Urchins and regime-shifts in marine ecosystems
A paper co-authored by Laura Blamey, an NRF ‘early career scientist’ at the Department of Biological Sciences highlights the important, yet underappreciated roles urchins play in marine ecosystems. Overgrazing of kelp beds by sea urchins is well known across the globe, resulting in shifts from diverse, productive kelp habitats to impoverished and persistent urchin barrens. The paper, which is published in the prestigious journal Philosophical Transactions of the Royal Society B, explores patterns and processes underlying shifts from kelp to urchin states across 13 rocky reef systems (spanning 11 different regions from both hemispheres). A clear, somewhat global pattern emerges in which urchin grazing is shown to cause a discontinuous ‘catastrophic’ shift from kelp- to urchin-dominated systems. To reverse this shift, urchin biomass needs to be reduced by roughly ten times. Unfortunately, once shifted, strong ecological mechanisms within the new system resist reversion to the original state. In addition, human-induced stressors (e.g. fishing) strengthen resilience of these urchin barrens and erode resilience of the kelp bed systems.
While urchin overgrazing is a common phenomenon in many parts of the world, the dynamics in South Africa are somewhat different. Here, in the Western Cape, the urchin Parechinus angulosus is one of the dominant subtidal herbivores, but its grazing abilities are nothing like that of urchins in other parts of the world. Instead of actively grazing kelp beds, our urchin traps drift kelp and in doing so, remains rather ‘immobile’, providing a refuge for juvenile abalone that shelter beneath the urchin spines to avoid being preyed upon by predators such as rock lobster and fish. In other parts of the world, some species of urchins are known to switch over to active grazing in the absence of drift kelp, and studies in South Africa suggest that our urchin can have a negative effect on kelp and other seaweed sporelings and thus possibly create or maintain small ‘barrens’. But because of its size and the fact that it plays more of a facilitative role instead of being a destructive grazer, urchin barrens in South Africa are small and patchy as opposed to large and widespread. A study is currently underway to examine, in detail, the occurrence of urchin barrens in South Africa and the point at which these small urchin barrens form, and kelps and other seaweeds disappear.