Local adaptation of fish consumers alters primary production through changes in algal community composition and diversity

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Abstract

Ecological research has focused on understanding how changes in consumer abundance affects community structure and ecosystem processes. However, there is increasing evidence that evolutionary changes in consumers can also alter community structure and ecosystem processes. Typically, the effects of consumer phenotype on communities and ecosystem processes are measured as net effects that integrate numerous ecological pathways. Here, we analyze new data from experimental manipulations of Trinidadian guppy Poecilia reticulata presence, density and phenotype to examine how effects on the algal community cause changes in gross-primary production (GPP). We combine analytical tools borrowed from path analysis with experimental exclosures in mesocosms to separate the ecological and evolutionary effects of guppies into direct and indirect components. We show that the evolutionary effects of guppy phenotype act through different ecological pathways than the effects of guppy presence and density on GPP. As reported in previous studies that used a different measure of algal biomass, adding guppies and doubling their densities decreased algal biovolume through direct effects. In contrast to these previously reported results, exchanging guppy phenotypes that live without predators for phenotypes that live with predators did not affect algal biovolume. Instead, guppies from populations that live with predators increased the diversity of algal species and increased GPP compared to guppies that live without predators. These changes in the algal community were driven primarily by guppy phenotypes that live with predators—algal communities in mesocosms without fish were similar to those with guppies from predator-free locations, but both were different from mesocosms with guppies from populations that live with predators. Changes in the algal community were driven directly by differences in foraging behavior between the two consumer phenotypes. We reconcile these results with our previous findings, thereby enhancing our understanding of the relationship between ecological and evolutionary processes.

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