Latitudinal variation in the competition-colonisation trade-off reveals rate-mediated mechanisms of coexistence

Theories of species coexistence often describe a trade-off between colonising and competitive abilities. In sessile marine invertebrates, this trade-off can manifest as trends in species distributions relative to the size of isolated patches of substrate. Based on their abilities to find available substrate and competitively exclude neighbours, good colonisers tend to dominate smaller patches, whereas better competitors tend to monopolise larger patches. In theory, species with equivalent colonising and competitive abilities should display similar distributions across patch sizes. We used patch size to observe this manifestation of the competition-colonisation trade-off over 20° of latitude. The trade-off was more readily observed at lower latitudes and was proportional to the 'ecological age' of communities (i.e. the degree of resource acquisition and likelihood of species interactions). Results suggest that ecological age may mediate the prominence of stochastic or deterministic coexistence mechanisms and will depend on the rate of ecological processes.