As recently as the 1980s, Tosa Bay on Japan’s southern coast was home to thick forests of kelp that provided food and shelter for an abundance of fish, invertebrates and marine mammal species. But, during a recent dive there, marine biologist Adriana Vergés found a very different underwater landscape.

“The kelp forests used to support fisheries of abalone and lobster,” Vergés says. “In the last two to three decades that's completely disappeared and if you go there now, it's all coral… It doesn't look like a temperate system any more.”

The reef-building corals have colonized areas stripped of their kelp cover by invasive tropical fish species. This phenomenon of tropical fish infiltrating once temperate aquatic ecosystems, wiping out kelp forests and radically changing life on the seafloor bed is known as “tropicalization.”

Vergés, a lecturer at the University of New South Wales in Australia, is one of the authors of a recent paper on the phenomenon off the southern coasts of Japan and Australia, the Mediterranean and the Gulf of Mexico.

These are “hotspots” where ocean currents flowing between warm and temperate waters are getting stronger. As well as warming these temperate habitats, the currents carry the larvae of tropical fish, such as rabbitfish, part of a global trend that is seeing species driven into new habitats by climate change.

“In areas [of the Mediterranean] where there are tropical rabbitfish, the overall levels of biomass and diversity have gone down,” Vergés says. “There’s a shift from algal forests to barrens. It’s really quite eerie to dive into some eastern parts of the Mediterranean because there is very little left there.”

Heading poleward

As the global climate heats up, it isn’t just marine species that are on the move. In 2011, a UK study found a mean global movement of terrestrial species towards the poles of close to 17 kilometers per decade – or around 4.5 meters every day.

This rapid range shift means that new combinations of species are interacting with native populations sometimes forced into a losing battle for resources with their new neighbors, which may be better adapted to the warmer conditions.

“When a northern species starts to decline as the climate gets warmer, it may be because they can't cope with the new physical climate. But, perhaps more likely is that they now have to face not only the physical climate, but they've got to deal with all these new species that are turning up,” Chris Thomas, an evolutionary biologist at the University of York and lead author of the report, says.

“Grolars” in the arctic

In the arctic, the impact of climate change on species range is also being driven by the rapid loss of sea ice that once formed a continent-sized barrier between different species populations. For arctic species like the minke whale, this presents an additional risk: hybridization.

“Minke whales occur in both the North Atlantic and the North Pacific but these populations have been isolated from one another by arctic sea ice,” explains Brendan Kelly, one of the authors of a 2010 Nature article warning that 22 arctic mammal species may be at risk of hybridization. “As that ice retreats you could expect those forms to come into contact with one another and distinctions between them will be erased.”

As yet, there has been no detailed research into the extent to which arctic hybridization is already occurring. But, there is anecdotal evidence of the phenomenon, including sightings of “grolars”– offspring resulting from interbreeding between polar and grizzly bears.

Kelly says hybridization could be the polar bear’s “last gasp”: As the sea ice they are specialized to hunt on is lost, they are forced onto the shore and into competition with more versatile grizzly bears. The resulting decline in the polar bear population then means remaining individuals are less likely to find a mate of their own species, and may instead reproduce with a grizzly.

Communities on a cliff edge

Still, while hybridization could see vulnerable species bred out of existence, scientists say it is loss of habitat that represents the biggest climate-related risk to arctic species like the polar bear – and to biodiversity on a global scale.

In this context, the poleward shift of some species could be their only chance at survival. But, geographical barriers often mean they can only go so far.

“The problem is what happens to those [ecological] communities that are on the edge of continents,” Vergés says, “for example, communities at the south end of Australia, in Tasmania – they have nowhere else to go. It's like they’re falling off a cliff edge. It’s a bit like in mountain systems, where species are [to higher latitudes] because of increases in temperature. The species at the top are the ones that are going to become extinct.”

Assisted migration

While the threat posed by invasive species like rabbitfish has prompted moves to reduce their population in the Mediterranean through fishing, some conservationists advocate “assisted migration” to actively facilitate the movement of certain endangered species to areas outside their normal range.

Torreya Guardians is a volunteer organization working to conserve Torreya taxifolia, an evergreen conifer that is endangered in its native Florida, partly as a result of climate change. The group is helping to plant it further north, in locations such as the Appalachian Mountains.

“Looking at all life-forms, it is trees that move the slowest. The majority of trees cannot keep pace with climate change,” Torreya Guardians’ founder Connie Barlow says, adding that the Florida torreya’s seeds are too large to be carried by the wind or most animals.

Assisted migration is controversial, but Barlow and others argue that on a continental landmass like Europe or North America, terrestrial species have shifted back and forth with climatic change over the millennia, so that what seem like “new” species combinations have actually existed in the past.

What is unprecedented is the rate at which climate change is now happening. Thomas says this means defending current species combinations may not always be the best approach.

“If all our biological communities are going to change anyway, why should we not think about including within those biological communities – even if it requires us to intervene – some of those species which are truly endangered?” asks Thomas. He says some may not think this is very natural. But, then neither is current climate change, he points out.