The role of sea urchins and kelp loss in changing the assemblage structure and trait diversity of shallow marine communities

Schuster, Jasmin M. (2023) The role of sea urchins and kelp loss in changing the assemblage structure and trait diversity of shallow marine communities. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Coastal habitats, such as kelp forests, are declining in many areas of the ocean. Change is driven by various factors, including increases in sea urchins, which are kelp consumers (grazers) that can rapidly remove kelp forests and create barrens devoid of macroalgae. At the same time, warming and cooling trends and extreme heat events due to anthropogenic climate change are restructuring reef communities around the globe. Biological assemblages found on shallow reefs are thus responding to the loss of complex habitat and ocean temperature changes simultaneously. The aims of this thesis were to: I) examine responses of shallow marine populations and communities to kelp loss, with a focus on urchin-driven kelp loss; and II) assess whether kelp loss alters assemblage vulnerability to anthropogenic temperature change. In my thesis I utilized measures of an organisms’ fundamental physiological response, and global biodiversity data from kelp ecosystems, to assess assemblage change at multiple levels of biological organization and from local to global spatiotemporal scales. I found that neighbouring kelp and sea urchin barren habitats have populations and communities with different physiological characteristics that relate to energetics. For example, green sea urchin (Strongylocentrotus droebachiensis) populations from kelp forests had higher routine metabolic rates across the range of temperatures tested than those from adjacent barrens. Sea urchins from kelp forests were also less sensitive to temperature increases than their barrens counterparts. Findings from subsequent experiments suggest that metabolic differences among sea urchin populations relate to the availability of kelp as a food source. At the community level, I report that kelp and barren habitats also host fish communities with distinct realized thermal affinities and geographic range sizes, with more warm-affinity species in regions close to the tropics present at barrens than in kelp habitats. Sea urchins, thus, facilitate tropicalization processes on rocky reefs in temperate regions. Similar tropicalization signals also emerged in a kelp forest ecosystem during two severe heat events in the Pacific Northwest, with stronger signals of tropicalization in urchin barrens than at kelp forested sites. Overall, loss of kelp (as habitat and food source) leads to a reorganization of reef assemblages, moving assemblages towards greater sensitivity to warming. My findings highlight the importance of protecting and restoring complex habitats to maintain the functioning and diversity of marine biological assemblages.

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