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Warming ocean waters, coupled with direct human actions such as pollution and overfishing, may threaten the rich diversity of life under the sea, a new U.S.-Canadian study suggests.

Researchers looked at how various factors have influenced the distribution of a spectrum of species — from seagrass to squid to sharks. Their findings help fill in the moving map of biodiversity across the world's oceans, knowledge that long has lagged behind that of diversity patterns on land.

"We wanted to find out which species were where, and why some places were greater hotspots of diversity than others," said lead researcher Derek Tittensor of Canada's Dalhousie University, in Halifax, Nova Scotia. "We were also interested in how these newly mapped hot spots related to human impacts on the oceans."

Species patterns

With the help of a public database created by the international network of researchers who conducted the Census of Marine Life, Tittensor and his colleagues identified the home waters of more than 11,000 species across 13 major groups. Then they began to uncover patterns.

Among coastal creatures, including corals and mangroves, the greatest area of diversity for most species groups was around the tropics of Southeast Asia. This was expected, given that terrestrial biodiversity is known to be highest around the equator and lowest at the poles.

Whales and other open-ocean species, on the other hand, were found in greatest concentrations along strips of sea at subtropical latitudes — those latitudes to the north and south of the tropical zone embracing the equator.

In addition to this "surprising pattern," Tittensor said, his team discovered a "worrying sign": These same hot spots of marine life overlapped areas with the largest human footprints, which raises the threat of severe species losses from pollution and other human action. Meanwhile, the combined effects of pollution, exploitation and habitat destruction put at risk the benefits humans gain from diverse ecosystems, such as water filtration and fish protein.

Warmer oceans

Another potentially devastating trend emerged when the team looked at environmental effects. Sea-surface temperature stood out as the only factor that consistently influenced all species groups, which suggests climate change could rearrange the distribution of oceanic life. Warmer parts of the ocean, for example, tended to sustain greater species diversity. But at the extremes of the temperature scale, Tittensor said, biodiversity may no longer be increasing. Instead it may be reaching a plateau or declining.

"In temperate regions, you may see more southern species coming in, due to warming, whereas in polar regions you tend to see a decline in diversity," he explained.

Still, the present picture is far from clear, leaving the future of the oceans unpredictable. Climate change affects more than just water temperature — ocean acidification and the bleaching of coral reefs are also global-warming-related problems. And these environmental effects probably interact with the array of other human actions.

The researchers hope their new map of diversity will provide a baseline that can be used to monitor future changes as the waters continue to warm, eventually eliciting a more complete understanding of what is going on. The map also could guide marine conservation by identifying areas where a large number of species could be protected at one time.

"There is amazing diversity in the oceans, and I'd like to see that continue," Tittensor said. "I think limiting warming and other human impacts is an important step."

The study is detailed in the July 28 edition of the journal Nature.

This article was provided by OurAmazingPlanet, sister site to LiveScience.