No welcome mat for climate migrants, new study finds

Umair Irfan, E&E reporter
Published: Tuesday, January 3, 2012

Climate change may lead to more extinctions than previously expected, according to new simulations that model how animals will get along with their new neighbors as warmer temperatures force them to migrate.

The research unveiled today adds a new wrinkle to the growing body of research showing, by modeling animal movement and competition, that climate change will force many species to migrate to new ranges or become extinct. Scientists found several species face an uphill battle for food and resources when pitted against animals already living in a particular range.

"There's no doubt that we're in the midst of a mass extinction. The question is what percentage of species will go extinct because of climate change," said Joshua Tewksbury, the Walker professor of natural
history in the Department of Biology and College of the Environment at the University of Washington. He noted that previous extinction estimates relied on models that didn't account for enough variables.

"We have very simple models to see how climate change will affect biodiversity in the future. These models tend to ignore species interaction and dispersal ability," said Mark Urban, an assistant professor
of ecology and evolutionary biology at the University of Connecticut. "I work on amphibians, and we know that some amphibians disperse a few meters every generation and some can disperse by a kilometer or more. As a result, some species cannot track [climate] changes as fast."

Urban and Tewksbury, along with Kimberly Sheldon, a doctoral student at the University of Washington, developed a simulation that includes how easily an animal can move and how well it would adapt to a new range with a different ecology in a climate change scenario. They published their findings in Proceedings of the Royal Society B: Biological Sciences.

When sweet spots sour

Competition is important to ecosystems and helps explain why organisms live in a certain area, scientists pointed out.

"Species don't exist everywhere they should. One of the main reasons is that someone else is living there," said Tewksbury. "When we start to see shifts in ranges of species, we start to see shifts in interactions between species." Warblers, for example, compete with other warbler species for nest sites in their current environment, he said, and without including these interactions, climate change extinction models are "fundamentally flawed."

By adding competition and species dispersion to their simulation, the researchers hope to develop a more accurate picture of survival. They created models in which species were distributed along a climate gradient, or a region where the climate gradually shifts, like a mountain on which the base might be temperate but the higher elevations experience colder average temperatures.

"Each of these species was able to survive under a specific set of conditions. We then let the climate change" by increasing the average temperature by 4 degrees Celsius, Urban said.

The species then shift to areas that remain hospitable. "We basically allow those organisms to move with the climate. The sweet spot for, say, the squirrel at the bottom of the mountain, will be moving up the side of the mountain," said Tewksbury.

The results showed that many species would not survive the fighting over food and habitat in a new range. "In the absence of competition, we actually see a lot less effects of climate change. As soon as you allow for competition, these species that are coming from behind begin to overrun those species already there. That competition can force those species to become extinct in the model," said Urban. He likened the competition scenario to cars traveling at different rates in the same direction on a single-lane road. Eventually, some of the cars will collide.

"The most surprising thing we found is how important the dispersal variation was. In the past, models assumed the same dispersal rates. When we incorporate very realistic rates, we find a whole lot more extinction risk," said Urban.

Tropics to see disproportionate impacts

But not all species face the same risks. Tewksbury explained that generalists --- species adapted to survive in a variety of conditions -- tend to fare better than specialists.

"What this means is that organisms that have this really specialized band of temperature, if [their adaptation] doesn't change, then they're going to be in more trouble. When a better competitor encroaches upon that zone, they don't have a lot of places to go," he said.

Tewksbury said this is especially important in the tropics, a region that accounts for most of the world's biodiversity and where the climate is relatively constant year-round. Climate models indicate that tropical regions will see smaller climate shifts than areas closer to the poles, but since life in the tropics is more specialized, they face disproportionate impacts from climate change.

These findings have consequences for conservationists.

"In the United States, for example, all of our conservation laws are based on current species boundaries," said Tewksbury, who said better projections are needed in order to offset any harm to ecosystems from climate change.

Urban emphasized that these results are by no means complete or comprehensive. "This is a very generic model. This allows us to think of this important system and apply lessons to other cases. The general conclusion here is that because species do interact, the current models that ignore these effects are inaccurate," he said. The researchers said the next step is to look at specific organisms, gathering hard data on where they live now and how they fit into their ecosystems, and projecting where they will go in the future.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. www.eenews.net. 202-628-6500

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