OLM 12.4. Kelp forests and urchin barrens as alternative stable states
The difference between the species diversities of forests and deserts is huge. It is no different in the oceans either. The highly productive “forests” or “beds” of brown macroalgae (Laminariales) are far more diverse than the deserts of crustose red algae (Corallinales) on rocky subtidal reefs. Evidence accumulates in support of the view that these two habitats might be two alternative stable states of the same community, any one transforming into the other by hysteresis. We have shown by the example of forests and savannas in the last chapter of our book (Ch12.3, pp.273-4) that a dynamical system admitting alternative stable states may show a patchy pattern even if the environment is spatially homogeneous. Multispecies kelp forest patches alternating with crustose coralline algal “urchin barrens” are examples of such systems. The patches are persistent if the transition between the alternative stable states admits hysteresis. It was possible to demonstrate that alternative stable states and hysteresis characterize a simplified three-component model system with an inedible but competitively inferior algal species facilitating the occurrence of a herbivore consuming the dominant competitor alga (Baskett and Salomon 2010). In contrast with the example presented in the previous OLM (OLM 12.3) pertaining to shallow lakes, changes in the physical environment are not indispensable for positive feedback and hysteresis to occur in this case (vs. Kéfi et al. 2016). Based on the model of Baskett and Salomon (2010) we show in the following that the shift between the alternative states of the system may be induced by changing the equilibrium density of the herbivore in itself.