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Hansen, Rebecca

University of British Columbia

Shifts in species distributions are one of the most anticipated impacts of climate change-associated warming. In the rocky intertidal zone, vertical species distributions may respond to warming through changes to upper distributional limits, which are typically determined by abiotic stress. Downward shifts in species upper limits could trigger major changes to intertidal communities, especially if lower limits, which are more often determined by biotic interactions, respond differently to warming. To investigate the effects of warming on intertidal species limits, I combined historical comparisons of intertidal distributions with an experimental thermal manipulation. Both my observational and experimental work took place along Juan de Fuca Strait, a region which has experienced warming over the past 65 years. Through my observational comparisons, I found that the upper limits of six intertidal foundation species (the sessile invertebrates Chthamalus dallii, Balanus glandula, Pollicipes polymerus, and Mytilus californianus, and the brown algae Fucus distichus and Hedophyllum sessile) were lower in 2022 than 1957-1958 and 2009-2010. Shifts in lower limits only occurred for sessile invertebrates between 2009-2010 and 2022, a period during which their major predator, sea stars, were suffering from disease-induced mass mortality. Across the intertidal assemblage as a whole, upper limits also shifted downwards between 1957-1958 and 2022. Shifts were largest in the high intertidal zone, an abiotically stressful environment which is mostly inhabited by sessile invertebrates, and at the western end of Juan de Fuca Strait, where lower air temperatures, higher wave action, and earlier low tide times lead to lower abiotic stress. Through my field experiments, which manipulated temperature at the upper limit of Hedophyllum sessile, I found that change in H. sessile percent cover of was higher when artificially shaded. This suggests that H. sessile, which provides habitat to invertebrates and other algae, could have persisted at higher intertidal heights under the cooler conditions of the past. My results suggest that shifting upper species limits in Juan de Fuca Strait are the result of climactic warming. These shifts are likely to continue with climate change and could have important consequences for the structure and function of temperate rocky intertidal ecosystems.

Text

2024-04-15

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87829

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/