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Climate models need to account for species interactions, researchers say

Nathanael Massey, E&E reporter
Published: Monday, June 25, 2012

Ecosystems are often described as vertical structures, their foundations composed of plant life and herbivores and omnivores stratified through the middle floors. For those species lucky enough to occupy the upper regions -- typically predators -- we even have a vertical-sounding designation: top of the food chain.

But unlike structures, the stability of an ecosystem is as dependent on its roof as it is on its foundations. Predators play an important role in curbing the population growth of the species below them, a kind of self-regulatory mechanism for life's propensity to multiply and increase. And as climate change affects the success of these "top consumers" around the globe, scientists worry that their decline could throw entire ecosystems out of balance.

"Everything in the chain is important," said Phoebe Zarnetske, a postdoctoral fellow at the Yale School of Forestry and Environmental Studies. "If you do something that affects one species -- particularly top consumers -- that can cause a cascade effect that ripples through the entire food system."

The climate-related decline of caribou and musk ox in Greenland has allowed tundra grass, a favorite food of both species, to overwhelm the island's other plant life. In certain regions of North America, overfishing has all but eliminated aquatic predators like salmon and sea bass, leading to a profusion of the smaller species on which they feed.

In a recent article in the journal Science, Zarnetske and Yale professor of ecology Dave Skelly argue that current climate models often neglect the complex interactions and interdependencies that exist within any given food chain. Taking that interplay into account will be critical to accurately forecasting future climate impacts, they say.

The wolves of Isle Royale

Among the cases cited by Zarnetske and Skelly is that of Isle Royale, an island national park in Lake Superior.

Several decades ago, during a winter of record cold, an ice bridge formed from the mainland, allowing the introduction of wolves and moose onto the island for the first time in recorded history. Both species were trapped there when the bridge thawed the next summer, and have remained so ever since.

As a closed ecosystem, Isle Royale presents scientists with an ideal case study in species interaction, and they have been tracking the ups and downs of the two populations for many years. But thanks in part to warming temperatures, that experiment may soon be coming to an end.

The wolf population is in decline, due in part to warmer temperatures -- deep snowfall grants a natural advantage to wolves, who move more quickly through it than do the moose. And while the loss of wolves may be good news for moose in the short term, it could also mean their eventual demise.

In the few recorded cases where the moose have surged in the past, it has led to the decimation of the island's balsam firs -- the moose's primary food source -- and ultimately, the moose population has collapsed, as well.

Zarnetske said that while climate wasn't the only factor destabilizing the island's balance -- disease carried over by parkgoers has also played a role in the wolves' slump -- it was probably exacerbating the problem.

"In many cases, there's a compounding effect where we as humans have already disrupted an ecosystem, and now climate change is introducing a new set of challenges," she said. "It's kind of like adding insult to injury."

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


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

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