Dynamic fragility of oceanic coral reef ecosystems

Abstract

As one of the most diverse and productive ecosystems known, and one of the first ecosystems to exhibit major climate-warming impacts (coral bleaching), coral reefs have drawn much scientific attention to what may prove to be their Achilles heel, the thermal sensitivity of reef-building corals. Here we show that climate change-driven loss of live coral, and ultimately structural complexity, in the Seychelles results in local extinctions, substantial reductions in species richness, reduced taxonomic distinctness, and a loss of species within key functional groups of reef fish. The importance of deteriorating physical structure to these patterns demonstrates the longer-term impacts of bleaching on reefs and raises questions over the potential for recovery. We suggest that isolated reef systems may be more susceptible to climate change, despite escaping many of the stressors impacting continental reefs.biodiversity ͉ climate change ͉ coral bleaching ͉ resilience ͉ reef fish G lobal warming is causing reef corals around the world to expel their photosynthetic symbionts, resulting in ''bleaching'' and extensive coral mortality (1-3). Widespread impacts of bleaching are predicted (4), although empirical data on the long-term effects on other components of the ecosystem are lacking. Of particular interest are reef fish, which support local fisheries and tourism (5), and are crucial for the resilience of coral reefs (6-8). Existing studies of the indirect effects of bleaching mediated coral mortality on reef fish assemblages have been short-term and indicated limited change (9-11); however, the longer-term implications of coral loss may be much more substantial (12). Long-term responses may result from changes in the physiological condition of species following coral loss (13) or through processes such as the breakdown of the physical matrix of the remaining reef structure.The global mass bleaching event of 1998 was devastating in the western Indian Ocean, where the El Niño event interacted with the Indian Ocean dipole (14), resulting in 75-99% loss of live coral (15). The Seychelles suffered particularly badly; live coral was reduced by Ͼ90% across the entire range of the inner islands (2, 16), with no apparent depth refuge. With widely accepted predictions of all reef regions of the world suffering similar large-scale degradation through bleaching in coming decades (1, 2), the study of such locations provides a unique opportunity to understand longer-term impacts on other components of the ecosystem, with implications for resilience and the persistence of vulnerable species.We surveyed coral and fish communities at 21 sites across the inner islands of the Seychelles in 1994 (17) and 2005. Over 50,000 m 2 of three distinct coral reef habitats were surveyed: fringing reefs with carbonate framework, coral growth on a granitic substrate, and patch reefs on a sand, rock, or rubble base (17). The study specifically aimed to assess changes in benthic variables after the 1998 bleaching event and relate these to changes in the ...

The future of evolutionary diversity in reef corals

“…For example, in southwest Western Australia, loss of 'complex' Acropora and Turbinaria species (figure 4c) with branching and laminar morphologies could reduce the complex three-dimensional architecture of reefs. This has negative consequences for the abundance, richness and functional diversity of reef inhabitants, including fish [88][89][90] and invertebrates [91][92][93]. Conversely, the preferential loss of 'robust' corals as projected for Pacific Costa Rica and Panama (figure 4a) may result in surviving assemblages dominated by Acropora valida, Gardineroseris planulata, Pavona and Porites spp., which are more susceptible to outbreaks of bleaching, disease and the crown-of-thorns seastar [20,34,36,94,95].…”

Perceptions of trends in Seychelles artisanal trap fisheries: comparing catch monitoring, underwater visual census and fishers' knowledge

“…At each site, 16 replicate 7-m radius point-count surveys of reef fishes were conducted. The abundance and size of 134 species of diurnally-active reef-associated fish were recorded at each count (Jennings et al 1995;Graham et al 2006). Length calibration was achieved by estimating the lengths of a random assortment of lengths of PVC pipe before each day's sampling, until the observer was within an error range of 1cm; mean errors for 1994 and 2005 were 3.1% and 2.2%, respectively (Graham et al 2007).…”

Biological community structure on patch reefs in Biscayne National Park, FL, USA

“…Generally, one body of literature supports niche diversification and predicts highly ordered communities (e.g., Connolly et al 2005), while another supports a more stochastic view, highlighting the importance of spatially and temporally variable recruitment of larvae (e.g., Sale and Douglas 1984;Doherty and Fowler 1994). However, with backgrounds of considerable temporal and spatial variability in fish and benthic community structure, many studies support human extraction and alteration of habitat and water quality as determinants of coral reef status (McClanahan 1994;Chapman and Kramer 2001;Halpern and Warner 2002;Graham et al 2006;Pandolfi and Jackson 2006;Maliao et al 2008). Human disturbance could, in effect, mask any natural relationships between species and their habitat by altering the limiting factors that control populations.…”