Adaptation mediates the impact of Daphnia populations on community and ecosystem variables: The relative importance of population size and grazing rates

Celia C. Symons, Ecology and Evolutionary Biology, University of California - Irvine, Irvine, CA and Eric P. Palkovacs, Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, CA

Background/Question/Methods
Trait variation within species can influence higher-level biological processes as much as species presence or absence. Though many studies document how species traits influence communities and ecosystems, we do not know when the effects of trait variation will be masked by variation in population abundance. Most evo-to-eco experiments place equal densities of individuals with different traits into experimental units to isolate the effect of trait differentiation on ecology. However, evolution also impacts population size. Because of past experimental designs, we do not know the relative importance of species traits (per-capita effect) and population abundances on community and ecosystem variables, a gap in our ability to predict the impact of evolution on ecosystem functioning. To begin answering this question we conducted a series of experiments. First, we conducted experimental evolution on Daphnia pulicaria x pulex populations from fishless and fish lakes, exposed to two different temperatures over 4 months in the lab. Second, we transplanted the populations into plankton communities in ambient or warmed mesocosms. With this design we asked: what impact does Daphnia thermal adaptation have on diversity and ecosystem function? What is the relative impact of Daphnia abundance and a functional trait (per capita grazing rate) on ecosystem variables?
Results/Conclusions
Daphnia populations varied in their life history traits, with Daphnia from fishless lakes having a larger size at maturity and a greater r (multivariate PERMANOVA, p=0.03). After selection at 19 and 21 C, life history difference between the fish and fishless Daphnia were maintained, with no difference in life histories between the two selection temperature treatments (multivariate PERMANOVA, p=0.07). However, there was variation in grazing rates between selective environments, with higher grazing rates in warm-selected populations from the fish lake (ANOVA, lake type × selection temperature, p<0.01). Once transplanted into plankton communities, there was a significant effect of Daphnia selection history on ecosystem function (chlorophyll-a concentration). Using variation partitioning, we show that the effect of selection history on ecosystem productivity is due to shifts in the per capita grazing rate (independently explains 14%), not variation in Daphnia abundance (2%). This series of experiments highlights that trait change within a species can exert ecosystem effects in the absence of life-history changes. This result (re-)emphasizes the need to measure effect traits to understand how evolution influences ecosystems. Finally, our results suggest that adaptation to warmer temperatures will increase the strength of top-down control, though this effect may be context-dependent.