The Arctic Melting

COMMENT

IS the presence of humans, from the hybridiza- orb tion of two other species. But hybridization c

on/ driven by human activities tends to occur I quickly and to reduce genomic and species diversity. When mallard ducks were intro- ence Fact ence I duced to New Zealand in the 1860s, they

/Sc began mating with native grey ducks. Now SKI few, if any, pure native populations remain. Diversity loss may be minor if, say, North

S. Kazlow S. Pacific and North Atlantic minke whale subspecies interbreed in an Arctic with diminished ice. Other crosses will be more problematic. Interbreeding between the North Pacific right whale, of which there are probably fewer than 200, and the more numerous bowhead whale could quickly push the former to extinction. If polar bears survive climate change in secluded refuges — which is far from certain — interbreeding The polar bear (possible hybrid pictured) is one of several species vulnerable to hybridization. could be the final straw. Cross-breeding might affect social and ecological interactions. The apparent narwhal–beluga hybrid discovered in The Arctic melting pot Greenland had teeth combining qualities of each species, but lacked the narwhal’s tusk — Hybridization in polar species could hit biodiversity hard, an important determinant of narwhal breed- say Brendan Kelly, Andrew Whiteley and David Tallmon. ing success. Polar–grizzly hybrid bears in a German zoo exhibited behaviour associated with seal hunting, but not the strong swim- n 2006, a white bear with patches of developed immediately to monitor the genet- ming abilities of polar bears. First-generation brown fur was shot by hunters in the ics of Arctic animals and to deal with hybrids crosses can have ‘hybrid vigour’, but later Arctic. DNA tests confirmed what many before currently discrete populations merge generations are likely to be less fit than their Isuspected — it was a hybrid of a polar bear and at-risk species are bred out of existence. ancestors (‘outbreeding depression’). and a grizzly. A media frenzy quoted biolo- We have counted at least 34 possible The International Union for the Con- gists as saying that although they knew in hybridizations between discrete populations, servation of Nature should develop a com- theory such cross-breeding could happen, species and genera of Arctic and near-Arctic prehensive policy for managing hybrids, they didn’t expect to see it in the wild. In marine mammals (see Supplementary Infor- including determining when it is practical 2010, another hybrid was killed by a hunter in mation). Of the 22 species involved, 14 are to prevent or limit hybridizations. Red wolf the western Canadian Arctic. This time, the listed — or are candidates for listing — as and coyote hybrids, for example, have been animal was a second-generation cross — its endangered, threatened or of special con- culled in the United States in the past decade mother was a hybrid and its father a grizzly. cern by one or more nations. Twelve cases to help preserve distinct species. More cases are probably out there. are of hybridization between different spe- Researchers should combine models of Biologists should not be surprised. There cies — half involving crosses between what sea-ice loss, oceanography and landscape have been hints of Arctic hybrids before. In the are normally classified as distinct genera. genomics to predict when and where hybrid- late 1980s, a whale thought to be a narwhal– Twenty-two cases involve isolated popula- ization is most likely, and to monitor the beluga mix was found in west Greenland. tions at risk of intra-species mixing, nine of genetics of at-risk populations. National and In 2009, an apparent bowhead–right-whale which are classified as distinct subspecies. tribal governments should work together: hybrid was photographed in the Bering Sea, The Arctic Ocean is predicted to be some indigenous groups actively monitor between Alaska and Russia. Dall’s porpoises ice-free in summer before the end of the the harvests of Arctic marine mammals, are known to be mating with harbour por- century, removing a continent-sized barrier and they could collect genetic samples in poises off the coast of British Columbia, and to interbreeding. Polar bears are spending remote areas. The rapid disappearance of seal hybrids have been identified in museum more time in the same areas as grizzlies; seals sea ice leaves little time to lose. ■ specimens and in the wild. and whales currently isolated by sea ice will These are just the first of many hybridiza- soon be likely to share the same waters. Brendan P. Kelly is at the National Marine tions that will threaten polar biodiversity. Not all cross-species matings will produce Mammal Laboratory National Oceanic Rapidly melting Arctic sea ice imperils viable — or indeed any — offspring. The and Atmospheric Administration, Juneau, species through interbreeding as well as chance is enhanced in Arctic marine mammals, Alaska 99801, USA. Andrew Whiteley through habitat loss. As more isolated popu- because their number of chromosomes has is in the Department of Environmental lations and species come into contact, they changed little over time. There is evidence of Conservation, University of Massachusetts, will mate, hybrids will form and rare species hybridization across species (such as between Amherst, Massachusetts 01003, USA. are likely to go extinct. As the genomes of spotted and harbour seals) as well as across David Tallmon is in the Faculty of Biology species become mixed, adaptive gene com- genera (such as harp and hooded seals). and Marine Biology, University of Alaska binations will be lost. Hybridization is not necessarily a bad thing. Southeast, Juneau, Alaska 99801, USA. Researchers have little idea how much It has been an important source of evolution- e-mail: [email protected] hybridization is occurring, let alone how ary novelty. For example, a new species of Further reading and Supplementary Information it will affect populations. Plans must be chub originated in the Colorado River before accompanies this article at go.nature.com/h4bksj