Eco-evo conflicts in a stage-structured mutualism: modeling the consequences of ontogenetic variation in host quality

Stage-structured mutualisms occur when mutualistic interactions vary across the ontogeny of one or both interacting species. They are widespread and can play important roles in ecological communities, yet little is known about the evolution of partner choice in stage-structured mutualisms. Inspired by branching corals and associated damselfish, we modeled a stage-structured mutualism between a large, long-lived host species and a smaller mutualistic partner. We explored the evolution of partner choice behavior (strength of partner’s preference for adult hosts) when host quality increased with host size and the partner either increased host survival (a defensive mutualism) or increased host growth (a nutritional mutualism). We found that while partners always evolved strong preferences for adult (high quality) hosts, evolved behaviors often resulted in lower equilibrium population sizes and longer post-disturbance recovery times relative to simulations in which partner preferences were fixed at weaker values. These differences were greatest for nutritional mutualisms, indicating that the type of mutualistic interaction was an important determinant of whether a strong preference for adult hosts had ecological consequences at the population level. Overall, our results suggest ontogenetic variation in host quality can create tension between what is best for individual partners (avoiding low quality hosts) and what is best for partner populations (spreading benefits across host life stages), highlighting the importance of considering both the evolutionary and ecological dynamics of stage-structured mutualisms.