Regime shift of a kelp-forest benthic community induced by an ‘invasion’ of the rock lobster Jasus lalandii

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An eastward shift in the center of abundance of the South African rock lobster Jasus lalandii occurred during the early 1990s into an area known as East of Cape Hangklip (EOCH). Given (1) the predatory capabilities of J. lalandii, (2) an intricate relationship between the urchin Parechinus angulosus and juveniles of the abalone Haliotis midae, and (3) existing over-exploitation of elements of the ecosystem, the ‘lobster invasion’ has major implications for the benthic ecosystem and associated fisheries. We surveyed the abundance of J. lalandii and the benthic community composition EOCH at six sites (three ‘invaded’ and three ‘non-invaded’ by J. lalandii), in three different depth zones (<5m, 6–12m and 13–20m). At all depths, J. lalandii was significantly more abundant in invaded areas than in non-invaded areas, and benthic communities were significantly different. The high densities of rock lobsters at invaded sites led to cascading effects including elimination of urchins and depletion of grazers, consequent enhancement of macroalgae, and diminishment of encrusting corallines. Non-invaded sites had few lobsters, abundant herbivores, less macroalgae and more encrusting corallines. Elimination of urchins in the invaded area has important implications, as juveniles of the commercially harvested abalone H. midae depend on shelter beneath urchins in this region. Floral species diversity was greater at invaded sites and increased with depth, whereas faunal species diversity was greater at non-invaded sites but also increased with depth. The depths at which strongest effects of J. lalandii were felt coincided with the depth of maximum abundance of the urchin P. angulosus, the abalone H. midae, the kelp Ecklonia maxima and encrusting corallines, with serious consequences for associated fisheries and the benthic community. We argue that the differences between invaded and non-invaded areas are sufficient to recognize that they are alternate stable-states and constitute a regime shift.