Estimating organisms' responses to environmental variables and taxon associations across broad spatial scales is vital for predicting their responses to climate change. Macroinvertebrates play a major role in wetland processes, but studies simultaneously exploring both community structure and community trait responses to environmental gradients are still lacking. We compiled a global dataset (six continents) from 756 depressional wetlands, including the occurrence of 96 macroinvertebrate families, their phylogenetic tree, and 19 biological traits. Using Bayesian hierarchical joint species distribution models (JSDMs), we estimated macroinvertebrate associations and compared the influences of local and climatic predictors on both individual macroinvertebrate families and their traits. While macroinvertebrate families were mainly related to broad-scale factors (maximum temperature and precipitation seasonality), macroinvertebrate traits were strongly related to local wetland hydroperiod. Interestingly, macroinvertebrate families and traits both showed positive and negative associations to the same environmental variables. As expected, many macroinvertebrate family occurrences were positively associated with temperature, but a few showed the opposite pattern and were found in cooler or montane regions. We also found that wetland macroinvertebrate communities would likely be affected by changing climates through alterations in traits related to precipitation seasonality, temperature seasonality, and wetland area. Temperature increases may negatively affect collector and shredder functional groups. A decrease in precipitation could lead to reductions in wetland area benefiting drought-tolerant macroinvertebrates, but it may negatively affect macroinvertebrates lacking those adaptations. Wetland processes may be compromised through broad-scale environmental changes altering macroinvertebrate family distributions and local hydroperiod shifts altering organism traits. Our complementary family-based and trait-based approaches elucidate the complex effects that climate change may produce on wetland ecosystems.

1. Climate change is leading many species to shift their geographical ranges. Species undergoing these range shifts often are moving into areas with heterogeneous abiotic conditions. Additionally, these range-shifting species will encounter resident species with whom they will compete for space and/or resources. However, the ways that these abiotic and biotic factors interact to influence the establishment and persistence of range-shifting species has received little attention. 2. Here, we conduct an in situ cage experiment examining how a local wetland hydroperiod gradient (i.e., temporary and semi-permanent ponds) and competition with a resident caddisfly species, Asynarchus nigriculus, influences the survival of the range-shifting species Limnephilus picturatus. We then use long-term survey data of population densities of these two species to determine whether pairwise interactions observed in the cage experiment translated into long-term dynamics. 3. The cage experiment revealed that A. nigriculus had a strong, negative effect on the survival of the range-shifting species L. picturatus, regardless of hydroperiod. However, we observed no relationship between the densities or occurrence of L. picturatus and A. nigriculus in long-term data for either temporary or semi-permanent ponds. 4. Our results suggest that landscape-level abiotic heterogeneity at range margins may not always be important for mediating antagonistic interactions between resident and range-shifting species. However, although an interaction appears ecologically significant in short-term field studies, broader context is needed to understand whether those types of interactions mediate species' distributions and abundance through time. 5. At face-value, our results from the field experiment and long-term data analysis did not align. This suggests that other factors such as additional competitive or trophic interactions may be more important drivers behind the population dynamics of this range-shifting species at its new upper-elevational limit.

Climate-driven species range shifts and expansions are changing community composition, yet the functional consequences in natural systems are mostly unknown. By combining a 30-year survey of subalpine pond larval caddisfly assemblages with species-specific functional traits (nitrogen and phosphorus excretion, and detritus processing rates), we tested how three upslope range expansions affected species' relative contributions to caddisfly-driven nutrient supply and detritus processing. A subdominant resident species (Ag. deflata) consistently made large relative contributions to caddisfly-driven nitrogen supply throughout all range expansions, thus "regulating" the caddisfly-driven nitrogen supply. Whereas, phosphorus supply and detritus processing were regulated by the dominant resident species (L. externus) until the third range expansion (by N. hostilis). Since the third range expansion, N. hostilis's relative contribution to caddisfly-driven phosphorus supply increased, displacing L. externus's role in regulating caddisfly-driven phosphorus supply. Meanwhile, detritus processing contributions became similar among the dominant resident, subdominant residents, and range expanding species. Total ecosystem process rates did not change throughout any of the range expansions. Thus, shifts in species' relative functional roles may occur before shifts in total ecosystem process rates, and changes in species' functional roles may stabilize processes in ecosystems undergoing change.

Functional trait diversity determines if ecosystem processes are sensitive to shifts in species abundances or composition. For example, trait variation suggests detritivores process detritus at different rates and make different contributions to whole-assemblage processing, which could be sensitive to compositional shifts. Here, we used a series of microcosm experiments to quantify species-specific coarse and fine particulate organic matter (CPOM and FPOM) processing for ten larval caddisfly species and three non-caddisfly species in high-elevation wetlands. We then compared trait-based models including life history, dietary, and extrinsic traits to determine which traits explained interspecific variation in detritus processing. Finally, we compared processing by mixed caddisfly assemblages in microcosms and natural ponds to additive predictions based on species-specific processing to determine if single-species effects are additive in multi-species assemblages. We found considerable interspecific variation in biomass-specific CPOM (13-fold differences) and FPOM (8-fold differences) processing. Furthermore, on a mass-specific basis, amphipods, chironomids, and caddisflies processed similar amounts of detritus, suggesting non-shredder taxa could process more than previously recognized. Trait models including dietary percent detritus, development rate, body size, and wetland hydroperiod explained 81 and 57% of interspecific variation in CPOM and FPOM processing, respectively. Finally, species-specific additive predictions were strikingly similar to mixed-assemblage processing in microcosms and natural ponds, with the largest difference being a 15% overestimate. Thus, additivity of species-specific processing suggests single-species rates may be useful for understanding functional consequences of shifting assemblages, and a trait-based approach to predicting species-specific processing could support generating additive predictions of whole-assemblage processing.

While many species distributions are shifting poleward or up in elevation in response to a changing climate, others are shifting their habitats along localized gradients in environmental conditions as abiotic conditions become more stressful. Whether species are moving across regional or local environmental gradients in response to climate change, range-shifting species become embedded in established communities of competitors and predators. The consequences of these shifts for both resident and shifting species are often unknown, as it can be difficult to isolate the effects of multiple species interactions. Using a model system of insects in high-elevation ponds in the Rocky Mountains of Colorado, we sought to disentangle the effects of predation and intraguild interactions on the survival and development of a semi-permanent pond resident caddisfly Limnephilus externus and the habitat-shifting caddis Asynarchus nigriculus that is being forced into semi-permanent ponds as temporary ponds dry too quickly to complete development. We conducted a manipulative in-situ pond cage experiment in which L. externus and A. nigriculus caddisfly larvae in single-species treatments and together were exposed to the presence/absence of predatory Dytiscus diving beetle larvae. This approach allowed us to isolate the effects of intraguild interactions and predation on the survival and development of both the resident and habitat-shifting species. We found that intraguild interactions had strong negative effects on the resident and habitat-shifting species. Intraguild interactions reduced the survival of the resident L. externus and increased the variation in survival of the shifting A. nigriculus. However, Dytiscus predators reduced these negative effects, stabilizing the community by increasing L. externus survival and reducing variation in A. nigriculus survival. We also found that intraguild interactions reduced L. externus biomass but resulted in increased A. nigriculus development. A. nigriculus development was also increased by predation. Our results show that strong intraguild interactions between resident and shifting species are likely to have negative consequences for both species. However, the presence of predators reduces these negative consequences of the habitat shift on both the resident and the shifting.

Climate change is rapidly driving global biodiversity declines. How wetland macroinvertebrate assemblages are responding is unclear, a concern given their vital function in these ecosystems. Using a data set from 769 minimally impacted depressional wetlands across the globe (467 temporary and 302 permanent), we evaluated how temperature and precipitation (average, range, variability) affects the richness and beta diversity of 144 macroinvertebrate families. To test the effects of climatic predictors on macroinvertebrate diversity, we fitted generalized additive mixed-effects models (GAMM) for family richness and generalized dissimilarity models (GDMs) for total beta diversity. We found non-linear relationships between family richness, beta diversity, and climate. Maximum temperature was the main climatic driver of wetland macroinvertebrate richness and beta diversity, but precipitation seasonality was also important. Assemblage responses to climatic variables also depended on wetland water permanency. Permanent wetlands from warmer regions had higher family richness than temporary wetlands. Interestingly, wetlands in cooler and dry-warm regions had the lowest taxonomic richness, but both kinds of wetlands supported unique assemblages. Our study suggests that climate change will have multiple effects on wetlands and their macroinvertebrate diversity, mostly via increases in maximum temperature, but also through changes in patterns of precipitation. The most vulnerable wetlands to climate change are likely those located in warm-dry regions, where entire macroinvertebrate assemblages would be extirpated. Montane and high-latitude wetlands (i.e., cooler regions) are also vulnerable to climate change, but we do not expect entire extirpations at the family level.

Certain habitat features of stream ecosystems can reduce their sensitivity to climate change and help protect the integrity of cold-water aquatic resources. Identifying such features is imperative for conserving the climate refugia of cold-water species. Using a combination of stream temperature and fish assemblage data, we quantified thermal sensitivities of 192 headwater streams in Northwest Pennsylvania to identify which landscape features best explained stream susceptibility to temperature change. We then projected changes in native brook trout (Salvelinus fontinalis) distributions, non-native brown trout (Salmo trutta) distributions and declines in cold-water thermal integrity under future climate warming and land use change scenarios. Brook trout were predicted to become increasingly relegated to smaller streams under future stream warming. However, we found that streams with intact forest cover at the watershed level had low thermal sensitivities, which slowed rates of projected warming. As a result, streams with forested watersheds were predicted to have smaller declines in thermal integrity and lower extirpation probabilities of brook trout. Additionally, non-native brown trout were not predicted to expand distributions under projected warming, suggesting minimal synergistic effects between non-native species and climate change. Forest cover buffers headwater streams from the effects of global change, similar to how groundwater inputs reduce the rate of stream warming. Forest restoration at riparian and watershed levels should help mitigate thermal-induced degradation of cold-water aquatic resources.

Species’ geographic range shifts toward higher latitudes and elevations are among the most frequently reported consequences of climate change. However, the role of species interactions in setting range margins remains poorly understood. We used cage experiments in ponds to test competing hypotheses about the role of abiotic and biotic mechanisms for structuring range boundaries of an upslope range‐shifting caddisfly Limnephilus picturatus . We found that competition with a ubiquitous species Limnephilus externus significantly decreased L. picturatus survival and emergence at subalpine elevations supporting the notion that species interactions play a critical role in determining upslope range limits. However, without competitors, L. picturatus survival was greater at high‐elevation than low‐elevation sites. This was contrary to decreases in body mass (a proxy for fecundity) with elevation regardless of the presence of competitors. We ultimately show that species interactions can be important for setting upslope range margins. Yet, our results also highlight the complications in defining what may be abiotically stressful for this species and the importance of considering multiple demographic variables. Understanding how species ranges will respond in a changing climate will require quantifying species interactions and how they are influenced by the abiotic context in which they play out.

In many lentic ecosystems, hydroperiod, or the duration of inundation, controls animal community composition and biomass. Although hydroperiod-imposed differences in wetland animal communities could cause differences in animal-driven nutrient supply, hydroperiod has not been considered as a template for investigating patterns of animal-driven nutrient cycling. Here, we use nutrient excretion rates (NH 4 -N and SRP) and biomasses of pelagic and benthic invertebrates and salamanders and nutrient uptake rates in a simulation model to estimate animal-driven nutrient supply and pond-level demand along a hydroperiod gradient of 12 subalpine ponds in the U.S. Rocky Mountains that are vulnerable to climate change. We found that animal biomass increased with hydroperiod duration and biomass predicted animal-driven supply contributions among hydroperiod classifications (temporary-permanent). Consequently, community-wide supply was greatest in permanent ponds. Animal-driven N supply exceeded demand in permanent and semi-permanent ponds, whereas P supply equaled demand in both. Conversely, temporary ponds had large deficits in N and P supply due to lower community biomass and hydroperiod-induced constraints on dominant suppliers (oligochaetes and chironomids). The distribution of taxon-specific supply also differed among hydroperiods, with supply dominated by a few taxa in permanent ponds and supply more evenly distributed among temporary pond taxa. The absence or lower biomass of dominant suppliers in temporary ponds creates nutrient deficits and possible limitation of productivity. Thus, as climate warming causes hydroperiods to become increasingly temporary and indirectly prompts biomass declines and compositional shifts, animal-driven nutrient supply will decrease and strong nutrient limitation may arise due to loss of animal-driven supply.

Freshwater wetlands can produce a large volume of detritus annually, including hydrophyte litter in herbaceous habitats and tree-leaf litter and dead wood in forested habitats. It has been assumed that detritus provides many ecological benefits to resident macroinvertebrates and, in turn, that these macroinvertebrates contribute substantially to the breakdown of wetland detritus. However, research conducted to date provides ambiguous conclusions, with reciprocal macroinvertebrate–detritus relationships apparently changing in importance across different global freshwater wetlands. We reviewed the body of research and assessed 96 empirical studies on detritus and macroinvertebrates in freshwater wetlands. Based on this review, we conclude that detritus provides many benefits to macroinvertebrates, both as a nutritional source (hydrophyte and tree-leaf litter) and as habitat (dead wood). Shredder macroinvertebrates, which are adapted to consume detritus, were commonly reported in some freshwater wetlands but were rare or absent in most. Where shredders were common, they contributed substantially to detritus breakdown. Where shredders were rare, macroinvertebrates played minor roles in detritus breakdown, which was instead processed by the microbial community. When assessing ecosystem functions in freshwater wetlands, it will be important to know the specific nature of the resident macroinvertebrate assemblages and the different ways that these organisms may or may not influence C flows.

Monitoring long-term changes in aquatic biodiversity requires the effective use of historical data that were collected with different methods and varying levels of effort. Aggregating data into different spatial scales can control for such differences and provide a robust framework for monitoring distribution trends. We used a quantitative, multi-scale assessment to evaluate the potential drivers of distribution change for 60 fish species at three spatial scales, using 503 unique sampling events conducted between 1931 and 2019 in a stream biodiversity hotspot (French Creek, Pennsylvania, U.S.A). Trends delineated at multiple scales demonstrated that only one cyprinid species consistently declined through time. In contrast, several species, particularly centrarchids (bass and sunfish), appeared to increase with time. However, evidence for species’ increases varied among the different spatial scales, and our observations suggest that differences in effort and detection across time periods may contribute to patterns of species increases. There was agreement among scales that agricultural land use, non-native brown trout (Salmo trutta), and anthropogenic barriers did not explain patterns in biodiversity change from the distribution trends in this study. The lack of species declines is likely due to the limited levels of historical impacts in the watershed compared with other locations in the region that experienced more acute pollution bottlenecks. Species increases were most prevalent for sportfish and baitfish species, suggesting that distribution increases were human mediated. Similar multi-scale assessments should provide more robust insight into patterns of biodiversity loss and distribution changes by maximizing the use of historical data.

Global climate change is expected to shorten hydroperiods and accelerate drying of ephemeral freshwater habitats, a shift that is likely to increase intraspecific competition and cannibalism in the aquatic animals that rely on those habitats. We experimentally examined the effects of simulated pond drying, tank size (initial larval density), and body size on survival and cannibalism in larvae of the dragonfly Anax junius, a species known to show frequent size-structured cannibalism. Thirty tanks of 3 different sizes were each stocked with 8 A. junius larvae (6 small, 1 medium, and 1 large) along with Enallagma damselfly larvae as prey. Anax junius survival and cannibalism were documented daily for 16 d. For tanks in the permanent hydroperiod treatment, we maintained water depth at a constant 14 cm for all 16 d, while we gradually reduced depth in the temporary hydroperiod tanks from 14 to 2 cm to simulate pond drying. We found that cannibalism was strongly size-dependent, as 31, 7, and 0% of small, medium, and large larvae, respectively, were cannibalized. Tank size (initial larval density) and hydroperiod treatment both affected larval survival and cannibalism. However, the effects of simulated pond drying were more pronounced than those of tank size. In addition, hydroperiod treatment was a predictor of daily risk of larval cannibalism, but daily volumetric larval density (number of A. junius alive divided by water volume present that day) was not. Our results, therefore, indicate that 1) pond drying can substantially increase cannibalism in larval odonates beyond its simple effect of producing high-density populations as water levels recede and 2) the effect of drying cues on the behavior and life-history characteristics of aquatic invertebrates merit increased attention from freshwater ecologists.

While inundated, small ponds (< 1000 m2 area) account for disproportionately large contributions of CO2 efflux to the global carbon budget and also store carbon in anoxic sediments. However, pond hydrology is shifting toward increasingly dry conditions in alpine and temperate zones, which might lead to increased exposure of shallow pond sediments. We analyzed sediment CO2 efflux rates in dried sediments of multiple ponds of varying hydrology and sediment characteristics at montane and subalpine elevations near the Rocky Mountain Biological Laboratory in Colorado. Average CO2 efflux rates from exposed sediments, 331.5 ± 11.5 mmol m−2 d−1 at the montane sites and 142.8 ± 45.1 mmol m−2 d−1 at the subalpine sites, were 10 times higher than average CO2 efflux rates from pond water. Principal components analysis to reduce dimensionality of sediment characteristics revealed that random inter‐pond differences rather than exposure timing or hydroperiod drove variation among sediments. In linear mixed effects models of CO2 flux rates, significant predictors included sediment moisture and temperature, pH, total organic carbon, and organic matter content at all pond hydroperiod classifications and sites. However, the sediment characteristics explaining the most variance differed among sites and hydroperiods and included nitrate concentrations, pH, bulk density, and temperature. We conclude that pond sediments are heterogeneous both within and among ponds in close proximity, and drivers of relatively high CO2 efflux rates differ among pond hydroperiods and elevations. This work emphasizes that local differences can impact predictions of CO2 flux from lentic sediments which are becoming increasingly exposed.

The efficacy of assessments that evaluate biological integrity can be improved by accounting for the ecological processes that influence assemblage composition. Many studies have emphasized that bioassessments need to account for natural environmental gradients, but there is little consensus on how bioassessments should account for the impacts of non-native species. In particular, non-native trout species have been introduced into many high-quality streams that probably meet the expected reference conditions for bioassessment in a given region. The goal of this study was to test whether the presence of large, piscivorous, non-native Brown Trout (Salmo trutta) at reference sites altered interpretations of taxonomic completeness indices (TCIs) based on fish assemblages. We used fish data from 215 sites in wadeable streams in northwestern Pennsylvania to compare the performance of 3 TCIs that used different modeling approaches to account for Brown Trout impacts. One model accounted for the presence of non-native Brown Trout as a covariate (covariate model), another model censored reference sites with non-native Brown Trout from the reference pool (censored model), and a final model used all reference sites without accounting for the presence of Brown Trout (unaccounted model). TCIs based on observed-to-expected ratios were able to distinguish reference conditions from altered conditions in the covariate and censored models. In contrast, the unaccounted index could not distinguish reference from altered conditions and was, thus, unable to accurately assess biological integrity, probably because large Brown Trout reduce native species richness. Our results provide a framework for how bioassessment practitioners can use different approaches to account for non-native species impacts, especially when considering which criteria are most important for defining reference conditions. Accounting for the effects of non-native species with these approaches should improve the ability of bioassessments designed to summarize the interactive effects of all potential human stressors on stream assemblages.

There is considerable variation among studies that evaluate how amphibian populations respond to global climate change. We used 23 years of annual survey data to test whether changes in climate have caused predictable shifts in the phenology and population characteristics of adult spotted salamanders (Ambystoma maculatum) during spring breeding migrations. Although we observed year-to-year correlation between seasonal climate variables and salamander population characteristics, there have not been long-term, directional shifts in phenological or population characteristics. Warm winters consistently resulted in early migration dates, but across the 23-year study, there was no overall shift towards warmer winters and thus no advanced migration timing. Warm summers and low variability in summer temperatures were correlated with large salamander body sizes, yet an overall shift towards increasing body sizes was not observed despite rising summer temperatures during the study. This was likely due to the absence of long-term changes of within-year variation in summer temperatures, which was a stronger determinant of body size than summer temperature alone. Climate-induced shifts in population characteristics were thus not observed for this species as long-term changes in important seasonal climate variables were not observed during the 23-years of the study. Different amphibian populations will likely be more resilient to climate change impacts than others, and the probability of amphibians exhibiting long-term population changes will depend on how seasonal climate change interacts with a species' life history, phenology, and geographic location. Linking a wide range of seasonal climatic conditions to species or population characteristics should thus improve our ability for explaining idiosyncratic responses of species to climate change.

Small ponds account for a disproportionately high percentage of carbon dioxide emissions relative to their small surface area. It is therefore crucial to understand carbon flow in these ponds to refine the current global carbon budget, especially because climate change is affecting pond hydrology. High elevation ponds in the Elk Mountains of western Colorado are drying more frequently as the timing of snowmelt advances. We compared CO2 concentrations and fluxes among ponds of different hydroperiods over diel sampling periods during the course of the 2017 open-water period. CO2 concentrations were significantly negatively correlated with pond depth and averaged 77.6 ± 24.5 μmol L−1 (mean ± S.E.) across all ponds and sampling events. Ponds were up to twenty times supersaturated in CO2 with respect to the atmosphere. Flux was highly variable within individual ponds but correlated with time of sampling and was highest at night. Flux averaged 19.7 ± 18.8 mg CO2 m−2 h−1 across all ponds and sampling events. We also compared flux values obtained using modeled and empirical methods and found that widely-applied models of gas exchange rates using wind-based gas exchange (K) values yielded estimates of CO2 flux that were significantly higher than those obtained using the floating chamber approach, but estimates of CO2 flux using globally averaged convection-based K values were lower than those obtained using the floating chambers. Lastly, we integrated soil vs. water efflux measurements with long-term patterns in hydrology to predict how total season-long efflux might change under the more rapid drying regimes and longer seasons that are already occurring in these systems. Because soil CO2 efflux averaged 277.0 ± 49.0 mg CO2 m−2 h−1, temporary ponds emitted 674.1 ± 99.4 kg CO2 m−2 over the course of the 2017 season from ice-out to refreezing, which was over twice as much as permanent and semi-permanent ponds. Our results emphasize that contributions of CO2 from small ponds to the global carbon budget estimates will vary with pond hydroperiod and sampling methodology, which have been overlooked given that most previous estimates were collected from limited sampling periods and from pond waters alone. Furthermore, pond CO2 contributions are predicted to increase over time as pond areas transition from efflux from water to efflux from soil.

Few in situ studies have investigated the biological drivers of detritus processing in shallow lentic systems, despite abundant evidence that vascular plant detritus is a primary source of nutrients and energy. In particular, the relative importance of microbial decomposers and animal detritivores to overall detritus breakdown is poorly documented. Caddisfly larvae (Trichoptera: Limnephilidae) are often the biomass-dominant animal detritivores in high-elevation and high-latitude ponds and wetlands in the northern hemisphere. The larvae of many limnephilid caddisfly species are shredders that rely on detritus as their primary food source, and they may therefore play an important role in litter breakdown in lentic systems. Here, we manipulated abundances (present/absent) of caddisfly larvae in shallow montane ponds in Colorado, and compared sedge detritus breakdown rates across treatments. We found that coarse particulate organic matter (CPOM) was converted to fine particulate organic matter (FPOM) 2 to 3 x faster when caddisflies were allowed access to the detritus than when not, indicating that caddisflies play a key role in litter breakdown in these temporary habitats. Dietary data from the 6 species of caddisflies in the ponds revealed that all primarily consume CPOM derived from vascular plants, although the ratios of CPOM and FPOM in the diets varied among species. The biomass of caddisflies relative to detrital inputs is particularly high at our study sites compared with other eutrophic, low-elevation wetlands. Thus, we suspect that animal detritivory relative to microbial processing may be especially high in these ponds. Future in situ, whole community studies in basins that differ in hydroperiod, nutrient status, and ratio of detrital inputs to detritivore biomass will be needed to construct a general model of detritus breakdown in shallow lentic freshwater habitats.

A popular conservation strategy for native trout species in western North America is to prevent invasions by nonnative trout by installing barriers that isolate native trout populations into headwater streams. In eastern North America, native Brook Trout Salvelinus fontinalis are frequently replaced in coolwater habitats by nonnative Brown Trout Salmo trutta and relegated to small headwater streams. In this study, we compared the effects of isolation and invasion by nonnative Brown Trout on the distribution and demographic structure of Brook Trout populations from 78 trout streams in northwestern Pennsylvania. The Brook Trout and Brown Trout distributions varied in predictable ways along the stream size gradient, with Brown Trout becoming dominant in larger streams. However, there was a prominent barrier effect, with streams 12 times more likely to have Brook Trout than Brown Trout when a downstream barrier was present between the sample site and the nearest Brown Trout stocking location. In comparison, 91% of the streams with Brown Trout had no downstream barrier, suggesting that barriers are important in creating refugia for Brook Trout. Brown Trout also appeared to have a negative impact on Brook Trout population demographics, as Brook Trout populations in sympatry with Brown Trout had fewer age-classes and lower population densities than allopatric Brook Trout populations. Isolating Brook Trout to small headwater streams with downstream barriers that prevent Brown Trout invasion could be a viable conservation strategy in regions where barriers would serve to reduce the negative impacts from Brown Trout. Since barriers could further fragment local Brook Trout populations, however, they would need to be strategically placed to allow for seasonal movements to maintain metapopulation structure and ensure population persistence.

Abstract Understanding the amount of variation in functional traits between closely related species within guilds is critical for understanding links between community composition and ecosystem processes. Nutrient excretion is an important link between animals and their environments, and aquatic invertebrate communities can supply a considerable proportion of ecosystem nutrient demand via excretion. We quantified nitrogen (N) and phosphorus (P) excretion rates of 10 species of larval caddisflies that inhabit high‐elevation ponds and wetlands to determine the magnitude of variation in nutrient excretion within this guild. We found considerable interspecific variation in biomass‐specific excretion of nitrogen (eightfold differences), phosphorus (sevenfold differences), and the stoichiometric N:P ratios (fivefold differences). Through a meta‐analysis, we compared the variation within this guild to the variation found in other family‐level species assemblages to determine the overall range in the variation of nutrient excretion that could be expected across guilds and to determine whether the variation in this caddisfly guild is comparatively extreme, average, or low. The meta‐analysis revealed a large range in variation among guilds, and comparatively, the variation within this caddisfly guild is high for N excretion and intermediate for P excretion. The considerable variation within guilds revealed by our meta‐analysis suggests that functional redundancy among guild members is difficult to predict. Thus, some natural or human‐caused species gains or losses within biological groupings such as guilds and trophic levels could have little or no effect on ecosystem processes, whereas others could have very large effects.

Summary Evaluating the biological integrity of stream ecosystems requires a clear understanding of biological responses to anthropogenic stressors. Co‐variation of stressors with natural landscape gradients has been shown to complicate the ability of biological assessments to detect community‐level responses to anthropogenic stressors. Similarly, the co‐varying occurrence of non‐native species would also likely confound the ability of biological assessments to accurately determine biological integrity, although these relationships have been less studied. We compared fish communities in 99 wadeable tributaries (with catchments of 10–35 km2) of the upper Allegheny River watershed, Pennsylvania, U.S.A., to disentangle the effects of human‐induced stressors associated with agricultural development, the presence of non‐native brown trout (Salmo trutta) and background variation in community composition associated with natural landscape features. Multivariate analyses using both taxonomic and trait‐based data revealed that environmental gradients sorted fish species into different thermal communities (coolwater versus warmwater). Layered over the shift in thermal communities was the presence of large‐bodied (piscivorous) brown trout, which along with agricultural land use, was a consistent determinant of community diversity and presence–absence of nine common native species. Small‐bodied taxa (minnows, darters) were the most responsive to the presence of these large non‐native predators. Our results highlight how the presence of a non‐native species can modify the interactions between natural and anthropogenic stressor gradients and therefore confound the ability to detect signals between land use stressors and local stream community composition. The complicated interaction between brown trout and other anthropogenic stressors documented herein is likely to be a widespread phenomenon given the global introductions of many salmonid species into systems with varying anthropogenic stressor gradients. Overall, understanding how non‐native species influence stream community composition should lead to biological assessments with greater capabilities for detecting non‐native effects on biological integrity relative to the more frequently evaluated land use stressors.