Despite widespread climate-driven reductions of coral cover on tropical reefs, little attention has been paid to the possibility that changes in the geographic distribution of coral recruitment could facilitate beneficial responses to the changing climate through latitudinal range shifts. To address this possibility, we compiled a global database of normalized densities of coral recruits on settlement tiles (corals m −2) deployed from 1974 to 2012, and used the data therein to test for latitudinal range shifts in the distribution of coral recruits. In total, 92 studies provided 1253 records of coral recruitment, with 77% origi nating from settlement tiles immersed for 3−24 mo, herein defined as long-immersion tiles (LITs); the limited temporal and geographic coverage of data from short-immersion tiles (SITs; deployed for < 3 mo) made them less suitable for the present purpose. The results from LITs show de clines in coral recruitment, on a global scale (i.e. 82% from 1974 to 2012) and throughout the tropics (85% reduction at < 20°latitude), and in creases in the sub-tropics (78% increase at > 20°latitude). These trends indicate that a global decline in coral recruitment has occurred since 1974, and the persistent reduction in the densities of recruits in equatorial latitudes, coupled with increased densities in subtropical latitudes, suggests that coral recruitment may be shifting poleward.

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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 more 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.

Summary 1.Corals are clonal organisms and show a plastic growth. We study a partial differential equation model for the dynamics of size distribution of corals and predict the trajectory of recovery after a catastrophic disturbance, such as the recent bleaching that killed most corals in southern Japan. 2. We assume that the mean growth rate of colony size, measured in projected area, is a linear function of colony size, and that the variance in growth rate is proportional to the size, which is consistent with the growth data of a coral Acropora hyacinthus Dana 1846. 3. The model incorporates the space-limitation in colony growth and recruitment. The growth rate and recruitment rate are proportional to the fraction of free space within the local habitat. 4. In many corals, including A. hyacinthus , recruitment occurs in a short period once a year. However, our model illustrates that the colony size distribution does not show distinguishable cohorts since the observed variance in growth rate is large. The model with discrete settlement and a large variance in growth rate results in size distributions that are very well approximated by an explicitly soluble model with constant recruitment and no growth variance. 5. When mortality is low, the dynamics of size distribution show two different phases in the recovery process. In the first phase, size distribution is determined by recruitment and growth, and can be predicted well by the case without mortality. After free space is depleted, recruitment and growth slow down and become balanced with mortality. The equilibrium size distribution is controlled by all the three processes. Both for the transient and the equilibrium size distribution, the average colony size increases with growth rate but decreases with recruitment rate. 6. Strongly skewed colony-size distributions in which small size classes have the largest numbers are generated for a wide range of parameters by the space-limitation in growth, even without partial death of colonies.

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