Is there a core set of organisms that structure macroinvertebrate assemblages in freshwater wetlands?

Abstract

Summary We analysed taxa lists from 447 individual wetlands from several ecoregions across the world using nestedness and similarity‐based multivariate analyses. We examined how similar wetland assemblages are across regions, whether variation in assemblages is ordered (nested) or unpredictable (idiosyncratic), whether individual taxa occur predictably or unpredictably across wetland habitats, and if any of these patterns differed between temporary‐ and permanent water habitats. We found that macroinvertebrate assemblages were highly nested (N = 0.947), but unexpectedly 37 of the 40 most widespread taxa (>10% occurrence) were idiosyncratic. Of the 447 wetlands, we identified 277 that shared more than 40% similarities in assemblages, were mostly nested, and clustered together in ordination space, and thus could be considered a core set of wetlands in terms of assemblage structure. Assemblages in the 170 wetlands outside this core (mostly idiosyncratic) tended to be depauperate sites in arid or high elevation areas, or alternatively taxonomically rich sites supporting numerous lotic or lacustrine organisms. The ‘Core’ itself split into two main parts, one comprised of wetlands from semi‐arid or mild climate areas dominated by strong flying insects, and the second comprised of wetlands from wetter, more northerly areas where non‐insects with passive dispersal were very prevalent. Climate and geology appear to be major controls on macroinvertebrate distributions across the set of 447 wetlands. Hydrology (temporary versus permanent) of wetlands was a lesser control on assemblage structure over the set of 447 wetlands. That wetlands are dominated by about 40 widely‐occurring macroinvertebrate taxa, and those taxa tend to occur idiosyncratically, suggests that overall assemblages across wetlands may share many similarities, but some of widespread taxa may still be missing from many individual wetlands. Why these otherwise fairly ubiquitous taxa do not occur in specific wetlands may shed important light on how those wetlands are controlled ecologically; in other words, do sites lack specific factors required by these taxa?