The overall caribou population in Alaska is 700,000 to one million. The Porcupine herd, which Arthur has been monitoring, spans the Alaska-Canada border. The herd's population peaked in 1989. Since then its numbers have declined by 3 percent per year.
Warmer weather may actually have led to greater reproductive success for the caribou. The caribou calving season is in early June, immediately after the snow disappears, a time when nutrients in the vegetation are most concentrated. Rising temperatures mean the date of initial "green-up" of vegetation has been earlier each year, which is helping the newborn calves.
However, the earlier green-up also means the vegetations' nutrient levels eventually also decline earlier. If caribou don't get enough nutrient-rich food, they may not be able to build up enough energy to survive the winter.
"The total amount of food doesn't change," Arthur said. "There's only a shift in time [when the food is available], and that doesn't help the caribou in the long run."
Warmer weather can also lead to longer breeding periods for mosquitoes. To escape growing swarms of mosquitoes, the caribou herds may move up on rocky hillsides, where the grazing is poor, expanding precious energy and taking in less energy by way of feeding.
Rising temperatures may also bring more precipitation. More snow not only buries food deeper but also makes avoiding predators like wolves much more difficult. Arthur warns that the caribou are becoming more vulnerable to big storms.
"It's possible that we could be looking at a threshold, where the caribou could no longer recover [from natural events like big storms]," Arthur said.
Other species may already be living close to the edge. Take, for example, the crabs, snails, and mussels found in the rocky tidal pools along the California coast.
These creatures experience extreme temperature swings, especially those animals that are exposed to air. Their body temperatures can change from lows of 55 to 57 degrees Fahrenheit (13 to 14 degrees Celsius) to highs above 86 degrees Fahrenheit (30 degrees Celsius).
To find out how rising temperatures could affect these animals, scientists tested the thermal tolerance of porcelain crabs by hooking them up to a heart-rate monitor, immersing them in water, and gradually increasing the temperature.
"We found that crabs that are experiencing the highest temperatures up on the rocks are already living close to their limit," said George Somero, a marine science professor at Stanford University's Hopkins Marine Station in Pacific Grove, California.
"Their hearts stop beating in water only slightly warmer than what they currently experience in the wild," he said.
Testing has shown the same thing in mussels and snails.
Somero says the animals that occur highest on the rock have already evolved a higher degree of heat tolerance than those found underwater. With global warming increasing, the animals may be edging closer to their absolute limits.
"They don't have a reserve capacity to cope with higher temperatures," he said. "They're already pushing the envelope in terms of what they can do about their physiology."
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