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Understanding the mechanisms producing and maintaining discontinuities between patches in mosaics of habitats is necessary to predict changes in patterns of abundance and distribution of species. On temperate rocky reefs, physical and biological disturbance can result in a mosaic of patches of encrusting coralline, turf- or canopy-forming algae. We experimentally investigated the effects of disturbance within and at the boundary between these patches, in order to identify mechanisms accounting for re-colonisation of space and to assess whether the response of boundaries can alter the relative size of contrasting habitats. Also, we tested whether the resilience of the different types of assemblages depends on species richness (i.e. number of taxa present) of habitats or, alternatively, on other properties of systems like differences in life-history traits of dominating species. Although the nature of the mechanisms generating differences among habitats changed among different stages of the colonisation, local processes (within patches) prevailed over larger scale processes (among patches) in determining early patterns of colonisation of space in mosaics. By the end of the experiment, assemblages in clearings at boundaries had recovered to the undisturbed reference condition, in contrast to clearings within barren patches or algal turfs. Boundaries represent, therefore, relatively more stable components of the mosaic, with greater resilience than adjacent habitats. Although sea urchins are often indicated as the main force regulating the proportion of contrasting habitats on shallow rocky reefs, determining the nature of variation in the interaction between species dominating each type of habitat is crucial to understand dynamics of mosaics. Finally, our study provides evidence that resilience could not be entirely controlled by initial species diversity, suggesting that different proportions of dominant taxa could influence the stability of natural systems.

It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17 degrees of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.