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A map of the Verde River watershed located in central Arizona, USA. Points represent the communitylevel sampling efforts from discrete NHD+ reachcode units that match our data requirements. Point size is proportional to the degree of community-level non-native dominance (percentage abundance, 0–100%) and the black rectangle represents the approximate study area.
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A contemporary challenge in ecology is assessing the ecosystem effects of multispecies introductions. Quantifying shifts in body sizes, a common trait with which many per capita rates of ecosystems functioning scales, provide an important way forward. Evidence suggests that freshwater fish introductions have altered species-level body size distribu...
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Although species with larger body size and slow pace of life have a higher risk of extinction at a global scale, it is unclear whether this global trend will be consistent across biogeographic realms. Here we measure the functional diversity of terrestrial and freshwater vertebrates in the six terrestrial biogeographic realms and predict their futu...
Citations
... The need to use the same trait in two different test systems dictated our choice of body size. Conveniently, size is a universal trait influencing many other traits and population dynamics (Fritschie & Olden, 2016) and serves here to conduct a proof of concept exercise. Others (e.g., Padisak et al., 2009) considered body size to define functional groups in phytoplankton or reported sizedependent regularities in zooplankton communities (Gianuca et al., 2018). ...
A functional group approach to analyzing ecological processes calls for aggregating species populations' data (e.g., biomass, abundance) into larger functional units. This method offers a more direct insight into the coarse community and ecosystem dynamics than a species level. Since aggregation changes the scale of analysis, it is reasonable to expect that the results and inferences will also change. To gauge the nature and size of this effect, we examined two sets of communities using the same methodology at taxonomic and functional levels. We asked how asynchrony among constituent populations stabilizes the regional biomass or abundance in two systems: zooplankton in North American temperate lakes and rock pool invertebrates in Jamaica. We aggregated species into pairs by organism size for the functional level analysis because it correlates well with life history traits. We hypothesized that taxonomical and functional level analyses would yield different pictures and inferences. We analyzed asynchrony among local populations, metapopulations, and populations of different species at different locations. We found that in lakes, different asynchrony classes contributed to the stabilization of the regional metric than in the rock pools. We further found that aggregation of taxa into functional units changed the pattern of differences between lakes and rock pools revealed by taxonomical analysis and that each set of communities responded differently to species lumping. While aggregating taxa leads to different results, the taxonomic and functional lenses add new insights to the biological interpretation of mechanisms stabilizing communities when interpreted jointly. Specifically, population asynchrony—a known stabilizing factor—can provide complementary stabilizing mechanisms when seen through taxonomic and functional lenses. Without exposing these modes, inferences about abundance/biomass/richness stabilizing contributions arising from asynchrony will remain tenuous. Exposing these modes offers a path for significant breakthroughs in linking local, spatial, and functional structures to multispecies system dynamics.
... By examining changes in the individual size distribution (ISD) at the community level (vs. average size distribution at the species level), patterns may emerge in community response to species loss (Balvanera et al. 2006;Fritschie and Olden 2016). ...
Resilience of ecosystems to the sudden decline of large‐bodied species is dependent on characteristics of surviving guild members. However, that response may also be mediated by local habitat conditions. Here, we examine the mechanisms behind the observed lack of functional compensation in the algal‐grazing guild by insect grazers following the decline of tadpole grazers in a forested Panamanian stream. We examined: (1) shifts to the individual size distribution of insect grazers between pre‐ and post‐tadpole declines in pool and riffle habitats; (2) tadpole and insect preferences for small‐, medium‐, and large‐sized diatoms; and (3) a causal explanation for why insects did not functionally compensate for tadpole declines. The size distribution of insect grazers following tadpole declines differed between habitats, becoming uniform in pools and more right skewed toward a smaller size class in riffles. In both habitats, tadpoles selectively consumed medium‐sized diatoms but avoided the largest‐sized diatoms. In contrast, grazing insects selectively consumed small‐sized diatoms, but switched to medium‐sized diatoms after tadpole declines. Tadpole declines led to the loss of the strongest interactions between consumers and diatoms. Smaller‐bodied grazing insects could not duplicate these interactions, even with a shift in resource use, providing an explanation for the lack of functional compensation. Furthermore, tadpole declines led to different community structures in each habitat indicating that local habitat conditions mediated the response of surviving guild members. This suggests that the sudden decline of a large‐bodied species does not lead to a singular outcome for the surviving community.
... A relatively new empirical framework combining species composition and body size distributions by comparing cumulative abundance profiles in resemblance measurements (De Cáceres et al., 2013) shows promise for addressing this challenge (Appendix S1), but how broadly applicable this approach will be for assessing the status of anthropogenically impacted catchments remains to be analysed. In this vein, Fritschie and Olden (2016) concluded that exotic species introductions shifted fish body size in the Lower Colorado River Basin, leading to the question of whether resemblance in terms of size structure can also be used to assess the effects of land-use degradation on stream ecosystems. ...
... Following De Cáceres et al. (2013), cumulative abundance values were also log-transformed for further analyses. We re-ran our analyses with a bin size of 0.3 log units and found no qualitative changes in the resulting CAPs (Appendix S4), supporting previous research suggesting that this framework is robust to variations in the distribution of bin sizes (De Cáceres et al., 2013;Fritschie and Olden, 2016). We stratified the analyses by stream groups based on anthropogenic land-use degradation (i.e., NP, MI and SI; see above) to test whether resemblance differed between near-pristine and anthropogenically impacted sites. ...
... Hence, it may be particularly useful to address taxon-specific variation in individual body sizes between sites, and the response of assemblage size structure to different catchment management practices intended to reduce land-use impacts on stream ecosystems. Recent work in this area of research is promising (Basset et al., 2012;Siqueira et al., 2015;Fritschie and Olden, 2016), and it might be examined best by setting much needed expectations from biogeographically distinct regions with a range of human disturbances. ...
Body size descriptors and associated resemblance measurements may provide useful tools for forecasting ecological responses to increasing anthropogenic land‒use disturbances. Yet, the influences of agriculture and urbanisation on the size structure of biotic assemblages have seldom been investigated in running waters. Using a comprehensive dataset on stream macroinvertebrates from 21 river basins across Western Finland, we assessed whether the structure of assemblages via changes in taxonomic composition and body size distributions responded predictably to anthropogenic land‒use impacts. Specifically, we applied a combination of resemblance measurements based on cumulative abundance profiles and spatially constrained null models to understand faunal impairment by agricultural and urban development, and the most likely mechanisms underlying the observed shifts in assemblage size structure. Anthropogenically impacted stream sites showed less variation in assemblage composition and size distributions compared with least‒disturbed sites, with strong declines in internal variation also occurring for the transition from near‒pristine to moderately impacted landscapes. These results were consistent whether based on species‒level or genus‒level data. Variation in assemblage size structure seemed to be more predictable than taxonomic composition, supporting the notion that resemblance measurements based on body size distributions can represent an improvement to more traditional approaches based on taxonomic identities alone. In addition, we showed that macroinvertebrate assemblages resulted from effects of land‒use degradation mediated through local conditions and spurious spatial structures in the distribution of anthropogenic activities across the landscape. Overall, our findings suggest that existing water policies and agri‒environment schemes should be guided not only by understanding the individual effects of agricultural and urban development on taxonomic composition at a given stream site. Rather, we should also acknowledge the size structure of stream assemblages and whether concomitant changes in local conditions and the non‒random distribution of human infrastructures are likely to mitigate or accelerate these effects.
... Invasive species (Strayer 2010) and river flow alteration via dams (Bunn and Arthington 2002;Nilsson et al. 2005) represent two of the major drivers of biodiversity change in aquatic ecosystems worldwide. These anthropogenic pressures can adversely affect the biomass and abundance of native species (Gehrke et al. 1995;Propst et al. 2008), thereby altering species and community size structure at varying spatial scales (Blanchet et al. 2010;Jellyman et al. 2017;Fritschie and Olden 2016). However, no studies have examined whether these anthropogenic pressures affect community biomass -body size scaling relationships. ...
... Different types of changes to community biomass -body mass scaling exponents and trophic structure may be expected in systems where the most prolific invasive species are large-bodied piscivores (e.g., salmonidae) or where the most prolific invasive species are small-bodied and feed at low trophic levels (e.g., Gambusia spp.). For example, Fritschie and Olden (2016) found that individual size distributions nonnative species were generally smaller than native fishes in dryland rivers of Arizona but their dominance did not influence community trophic structure. Conversely, nonnative salmonids have had profound impacts on food webs and have caused trophic cascades affecting native species (Crowl et al. 1992;Jellyman et al. 2017), most likely with alterations to the community biomasssize structure, trophic energy transfer efficiency, and predator-prey mass ratios. ...
The biomass of organisms of different sizes is increasingly being used to explore macroscale variation in food-web and community structure. Here we examine how invasive species and river flow regulation affect native fish biomass and fish community log10 biomass – body mass scaling relationships in Australia’s largest river system, the Murray–Darling. The log10 biomass – body mass scaling exponent (scaling B) of invasive fishes (95% CI: −0.14 to −0.18) was less negative than for native fishes (95% CI: −0.20 to −0.25), meaning that invasive species attained a higher biomass in larger size-classes compared to native species. Flow alteration and invasive common carp (Cyprinus carpio) biomass were correlated with severe reductions in native fish biomass ranging from −47% to −68% (95% CI). Our study provides novel evidence suggesting that invasive and native communities have different biomass – body mass scaling patterns, which likely depend on differences in their trophic ecology and body size distributions. Our results suggest that restoration efforts using environmental flows and common carp control has potential to boost native fish biomass to more than double the current level.
... During the disturbance, key habitat changes should be evaluated in combination with an assessment of trait changes and community interactions. Planning for habitat restoration should involve extensive review of evolutionary and plastic responses to environmental variables, allowing management plans to be implemented at an appropriate timescale for recovery assessment of species assemblages in a dryland river in the USA found that introduced species tend to have smaller size distributions than native species, but in fish members of the invertivore and piscivore guilds, non-native fishes are overall larger (Fritschie & Olden, 2016). Although size structuring is not influenced by predatory or competitive interactions between native and introduced fishes in this system, it is likely that a variety of ecological and evolutionary mechanisms influence community dynamics and ecosystem processes (Fritschie & Olden, 2016). ...
... Planning for habitat restoration should involve extensive review of evolutionary and plastic responses to environmental variables, allowing management plans to be implemented at an appropriate timescale for recovery assessment of species assemblages in a dryland river in the USA found that introduced species tend to have smaller size distributions than native species, but in fish members of the invertivore and piscivore guilds, non-native fishes are overall larger (Fritschie & Olden, 2016). Although size structuring is not influenced by predatory or competitive interactions between native and introduced fishes in this system, it is likely that a variety of ecological and evolutionary mechanisms influence community dynamics and ecosystem processes (Fritschie & Olden, 2016). Regular evaluation of the population size structure may provide a cheap and simple way of monitoring food web structure, as body size is usually positively (Romanuk, Hayward, & Hutchings, 2011), but sometimes negatively (Burress, Holcomb, Bonato, & Armbruster, 2016) correlated with a species' trophic position. ...
Aquatic ecosystems are facing escalating threats from urbanization, habitat loss and projected impacts of climate change, which both individually and in combination have the potential to fundamentally alter ecosystem functioning. While it is well established that habitat disturbances can affect the composition and diversity of aquatic communities, only recently have studies considered whether such impacts result in changes in species’ functional traits. We consider how functional traits of freshwater and marine fishes respond to environmental change, and how shifts in the expression of these traits can impact community dynamics and key ecological processes, including trophic interactions and nutrient transfer. We find that a multitude of functional traits, including behavioural and sensory traits, is sensitive to habitat disturbances. We demonstrate how these trait changes can be used to reveal hidden “ecological diversity” as well as serving as early indicators of environmental perturbation. We conclude that management strategies that consider the fundamental biological responses of fishes to habitat disturbance will be particularly effective in determining causal relationships within the ecological network. While detailed information on trait function is often lacking, even some general understanding of trait function and importance will facilitate targeted and efficient ecosystem management. We urge fisheries biologists and aquatic ecosystem managers to consider the role of functional traits in facilitating effective habitat restoration and management.
... This simulation method best reproduced the empirical data to which allometric models were originally We calculated the average body size (g), total biomass (g) and nonnative dominance (proportion of total biomass attributed to nonnative species) for each fish community. We were primarily interested in the effects of non-native dominance on nutrient recycling, but total site biomass and average body size are also important predictors of aggregate recycling rates (Hall, Koch, Marshall, Taylor, & Tronstad, 2007) and are known to vary with non-native introductions in the Verde River (Fritschie & Olden, 2016). We tested competing multiple regression models relating the main effects of nonnative dominance, total mass and average body size to aggregate recycling rates and ratios. ...
• Non‐native species are now common in community assemblages, but the influence of multiple introductions on ecosystem functioning remains poorly understood. In highly invaded systems, one promising approach is to use functional traits to scale measured individuals’ effects on ecosystem function up to the community level. This approach assumes that functional traits provide a common currency among species to relate individuals to ecosystem functioning.
• The goals of this study were to (i) test whether the relationship between body size and ecosystem functioning (per capita nutrient recycling) was best described by general or species‐specific scaling models; (ii) relate community structure (total biomass, average body size, non‐native dominance) to aggregated, community‐level nutrient recycling rates and ratios; and (iii) determine whether conclusions regarding the relationships between community structure and aggregate ecosystem functioning differed between species‐specific and general scaling approaches.
• By combining experimental incubations and field surveys, we compare consumer‐mediated nutrient recycling of fish communities along a non‐native dominance gradient in the Verde River watershed of central Arizona, USA. Data from ˜340 field‐sampled freshwater fish demonstrated support for general allometric relationships predicted by the metabolic theory of ecology (NH4‐N scaling coefficient = 0.72 [0.64–0.80]; PO4‐P = 0.67 [0.47–0.86]). However, the best‐fit models for N and P included species‐specific random effects for both allometric slopes and intercepts.
• According to species‐specific models, stream fish communities recycled 1–12 mmol NH4‐N/hr (median = 2.8 mmol/hr) and 0.02–0.74 mmol PO4‐P/hr (median = 0.07 mmol/hr) at N:P ratios between 13.3 and 83.5 (median = 28.8). General models generated similar estimates for NH4‐N recycling but less accurate estimates for PO4‐P. Stochastic simulations that incorporated error around allometric parameter estimates led to qualitatively similar but larger differences between general and species‐specific results.
• Community structure influenced aggregate nutrient recycling, but specific conclusions depended on the scaling approach. Total biomass explained much of the among‐community variation in aggregate NH4‐N and PO4‐P for both model types, whereas non‐native dominance alone best predicted variation in aggregate N:P. Surprisingly, species‐specific and general models both reached significant yet quantitatively opposing conclusions regarding the relationship between N:P supply and non‐native dominance.
• Study results indicate that shifting fish community structure can substantially alter ecosystem functioning in this river system. However, some inferred relationships between community structure and aggregate nutrient recycling varied depending on whether general or species‐specific scaling approaches were taken. Although trait‐based approaches to link environmental change, community structure and ecosystem function hold much promise, it will be important to consider when species‐specific versus general models are necessary to scale from individuals to ecosystems.
Predicting the impacts of species introductions long has attracted the attention of ecologists yet there still is limited insight into how impacts on native assemblages vary with the degree of shared evolutionary context. Here, we used data from 535 stream-fish surveys from 15 catchments in north-eastern Spain (99,700 km2) to explore whether the relative effects on native fishes differ between fish introductions from two different ecoregions (i.e., evolutionary contexts), namely, catchments within Iberian Peninsula (i.e., ‘translocated species’) and catchments beyond Iberian Peninsula (i.e., ‘exotic fishes’). We used hierarchical Bayesian models to relate taxon richness, abundance, and the individual-size distributions (ISDs) of native fishes to the presence, abundance, and weighted trophic level (TL) of translocated and exotic fishes, conditional on geographic and habitat covariates. Environmental covariates dominated the percentage of explained variance (≥ 65%) for all responses. Translocated fishes accounted for more of the explained variance than did exotic fishes for ISDs and abundance, but not for native fish species richness. The presence of translocated fishes was associated with lower abundance and richness of native fishes, with individuals being smaller in the presence of translocated fishes of higher TL. The presence of exotic fishes was associated with a greater abundance and richness of native fishes, with individuals generally being larger in the presence of exotic fishes. Our study suggests that translocated fishes could be as problematic as exotic fishes when angling and water transfers among catchments to deal with climate change may increase the establishment of translocated fishes. We also discuss the difficulties of using fish body size as species-blind, transferable assemblage-level trait in fish monitoring.
