Species and environmental datasets from Sierra Nevada, CA (USA) streams in lake-stream networks

A major goal of community ecology is understanding the processes
responsible for generating biodiversity patterns along spatial and
environmental gradients. In stream ecosystems, system specific conceptual
frameworks have dominated research describing biodiversity change along
longitudinal gradients of river networks. However, support for these
conceptual frameworks has been mixed, mainly applicable to specific stream
ecosystems and biomes, and these frameworks have placed less emphasis on
general mechanisms driving biodiversity patterns. Rethinking biodiversity
patterns and processes in stream ecosystems with a focus on the
overarching mechanisms common across ecosystems will provide a more
holistic understanding of why biodiversity patterns vary along river
networks. In this study, we apply the Theory of Ecological Communities
(TEC) conceptual framework to stream ecosystems to focus explicitly on the
core ecological processes structuring communities: dispersal, speciation,
niche selection, and ecological drift. Using a unique case study from high
elevation networks of connected lakes and streams, we sampled stream
invertebrate communities in the Sierra Nevada, CA to test established
stream ecology frameworks and compared them to the TEC framework. Local
diversity increased and β-diversity decreased moving downstream from the
headwaters, consistent with the river continuum concept and the small but
mighty framework of mountain stream biodiversity. Local diversity was also
structured by distance below upstream lakes, where diversity increased
with distance below upstream lakes, in support of the serial discontinuity
concept. Despite some support for the biodiversity patterns predicted from
the stream ecology frameworks, no single framework was fully supported,
suggesting “context dependence”. By framing our results under the TEC, we
found species diversity was structured by niche selection, where local
diversity was highest in environmentally favorable sites. Local diversity
was also highest in sites with small community sizes, countering predicted
effects of ecological drift. Moreover, higher β-diversity in the
headwaters was influenced by dispersal and niche selection, where
environmentally harsh and spatially isolated sites exhibit higher
community variation. Taken together our results suggest that combining
system specific ecological frameworks with the TEC provides a powerful
approach for inferring the mechanisms driving biodiversity patterns and
provides a path toward generalization of biodiversity research across
ecosystems.