinds of linear infrastructure in landscape connectivity modelling. We utilised presence data of red deer Cervus elaphus and wild boar Sus scrofa in Doñana Biosphere Reserve (Spain) to generate a sequential approach of scenarios of landscape connectivity; firstly only with environmental variables, secondly with roads as the sole infrastructure, thirdly with the addition of fences, and finally with the further addition of fences and wildlife road‐crossing structures. We found that the connectivity of the landscape was greatly affected by the addition of fences and wildlife road‐crossing structures in both species, with fences in particular causing considerable alterations to estimated movement pathways. Our finding impresses a need to consider multiple different types of linear infrastructure when modelling landscape connectivity to enable a more realistic view of wildlife movement and inform mitigation and conservation measures more accurately.
One of the biggest threats to the survival of many plant and animal species is the destruction or fragmentation of their natural habitats. The conservation of landscape connections, where animals, plants, and ecological processes can move freely from one habitat to another, is therefore an essential part of any new conservation or environmental protection plan. In practice, however, maintaining, creating, and protecting connectivity in our increasingly dissected world is a daunting challenge. This fascinating volume provides a synthesis on the current status and literature of connectivity conservation research and implementation. It shows the challenges involved in applying existing knowledge to real-world examples and highlights areas in need of further study. Containing contributions from leading scientists and practitioners, this topical and thought-provoking volume will be essential reading for graduate students, researchers, and practitioners working in conservation biology and natural resource management.
We describe a model-based clustering method for using multilocus genotype data to infer population structure and assign individuals to populations. We assume a model in which there are K populations (where K may be unknown), each of which is characterized by a set of allele frequencies at each locus. Individuals in the sample are assigned (probabilistically) to populations, or jointly to two or more populations if their genotypes indicate that they are admixed. Our model does not assume a particular mutation process, and it can be applied to most of the commonly used genetic markers, provided that they are not closely linked. Applications of our method include demonstrating the presence of population structure, assigning individuals to populations, studying hybrid zones, and identifying migrants and admixed individuals. We show that the method can produce highly accurate assignments using modest numbers of loci—e.g., seven microsatellite loci in an example using genotype data from an endangered bird species. The software used for this article is available from http://www.stats.ox.ac.uk/~pritch/home.html.
In ecological applications of large-scale spatial data to management decisions concerning land planning and conservation, errors and biases may creep into the analysis and decision making at several steps (see Chaps. 1, 2, and 3), including:
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Uncertainty in positions of spatial locations of relevant ecological and physiographic features of the landscape.
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Uncertainty of the type and attributes of land cover at a particular location.
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Uncertainty in how different land covers at a position in space and the geometric arrangement of land covers nearby might influence an animal species occurrence or distribution, or the magnitude of some ecological process.
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Uncertainty about the relative importance of each spatial location to the overall success or persistence of a population or ecological process.
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Uncertainty about how to weight each species or ecological process in determining the overall biodiversity and functioning of ecosystems, local and national resource priorities, and consistency with legislative mandates. We would like to be able to quantify the errors at each step, identify biases, and pass these along to the next analysis step so that our degree of uncertainty regarding potential outcomes is evident at each level (e.g., Stoms et al. 1992).
Abstract A huge road network with vehicles ramifies across the land, representing a surprising frontier of ecology. Species-rich roadsides are conduits for few species. Roadkills are a premier mortality source, yet except for local spots, rates rarely limit population size. Road avoidance, especially due to traffic noise, has a greater ecological impact. The still-more-important barrier effect subdivides populations, with demographic and probably genetic consequences. Road networks crossing landscapes cause local hydrologic and erosion effects, whereas stream networks and distant valleys receive major peak-flow and sediment impacts. Chemical effects mainly occur near roads. Road networks interrupt horizontal ecological flows, alter landscape spatial pattern, and therefore inhibit important interior species. Thus, road density and network structure are informative landscape ecology assays. Australia has huge road-reserve networks of native vegetation, whereas the Dutch have tunnels and overpasses perforating road barriers to enhance ecological flows. Based on road-effect zones, an estimated 15–20% of the United States is ecologically impacted by roads.
Hatchling dispersal was measured from 1989-1991 in two populations of the side-blotched lizard, Uta stansburiana, in central California. Hatchlings from eggs incubated in the laboratory were released on site and were recaptured throughout the summer and the following spring. Median dispersal was approximately five times greater at Los Baños than at Del Puerto Canyon, and was likely due to different spatial distributions of microhabitats. Body size did not affect dispersal distance at either site despite an experimental increase in the range of hatchling body sizes. At Del Puerto Canyon in the summer, dispersal distances were greater in males than in females, but were not affected by the time of hatching. At Los Baños in the summer, dispersal distances were greater in males and late season hatchlings. Most trends were not significant in the spring at either site. Overall, there were large overlaps in dispersal distributions for all factors studied indicating a large stochastic component to lizard dispersal.