Climate change will directly affect species' population growth rates, but will also have indirect effects mediated by the response of species' competitors and facilitators to climate change. However, many models of species' responses to climate change do not include interspecific interaction. We addressed two questions about the role of interspecific interactions in mediating the population impacts of climate change: 1. Are interspecific interactions strong enough to tip the balance from population persistence to local extinction? 2. Will climate change alter the direction or strength of interspecific interactions? To answer these questions, we manipulated precipitation in a Kansas mixed prairie and measured the performance of transplanted plugs of three perennial grasses, Bouteloua curtipendula, Bouteloua hirsuta, and Schizachyrium scoparium. The plugs were transplanted into local neighborhoods composed entirely of heterospecifics or with all neighbors removed, allowing us to quantify the effect of interspecific interactions and precipitation on low density growth rates, the critical measure of population persistence.  

Results/Conclusions

In the absence of interspecific neighbors, all three grasses had high tiller growth rates. However, with neighbors intact, the tiller growth rates of all three species were close to, or below, zero. This result demonstrates the potential for interspecific interactions to have strong effects on population persistence. We did not find evidence that the direction or strength of interspecific interactions varied with precipitation: The interaction between precipitation treatment and the presence of neighbors had no significant effect on either tiller growth rates or biomass. Our results imply that models predicting species' responses to climate change should include the effects of interspecific interactions, but could ignore the effect of climate change on the direction and strength of those interactions.