Thresholds and the resilience of Caribbean coral reefs

Abstract

The deteriorating health of the world's coral reefs threatens global biodiversity, ecosystem function, and the livelihoods of millions of people living in tropical coastal regions. Reefs in the Caribbean are among the most heavily affected, having experienced mass disease-induced mortality of the herbivorous urchin Diadema antillarum in 1983 and two framework-building species of coral. Declining reef health is characterized by increases in macroalgae. A critical question is whether the observed macroalgal bloom on Caribbean reefs is easily reversible. To answer this question, we must resolve whether algal-dominated reefs are an alternative stable state of the ecosystem or simply the readily reversible result of a phase change along a gradient of some environmental or ecological parameter. Here, using a fully parameterized simulation model in combination with a simple analytical model, we show that Caribbean reefs became susceptible to alternative stable states once the urchin mortality event of 1983 confined the majority of grazing to parrotfishes. We reveal dramatic hysteresis in a natural system and define critical thresholds of grazing and coral cover beyond which resilience is lost. Most grazing thresholds lie near the upper level observed for parrotfishes in nature, suggesting that reefs are highly sensitive to parrotfish exploitation. Ecosystem thresholds can be combined with stochastic models of disturbance to identify targets for the restoration of ecosystem processes. We illustrate this principle by estimating the relationship between current reef state (coral cover and grazing) and the probability that the reef will withstand moderate hurricane intensity for two decades without becoming entrained in a shift towards a stable macroalgal-dominated state. Such targets may help reef managers face the challenge of addressing global disturbance at local scales.

Transitional states in marine fisheries: adapting to predicted global change

“…Monitoring and indicators: deciding what, where and when to monitor are among the most important decisions in the development of any fisheries adaptation plan. To act, regulators must understand the current state of the ecosystem (Vilhjálmsson et al 2005) and this understanding can only come through monitoring of indicators showing changes in state (King & McFarlane 2006) that can be related to unfavourable thresholds (Mumby et al 2007) and show how close the system is to reaching them (Scheffer & Carpenter 2003). Often quite simple biological indicators such as population variance (Brooks et al 2006) or current abundance as a fraction of unfished abundance (Worm et al 2009) can provide useful information but many of these are underused (Brander 2010).…”

Response Diversity Can Increase Ecological Resilience to Disturbance in Coral Reefs

“…Whether coral reefs exhibit alternative stable states between coraldominated states and those dominated by other species such as macroalgae has mixed evidence (e.g., Dudgeon et al 2010;Zychaluk et al 2012;Mumby et al 2013), with greater support for alternative stable states in Caribbean reefs (Bruno et al 2009;Hughes et al 2010;Roff and Mumby 2012). One mechanism that can theoretically drive alternative stable states is a positive association between coral cover and grazing rates on macroalgae that would otherwise overgrow corals (Mumby et al 2007). Empirical evidence supports the potential for corals to attract grazers that would otherwise avoid macroalgal-dominated patches (Williams and Polunin 2001;Hoey and Bellwood 2011), macroalgal overgrowth of corals in the absence of grazers (Hughes et al 2007), and macroalgae prevention of coral recruitment (McCook et al 2001); corals also compete directly with one another via injuries to soft tissue and overtopping (Lang and Chornesky 1990).…”

“…We build on an existing theoretical framework of coral-macroalgal interactions by Mumby et al (2007) that has been extended in a number of ways (e.g., to include multiple anthropogenic stressors and dynamical grazer populations; Anthony et al 2011;Blackwood et al 2012). Here, we extend it to include two contrasting coral types: a resistant coral that tolerates stress and a resilient coral that recovers quickly.…”

Limited flexibility in resource use in a coral reef grazer foraging on seasonally changing algal communities

“…Parrotfishes (family Labridae) are probably among the most important components of the grazing guild owing to their high biomass and wide distribution and a feeding mechanism that is highly specialized for scraping algae and detritus from carbonate substrates (Russ 1984;Choat 1991;Mumby et al 2007;Fox and Bellwood 2007;Sotka and Hay 2009). Within this family, species differ in their feeding mode (for example excavator vs. scraper), microhabitat use, and food selection (Bellwood and Choat 1990;Bruggemann et al 1994b;Hoey and Bellwood 2008).…”