Macroalgal forests and sea urchin barrens: Structural complexity loss, fisheries exploitation and catastrophic regime shifts

Abstract

In contrast to ecosystems that change smoothly and continuously in response to various stressors, some transitions between states with radically different properties can occur abruptly. An example are the sea urchin barrens and canopy algae (e.g. kelp beds) which represent alternative stable states. More precisely, the variation in grazing intensity in coastal rocky system may drive switches between one complex state into the barren state, the former dominated by erect algae and the latter by encrusting coralline algae and bare rock. Identifying the causes that drive a complex system towards a phase-shift becomes crucial for implementing strategies for the successful conservation and/or recovery of marine forests. Mathematical models that aim to assess effects of fisheries and sea urchins-seaweeds interaction may contribute to understand mechanisms driving transitions between alternative states. Fisheries exploitation has been considered the main driver of urchin population density transitions, with consequent effects on canopy algae distribution. The major novelty of the model here presented is the incorporation of habitat structural complexity, which explains the effect of algal biomass loss on coastal fish assemblages and the strong irreversibility of the system. We have found that as some critical parameters change macroalgae are more resilient and this may give rise to new scenarios, such as the emergence of new stationary states.