Predators shape river world top-down.

In an anti-intuitive tale of predators and prey, riverside birds are prospering more outside a national park designed to protect them than inside the park, say wildlife biologists.

Strips of land cradling waterways, or riparian zones, that are outside the boundaries of Grand Teton National Park have more-diverse bird populations, says peter B. Stacey of the University of New Mexico in Albuquerque. He and Joel Berger of the University of Nevada at Reno report that these unprotected zones also have higher numbers of certain species than comparable spots inside the park.

The researchers argue that this apparent quirk makes sense in terms of a much-debated view that predators at the top of a food chain influence its character more than their prey do.

Inside the park, moose abound, feasting on rich willow patches and other waterside growth that birds need as well. Predators that kill moose, such as grizzlies, have long been gone from the region. Outside the park, human hunters keep moose numbers in check, which ends up preserving more waterside habitat for bird's than is available inside.

This concept of top-down effects from predators sounds reasonable, but it's been hard to prove. For one thing, it's not easy to add and subtract bears just for the sake of experiments.

Stacey and Berger realized, however, that people effectively served as experimental stand-ins for the other top guns in the ecosystem. With this part of the equation, the finding of higher bird diversity on the riparian land outside the park fits nicely with predictions about top-down effects of predators, even two-legged ones with orange hats.