20 April 2005The Daily CaliforniaOmeed Elboudwarej
The re-emergence of gray wolves at Yellowstone National Park is boosting the park's biodiversity by mitigating the effects of climate change on food availability, according to a new study by researchers at UC Berkeley.
Global warming has created shorter, warmer winters that have improved the survival rate of elk over wintertime in Yellowstone, relative to past decades in which the longer winter season spelled doom for many of them. The change previously denied food for many of the scavengers in the area that depend on elk carcasses as a part of their diet, including coyotes, eagles, ravens and bears.
Since wolves were introduced to Yellowstone in 1995, however, the distribution and availability of food for scavengers has shifted from being primarily a function of winter severity to a more constant resource that is available on a year-round basis.
"With wolves in the system, elk no longer starve to death but rather succumb to predation by wolves that leave their leftovers for scavengers," said Chris Wilmers, who conducted the study as a graduate student in ecosystem sciences at UC Berkeley's College of Natural Resources. "As winters shorten, therefore, the effect on scavengers is not as great with wolves in the system because the availability of winter carrion is now more dependent of wolf predation."
Unlike other predators such as mountain lions, gray wolves are willing to share the leftovers with other animals, especially in the wintertime when the benefits of attacking the next available prey outweigh the marginal benefit of guarding a carcass.
"After gorging themselves at a fresh kill, it seems that the better option for wolves is to kill again rather than defend the remains, which are now of much lower quality as a resource than a fresh carcass," said Wayne Getz, UC Berkeley professor of environmental science, policy and management and co-author of the study. "This is especially true for larger packs which eat a much greater proportion of the high-quality resources after a first feeding, and can more easily kill again than smaller groups of wolves."
This constant resource subsidy may in turn increase biodiversity and lead to larger populations of scavenger species, as over 30 species of avian and mammalian scavengers benefit from wolf kills in the Northern Rocky Mountains.
The researchers studied 55 years of data from Yellowstone to determine trends in weather conditions. They determined that although the onset of winter has not changed significantly over time, the winter period is shortening as midwinter snowfall decreases along with the duration of snow cover on the ground. Compounding these effects is an increase in the number of days with maximum temperatures above freezing from January through March.
Using statistical analysis, the researchers calculated that wolves boost food availability from a reduction of 27 percent in March in a climate change scenario without wolves in the system to a reduction of only 4 percent in a climate change scenario with wolves in the system. Likewise, there is a significant increase in food availability in April from a reduction of 66 percent to 11 percent in their presence.
The presence of wolves was much less pronounced during the early and midwinter months when food shortage was not a factor.
The results of the study demonstrate the importance of intact food chains in alleviating the effects of climate change since ecosystems without top predators may be more vulnerable to species loss as a result of global warming than those with predators, Wilmers said.
Exterminated from Yellowstone in the 1920s by hunters, the current 31 wolves have had a greater impact on resource availability for scavengers with small home ranges, such as coyotes and magpies, than highly mobile scavengers, such as bald eagles, since the latter have access to a greater range of feeding grounds, studies show.
Understanding how climate and predation patterns affect community structure gains added importance as the combination of global warming and high animal extinction rates continue to generate changes in many food web cycles.
"The issue is very complex," Getz said. "In some cases, global warming can lead to increased biological diversity and in others reduced diversity."
While scientists thus far have concentrated on the effects of climate change on a single species, there has been little work done on how whole food chains are impacted by fluctuating weather patterns.
"We hope that our study will stimulate ecologists to look at the impact of climate change on scavenger communities in other ecosystems and to think about the importance of top predators in maintaining biodiversity at lower levels in the food chain," Getz said.
