The university has an e-subscription to the journal Science, so each week we get details of the latest issue via e-mail. I was scrolling through one of the July issues when an article's title caught my eye: Aspens return to Yellowstone, with help from some wolves. Really? I thought. What have wolves got to do with trees growing??
Wolves used to be locally extinct in the US's Yellowstone National Park; they were re-introduced in 1995 (in the face of concerns from local farmers with land bordering onto the park, who thought that the wolves would target livestock). By 1995 aspens and other trees like cottonwoods and willows had been rare in the park for around 50 years – they were being overbrowsed by elk and couldn't regenerate fast enough to keep up with the grazing pressure. So ecologists were keen to see whether the wolves' return would have an effect on vegetation in the valley. This was a wonderful one-off natural experiment in ecology. Would the wolves be able to kill enough elk (red deer) to give the trees a chance to regenerate?
The answer was yes. The park's wolves kill an elk (usually one that's too young, too sick or too old to get away from them) every few days, and as the elk population has dropped the trees have made a comeback. And while there are still a lot of deer in the park, the trees are growing thickly in the riparian zones around waterways. It seems that the deer are nervous now that the wolves have returned – the thick regrowth makes it hard for them to spot wolves lying in wait, and as a result they're too nervous to stay feeding there for long. This reinforces the concept that top predators play a crucial role in maintaining ecosystem diversity.
Virginia Morell (2007) Aspens return to Yellowstone, with help from some wolves. Science 317: 438-439