Examining the ecosystem-level consequences of local adaptation in Trinidadian guppies across natural levels of environmental heterogeneity in montane streams of northern Trinidad

Background/Question/Methods

Despite empirical evidence that evolution can influence populations, communities and ecosystems, we do not know if the ecological effects of evolution are frequent and strong enough to regulate communities and ecosystems in nature.  We examine how trait variation (diet selectivity and life history) in locally adapted Trinidadian guppies affects stream ecosystems at local and landscape-scales to address the question: what is the ecosystem-level significance of evolution across natural levels of environmental heterogeneity?  We focus on two locally adapted guppy phenotypes, which previous studies have shown to have distinct ecosystem-level impacts on primary production and nutrient cycling in ex-situ common garden conditions. At the local reach-scale we ran two in-situ macroconsumer exclusion experiments to examine the top-down effects of each guppy phenotype on ecosystem properties (invertebrate and algal biomass).  Experiments were conducted in separate 150 m reaches in a single stream, with one reach dominated by guppy phenotype-1 (G_P1) and a second reach dominated by guppy phenotype-2 (G_P2).  We then extended our studies to a broader landscape-scale by measuring invertebrate and algal biomass in 16, 150 m stream reaches, across 8 subwatersheds characterized by either G_P1 guppies (n=4) or G_P2 guppies (n=4), where the ecosystem effects of guppy presence was estimated relative to paired guppy-free reaches (n=8). 

Results/Conclusions

Reach-scale experiments showed distinct top-down effects of each guppy phenotype on both algae and invertebrate biomass.  Moreover, our landscape-scale assessment of ecosystem properties reflected experimental differences in how guppy phenotype affected algae and invertebrates.  Compared to previous ex-situ experiments, the effect of guppy phenotype in natural streams at the reach-scale reduced the effect size of G_P1 guppies on algal biomass (46% reduction in-situ compared to 500% reductions ex-situ), and increased the effect size of G_P2 guppies on invertebrate biomass (84% reduction in-situ compared to no effect ex-situ).  We found similar effects of guppy phenotype at the landscape-scale; however, there were consistent decreases in the effect size, but not the biological significance of each guppy phenotype on the structure of the ecosystem.  Top-down effects of guppy phenotype on algal and invertebrate biomass at the landscape-level were similar in magnitude to the bottom-up effects of light availability, an important abiotic driver of benthic resources.  Our findings strongly suggest that estimating the realized ecosystem consequences of evolution requires measuring the strength of the evolution-to-ecosystem pathway in natural communities, where complex species interactions and environmental variation may greatly alter the ecosystem effects of evolution in nature.