Colonization limitation of specialized enemies reduces species richness

Abstract

The densities of a species and its specialist enemies are expected to covary, giving rare species an advantage that may promote species coexistence. This hypothesis has been particularly applied to tree species coexistence, and theoretical work has investigated how the spatial scale of dispersal and degree of enemy specialization influence coexistence outcomes. A simplifying assumption of this modeling work is that enemies are ubiquitous and host individuals always colonized. Yet recent studies have suggested that this assumption may not be justified in natural systems. Here we build a multispecies spatial model in which both the host species and enemy species are demographically dynamic. Exploring a range of spatial scales of enemy dispersal and seed dispersal, we find that the lag between seedling recruitment and colonization by enemies generally results in reduced species richness compared with models assuming ubiquitous enemies. This result depends on the spatial dynamics of enemy dispersal, and holds whether or not species diversity is supported by migration from outside the study area, as is often assumed in community ecology models. In light of our findings, we suggest that a full understanding of how enemies promote species coexistence must account for the colonization dynamics of enemies.