Background/Question/Methods: The role of positive interactions in structuring ecological communities is relatively underexplored despite their prevalence in ecological systems. For example, recruitment facilitation is a common characteristic of marine systems such as temperate rocky reefs. Here we explore the ecological consequences of incorporating recruitment facilitation in a simple model of sea urchin-algae interactions. Specifically, the model represents sea urchin grazing on macroalgae, macroalgae competition with crustose coralline algae (CCA), and sea urchin recruitment facilitation to CCA.

Results/Conclusions: These interactions generate alternative stable states, one dominated by macroalgae and the other by urchins, which do not occur when recruitment facilitation of urchins to CCA is ignored. Therefore, recruitment facilitation provides a possible mechanism for the existence of alternative kelp forest and urchin barren states in temperate marine systems, where storm events or harvesting of urchins or their predators can drive switches between states that are difficult to reverse. In systems with such dynamics, spatial management such as no-take marine reserves may play a crucial role in protecting community structure by increasing the resilience to shifts between states.