Using aquatic productivity gradients in the Sierra Nevada mountains to understand spatial patterns of biodiversity

Background/Question/Methods

Variability in species composition across sites, or beta diversity, often shows a strong correlation with ecosystem productivity. Ecological theory and experiments point to two potential but contrasting mechanisms for how increasing productivity drives ecosystems to contain more dissimilar communities: Dynamic Community Assembly (DCA), and Dynamic Instability (DI). DCA is driven by species turnover through time, with more persistent turnover in higher productivity ecosystems. DI is driven by the order in which species colonize an ecosystem to create a stable community, with more possible configurations of stable communities in higher productivity ecosystems, but minimal species turnover. Despite their incompatibilities, both DCA and DI have received experimental support, and it remains unclear how these mechanisms operate in natural ecosystems. We estimated benthic productivity rates and macroinvertebrate diversity in lakes spanning a large productivity gradient in Yosemite National Park. Lakes were chosen to minimize differences in size, depth, and morphometry, and were also sampled historically for macroinvertebrates. We used a multiple regression framework to model how beta diversity and species turnover vary with productivity, and we distinguished changes in beta diversity from changes in alpha diversity by comparing null model simulations of stochastic community assembly with observed diversity patterns.

Results/Conclusions

While we plan to increase the number of lakes in the study to improve model robustness, preliminary results indicate that 1) spatial beta diversity increases with productivity, and 2) species turnover increases with productivity when comparing contemporary and historical samples. Comparisons of null model simulations and observed diversity patterns also indicate that changes in beta diversity are not artifacts of changes in alpha diversity across sites. Overall, we demonstrate how environmental gradients in mountain aquatic systems can be used to address basic drivers of biodiversity patterns.