Community trajectories in fragmented arid-land streams: Combined effects of extreme environmental conditions and aerial dispersal

Background/Question/Methods

Ecological theory predicts that species composition in fragmented communities may become more similar (convergent) or more distinct (divergent) through time. Fragmented communities may converge in composition due strong reductions in the overall number of species, loss of keystone species that facilitate persistence of weak competitors, or high levels of dispersal between fragments, among other drivers. On the other hand, fragments may diverge in composition due to stochastic loss of functionally-equivalent species, low among-fragment migration rates, or high variability in environmental conditions among fragments. Elucidating the mechanisms behind community convergence or divergence is of imminent conservation concern in arid-land stream habitats. Climate change and anthropogenic water withdrawals are causing once-contiguous arid-land streams to fragment, resulting in unprecedented isolation of aquatic communities and deterioration of abiotic conditions. In such streams, extreme abiotic conditions and aerial dispersal are two important drivers of local community structure, but it is unknown how these factors interact to impact community structure in stream fragments. We combined data from several independent studies of aquatic invertebrate community structure in fragmented streams in southeastern Arizona to measure trajectories of community convergence or divergence and examined how aerial dispersal and abiotic conditions were associated with these trajectories.

Results/Conclusions

In observational and manipulative studies conducted over three years, we sampled aquatic invertebrate community structure at the beginning and end of the harsh dry season, quantified aerial dispersal, and measured environmental variables. While aquatic invertebrate communities exhibited great interannual variability among the four studies, several consistent patterns emerged. We found that extreme environmental conditions were associated with community convergence through time as vulnerable taxa were eliminated and replaced by a small subset of tolerant taxa. When aerial dispersal rates were high, however, we observed divergence of taxonomic composition through time. We expected similar patterns in trait diversity but instead found that both extreme environmental conditions and aerial dispersal led to convergence in trophic trait composition. This trophic trait convergence was mainly due to the prevalence of large predators amongst the aerially dispersing taxa. Given predicted increases in both abiotic extremes and rates of large predator extinctions in arid-land ecosystems, our research suggests increased community homogenization in arid-land streams in the near future. We believe that aerial dispersal may be a mechanism to maintain diversity in these imperiled habitats and should be considered in future biodiversity conservation initiatives.