A model-based approach to determine the long-term effects of multiple interacting stressors on coral reefs

Abstract

The interaction between multiple stressors on Caribbean coral reefs, namely, fishing effort and hurricane impacts, is a key element in the future sustainability of reefs. We develop an analytic model of coral-algal interactions and explicitly consider grazing by herbivorous reef fish. Further, we consider changes in structural complexity, or rugosity, in addition to the direct impacts of hurricanes, which are implemented as stochastic jump processes. The model simulations consider various levels of fishing effort corresponding to' several hurricane frequencies and impact levels dependent on geographic location. We focus on relatively short time scales so we do not explicitly include changes in ocean temperature, chemistry, or sea level rise. The general features of our approach would, however, apply to these other stressors and to the management of other systems in the face of multiple stressors. It is determined that the appropriate management policy, either local reef restoration or fisheries management, greatly depends on hurricane frequency and impact level. For sufficiently low hurricane impact and macroalgal growth rate, our results indicate that regions with lower-frequency hurricanes require stricter fishing regulations, whereas management in regions with higher-frequency hurricanes might be less concerned with enhancing grazing and instead consider whether local-scale restorative activities to increase vertical structure are cost-effective.

Response Diversity Can Increase Ecological Resilience to Disturbance in Coral Reefs

“…In the absence of external recruitment or multipatch dynamics, even if disturbance levels are high enough to cause the resilient coral to collapse, the resilient coral can have a long-term (decadal-scale) legacy effect of reduced macroalgal cover that benefits the resistant coral due to the slow transients inherent to the model (app. C.2; analogous to the slow transients observed in Blackwood et al 2012). The simulations with external recruitment or exchange between patches indicate the potential for competitor-enabled rescue (using "rescue effect" in the broad sense of Brown and Kodric-Brown 1977) where the role of each coral type in reducing the likelihood or rate of macroalgae invasion allows the local recovery of the other via recruitment from other sites ( fig.…”

“…Explicit herbivore dynamics in models of this type still lead to alternative stable states (Blackwood et al 2012). Such dynamics might include a time lag between coral loss and the loss of grazing on macroalgae, which can shift the basins of attraction ; time lags would increase the time over which competitor-enabled rescue could occur in our model.…”

“…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.…”

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“…Manuscript to be reviewed 77 under various conditions of conservation and restoration; and 2) performed a cost-benefit 78 analysis to compare the financial implications of restocking over time according to the model; 79 that is to determine whether some of the limited funds available for reef conservation should be 80 allocated to restoration or rather solely to conservation. 92 The dynamic model we use here has been adapted from (Blackwood et al 2011), the most recent 93 version of the model first presented in (Mumby et al 2007) . This model enables us to follow the 94 dynamics of coral coverage, macroalgae, algal turfs, grazing fish, and terrain rugosity; denoted, 95 respectively, by the variables C,M,T,P and R. We assume that the corals, macroalgae, and algal 96 turfs are competing for seabed in a constant size location, and define the algal turf coverage to be Manuscript to be reviewed…”

Simplifying complexity : reflections on ecosystems, psychiatric disorders and biogeochemical pathways

“…Mumby et al 2007;Blackwood et al 2010), and the effect of multiple stressors (e.g. Blackwood et al 2011;Fung et al 2011). Here, we focus explicitly on a scenario in which multiple weak positive feedbacks interact.…”