Larval dispersal facilitates persistence of coral reef metacommunities
Connectivity through larval dispersal is thought to be an important determinant of coral persistence, especially in terms of the recovery potential for populations that suffer catastrophic mass bleaching events. However, our understanding of the interactions between coral recruitment and mortality on reefs is limited, particularly when reproduction occurs in a discrete, seasonal manner as in the majority of stony coral species in the Indo-Pacific. Here, we model coral-algal competition across multiple patches, accounting for the exchange of coral larvae among patches. We incorporate coral larvae into the model in two ways, with and without stage structure, to assess the effects of differential growth and mortality between newly settled and adult corals. In addition, we consider scenarios in which larval recruits are contributed both seasonally and continuously in time, in varying proportions by local and external sources. Our goals were to (1) determine the conditions under which connectivity can offset the effects of higher mortality due to bleaching and (2) evaluate the impacts of altered connectivity patterns and increased mortality on the stability of coral- vs. algal-dominated regimes.
Our results reveal a nuanced relationship between coral mortality, larval connectivity and coral persistence. In general, we see that incorporating coral recruitment, even seasonally, erodes the basin of attraction that is associated with algal-dominated states. This allows coral to persist in a parameter space that, without larval input, would lead to an algal-dominated regime in a patch. When there is very low to zero coral cover, there exists a threshold number of arriving coral larvae to a patch that is required to stimulate coral recovery; this threshold is highly dependent on the initial amount of algal cover and is significantly higher when we incorporate stage structure within the coral class. Beyond a critical algal cover, however, no amount of larval input can induce a regime shift to coral dominance. In this case, there is an increased importance of parameters that affect within-patch dynamics in setting the trajectory towards a particular reef state.