The dynamic biogeography of the anthropocene: The speed of recent range shifts in seaweeds

Abstract

The biogeographic boundaries of seaweeds are largely determined by temperature tolerances, physical barriers and limitations to dispersal. Anthropogenic ocean warming and increasing connectivity through human activities are now causing rapid changes in the biogeography of seaweeds. Globally, at least 346 non-native seaweed taxa have been introduced to new regions, and at least 31 species of seaweed have shifted their distributions in response to recent temperature changes. Range-shift speeds were determined for 40 taxa, and compared between three drivers: (I) range expansions caused by introductions, (II) range expansions and (III) contractions caused by climate change (warming/cooling). The speed of change in seaweed biogeography differed between these drivers of change, with expansions significantly faster than contractions, and climate-driven shifts significantly slower than introductions. Some of the best documented introduced species expansions include Sargassum muticum (4.4 km/year in Denmark), Undaria pinnatifida (35- 50 km/year in Argentina) and Caulerpa cylindracea (11.9 km/year in the Mediterranean Sea). Examples of seaweeds with recent climate-driven range shifts include Scytothalia dorycarpa, a native species in Western Australia, which retracted >100 km poleward as a consequence of a single event (a regional marine heat wave). However, climate-driven range shifts were generally assessed over long time periods (>10 years). Fucus serratus (1.7 km/year) and Himanthalia elongata (4.4 km/year) have slowly retracted westwards in northern Spain in response to warming in the Bay of Biscay. In England and South Africa, Laminaria ochroleuca (5.4 km/year) and Ecklonia maxima (36.5 km/year) have expanded their ranges in response to local warming and cooling, respectively. These changes in seaweed biogeography likely have had substantial implications for biodiversity and ecosystem processes, particularly where the shifting seaweeds have been canopy-forming foundation species. We discuss some of these consequences and different attributes of climate and invasion-driven range shifts in seaweeds.