Inducible defences and the paradox of enrichment

In order to evaluate the effects of inducible defences on community stability and persistence, we analyzed models of bitrophic and tritrophic food chains that incorporate consumer-induced polymorphisms. These models predict that intra-specific heterogeneity in defence levels resolves the paradox of enrichment for a range of top-down effects that affect consumer death rates and for all possible levels of primary productivity. We show analytically that this stability can be understood in terms of differences in handling times on the different prey types. Our predictions still hold when defences also affect consumer attack rates. The predicted stability occurs in both bitrophic and tritrophic food chains. Inducible defences may promote population persistence in tritrophic food chains. Here the minimum densities of cycling populations remain bound away from zero, thus decreasing the risk of population extinctions. However, the reverse can be true for the equivalent bitrophic predator-prey model. This shows that theoretical extrapolations from simple to complex communities should be made with caution. Our results show that inducible defences are among the ecological factors that promote stability in multitrophic communities.