Changes in food-web structure and energy flow in kelp forest ecosystems on the south-west coast of South Africa following the invasion of Jasus lalandii

Abstract

In the early 1990s, the West Coast rock lobster Jasus lalandii underwent an eastward shift in distribution into an area known as East of Cape Hangklip, where they had previously been rare. This shift has also been termed an invasion and resulted in a complete change in the benthic community structure, changing from a state dominated by herbivores and encrusting algae, to another state dominated by lobsters, sessile species, kelp and understory algae. Using the trophic modelling software Ecopath with Ecosim, baseline models of the pre- and post-invasion systems were created to better understand trophic pathways between the two different states and assess how fishing pressure may have driven the shift in ecosystem state. Using the baseline models, different fishing management strategies were tested to see whether the post lobster-invaded ecosystem could be shifted back or close to the pre-invasion state. Baseline models that were developed to describe the shift in ecosystem state reflected an increase in the presence of kelp, sessile species and lobsters, and the decline of encrusting algae and herbivores. Furthermore, the post-invasion system was more productive and less diverse than the pre-invasion system. Simulations using an Ecosim model showed that, with a reduction in fishing pressure on reef fish and abalone, and a simulated increase in fishing pressure on rock lobster, reef fish recovered to pre-invasion levels, but juvenile abalone, adult abalone and urchin biomass did not recover, probably due to the difficulty in capturing non-trophic interactions in the trophic models. Further functional groups such as sessile species, turf and foliose algae, also did not return to the pre-invasion state. This study concluded that a reduction in fishing pressure on abalone and reef fish, and the removal of rock lobster through increased fishing pressure, would allow the post-invasion ecosystem to partially recover to the pre-invasion state, but not completely, suggesting a hysteresis effect. Further exploration of management strategies through model simulations is needed, including those that can account for non-trophic processes.