Overthrowing a regime shift: displacement of sea urchins by abalone in a kelp forest ecosystem

Abstract

Interactions among sea urchin and abalone populations provide an important mechanism for maintaining segregation of habitat use at small spatial scales within kelp forest ecosystems. Here we provide evidence from a long‐term study of the sea urchin, Evechinus chloroticus, and the abalone, Haliotis iris, populations in a pristine rocky harbor of Stewart Island, in southern New Zealand. Between 1998 and 2014 we observe declines in density of abalone within fished regions of the Inlet and a coincident increase in density of sea urchins. In these regions, sea urchins progressively invaded the shallow (0–3 m) depth stratum formally occupied by abalone and dense, multi‐layered stands of macroalgae. Formation of the Ulva Island Marine Reserve in 2004 released abalone populations from fishing mortality, resulting in population increases within the shallow water depth stratum. Here we observed declines in sea urchin density as the growing abalone population displaced sea urchins from the shallow water habitats. The reestablishment of a depth‐stratified distribution of abalone and sea urchins within the kelp forest provides a likely mechanism for maintenance of habitat complexity and evidence for the important role of high‐density abalone populations within kelp forest ecosystems.