Effect of riverscape heterogeneity on animal communities
A) Ecologists have long focused on local scale phenomena (i.e. local environment variables) and assumed that spatial processes were unimportant factors influencing both the community structure and the functional diversity of aquatic communities. In this paper we used zooplankton assemblages in a typical large river (St. Lawrence River) as a biological model to examine the roles of (1) local environmental conditions (physicochemical characteristics of the water column), (2) broad-scale connectivity (a proxy for dispersion potential), and (3) habitat heterogeneity (a proxy for niche diversity) on the structure and the diversity of lotic communities. Together, these three sets of descriptors explained respectively 52, 49 and 59 % of the variation in zooplankton total biomass, functional diversity and community structure. After partitioning out the roles of local environmental conditions and broad-scale connectivity, we demonstrated that habitat heterogeneity alone is a key driver of zooplankton total biomass and functional evenness at the riverscape level. In homogeneous and temporally stable habitats, zooplankton communities had higher biomass and functional evenness but lower species richness. Conversely, zooplankton had lower biomass and higher species richness in heterogeneous and unstable habitats, suggesting that zooplankton species can coexist because disturbances prevent competitive exclusion from occurring. This is the first study to reveal how local environmental conditions, spatial connectivity and habitat heterogeneity operate jointly to determine the functional diversity and structure of aquatic communities in a natural ecosystem.
B) Loss of species richness in aquatic ecosystems is occurring rapidly and many factors, including habitat heterogeneity, have been suggested to affect the diversity of aquatic communities. We used fish community data (> 200 species) from extensive surveys conducted in two biogeographic provinces (extent > 1000 km) in North America to test the hypothesis that fish species richness is greater in more heterogeneous habitats (grain < 10 km2). Our tests are based on samples collected at nearly 800 stations over a period of five years. Using a set of environmental variables routinely measured by monitoring programs and simulations using a community assembly model, we demonstrate that fish species richness in coastal ecosystems is associated locally with the spatial heterogeneity of environmental variables but not their magnitude. The observed effect of heterogeneity on species richness was within the same range than that generated by simulations from a random placement model of community assembly. Our modeling framework opens avenues for targeted conservation of habitat heterogeneity at broader temporal and spatial scales.
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