For centuries, fishermen in Narrangansett, Rhode Island, have scoured the waters of the Northwest Atlantic in search of herring, a small fish that is also a staple food for ocean predators. But as climate change warms the world's seas, the herring these fishermen depend on is fading at the southern tip of their range and appearing more and more at their northern edges.
This situation is developing in ocean waters around the world: concentrations of populations of marine animals have been moving away from the equator towards the poles over the course of the last century, according to one of the most comprehensive analyzes of the distribution of marine species to date . According to the researchers, these movements could cause disruption to food webs and endanger the livelihoods of people who depend on key fisheries.
"These are changes that are really taking place in established local communities," says study co-author MartinGenner, a fish ecologist at the University of Bristol in England. "It is about changes in the species that people know in their environment, in the abundance of things that are already there."
The study, published in Current Biology, looked at how the number of 304 marine species, including small phytoplankton, sea grasses, algae, fish, reptiles, marine mammals, and seabirds, has changed over the past century. The researchers gathered data from 540 abundance measurements taken in the world's oceans since the late 1800s, from the Arctic Ocean north of Alaska and across the equator to the Southern Ocean off Antarctica. They found that studies done closer to the poles were more likely to show increases in the population of a species, and those done closer to the equator were more likely to show a decline.
Jennifer Sunday, a climate change ecologist at McGill University, who was not involved in the paper, says that many marine species are adapted and therefore sensitive to a narrower temperature range than terrestrial species. As a result, marine organisms are likely to be more impacted by warming. Terrestrial species may be less vulnerable to losing their equatorial ranges "because they have all kinds of microhabitats that they can fill," such as burrows or other places where they can cool off, he explains.
Although the study focused on the exact mechanisms that cause changes in marine abundance, Genner says that it is not about species simply migrating, but more about whether they are able to survive where they already are. While some species can adapt to changes in temperature by expanding toward the extreme pole of their range, many cannot. "For example," he says, "if you are some kind of rocky coast in southern Tasmania, and there is no rocky coast further south, where do you go?" Species that cannot adapt or move poleward may ultimately face extinction.
The change in species concentrations can have cascading effects on ecosystems. For example, some species that expand toward the regions toward the poles as the waters warm can endanger local fisheries, Genner says. In one case, a type of parasitic crustacean called sea louse could become a problem for fish. UK salmon farmers. “Right now, they can keep them in check,” Genner says, but clarifies: “If you raise the temperature by a degree or two, those populations (sea lice) might well start to do much better. And as a consequence, the viability of the industry could be threatened ”.
Steve Murawski, a fisheries biologist at the University of South Florida, who was not involved in the study, says that as species disappear from parts of their range, the change affects the resources available to people living in the areas. affected. It points out that a large fraction of the world's population depends on fish as its main source of protein. The possibility of this problem is particularly true in the tropics, where animals have a narrower range to adapt to warming oceans because they are already very close to the upper limits of their thermal tolerance.
“You are moving away from the equatorial areas of the world, which tend to be in the developing world,” says Murawski. “The further we move these resources away from traditional communities and population centers, the more we will also create food insecurity. There is a crucial need to monitor the distribution and abundance of animals in the developing world ”.
The changes also mean that some predators could lose their prey, Murawski says, because many of the most temperature-sensitive animals tend to be small species that form the bases of food chains. The loss of such species could break these chains, making it difficult for predators to survive.
Since marine species are subject to many stressors in addition to climate change, having conservation programs that address some of those other factors, such as overfishing or habitat loss, can provide a buffer against the impacts of rising temperatures, he says. Genner. "We don't know how much it will actually work in the marine environment at this point, but it really can't hurt," he says. "It will require things like reducing harvests and changing our ways of using the marine environment.