Space preemption, size-dependent competition, and the coexistence of clonal growth forms

Interspecific interactions that produce nontransitive competitive networks have been proposed to promote diversity in a broad range of systems, including coral reefs. In this paper, we model the effect of size-dependent shifts in competitive ability on the coexistence of canopy-forming and understory coral species, and we predict how these shifts influence patterns of community structure along large-scale gradients in disturbance and recruitment limitation. We consider three models, representing a gradient from purely hierarchical competition in which the canopy-former is dominant, to competition involving standoffs and reversals between the understory species and juvenile canopy-formers. Analysis of these models leads to two key conclusions. First, as competition becomes less transitive, coexistence may be promoted or inhibited, depending upon the extent to which the canopy-former can sustain itself by clonal propagation. Specifically, when clonal growth alone is adequate to sustain the canopy-former, increasing nontransitivity promotes coexistence. When it is not, nontransitivity inhibits coexistence. Secondly, size-dependent nontransitivity dramatically changes how gradients in disturbance and recruitment affect species coexistence. In contrast to hierarchical interactions, standoffs and reversals do not show an "intermediate recruitment" phenomenon, in which coexistence is facilitated at intermediate levels of recruitment. Moreover, under hierarchical competition, the dominant always benefits mote than the subordinate as recruitment is increasingly facilitated. Under standoffs and reversals, however, increasing recruitment often favors the canopy-former at some levels of disturbance, but the understory species at other levels of disturbance. These results differ markedly from previous models of stage-dependent competition, suggesting that promotion of coexistence by ontogenetic shifts in competitive ability depends upon the mechanisms by which competition occurs in particular ecological contexts. The results also indicate that the effects of gradients in disturbance and recruitment on community structure depend fundamentally on how species compete for space.