Direct and indirect effects of climate change squeeze the local distribution of a habitat-forming seaweed

Predictions concerning biological responses to climate change are based primarily on environmental tolerances of individual species. However, biological interactions and the way these may be altered by changes in climate can also be important drivers of ecological change. A combination of observational and experimental studies was conducted to test the factors controlling the vertical distribution of the canopy-forming alga Fucus guiryi, a cold-water seaweed and ecological engineer. We found that F. guiryi transplanted to areas just above its upper limit of distribution died rapidly, whereas survivorship was high in unmanipulated controls and individuals translocated to areas within the species’ vertical range. Towards the lower limit of its vertical range, unprotected F. guiryi were heavily grazed by a warm-water fish, Sarpa salpa, and percent cover, frond length, and number of reproductive structures were all reduced compared to experimentally protected individuals. Observations over a period of 2 yr suggest that grazing is stronger towards the lower limit of distribution, especially during the warmer seasons of the year. Commercial landing records indicate that the abundance of S. salpa was positively related to warmer sea surface temperatures. Our results highlight the interplay between abiotic and biotic factors in determining the vertical distribution of F. guiryi and suggest that predicted increases in temperature during this century may lead to local extinctions of this ecosystem engineer through the combination of increased physiological stress at its upper limit of distribution and grazing at its lower limit, which, together, act to squeeze its vertical distribution in the rocky intertidal