Outcomes of species interactions are commonly dependent on environmental factors, a phenomenon known as conditionality. A well-known manifestation of conditionality is when interactions have positive outcomes under stressful or limiting conditions, but antagonistic or neutral outcomes under benign conditions. Such patterns have been extensively described by economic models of plant-microbe mutualisms and the Stress-Gradient Hypothesis. While conditionality itself is well-documented, the evolutionary implications of conditionality are much less studied, though with notable exceptions. Here, we merge existing frameworks for conditionality and evolution in species interactions to predict patterns of adaptation and co-adaptation along abiotic gradients.
Results/Conclusions
Conditionality predicts that mutualisms will be most mutually positive in stressful environments, and this aligns the fitness interests of interactors. We posit that selection in stressful environments will drive mutualistic adaptation and co-adaptation: mutations in one partner increasing benefits conferred to the other partner will feed back to increase fitness in the first. As a corollary, conditionality predicts antagonisms will be most exacerbated in benign environments, and we posit that adaptation and co-adaptation will also be stronger in benign environments, resulting in Red-Queen adaptation dynamics, or niche partitioning. Finally, if interactions instead become neutral in benign environments, no adaptation is predicted. We further develop experimental designs and a linear model framework to test predictions of our hypothesis, with a focus on interactions that include positive outcomes.
Our hypothesis has implications for understanding species current and future range limits. On a broader scale, our hypothesis joins a growing literature demonstrating biotic influence on species range limits, even when those limits appear to be patterned by climate. Under climate change, stress experienced by species may increase. In turn, fitness impacts of species interactions ameliorating stress would increase, with consequences for adaptation and range expansion or contractions.