The biogeography of Scleractinian reef corals: Evidence from beta diversity

Abstract

Across the globe, hundreds of millions of people and even greater numbers of marine organisms depend on coral reef ecosystems. However, these ecosystems are increasingly harmed by human activities that are transforming the environment in a variety of ways. Corals are particularly vulnerable to these changes, with many reefs having lost over half of their coral cover in the last three decades. These trends threaten people’s livelihoods and even the continued existence of many species. Implementing effective conservation action with limited resources requires prioritisation. Prioritisation, however, depends on knowledge of how organisms will respond to the various pressure placed on them and which of those are most destructive. Experiments offer one avenue to pursue such understanding, but developing an experimental understanding of coral ecology is hindered by the richness of corals, their phenotypic plasticity, their diversity of traits and the numerous ways that the environment is being modified. Moreover, certain factors may have influences that are only detectable at very large spatio-temporal scales, and which are essentially impossible to manipulate experimentally. Biogeography constitutes an alternative method to understanding the factors that are most limiting to corals. Biogeographers take a broad-scale approach to understanding biodiversity, exploring patterns in the distribution of life and the structure of assemblages and relating them to putative drivers. Identifying what generates these present-day patterns provides an indication of the factors most relevant to the ecology of the organisms and may improve predictions of the impacts and responses of organisms and ecosystems to future climate change. The work presented in this thesis applies such a biogeographic approach to reef-building corals. In Chapter 3, I attempt to identify the key factors generating differences in species composition (i.e., beta diversity) of coral assemblages across oceans basins and spatial scales. To achieve this objective I assess the influence and relative importance of a number of variables, including environmental conditions, dispersal limitations and geographic factors on global patterns of coral beta diversity. My results demonstrate that Indo-Pacific and Atlantic corals respond to similar factors, especially dispersal limitation, sea surface temperature and nutrient concentrations. Previous large-scale analyses of coral diversity have typically failed to identify a strong influence of dispersal limitation, but the integration of realistic estimates of coral connectivity into this analysis reveals that it is a key determinant of coral distributions and beta diversity. The influence of environmental factors are particularly apparent at small spatial scales, whereas the size of reef areas emerges as an important factor at large spatial scales. Chapter 4 builds on the conclusions from the third chapter and explores the influence of dispersal limitation on coral assemblages in more detail. The life-history traits of organisms dictate how they interact with their environment and in this chapter I investigate how different modes of reproduction (i.e., spawning of gametes and brooding larvae), with different longrange dispersal abilities, may affect the biogeography of corals. I begin by investigating the consequences of different reproductive modes on the range sizes of corals using permutation tests. I then determine how the structure of coral assemblages change across distance depending on the mode of reproduction. My results do not show that species’ range sizes differ significantly between reproductive groups, despite a tendency for spawning corals to have larger ranges. However, the spatial structure of assemblages does differ significantly between reproductive groups. Assemblages of corals that brood larvae are more dissimilar across space than assemblages of broadcast spawning corals, which display greater spatial overlap. These results suggest that differences in dispersal ability resulting from distinct reproductive strategies influences large-scale biogeographic patterns. This thesis demonstrated that dispersal limitation is an important challenge for corals, suggesting that there may be a delay, or even a failure, to track suitable environmental conditions under climate change. This problem may not be of equal importance to all coral species, since species with spawning reproduction appear to be less dispersal limited. Managers may therefore need to focus extra attention on brooding corals. In addition, the primary environmental factors that influence coral distributions (i.e., temperature and nutrient concentrations) are strongly impacted by human activities and worsening conditions are virtually guaranteed. The importance of coral reef area at very broad scales emphasises the necessity of coordinated and decisive action to limit the multiple stresses being placed on coral reef ecosystems to halt the degeneration and destruction of these precious places. I hope that this thesis will be inspiring to other researchers and prove useful in tackling one the most pressing matters of our time, namely the preservation of coral reefs.