Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

Abstract

On temperate rocky reefs, overexploitation of high-trophic-level omnivores can result in the decimation of kelp forest habitats by releasing sea urchin population networks from top-down control. However, the local dynamics of the resulting trophic cascades are context-dependent. Here, we investigate the community-wide patterns associated with both diffuse stressors and localized protection of high-trophic-level omnivores in kelp forest ecosystems by comparing communities among marine reserves and fished areas in two contrasting regions in terms of fishing intensity and land-based stressors, Fiordland and the Marlborough Sounds, New Zealand. We find higher densities of the potential sea urchin predators, red rock lobsters (Jasus edwardsii) and banded wrasse (Notolabrus fucicola), in the Fiordland region, and larger effect sizes of fishing on the exploited fish communities in the Marlborough Sounds. Patterns in sea urchin density were consistent with the idea that high-trophic-level species, such as large fish and rock lobsters, regulate sea urchin population density, with lower densities of Evechinus chloroticus observed inside marine reserves, in both regions. Nevertheless, densities of E. chloroticus were generally high (>3 m2) in the Marlborough Sounds, likely above a grazing threshold in both fished and reserve sites. The proportion of habitat where sea urchins were absent was 29% in Marlborough Sounds and 90% in Fiordland. Consequently, we observe 49% barren habitat in Fiordland vs. 70%, and a larger effect of fishing on kelp community structure, in the Marlborough Sounds, where fishing effect sizes and land-based stressors were more severe. We propose that a combination of diffuse stressors including regional overexploitation of important sea urchin predators, sedimentation, and warming of coastal waters likely contributed to regional differences in the responses of trophic interaction chains to localized reductions in fishing within marine reserves. The present study highlights how physiologically stressed and modified kelp forest ecosystems are more susceptible to detrimental phase shifts at a regional spatial scale.