Herbivory-pollination coupling mediated by ontogenetic change in interaction type stabilizes community dynamics
Species change their ecology over the course of development, a phenomenon known as ontogenetic niche shift (ONS). There is an increasing interest in how ONS would affect community dynamics. Models have predicted that ONS may cause alternative stable states (ASS) in food webs (i.e., communities structured only by trophic interactions). However, interaction type among species can completely changes as a result of ONS. One example is the change from herbivory (i.e. antagonistic interaction) to pollination (i.e. mutualistic interaction) mediated by metamorphosis of pollinating insects (e.g. Lepidoptera and Coleopteron). How the addition of mutualistic interactions to food webs with ONS influence community stability remain unexplored, despite its ubiquity in nature. Here, we construct a community module model to incorporate ontogenetic change in interaction type, where juveniles act as consumers while matured adults engage in mutualism with another resource. We then compare community stability of our module with that of food-web modules which focus only on trophic interaction shifts.
We show that the emergence of ASS tends to be suppressed in communities with ontogenetic change in interaction types. This is because the positive feedback underlying ASS in food webs is reversed to be negative feedback if the adults become mutualists. We further investigated how the likelihood of ASS depends on functional forms of juvenile consumption and adult mutualism. When the mutualistic benefit is a non-linear function that saturates at high adult resource abundance, the strength of negative feedback is weakened and thus ASS reemerges. Under this condition, a nonlinear consumption could reduce the likelihood of ASS. Furthermore, nullcline analysis suggests that mutualistic interactions in the ontogeny-mediated interaction change module could increase the likelihood of consumer persistence even if the consumer reproductive number is less than one. Overall, our result suggests that although both stage-structured food-web modules and mutualism modules have the potential of ASS, their combination is stabilizing which can improve community resilience and enhance species persistence. Our simple community module is a new building block of complex network models involving both trophic and mutualistic interactions.