The dynamic biogeography of the Anthropocene: recent range-shifts in seaweeds

Sandra C. Straub (1) , Mads S. Thomsen (2), Thomas Wernberg (3)

1 UWA Oceans Institute and School of Plant Biology, The University of Western Australia, 39 Fairway, Crawley 6009, Western

Australia, Sandra.straub@research.uwa.edu.au

2 Marine Ecology Research Group, School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New

Zealand, mads.solgaard.thomsen@gmail.com

3 UWA Oceans Institute and School of Plant Biology, The University of Western Australia, 39 Fairway, Crawley 6009, Western

Australia, Thomas.wernberg@uwa.edu.au

The biogeographic boundaries of seaweeds, like many other marine organisms, are largely determined by temperature tolerances, physical barriers and limitations to dispersal. Anthropogenic ocean warming and increasing connectivity through ships and aquaculture are now causing rapid changes in the biogeography of seaweeds. We reviewed range shifts of seaweeds worldwide and present illustrative examples. Globally, more than 300 non-native seaweeds have been introduced to new regions, and at least 27 species have shifted their distributions poleward in response to recent warming. Scytothalia dorycarpa, a native to Western Australia, retracted >100 km poleward as a consequence of a single event (a marine heatwave). By contrast, Fucus serratus (1.7 km/year) and Himanthalia elongata (4.4 km/year) from northern Spain have slowly retracted poleward over a century. The best documented introduced species expansions include Sargassum muticum (4.4 km/year  in Denmark), Undaria pinnatifica (35–50 km/year  in Argentina) and Caulerpa racemosa var. cylindracea (11.9 km/year in the Mediterranean Sea). Our analyses show that, in general, rates of biogeographic reconfiguration are within the same order of magnitude for climate and invasion driven shifts, although the latter tend to be fastest. These changes likely have strong implications for biodiversity and ecosystem processes, particularly when the moving seaweeds are canopy-forming foundation species. We discuss some of these consequences and the different attributes of climate and invasion driven range shifts in seaweeds.

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