Interactions in a warmer world: The relative effects of experimental warming, intraspecific density, and insect herbivory on seedling dynamics

Many effects of a changing climate for organisms, populations, and ecosystems are already apparent. Less studied are the effects of increases in temperature on species interactions. While warming may potentially alter interactions among species, species interactions may also mediate individual species responses to ongoing climatic change. In this experiment we manipulated temperature in field-based, open-top chambers for three years to examine the relationship between biotic interactions and climatic warming on the population dynamics of seedlings of Quercus alba. We investigated the effect of warming on rates of insect herbivory on Q. alba seedlings. Additionally, we assessed the relative effects of increasing temperature, insect herbivory, and conspecific density on seedling survival. We found two unexpected results. First, we observed a negative relationship between temperature and levels of insect herbivory during each year of the experiment. Second, higher levels of herbivory were associated with higher rates of survival to the second year of the study. Although we never detected a direct effect of conspecific density on seedling survival, herbivory and conspecific seedling density did interact to influence Q. alba seedling survival early in the experiment. Taken together, our results indicate species responses to climatic warming may be contingent on intra- and interspecific interactions, sometimes in complicated and counter-intuitive ways.