RANGE EXTENSION OF INVASIVE CRABS IN NORTH-WEST ICELAND

Range extension of invasive crabs in North-West Iceland
A study conducted by Jón T. Magnússon, Stephen J. Hawkins, Lilja Gunnarsdóttir, Jörundur Svavarsson and Karl Gunnarsson, published in Journal of the Marine Biological Association of the United Kingdom reveals that Species distributions are changing in response to global warming and the introduction of non-native species often acting in combination. Karl Gunnarsson and Lilja Gunnarsdóttir are employed as marine biologist at the Marine and Freshwater Research Institute.
Ranges are expected to move poleward as temperatures rise, with effects likely to be greater in low diversity systems in higher latitude areas. Such spread can potentially have consequences for resident species, especially in disturbed systems.
One recent case of invasion by a non-native species in Iceland is that of the Atlantic rock crab (Cancer irroratus) Say, 1817 from the east coast of North. First sightings date back to 2006 in Hvalfjörður, South-West Iceland; since then C. irroratus has spread to North and East Iceland, primarily in the subtidal zone but also occurring intertidally. The European green crab Carcinus maenas (Linnaeus, 1758), native to South-West Iceland, was later also found further north, in the Breiðafjörður area as its range extended poleward, probably in response to warming waters as a result of climate change.
Despite numerous studies of C. irroratus in the subtidal zone in Iceland, where it has been shown to outcompete its native counterparts, the prevalence of this invasive species in the intertidal zone is yet unknown. Furthermore, in the context of marine ecosystem dynamics, there is a significant knowledge gap concerning the effects of Ascophyllum nodosum (L.) Le Jolis harvesting on associated mobile predators, and specifically crabs. Having a holistic understanding of the ecological consequences of seaweed harvesting, such as influences on the survival and distribution of associated fauna, can contribute to sustainable resource use and management.
Here, the distribution is described, relative abundance and population structure of crab species occupying the intertidal zone at eight sites within Ascophyllum-dominated sheltered shores of Breiðafjörður, where industrial-scale harvesting (~15.000 tons of algae/year) occurs. They are compared to two sites further south in Faxaflói, South-West Iceland, with long-established populations of C. maenas and large adjacent subtidal populations of C. irroratus. Based on previous observations of population changes in the subtidal zone, the hypothesis is tested that H. araneus would diminish in abundance and show changed population structure in the face of spread by the other two species. The relationship between Ascophyllum harvesting, intertidal crab abundance and assemblage composition in Breiðafjörður is also examined as a preliminary test of whether such disturbance affected the spread of C. maenas and C. irroratus.
Quantifying and mapping species distribution are essential for understanding the interplay of anthropogenic impacts such as climate change, introduction of non-native species and harvesting of habitat-forming species. In the study it is demonstrated that, currently, the presence of C. irroratus in the intertidal zone was negligible and that C. maenas is advancing northwards. Successful establishment of C. maenas populations in the intertidal zone in Breiðafjörður bay has led to a displacement and substantial reduction of H. araneus probably by competition for resources. Given its displacement in the subtidal by C. irroratus, current squeezing of H. araneus may accelerate with warming and further spread of C. maenas. This warrants further tracking. Ascophyllum harvesting was not linked to effects on crab species. Nevertheless, a precautionary approach should always be practised, as juvenile individuals are susceptible to nursery habitat degradation should harvesting intensify.