PerspectiveEcology

The Coming Arctic Invasion

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Science  08 Aug 2008:
Vol. 321, Issue 5890, pp. 780-781
DOI: 10.1126/science.1160852

The current episode of climate warming is having drastic consequences for animal and plant life worldwide. Besides the expected poleward expansion of temperate and tropical species and the latitudinal contraction of cold-adapted ones, an even more dramatic interoceanic invasion will ensue in the Arctic: North Pacific lineages will resume spreading through the Bering Strait into a warmer Arctic Ocean and eventually into the temperate North Atlantic.

Trans-Arctic invasion began about 3.5 million years ago during the warm mid-Pliocene epoch (1). A combination of northward flow through the Bering Strait, high productivity in the Bering Sea (the geographic source of trans-Arctic invaders) (2), favorable conditions for rapid growth and dispersal in the Arctic Ocean, and the removal through extinction of many species during the mid-Pliocene in the North Atlantic (1) enabled hundreds of marine lineages to colonize and enrich the biotas of the Arctic and North Atlantic. Although geochemical evidence from a core drilled near the North Pole points to perennial sea-ice cover in the Arctic beginning in the middle Miocene (14 million years ago) (3), the presence of mid-Pliocene temperate marine mollusks in northern Alaska and Greenland (4) indicates that coastal sectors of the Arctic Ocean were seasonally or perennially ice-free at that time.

Source regions of potential trans-Arctic invaders.

Fifty-six molluscan lineages present in the Bering and Chukchi seas (light-blue region) have not yet participated in trans-Arctic expansion but have the potential to do so; 28 of these species extend as far north as the Pribilof Islands and Anadyrski Gulf. Another 19 mollusk species are separated from related temperate Atlantic relatives by a genetic and geographic gap. These numbers exclude North Pacific lineages whose participation in the trans-Arctic interchange during the Pliocene led to the formation of species still living in the high Arctic.

CREDIT: MAP ADAPTED FROM NASA'S EARTH OBSERVATORY (9)

In much of today's ice-bound nearshore Arctic Ocean, annual phytoplankton production is a factor of 8 to 30 lower than in the Bering Sea (2), with production beneath the ice accounting for 1 to 33% of annual Arctic production of phytoplankton (5). In the relatively ice-free mid-Pliocene Arctic Ocean, food for suspension-feeding animals would have been much more abundant, allowing many planktonically dispersing, large-bodied, fast-growing species that require high productivity to survive in that ocean and to seed populations in the temperate Atlantic. Most trans-Arctic lineages with temperate Atlantic members show genetic and geographic gaps between Pacific and Atlantic populations, indicating that post-Pliocene sea-ice expansion in the coastal Arctic Ocean ended trans-Arctic dispersals in these lineages.

Climate models and recently observed trends toward contraction and thinning of Arctic sea ice predict seasonally or perennially ice-free conditions in the nearshore Arctic Ocean by 2050 or even earlier (6), reestablishing a regime of temperature and productivity similar to that of the mid-Pliocene. Marine mollusks, whose past and present distributions are well documented, offer unparalleled insight into how marine species and communities are likely to respond to these future conditions.

At least 77 molluscan lineages (35% of 219 shell-bearing, shallow-water mollusk species in the northern Bering Sea) have the potential to extend to the North Atlantic via the warmer Arctic Ocean without direct human assistance (7). Of these, 19 have Atlantic members but are separated from them by wide geographic and genetic gaps; 2 have extinct but no living North Atlantic representatives; and 56 have not yet extended beyond the Bering Sea or the Chukchi Sea just north of Bering Strait (see the figure). The remaining 142 Bering Sea lineages are distributed throughout the Arctic and subpolar North Atlantic Oceans. The number of would-be interoceanic invaders could well be much higher, because many species with northern limits in Kamchatka and the Aleutian-Commander island arc can expand northward and therefore also become candidates for trans-Arctic invasion.

Pacific-derived species already have the largest body sizes in all ecological guilds in the Arctic and in many on the east and west sides of the North Atlantic, including mussels, barnacles, coiled grazing snails, and predatory whelks. The Bering Sea source pool contains many additional large-bodied species that may establish viable populations in the temperate North Atlantic. Given that marine invasions rarely lead to extinctions in recipient ecosystems, these trans-Arctic invaders of the future will likely enrich Atlantic biotas both by adding new lineages and by hybridizing with established species. Competitive standards in the North Atlantic will rise because of the addition of large-bodied, fast-growing species to which natives must adapt.

As in the past, few Atlantic to Pacific invasions are expected. Most of the 50 shallow-water, Atlantic-derived Arctic mollusks (out of about 180 species in the American Arctic) are small-bodied compared to both the Pacific-derived members of the Arctic fauna and to potential Pacific invaders in the Bering Sea. With the exception of the largest Atlantic-derived species (bivalves in the genera Tridonta and Cyrtodaria, with shell lengths of 38 to 50 mm), which arrived in the North Pacific when the Bering Strait first opened 5.3 to 5.4 million years ago (8), these species do not exceed 30 mm in maximum shell dimension and might be unable to establish populations in the Bering Sea, where competition and predation are intense.

Geographic expansions within and between oceans are generally concentrated during warm periods even in areas far from the poles. The coming warmth may therefore initiate an age of renewed interoceanic dispersal worldwide, a natural experiment that we should all anticipate with great interest.

References and Notes

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