Alternative stable states and phase shifts in coral reefs under anthropogenic stress

Abstract

Ecosystems with alternative stable states (ASS) may shift discontinuously from one stable state to another as environmental parameters cross a threshold. Reversal can then be difficult due to hysteresis effects. This contrasts with continuous state changes in response to changing environmental parameters, which are less difficult to reverse. Worldwide degradation of coral reefs, involving "phase shifts" from coral to algal dominance, highlights the pressing need to determine the likelihood of discontinuous phase shifts in coral reefs, in contrast to continuous shifts with no ASS. However, there is little evidence either for or against the existence of ASS for coral reefs. We use dynamic models to investigate the likelihood of continuous and discontinuous phase shifts in coral reefs subject to sustained environmental perturbation by fishing, nutrification, and sedimentation. Our modeling results suggest that coral reefs with or without anthropogenic stress can exhibit ASS, such that discontinuous phase shifts can occur. We also find evidence to support the view that high macroalgal growth rates and low grazing rates on macroalgae favor ASS in coral reefs. Further, our results suggest that the three stressors studied, either alone or in combination, can increase the likelihood of both continuous and discontinuous phase shifts by altering the competitive balance between corals and algae. However, in contrast to continuous phase shifts, we find that discontinuous shifts occur only in model coral reefs with parameter values near the extremes of their empirically determined ranges. This suggests that continuous shifts are more likely than discontinuous shifts in coral reefs. Our results also suggest that, for ecosystems in general, tackling multiple human stressors simultaneously maximizes resilience to phase shifts, ASS, and hysteresis, leading to improvements in ecosystem health and functioning.

Cryptic regime shift in benthic community structure on shallow reefs in St. John, US Virgin Islands

“…This distinction has been debated for over a decade (Fung et al 2011), and both concepts remain in use. The phrase 'regime shifts' provides an alternative terminology to describe large changes in community structure independent of the causes and the ecological stability of the phases (Conversi et al 2015).…”

Response Diversity Can Increase Ecological Resilience to Disturbance in Coral Reefs

“…We focus our model parameterization on parameter space with bistability in the one-coral-type plus macroalgae cases (i.e., for each of the resistant or resilient coral considered separately) because alternative stable states are necessary for the effect of community composition on ecological resilience to be relevant (see Fung et al 2011 for an exploration of the parameter range where alternative stable states occur in a model of this type). Within this space, we use the size of the basin of attraction to define the parameters for each coral, with a large basin for the resilient coral C L and a small basin for the resistant coral C S ( fig.…”

“…Whether alternative stable states occur in a model of this type depends on a variety of parameter values, especially macroalgal growth and grazing rates (Fung et al 2011). Empirically, the existence of alternative stable states is difficult to demonstrate (Petraitis and Dudgeon 2004), and the evidence for them in tropical reefs is greater for Caribbean than Indo-Pacific reefs due to a combination of potential factors such as lower herbivore diversity and abundance, faster macroalgal growth and recruitment, and loss of faster-growing corals in the Caribbean (Roff and Mumby 2012).…”

Coupling Biophysical and Socioeconomic Models for Coral Reef Systems in Quintana Roo, Mexican Caribbean

“…The environmental impact of resorts and urban areas is represented in the model by means of a simple "environmental impact index" derived from the number of resorts and the extent of urban areas http://www.ecologyandsociety.org/vol16/iss3/art23/ in each economic development area. Finally, the reef ecology component of SimReef uses a highly simplified representation of local-scale ecological dynamics to model changes in coral cover, algal cover, herbivorous fish biomass, and piscivorous fish biomass, after the models of Fung (2009) and Fung et al (2011). Fishing activity depletes herbivorous and piscivorous fish stocks, and fishermen are presumed to target piscivorous fish preferentially (Pauly et al 1998).…”