ABSTRACT
Understanding species distributions and patterns of coexistence is a basic aim of ecology and fundamental to understanding not only how communities have come to be as they are but also how they will change in the future. This is particularly important when trying to predict biological responses to rapid and extreme changes in environmental conditions. Our perception of community dynamics depends on the temporal and the spatial scales at which we make our observations and the taxonomic resolution we use. Thus, observations and empirical approaches that allow for variability in time and space of multiple, interacting drivers are essential to counteract the assumption that simple patterns of species’ arrangement are driven by similarly simple processes. By altering the structure of resident communities, invasive species allow us to observe the development of new distributional equilibria and identify how variation in time and space of biotic and abiotic factors determines patterns of coexistence with native species. Here, we use the example of an invasive and an indigenous intertidal mussel to achieve this. We identify rather simple patterns of distribution of an exceptionally successful marine invasive species, the mussel Mytilus galloprovincialis, in its distribution along the coast of South Africa, examining relevant processes at multiple scales. In particular, we draw together research on its coexistence with an indigenous mussel, Perna perna. Over the last 40 years, Mytilus galloprovincialis has spread along thousands of kilometres of the southern African coast. Mytilus galloprovincialis has now reached at least a temporary equilibrium in its eastern limit and in its pattern of coexistence with the native mussel Perna perna. We synthesize a comprehensive body of literature on these two species that addresses processes occurring from centimetres to thousands of kilometres and from minutes to evolutionary timescales to develop a more robust understanding of the relative contributions of the deterministic and stochastic processes that structure the dynamics of coexistence. The review shows that the development and maintenance of simple distributional patterns emerge from and are maintained by variations in multiple biotic and abiotic interactions that occur at different and sometimes-nested scales. These include species-specic effects of environmental factors, such as not only high temperatures, wave action, desiccation, and sand scour or sand burial but also biological effects such as predation, parasitism, larval supply, and recruitment. Importantly, abiotic conditions can modulate species interactions, including interference and exploitation competition, so that species-specic responses
to environmental conditions are important. Viewed across the environmental gradient offered by the intertidal landscape, direct interactions include examples of initial facilitation followed by later competitive exclusion. Although in a state of ux at small scales, the present patterns of distribution and coexistence have been relatively stable at larger scales for decades and reect place-specic balances among the interacting factors, with different factors assuming prominence in different places. Detailed observation and experimentation are necessary to avoid assuming that species’ arrangements emerge from a single driver or a few simple drivers, and this has clear implications for attempts to predict species distributions under conditions of climate change. In addition, the same pattern can emerge for different reasons. For example, recruitment of both mussel species is weak in the high mussel zone, but for different reasons. For Perna perna, this is because of poor survival of few settlers, while for Mytilus galloprovincialis, it reects good survival of few settlers. With so many interacting factors achieving uctuating local balances, the surprise is not that the pattern is so clear, but that there is any pattern at all.
