Tuesday, March 24, 2009

Are Tropical Species More Threatened than Arctic Ones from Global Warming?


A team of researchers say that in spite of all the media attention given to the Arctic region and polar bears, species living in the tropics may face an even greater risk as the world warms up. Shrinking polar ice has concerned ecologists that polar bears will soon start dying off as their hunting ground literally melts away.

However, according to a team led by University of Washington, while temperature changes will be much more extreme at high latitudes, tropical species have a far greater risk of extinction since even relatively slight warming of just a degree or two can have a devastating impact. The Daily Galaxy asked Joshua Tewksbury, a biologist at the University of Washington who is studying tropic species, why these warm weather species are in greater danger. After all, it’s already warm where they live, so how could just a degree or two of warming make much of a difference?

“We’re looking specifically at the intersection between where an organism lives, and how susceptible they are to change. What we found is that organisms in the tropics are much less resilient to heat change,” Tewksbury explained to The Daily Galaxy.

Why? Because tropic species are adapted to living within a much more narrow temperature range. Once temperatures get beyond that comfortable range, many species will likely have a difficult time coping.

Geographically, Earth’s tropical region is a giant belt that stretches from the Tropic of Cancer to the Tropic of Capricorn; or, in actual terms, just south of Miami to half way through Australia.

However a more scientific definition is taken by meteorologists who define the tropics as a region defined by a long term climate. It is this definition that has shown the shift in the width of the tropics towards our planet’s poles.

"There's a strong relationship between your physiology and the climate you live in," said Tewksbury, "In the tropics many species appear to be living at or near their thermal optimum, a temperature that lets them thrive. But once temperature gets above the thermal optimum, fitness levels most likely decline quickly and there may not be much they can do about it."

Arctic species, on the other hand, often experience temperatures ranging from subzero to a comparatively warm 60 degrees Fahrenheit. They typically live at temperatures well below their thermal limit, and most will continue to do so even with climate change.

"Many tropical species can only tolerate a narrow range of temperatures because the climate they experience is pretty constant throughout the year," said Curtis Deutsch, an assistant professor of atmospheric and oceanic sciences at the University of California, Los Angeles. "Our calculations show that they will be harmed by rising temperatures more than would species in cold climates.

"Unfortunately, the tropics also hold the large majority of species on the planet," he said.

Tewksbury and Deutsch are lead authors of a paper detailing the research, published in the May 6 print edition of the Proceedings of the National Academy of Sciences. The scientists compared data describing the relationship between temperatures and fitness for a variety of temperate and tropical insect species, as well as frogs, lizards and turtles. Fitness levels were measured by examining population growth rates in combination with physical performance.

"The direct effects of climate change on the organisms we studied appear to depend a lot more on the organisms' flexibility than on the amount of warming predicted for where they live," Tewksbury said. "The tropical species in our data were mostly thermal specialists, meaning that their current climate is nearly ideal and any temperature increases will spell trouble for them."

So does that mean that we should turn our focus away from the Arctic? Definitely not, says Tewksbury. Polar bears are in danger, but its just for different reasons than for tropic species. It’s not the temperature itself that will harm the bears—they already live in a climate that varies wildly throughout the year—what will harm them is the loss of habitat they will face as polar ice disappears. Many tropic species, on the other hand, will actually likely be harmed directly from the rising temperature itself since their physiology cannot handle the vastly swaying temperatures like the bears can.

“The polar bears are in trouble, but what many don’t know is that even a small amount of change in the tropics could effect a vast number of species,” Tewksbury told The Daily Galaxy. “People won’t see as dramatic decline, because many tropic species that will be effected like insects and lizards, simply don’t have the appeal that the polar bear has. Yet they are still vitally important to their ecosystems.”

Independent teams set out to measure the atmospheric data available, and using four different meteorological measurements, found that the tropical belt has grown by between 2 and 4.8 degrees of latitude since 1979. This measurement translates to a total expansion – north and south – of 140 to 330 miles.

Climate scientists have long predicted that, by the end of the 21st century, a growth of the tropical belt was expected. But the growth that has taken place over the last quarter-century is puzzling, and not part of their theories.

Dian Seidel, a research meteorologist with the National Oceanic and Atmospheric Administration lab in Silver Spring, Md, is confused. "They are big changes," she said. "It's a little puzzling."

And while one explanation for the expanding tropics is indeed global warming, others vie for ranking as another explanation. Depletions in the ozone layer and changes in El Nino are both other options that could explain what has happened.

So while much of the tropics are thought to be just that – tropical – there are great swathes of desert as well. One only needs to look at Australia to see that played out, with the ‘Top End’ dominated by rain forests and rainy seasons, that sit just on top of the massive desert center.

It is these desert areas that sit on the edge of tropical locations, such as the U.S. Southwest, parts of the Mediterranean, and of course Australia, that are at the most risk, according to the experts.

While warming is happening much faster at higher elevations, it is also occurring at a slower rate in tropic zones, which over time will likely just as severe of an impact, but for different reasons. They may not be as majestic as polar bears, says Tewksbury, but we can’t forget about the little guys.

Posted by Rebecca Sato.

Original here

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