Reef Resilience

Reef Resilience

Resilience describes the ability of a system to absorb shocks without fundamentally shifting to a different community state. Caribbean reefs are very likely to have at least two alternate community states, one rich in coral and the other lacking much coral and usually possessing much macroalgae. Feedback processes determine whether a reef will move towards one or other of these extremes (see diagram below).

We use ecological models to explore the conditions under which reefs move from one state to another. In general, trajectories towards the ‘high coral’ state represent recovery whereas trajectories towards the ‘coral depleted’ state represent degradation. A series of thresholds distinguish these two forms of trajectory and we are interested in knowing what controls these thresholds and predicting their locations.

Thresholds are influenced by the local physical environment, which in turn influences the productivity of the ecosystem. Other factors such as coral recruitment rate, coral growth rates, and so on also influence the potential recovery of the system.

For convenience, we often plot these thresholds as a function of grazing which is often depleted artificially by fishing or disease. Models of disturbance are then used to ask whether a reef in a given state today are likely to be pushed beyond a threshold. If reefs are pushed from the ‘recovery’ phase to the ‘degradation’ phase then natural processes will tend to reinforce the decline and reef health will continue to deteriorate. Clearly, this is a situation that managers wish to avoid so we look for ways to reduce the chances of this eventuality. So far, we’ve investigated a few impacts on resilience. These include fishing herbivorous parrotfish, removing mangrove nursery habitats (which also influences the number of herbivores), reducing nutrient runoff into the watershed, and mitigating the effects of ocean acidification.

Stable and unstable equilibria for Caribbean coral reefs at two levels of algal-coral overgrowth. Stable and unstable equilibria denoted (n) and (o) respectively. Black denotes 8 cm2 yr-1, red denotes 14 cm2 yr-1. Blue lines, marked with appropriate dates, represent model predictions of the trajectory of reefs in Jamaica

Probability that reefs of given initial state will remain above the unstable equilibrium during a 25 year period. The physical disturbance regime includes stochastic hurricanes with a 20 year periodicity and the algal-coral overgrowth rate is 8 cm2 yr-1. The unstable equilibrium is denoted (—).

References:

Mumby PJ & Steneck RS (2010) The resilience of coral reefs and its implications for reef management. In: Dubinsky, Z (ed.) Coral Reefs. Springer, Amsterdam (in press)

Blackmore J, Hastings A, Mumby PJ (2011) The effect of fishing on hysteresis in Caribbean coral reefs. Journal of Theoretical Ecology (in press)

Anthony K, Maynard J, Diaz-Pulido G, Mumby PJ, Cao L, Hoegh-Guldberg O (2011) Ocean acidification and warming will lower coral reef resilience. Global Change Biology (in press)

Mumby PJ (2009) Phase shifts and the stability of macroalgal communities on Caribbean coral reefs. Coral Reefs 28: 683-690

Mumby PJ, Steneck RS (2008) Coral reef management and conservation in the light of rapidly-evolving ecological paradigms. Trends in Ecology and Evolution 23: 555-563

Mumby PJ, Hastings A (2008) The impact of ecosystem connectivity on coral reef resilience. Journal of Applied Ecology 45: 854-862

Mumby PJ, Hastings A, Edwards HJ (2007) Thresholds and the resilience of Caribbean coral reefs. Nature 450: 98-101