Predicting global dynamics from local interactions: individual-based models predict complex features of marine epibenthic communities

Spatially explicit community models often generate a wide range of complex dynamics and behaviours, but the predictions of community structure and dynamics from many of these models are rarely compared with the natural communities they are intended to represent. Here, we develop a spatially explicit individual-based model of a complex marine epibenthic community and test its ability to predict the dynamics and structure of the natural community on which the model is based. We studied a natural epibenthic community on small-scale patches of jetty wall to estimate the outcomes of pair-wise interactions among individuals of different species, neighbour-specific growth rates, and species-specific recruitment and mortality rates. The model is defined with rules acting at two spatial scales: (1) between individual cells on the spatial landscape that define the nature of interactions, growth and recruitment at a scale of 1 cm2, and (2) at the scale of whole colonies (blocks of contiguous cells) that define size-specific mortality and limitations to the maximum size of colonies for some species for scales up to 1000 cm2. The model is compared to the existing patches on the jetty wall and proves to be a good descriptor of the large range of possible communities on the jetty, and of the multivariate variances of the patches. The high variability in community structure predicted by the model, which is similar to that observed in the natural community, arises from observed variability in parameters of interaction outcomes, growth, recruitment, and mortality of each species. Thus if the processes we modelled operate similarly in nature, our results suggest that it is difficult to attempt to predict the precise trajectory of the community in a particular patch. Our results show that it is possible to develop a testable, predictive spatial model where the patch-scale community patterns of structure and dynamics are emergent, arising from local processes between colonies and species-specific demography.