Eco-evolutionary response of communities to nutrient enrichment and warming

Background/Question/Methods

            Global change caused by human activities, such as increased average temperatures and increased nutrient input into ecosystems, is likely to affect most natural communities. However, making predictions about how communities may be affected can be complex because multiple stressors may have non-additive effects, some trophic levels may be more sensitive than others, and evolutionary changes may mediate ecological consequences. Here, we ask how the interaction between warming and increased nutrient input affects different trophic levels both ecologically and evolutionarily.

            We used microcosms consisting of a single species of bacterivorous protist, Colpidium spp., and a community of bacteria for our experiments. We factorially manipulated low and high nutrient enrichment and warming treatments for a total of four possible “historical environments” and allow communities to evolve for four weeks (~150 protist generations). We then performed reciprocal transplants to and from each environment and allow communities to remain in these “contemporary environments” for three days. We measured protist traits (cell size and peak density) and fitness (per-capita growth rates) as well as features of bacterial ecology (community composition and abundance). Structural equation modeling was used to evaluate the causal effects of treatments, protist ecology/evolution and bacterial ecology.

Results/Conclusions

            Protists that evolved in low nutrient treatments have higher growth rates in both low and high contemporary nutrient environments. This effect changes slightly, however, when historical temperature regime is considered. We find that protists that evolve in low nutrient environments evolve to have a higher peak density, indicating evolution toward higher resource efficiency. Bacterial community composition and abundance differ strongly between historical and contemporary treatments. There are direct effects of historical and contemporary treatments on bacterial community composition and abundance. In addition, there are indirect effects on bacterial ecology that are mediated by protist trait evolution.

            Our results show that non-additivity between environmental factors, like temperature and nutrients, can cause populations and communities to change in unexpected ways. Trophic levels also differ in their responses to these factors so their individual responses must be considered. Finally, no trophic level can be considered in isolation. Here, we find that evolution at higher trophic levels can alter the response of those below it. Many factors, including those presented here, should be considered in understanding how food webs are expected to respond to different environments.