Coral reef ecosystems
Tropical coral reefs are among the most productive and biologically diverse ecosystems found on earth (Odum and Odum 1955; Connell 1978; Moberg and Rönnbäck 2003). Although these reefs only cover 0.1 – 0.5% of the ocean floor they provide a home to almost one third of the marine fish species and other marine biota (Mcallister 1991; Spalding and Grenfell 1997; Spalding et al. 2001). Like rainforests, their terrestrial equivalent, the three-dimensional habitat complexity underpins the biological success of coral reef systems (Connell 1978; Grigg et al. 1984; Reaka-Kudla 1997). This structural framework is primarily provided through the precipitation of vast quantities of calcium carbonate by scleractinian corals (Goreau 1959b; Goreau and Goreau 1959; Smith and Kinsey 1976). Basic growth of coral skeleton forms the fundament of the reef and facilitates complex ecosystem functioning and niche partitioning to harbour an exceptional heterogeneity of associated biota (Connell 1978; Graham and Nash 2012; Kennedy et al. 2013; Newman et al. 2015). Ancillary to the inexpressible biological value, millions of people worldwide rely in some way on the services provided by coral reefs, most notably for nourishment, but also for services associated with tourism and coastal protection (Costanza et al. 1997; Moberg and Folke 1999; Moberg and Rönnbäck 2003). By increasing frictional dissipation of wave energy, the complex physical structure created by corals protects coastal shorelines from erosion. This has allowed humans to settle and develop coastal areas throughout the tropics. Yet, coral reefs are at present ubiquitously under pressure due to a variety of stressors associated with increased anthropogenic activity on a global and local scale.
The marine environment is continuously exposed to change, but currently this change is more and more the result of human actions (Harvell et al. 1999; Derraik 2002; Orr et al. 2005; HoeghGuldberg and Bruno 2010). The stress exerted by the natural and anthropogenic induced changing global environment works in synergy with stressors that act on a finer spatial scale. Factors such as the overharvesting of fish, pollution, eutrophication, coastal development and the introduction of invasive species can locally trigger shifts in community composition and trophic hierarchy (Hughes 1994; Hughes et al. 2003; Pandolfi et al. 2003; Hughes et al. 2007; Hughes et al. 2017). By destabilising ecosystem functioning and interactions between key species, these stressors reduce reef resilience and therewith the capacity of coral reefs to cope with globally induced sea surface temperature anomalies or ocean acidification (Pandolfi et al. 2003; Bellwood et al. 2004; Hughes et al. 2017). Reefs in the wider Caribbean region seem particularly vulnerable to anthropogenic impact (Jackson et al. 2014). By large this can be ascribed to increased local pressures associated with the unprecedented human population expansion in the region. Since the 1950s, the total population in the Caribbean has more than doubled (United Nations, Department of Economic and Social Affairs, Population, Division, 2015). Natural biological and hydrological conditions are also less favourable compared to, for instance, the Indo-Pacific region (Roff and Mumby 2012). Biological diversity in the IndoPacific exceeds 10-fold the diversity found in the Caribbean (Spalding et al. 2001; Hoeksema et al. 2017), implying limited functional redundancy in the latter (Bellwood et al. 2003; Bellwood et al. 2004; Jackson et al. 2014). In addition, the quality of Caribbean surface water is significantly impacted by discharge from major South-American rivers like the Amazon and Orinoco as well as the North-American Mississippi river. The residence time of the polluted and eutrophic water from these rivers, combined with run-off and sewage water from the numerous islands is relatively long in the Caribbean Sea due to its distinct basin-like morphological and hydrological features (Roff and Mumby 2012). As a consequence of the rapid anthropogenic alteration of the marine environment we now see an ecological degradation of Caribbean coral reef habitats that has not occurred for over 200.000 years (Pandolfi and Jackson 2006).