Intercontinental comparison of small-lake fish assemblages: the balance between local and regional processes
We compared characteristics of fish assemblages from Finland and northern Wisconsin on several scales to investigate community-level similarity in environmentally similar but faunistically different small lakes. Although Finland has only half as many fishes in its regional species pool, local species richness did not differ between the two regions. Variability in species composition among assemblages was lower in Finland, suggesting that a large proportion of the fauna could maintain populations across a broad range of environmental conditions, whereas the Wisconsin fauna was composed of more specialized species. Three types of fish assemblages were identified in Wisconsin based on species’ presence or absence, whereas on the same scale, Finnish assemblages presented a hierarchical continuum based on species addition. These and other patterns suggest that habitat specialization and the exclusion of small prey species from lakes with piscivores contribute to the occurrence of presence-absence assemblage types in Wisconsin. Biotic interactions can affect population densities of Finnish species without exclusion, and assemblage types could be identified in Finland only on the basis of species’ relative abundance. Finnish assemblages dominated by perch, roach, and crucian carp occurred in lakes that were environmentally similar to lakes in Wisconsin with bass, pike, and mudminnow assemblages, respectively. Because of these similar relations, discriminant functions from one region could successfully identify the assemblage types that occurred in lakes from the other region; this community-level predictability suggests that fish assemblage structures in northern forest lakes are influenced by common sets of environmental factors. Similarities and differences on different scales reflect the balance between local and regional processes; although local characteristics are well matched between regions, many regional and historical phenomena are dissimilar. Because of influences operating on several scales, viewed as a series of filters, community ecologists should not be content with simple acceptance or rejection of the convergence hypothesis. Greater understanding of communities will come from comprehensive studies that encompass a hierarchy of causes and effects.