AimThe influences of environmental and spatial processes on species composition have been at the center of metacommunity ecology. Conversely, the relative importance of these processes for species co-occurrences and taxonomic similarity has remained poorly understood. We hypothesised that at a subcontinental scale, shared environmental preference would be the major driver of co-occurrences across species groups. In contrast, co-occurrences due to shared dispersal history were more likely in dispersal-limited taxa. Finally, we tested whether taxa co-occurring due to similar responses to environmental and spatial processes were more taxonomically similar than expected by chance.LocationThe conterminous United States.Time Period1993-2019.Major Taxa StudiedStream diatoms, insects and fish.MethodsWe generated co-occurrence networks and developed methodology to determine the proportions of nodes and edges explained by pure environment alone (after accounting for space), pure space alone (after accounting for the environment), pure environment and pure space together, and spatially structured environment. Taxonomic similarity of taxa co-occurring because of environmental and/or spatial controls or because of unmeasured processes was compared to that of a null model.ResultsPure environment alone, spatially structured environment, and pure environment and pure space together explained the greatest proportion of nodes and edges in the co-occurrence networks of diatom species and genera, and insect genera. Conversely, pure environment and pure space together best explained the nodes and edges in the co-occurrence network of fish species and genera. Co-occurring taxa were more closely related than the random expectation in all comparisons.Main ConclusionsThe environment controlled co-occurrences in all groups, while the influence of space was the strongest in fish, the most dispersal-limited group in our study. All co-occurring taxa were more taxonomically related than expected by chance due to environmental or spatial overlap or unaccounted factors.

Motivation: Freshwater ecosystems have been heavily impacted by land-use changes, but data syntheses on these impacts are still limited. Here, we compiled a global database encompassing 241 studies with species abundance data (from multiple biological groups and geographic locations) across sites with different land-use categories. This compilation will be useful for addressing questions regarding land-use change and its impact on freshwater biodiversity. Main Types of Variables Contained: The database includes metadata of each study, sites location, sample methods, sample time, land-use category and abundance of each taxon. Spatial Location and Grain: The database contains data from across the globe, with 85% of the sites having well-defined geographical coordinates. Major Taxa and Level of Measurement: The database covers all major freshwater biological groups including algae, macrophytes, zooplankton, macroinvertebrates, fish and amphibians.

Aim: Understanding the roles of deterministic and stochastic processes in community assembly is essential for gaining insights into the biogeographical patterns of biodiversity. However, the way community assembly processes operate is still not fully understood, especially in oceanic islands. In this study, we examine the importance of assembly processes in shaping diatom communities in islands and continents, while also investigating the influence of climate and local water chemistry variables on species distributions.Location: Global.Taxon: Stream benthic diatoms.Methods: We used diatom datasets from five continents and 19 islands and applied beta diversity analyses with a null model approach and hierarchical joint species distribution modelling. To facilitate comparisons with continents, we used continental area equivalents (CAEs), which represent continental subsets with comparable areas and the same number of study sites as their corresponding islands counterparts.Results: We found that homogeneous selection (i.e., communities being more similar than the random expectation) was the dominant assembly process within islands whereas stochastic processes tended to be more important within continents. In addition, assembly processes were influenced by study scale and island isolation. Climatic variables showed a greater influence on species distribution than local factors. However, in islands, local environmental variables had a greater impact on the distributions of unique taxa as opposed to non-unique taxa.Main Conclusions: We observed that the assembly processes of diatom communities were complex and influenced by a combination of deterministic and stochastic forces, which varied across spatial scales. In islands, there was no universal pattern of assembly processes, given that their influence depends on abiotic conditions such as area, isolation, and environmental heterogeneity. In addition, the sensitivity of species occurring uniquely in islands to local environmental variables suggests that they are perhaps less vulnerable to climatic changes but may be more influenced by changes in local physicochemistry.

Red Swamp Crayfish Procambarus clarkii (Girard, 1852) (RSC) are a highly invasive species established on five continents. Recent detections of RSC populations in several locations in Michigan, USA spurred action to develop trap-based early detection and response plans based on the best available science, however, few recommendations are available concerning important sampling logistics such as timing and density of deployed traps. We surveyed 231 waterbodies (mainly lentic systems with relatively few small lotic systems) for RSC in two invasion zones in southern Michigan. We used Bayesian hierarchical models to evaluate baited traps for detection of RSC invasions, and qualitatively evaluated feasibility of eDNA metabarcoding for enhancing trap-based detection strategies. While overall occupancy was low, predicted detection probabilities with baited traps ranged from 27 to 98% and depended on season, density of traps, and relative crayfish abundance. There was high agreement between trapping and eDNA detections (94%). However, an improved eDNA sampling design that increases the number of samples is needed. We conclude that trap-based detections of RSC in Michigan are most likely to occur mid-summer, trapping densities should be increased in the spring and fall to offset lower detection probabilities, and eDNA could feasibly supplement trapping in spring and fall.

Influential ecological research in the 1980s, elucidating that local biodiversity (LB) is a function of local ecological factors and the size of the regional species pool (γ-diversity), has prompted numerous investigations on the local and regional origins of LB. These investigations, however, have been mostly limited to single scales and target groups and centered exclusively on γ-diversity. Here we developed a unified framework including scale, environmental factors (heterogeneity and ambient levels), and metacommunity properties (intraspecific spatial aggregation, regional evenness, and γ-diversity) as hierarchical predictors of LB. We tested this framework with variance partitioning and structural equation modeling using subcontinental data on stream diatoms, insects, and fish as well as local physicochemistry, climate, and land use. Pure aggregation + regional evenness outperformed pure γ-diversity in explaining LB across groups. The covariance of the environment with aggregation + regional evenness rather than with γ-diversity generally explained a much greater proportion of the variance in diatom and insect LB, especially at smaller scales. Thus, disregarding aggregation and regional evenness, as commonly done, may lead to gross underestimation of the pure metacommunity effects and the indirect environmental effects on LB. We examined the shape of the local-regional species richness relationship, which has been widely used to infer local vs. regional effects on LB. We showed that this shape has an ecological basis, but its interpretation is not straightforward. Therefore, we advocate that the variance partitioning analysis under the proposed framework is adopted instead. In diatoms, metacommunity properties had the greatest total effects on LB, while in insects and fish, it was the environment, suggesting that larger organisms are more strongly controlled by the environment. Broader use of our framework may lead to novel biogeographical insights into the drivers of LB and improved projections of its trends along current and future environmental gradients.

Aim: To evaluate the patterns of stream diatom beta diversity in islands versus continents across scales, to relate community similarities with spatial and environmental distances and to investigate the role of island characteristics in shaping insular diatom beta diversity. Location: Africa, America, Europe and the Pacific. Time Period: Present. Major Taxa Studied: Stream diatoms. Methods: We compared diatom beta diversity between islands and continents at large scales (within biogeographic regions) in two study regions (America and Europe) and at small scales (within islands/equivalent areas in continents) in three regions (Africa, America and Europe) partitioning beta diversity into turnover and nestedness components. We used a partial Mantel test and distance-decay curves to assess how diatom beta diversity on islands and continents is affected by spatial and environmental distances. Finally, using island data from all four regions, we evaluated the relationship between island beta diversity and island latitude, area, age and isolation using linear models. Results: At large scales, mean dissimilarities were higher on islands than in continents in Europe but lower in America. At smaller scales, the differences varied mostly depending on island isolation. Beta diversity was mainly caused by species turnover. Partial Mantel test and distance-decay curves revealed that spatial and environmental distances shaped diatom beta diversity at large, but not at small scales. Moreover, diatom beta diversity on islands was affected by island latitude, age and isolation, but not by island area. Main Conclusions: Diatom beta diversity on islands versus continents and its responses to spatial and environmental factors are scale and region dependent. Incomplete colonisation, evolutionary processes and environmental filtering likely contribute to insular beta diversity, which further varies with island latitude, age and isolation. This study sheds new light on beta diversity of microorganisms on islands and suggests that beta diversity should be explicitly considered in island biogeographical research.

The species-area relationship (SAR) has over a 150-year-long history in ecology, but how its shape and origins vary across scales and organisms remains incompletely understood. This is the first subcontinental freshwater study to examine both these properties of the SAR in a spatially explicit way across major organismal groups (diatoms, insects, and fish) that differ in body size and dispersal capacity. First, to describe the SAR shape, we evaluated the fit of three commonly used models, logarithmic, power, and Michaelis-Menten. Second, we proposed a hierarchical framework to explain the variability in the SAR shape, captured by the parameters of the SAR model. According to this framework, scale and species group were the top predictors of the SAR shape, climatic factors (heterogeneity and median conditions) represented the second predictor level, and metacommunity properties (intraspecific spatial aggregation, γ-diversity, and species abundance distribution) the third predictor level. We calculated the SAR as a sample-based rarefaction curve using 60 streams within landscape windows (scales) in the United States, ranging from 160,000 to 6,760,000 km2 . First, we found that all models provided good fits (R2  ≥ 0.93), but the frequency of the best-fitting model was strongly dependent on organism, scale, and metacommunity properties. The Michaelis-Menten model was most common in fish, at the largest scales, and at the highest levels of intraspecific spatial aggregation. The power model was most frequent in diatoms and insects, at smaller scales, and in metacommunities with the lowest evenness. The logarithmic model fit best exclusively at the smallest scales and in species-poor metacommunities, primarily fish. Second, we tested our framework with the parameters of the most broadly used SAR model, the log-log form of the power model, using a structural equation model. This model supported our framework and revealed that the SAR slope was best predicted by scale- and organism-dependent metacommunity properties, particularly spatial aggregation, whereas the intercept responded most strongly to species group and γ-diversity. Future research should investigate from the perspective of our framework how shifts in metacommunity properties due to climate change may alter the SAR.

Aim The species-area relationship (SAR) is one of the most distinctive biogeographic patterns, but global comparisons of the SARs between island and mainland are lacking for microbial taxa. Here, we explore whether the form of the SAR and the drivers of species richness, including area, environmental heterogeneity, climate and physico-chemistry, differ between islands and similarly sized areas on mainland, referred to as continental area equivalents (CAEs). Location Global. Taxon Stream benthic diatoms. Methods We generated CAEs on six continental datasets and examined the SARs of CAEs and islands (ISAR). Then, we compared CAEs and islands in terms of total richness and richness of different ecological guilds. We tested the factors contributing to richness in islands and CAEs with regressions. We used structural equation models to determine the effects of area versus environmental heterogeneity, climate and local conditions on species richness. Results We found a non-significant ISAR, but a significant positive SAR in CAEs. Richness in islands was related to productivity. Richness in CAEs was mainly dependent on area and climate, but not directly on environmental heterogeneity. Species richness within guilds exhibited inconsistent relationships with island isolation and area. Main conclusions Ecological and evolutionary processes shaping diatom island biogeography do not depend on area at the worldwide scale probably due to the presence of distinct species pool across islands. Conversely, area was an important driver of diatom richness in continents, and this effect could be attributed to dispersal. Continents had greater richness than islands, but this was a consequence of differences in environmental conditions such as specific island climatic conditions. We stress the need for more island data on benthic diatoms, particularly from archipelagos, to better understand the biogeography of this most speciose group of algae.

Trapping and removing invasive crayfish is one of the most common forms of control, however gear-specific biases can limit effectiveness of such methods. We evaluated five trapping gears (Gee’s minnow traps [GMT], pyramid traps [PYR], artificial refuge traps [ART], additional partition refuge traps [APART], and juvenile traps [JUV]) for their effectiveness in a Procambarus clarkii Girard, 1852 (red swamp crayfish) removal effort among four Southern Michigan ponds between May and October 2019. Our objectives were 1) determine which gear(s) produce the highest catch per unit effort (CPUE); 2) assess gears for body size and sex biases; 3) ascertain the degree of seasonality in gear-specific catches and biases. We found that baited GMT and PYR traps substantially outperformed the ART, APART, and JUV traps with respect to CPUE. However, catches of refuge-style traps trended positively over the season. Body size biases were prevalent, with GMT and PYR traps consistently recording individuals > 30 mm carapace length. The ART and APART traps caught relatively smaller individuals but trended towards capturing larger individuals later in the season. We observed no sex biases or biases between Form 1 and Form 2 males among the gears. Our findings support employing multiple gears for crayfish removals but demonstrated that baited traps can remove more crayfish than unbaited traps (on a CPUE basis) and both can capture body size trends by accounting for the timing of capture. Further, the lack of sex bias suggests any of these commonly used traps can be used to broadly sample females or male reproductive forms. Based on our results and recommendations, the Michigan adaptive response strategy has adopted a combination GMT-APART strategy to maximize CPUE while also targeting a wide range of crayfish sizes.

Aim Niche and dispersal processes influence biodiversity, but their relative importance along latitude is unclear. We predicted that: (a) niche processes would dominate at high latitudes due to increased climatic stress, consistent with the physiological tolerance hypothesis and the Dobzhansky-MacArthur hypothesis and (b) dispersal limitation would prevail at low latitudes due to narrower niches and smaller range sizes, as postulated by the dispersal-ecological specialization trade-off hypothesis, the latitude-niche breadth hypothesis, and Rapoport's rule. Location Central United States. Time period 1993-2019. Major taxa studied Stream algae, insects, and fish. Methods We examined the relative effects of environment (climate and physicochemistry) versus space on stream biodiversity in seven latitudinal zones, spanning 19 latitudinal degrees. In each zone, species richness (alpha-diversity) was analysed with multiple regression and variance partitioning. Compositional dissimilarity (beta-diversity) within zones was assessed with distance-based redundancy analysis (dbRDA) and variance partitioning. Results For alpha-diversity, latitudinal variability of niche and dispersal processes conformed to our predictions in all three groups, except for dispersal processes in insects. However, the drivers of beta-diversity did not follow our predictions. The latitude-niche breadth hypothesis and Rapoport's rule were weakly supported only in fish. Main conclusions The importance of niche and dispersal processes varied predictably along the latitudinal gradient only for alpha-diversity. However, the niche effects were driven mostly by physicochemistry, and the dispersal effects were not always linked with ecological specialization and range size. This suggests that climate-based biodiversity theories do not have particular relevance for the streams in our study. Niche processes had a greater impact than dispersal processes across species groups and diversity metrics, emphasizing the primary role of the environment.

The amounts and ratios of nutrients (nitrogen and phosphorus) are important determinants of producer community biodiversity and composition and their responses to climate and dispersal. However, the nutrient effects on co-occurrence network topology, particularly in freshwaters, are understudied. Here, we investigate 1) whether nutrient supply and ratio constrain topological properties of algal co-occurrence networks in streams and 2) to what extent climate and space (a surrogate for dispersal) affect co-occurrence network topology versus metacommunity composition across nutrient supply and ratio contexts. We used a subcontinental dataset of benthic algae from 840 stream localities in the conterminous US. We constructed co-occurrence networks representing nutrient supply contexts (oligotrophic versus eutrophic) and nutrient ratio contexts (N-limited versus P-limited) and statistically assessed topological variability within each pair via randomization. We then used a null model framework and direct gradient analysis to ascertain the importance of climate and space in driving, respectively, network topology and metacommunity composition. Nutrient supply was only positively related to network size (species node counts), which was driven by motile species, while other topological differences were non-significant. Climatic and spatial variables had pronounced and for the most part comparable effects on network topology that further depended on nutrient context. A comparative assessment of topological versus compositional responses to climate and space across nutrient contexts identified both similarities and differences. While climate and space contributed to both network topology and metacommunity composition, space was a stronger predictor of compositional variability than climate, regardless of nutrient context. Our findings highlight the need for developing integrative multi-level approaches (from metacommunities to co-occurrence networks) to fully understand biological responses to complex and interactive abiotic forces.

Mass effect, allowing species to persist in unfavourable habitats, and dispersal limitation, preventing species from reaching favourable habitats, are the two major dispersal processes. While dispersal limitation can be detected by experimental or modelling techniques, mass effect is more challenging to evaluate, which hampers our ability to disentangle the influence of the environment versus dispersal on species distribution. This is undesirable for biomonitoring programs built on known species-environment relationships. We developed an approach for detection of species influenced by mass effect. We tested it on stream diatoms, a widely used taxonomic group for stream biomonitoring, from four French watersheds. This approach combined (a) an appropriate spatial framework, the asymmetric eigenvector map (AEM), used in species distribution modelling to measure the relative influence of dispersal versus niche processes, (b) an analysis of negative co-occurrence patterns to separate mass effect from dispersal limitation and (c) a measurement of niche breadths to distinguish between non-spatially structured generalists and species influenced by mass effect. We propose that species characterized by low negative co-occurrence values, a high correlation to spatial factors and average to low niche breadths are sensitive to mass effect. Synthesis and applications. We suggest that the sensitivity of species towards mass effect should represent a new ecological trait to be considered for fundamental and applied issues concerning ecology and water quality assessment. Almost all of the species identified here as influenced by mass effect are contributing to the calculation of different diatom-based indices (e.g. Biological Diatom Index or Specific Pollution-sensitivity Index) and should be treated with caution when assigning ecological status classes to water bodies.

Stream community surveys from 2008 to 2015 in the central Louisiana river basins (Red, Calcasieu, Vermillion-Teche, Mermentau, Ouachita, and Atchafalaya rivers) have resulted in new distributional data for 2 subspecies of Faxonius palmeri (Lowland Painted Crayfish). We also report unidentified Faxonius specimens appearing in streams of the northern Calcasieu River basin as well as a stream crossing the Mermentau-Calcaiseu River basin boundary, which warrant further investigation as to their identity. We determined that F. palmeri cannot be assumed to be separated along basin and river boundaries, and that the Mississippi River is probably not a biogeographic barrier for F. p. palmeri. We believe further species overlaps caused by range expansions of F. palmeri subspecies with endemic species of state conservation interest are likely.

Aim The interaction of land use with local versus regional processes driving biological homogenization (β‐diversity loss) is poorly understood. We explored: (a) stream β‐diversity responses to land cover (forest versus agriculture) in terms of physicochemistry and physicochemical heterogeneity; (b) whether these responses were constrained by the regional species pool, i.e. γ‐diversity, or local assembly processes through local (α) diversity; (c) whether local assembly operated through the regional species abundance distribution (SAD) or intraspecific spatial aggregation; and (d) the dependence on body size, dispersal capacity and trophic level (producer versus consumer). Location USA, Canada and France. Time period 1993–2012. Major taxa studied Stream diatoms, insects and fish. Methods We analysed six datasets totalling 1,225 stream samples. We compared diversity responses to eutrophication and physicochemical heterogeneity in forested versus agricultural streams with regression methods. Null models quantified the contribution of local assembly to β‐diversity (β‐deviance, βDEV) for both types of land covers and partitioned it into fractions explained by the regional SAD (βSAD) versus aggregation (βAGG). Results Eutrophication explained homogenization and more uneven regional SADs across groups, but local and regional biodiversity responses differed across taxa. The βDEV was insensitive to land use. The βSAD largely exceeded βAGG and was higher in agriculture. Main conclusions Eutrophication but not physicochemical heterogeneity of agricultural streams underlay β‐diversity loss in diatoms, insects and fish. Agriculture did not constrain the magnitude of local versus regional effects on β‐diversity but controlled the local assembly mechanisms. Although the SAD fraction dominated in both land covers, it increased further in agriculture at the expense of aggregation. Notably, the regional SADs were more uneven in agriculture, exhibiting excess common species or stronger dominance. Diatoms and insects diverged from fish in terms of biodiversity, SAD shape and βDEV patterns, suggesting an overriding role of body size and/or dispersal capacity compared with trophic position.

Aim To quantify the relative contributions of local community assembly processes versus gamma-diversity to beta-diversity, and to assess how spatial scale and anthropogenic disturbance (i.e. nutrient enrichment) interact to dictate which driver dominates. Location France and the United States. Time period 1993-2011. Major taxa studied Freshwater stream diatoms. Methods beta-diversity along a nutrient enrichment gradient was examined across multiple spatial scales. beta-diversity was estimated using multi-site Sorensen dissimilarity. We assessed the relative importance of specialists versus generalists using Friedley coefficient, and the contribution of local community assembly versus gamma-diversity to beta-diversity across spatial scales, with a null model. Finally, we estimated the response of beta-diversity to environmental and spatial factors by testing the correlations between community, environmental and geographical distance matrices with partial Mantel tests. Results beta-diversity generally increased with spatial scale but the rate of increase depended on nutrient enrichment level. beta-diversity decreased significantly with increasing nutrient enrichment level due to the loss of specialist species. Local assembly was an important driver of beta-diversity especially under low nutrient enrichment. Significant partial Mantel correlations were observed between diatom beta-diversity and pure environmental distances under these conditions, highlighting the role of species sorting in local assembly processes. Conversely, in heavily enriched sites, only spatial distances were significantly correlated with beta-diversity, which indicated a substantial role of dispersal processes. Main conclusions Nutrient concentration mediated the expected increase in beta-diversity with spatial scales. Across spatial scales, beta-diversity was more influenced by local assembly processes rather than by gamma-diversity. Nutrient enrichment was associated with an overall decline in diatom beta-diversity and a shift in assembly processes from species sorting to dispersal, notably due to the elimination of some specialists and their subsequent replacement by generalists.

Identifying an effective protocol for sampling crayfish in streams that vary in habitat and physical/chemical characteristics has proven problematic. We evaluated an active, combined-gear (backpack electrofishing and dipnetting) sampling protocol in 20 Coastal Plain streams in Louisiana. Using generalized linear models and rarefaction curves, we evaluated environmental and gear (separate and combined) effects on crayfish catch-per-unit-effort (CPUE), orbital carapace lengths, sex ratios, frequencies of rare species, and sample richness. Although pooled data from combined gears showed greater total numbers of crayfishes, CPUE, and richness compared to either gear individually, combined gear and backpack electrofisher results differed minimally. Overall, richness was negatively related to specific conductance, indicating potential agricultural influence. Neither crayfish sex ratios, lengths, nor frequencies of rare species differed by gear; however, combining data from both gears ensured crayfish were captured in all study streams, which was not found for electrofishing or dipnetting alone. Species accumulation and rarefaction curves indicated sampling was sufficient for recording crayfish diversity at the scale of the study and that adding streams (versus sites within streams) would be most effective for watershed-scale studies. Our results suggested the combined gear protocol was effective for assessing crayfish population and assemblage characteristics in these Coastal Plain streams.

In this intercontinental study of stream diatoms, we asked three important but still unresolved ecological questions: (1) What factors drive the biogeography of species richness and species abundance distribution (SAD)? (2) Are climate-related hypotheses, which have dominated the research on the latitudinal and altitudinal diversity gradients, adequate in explaining spatial biotic variability? and (3) Is the SAD response to the environment independent of richness? We tested a number of climatic theories and hypotheses (i.e., the species-energy theory, the metabolic theory, the energy variability hypothesis, and the climatic tolerance hypothesis) but found no support for any of these concepts, as the relationships of richness with explanatory variables were nonexistent, weak, or unexpected. Instead, we demonstrated that diatom richness and SAD evenness generally increased with temperature seasonality and at mid- to high total phosphorus concentrations. The spatial patterns of diatom richness and the SAD—mainly longitudinal in the United States but latitudinal in Finland—were defined primarily by the covariance of climate and water chemistry with space. The SAD was not entirely controlled by richness, emphasizing its utility for ecological research. Thus, we found support for the operation of both climate and water chemistry mechanisms in structuring diatom communities, which underscores their complex response to the environment and the necessity for novel predictive frameworks.

Burrowing crayfish are potentially important ecological links between terrestrial and aquatic systems, but little is known about what drives their local distribution patterns. We investigated potential mechanisms underlying the patchy local distribution of the devil crayfish (Cambarus diogenes Girard) in a stream floodplain ecosystem of the coastal plain of Alabama. We used a series of field surveys and laboratory trials to: 1) quantify local distributions and soil-type associations, 2) identify soil preferences of juveniles and adults, 3) examine the effects of biotic (adult occupancy) and abiotic (soil compaction) factors on juvenile burrowing preferences, and 4) investigate the role of floodplain connectivity on burrow density. Juvenile crayfish were more abundant in sandy streamside soils, whereas adults were more abundant in clay-based floodplain soils. In laboratory preference trials in artificial burrowing chambers, adults and juveniles showed affinity for floodplain over streamside soils, a result suggesting that factors other than soil preference influence field patterns. When juveniles were presented with situations where adults were established, they initiated burrows in association with the adult burrows regardless of soil type. Soil compaction did not influence juvenile burrowing preference, but did result in smaller burrows in laboratory trials. Adult burrow density in the floodplain was negatively related to stream bank angle and top-of-bank height. Juvenile C. diogenes took longer to climb banks with high bank angles than those with low bank angles, a result suggesting increased risk of predation and desiccation associated with disconnected floodplain conditions. Local distribution of C. diogenes burrows in stream/floodplain systems appears to be a product of interacting physical and biological factors rather than of soil-type preferences. Stream-floodplain connectivity may affect recruitment and population viability of burrowing crayfish.