Emergence of the metacommunity concept has made a substantial contribution to better understanding of the community composition and dynamics in a regional context. However, long-term field data for testing of available metacommunity models are still scarce, and the extent to which these models apply to the real world remains unknown. Tests conducted so far have largely sought to fit data on the entire regional set of species to one of several metacommunity models, implicitly assuming that all species operate similarly over the same set of sites. However, species differ in their habitat use. These differences can, in the most general terms, be expressed as a gradient of habitat specialization (ranging from habitat specialists to habitat generalists). We postulate that such differences in habitat specialization will have implications for metacommunity dynamics. Specifically, we predict that specialists respond more to local processes and generalists respond to regional spatial processes. We tested these predictions using natural microcosm communities for which long-term (nine-year) environmental and population dynamics data were available. We used redundancy analysis to determine the proportion of variation explained by environmental and spatial factors. We repeated this analysis to explain variation in the entire regional set of species, in generalist species only, and in specialists only. We further used ANOVA to test for differences in the proportions of explained variation. We found that habitat specialists responded primarily to environmental factors and habitat generalists responded mainly to spatial factors. Thus, from the metacommunity perspective, the dynamics of habitat specialists are best explained by a combination of species sorting and mass effects, while that of habitat generalists are best explained by patch dynamics and neutral models. Consequently, we infer that a natural metacommunity can exhibit complicated dynamics, with some groups of species (e.g., habitat specialists) governed according to environmental processes and other groups (e.g., habitat generalists) governed mainly by dispersal processes.

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Summary Stream crossing structures are an increasingly prevalent anthropogenic feature on North American riverscapes, particularly in watersheds affected by industrial resource development in sensitive boreal environments. If improperly managed, stream crossings have the potential to alter fish habitat and impede fish movement. This study assessed instream habitat characteristics and fish communities from 33 culverted, bridged and reference streams in an industrialising region of the boreal forest in west‐central Alberta. Mixed‐effects modelling and multivariate analysis were used to determine impacts of stream crossings at three scales: whole‐stream scale, within‐stream scale and the interaction of scales. Instream habitat characteristics such as mean depth, water velocity, percent fines, turbidity, water temperature and dissolved oxygen showed significant between‐stream as well as within‐stream differences among stream crossings. The majority of fish species exhibited significantly lower densities (n m−2) in upstream habitats as compared to downstream habitats, including a significant reduction in Slimy Sculpin densities in culverted streams. Multivariate tests corroborated these results, showing that fish assemblages differ as a function of stream type. This study suggests industrial stream crossings influence abiotic habitat characteristics in freshwater ecosystems, restrict biotic connectivity and impact fish community structure at the whole‐stream and within‐stream scales. Alterations to stream ecosystems associated with stream crossings may be driving large‐scale changes in stream fish communities in the boreal forest. With expanded development expected in much of North America's boreal region, mitigation measures which limit impacts from stream crossings are needed to ensure proper ecosystem function in freshwater systems.

Fluctuations of local but connected populations may show correlation or synchrony whenever they experience significant dispersal or correlated environmental biotic and abiotic variability. Synchrony may be an important variable in multispecies systems, but its nature and implications have not been explicitly examined. Because the number of locally coexisting species (richness) affects the population variability of community members, we manipulated richness under different regimes of environmental fluctuation (EF). We predicted that the temporal synchrony of populations in a species should decline with increasing richness of the metacommunity they live in. Additionally, we predicted that specialist species that are sensitive to a specific environmental factor would show higher synchronization when EF increases. We thus created experimental communities with varied richness, EF, and species specialization to examine the synchronizing effects of these factors on three aquatic invertebrate species. We created four levels of richness and three levels of EF by manipulating the salinity of the culture media. Monocultures exhibited higher population synchrony than metacommunities of 2-4 species. Furthermore, we found that species responded differently to EF treatments: high EF enhanced population synchrony for the specialist and intermediate species, but not for the generalist species. Our findings emphasize that the magnitude of EF and species richness both contribute to determine population synchrony, and importantly, our results suggest that biotic diversity may actually stabilize metacommunities by disrupting synchrony.

Species are continuously lost and added to a local community. Dynamics of this process in a complex habitat mosaic (multiple habitats in a landscape), particularly of its rates (species turnover) are of primary concern for biodiversity conservation. Various studies suggest that species traits such as habitat specialization should affect species turnover. In communities where habitat specialization is a function of abiotic constraints, habitat specialists should respond faster to changing environment than generalists. We thus predicted a higher temporal turnover for specialists than for generalists in the presence of environmental variability (EV). In addition, we predicted that temporal turnover should decrease with increasing species richness of the communities they live in. We tested these predictions in a model system of 49 natural rock pools inhabited by 70 invertebrate species for which long-term (9 years) environmental and population dynamics data are available. We computed standard deviation of salinity measurements to represent EV for each pool. We further obtained the number of combined colonization and extinction events weighted by the number of years a species was recorded as a temporal turnover for each species in individual pools. We found that EV induced greater temporal turnover, however, the turnover depended on the species habitat traits (habitat specialization)-it has been higher in specialists but that relationship between EV and temporal turnover dissolved with increasing niche breadth (generalists). We further found that for some species, temporal turnover decreased with higher species richness and for other species, temporal turnover increased with higher species richness. The effect of species richness on temporal turnover was unrelated to species traits. This study suggests that whenever habitat is complex and heterogeneous and species pool diversified, local community dynamics becomes a composite of differential responses.

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Citation: Pandit, S. N., K. Cottenie, E. C. Enders, and J. Kolasa. 2016. The role of local and regional processes on population synchrony along the gradients of habitat specialization. Ecosphere 7(5):e01217. 10. 1002/ecs2.1217 Abstract. Metacommunity perspective highlights the role of space as a factor contributing to local community structure and dynamics. Often tests of metacommunity models rely on snapshot patterns of species distribution and abundance. Such patterns may introduce biases because they overlook differences in responses of constituent species to changing environmental conditions, particularly when such responses override patterns predicted from dispersal and biotic interactions alone. This applies, for example, to habitat generalists, whose responses to environmental variations differ from those of habitat specialists, resulting in different snapshot patterns at different times. Synchronized (i.e., correlated among sites) environmental variation is perhaps the most obvious case of environmental variation that could generate regular differences among species of different specialization. We hypothesized that synchronized environmental variation synchronizes local populations of habitat specialists to a greater degree than it does for habitat generalists as experiments have shown that habitat generalists are less sensitive to changing environmental conditions. To test this in an empirical system, we used time series data (nine annual surveys) on an invertebrate metacommunity of 49 rock pools on the coast of Jamaica. We found that population synchrony of a significant portion of 24 species sharing these rock pools increased with habitat specialization in response to environmental synchrony (represented by synchrony of environment). However, distance among rock pools, which may affect dispersal or shared perturbations, was negatively correlated with population synchrony of individual species, irrespective of their specialization. As only habitat specialists showed increasing synchrony with environmental synchrony, and both generalist and specialist synchrony were negatively correlated with distance, we infer that habitat generalists must be synchronized more by space-related processes while habitat specialists by a combination of environmental forcing and dispersal. Overall, the study suggests that species of different habitat specialization show consistent differences with respect to local processes involving environmental variations but show fewer differences with respect to regional processes involving distance, at least when long-term dynamics are concerned. Furthermore, the study identifies a rarely recognized link between variation in space and its consequences for variation in time-a link much more expressed among spatially restricted species such as habitat specialists.