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Species Dispersal and Invasions

As global temperatures increase many species' distributions are expected to expand pole-wards as temperature limitations are eased in that direction. Although local factors will influence individual species, in general this pattern holds for marine and terrestrial species: 75% of marine species investigated1 had shifted their range in the pole-ward direction, mirroring a similar finding (76% changed latitude or elevation) for terrestrial and aquatic species2. One marked difference between the marine and terrestrial shifts however is the rate at which they occur. While terrestrial range margins have shifted at approximately the same rate as temperature (0.61 - 0.24 km year-1), marine expansions occurred much faster at 19.4 km year-1 (± 3.8 SE) on average. One reason for this may be locally rapid changes in oceanic currents, as have helped blue mussels (M. edulis) to become re-established on Svalbard in 2002 after an absence of at least 1000 years3. In the Irish Sea this may mean species from further South or the Mediterranean are more frequently seen over the coming years.   Invasions by non-native species involve the same suite of biological processes as range expansions; competitive advantage over resident species, release from predation and exploitation of naive prey if the recipient physical environment is amenable. Warming in the Irish Sea is likely to increase the number of species for whom the environment is suitable to become established and so may increase the risk of species invasions.

One alien species that could benefit is the pacific oyster, C. gigas, (Fig. 1) currently the main commercial  oyster species. Since the 1990s naturalised C. gigas populations have already started to naturally re-seed in favourable years and are predicted to do so annually by 20401. However, any benefit may be negated by increased disease incidence. Range expansion will be a necessary adaptation to increasing temperature and species invasions may occasionally bring benefits, as seen in C. gigas or by bringing other commercially exploitable stocks. However, neither is very likely to occur without some negative impacts on the Irish Sea environment and its shellfisheries. While invasive species have long been recognised to frequently impact on the recipient communities, similar sized effects have recently been described for species newly spreading into adjacent habitats1.

Figure 1. Crassostrea gigas

1. Sorte, C. J. B., Williams, S. L., & Carlton, J. T. 2010. Marine range shifts and species introductions: comparative spread rates and community impacts. Global Ecology and Biogeography 19: 303-316.

2. Root, T. L., Price, J. T., Hall, K. R., & Schneider, S. H. 2003. Fingerprints of global warming on wild animals and plants. Nature: 57-60.

3. Berge, J., Johnsen, G., Nilsen, F., Gulliksen, B., & Slagstad, D. 2005. Ocean temperature oscillations enable reappearance of blue mussels Mytilus edulis in Svalbard after a 1000 year absence. Marine Ecology Progress Series 303: 167-175