Assessing the effects of guppy life history evolution on nutrient recycling: from experiments to the field

Trait evolution can occur in response to anthropogenic alterations to ecosystems and can occur on timescales similar to those of ecological processes suggesting that it could alter ecosystem function. In this study, we characterise the effects of life history evolution on nutrient recycling using the Trinidadian guppy (Poecilia reticulata) as a model system. Guppy life history traits evolve in response to predation pressure. When predation pressure is removed, guppy population density and average body size of the population also increase. Therefore, the evolution of guppy life histories involves changes in individual traits and demographic characteristics, both of which can alter nutrient recycling. The relative contributions of these variables to guppy-driven nutrient recycling are unknown. We synthesise data from published experiments to disentangle how differences in individual traits, population characteristics and environmental conditions contribute to differences in guppy excretion rates. Individual guppies adapted to the absence of predators [low-predation (LP) guppies] have lower nitrogen and phosphorus excretion rates than individual guppies adapted to predators [high-predation (HP) guppies]. However, LP guppy populations excrete twice as much nitrogen as HP populations because of their larger average body size and higher population densities. We compare these findings to guppy excretion data collected from HP and LP sites in four rivers in Trinidad. Phenotypic and population differences in excretion rates are consistent with those observed in the experiments. Our study demonstrates that life history evolution can alter nutrient recycling in freshwater ecosystems. Characterizing the combined effects of traits and demographics is essential for understanding the effects of life history evolution on ecosystem processes.