The keystone concept has been applied to a variety of species, from large carnivores to herbivores and even plants. And it has created disagreement among ecologists, who have questioned this term's usefulness and generality. In response to mounting criticism of the overly broad use of this popular term, marine ecologist Bruce Menge defined a keystone species as "one of several predators in a community that alone determines most patterns of prey community structure, including distribution, abundance, composition, size, and diversity." Keystones selectively prey on dominant prey, have large body size relative to prey size, and are highly mobile, with a large foraging range. More recently Michael Soulé and his colleagues proposed the term strongly interacting species for those that have a strong ecological effect on communities. Next we will see how fear drives some of these effects.
The Ecology of Fear
Mid-May in Glacier National Park, Montana, is not a time or place for the fainthearted. The gunmetal sky was beginning to spit snow at me and my field technician, Dave Moskowitz, as we hurried to finish a track transect before a late season blizzard broke out. All day we had been hearing wolves howling. It had been difficult to pinpoint their location over the rising wind, but it seemed as if they were moving along a benchland one-tenth of a mile east of us. Hearing them like this provided a powerful reminder of the wildness of the system I was studying.
My work involved putting in fifty-seven miles of track transects in Glacier National Park's North Fork, arguably one of the most intact systems in the lower forty-eight states. This place harbored a full suite of large predators in one of the highest densities found south of Alaska, as well as abundant elk (Cervus elaphus) and deer (Odocoileus spp.). An expert tracker, Dave was writing a field guide to tracks of the Pacific Northwest. I was fortunate to have his help. Using track transects in which we were measuring all occurrences of elk, deer, moose (Alces alces), wolves, cougars, coyotes (Canis latrans), and bears (Ursus spp.), I was mapping their interactions and determining game densities in a system with so many predators. I was also measuring predation risk, to see whether wolves and other predators influence elk movements by causing them to avoid areas with escape impediments, such as downed wood and thick brush. In a 1999 article wildlife ecologist Joel Brown noted that the nonlethal effects of predators can be ecologically more important than the direct mortality they inflict. Evidence I'd found thus far created a compelling picture of the same sort of ecologically complete system Aldo Leopold had observed in 1936 in Mexico. I was also seeing patterns sharply etched on the landscape. These elk were spending more time on flat, open ground and less in riskier terrain, where they might have had more difficulty escaping predators.
As we raced to finish before the weather deteriorated further, we encountered a bachelor herd of a dozen skittish elk at a distance of fifty yards. They had shed their antlers and sported velvety antler buds. I gauged their ages from their body size and behavior. The younger bulls nervously ran around in circles. The mature bulls eyed us warily but stood their ground. Eventually they left, the young bulls taking cues from their elders, moving in the elegant head-high trot characteristic of their kind. As they vanished into the inky conifers below the benchland, I was left with an ominous feeling in the pit of my stomach. Something about their behavior made me uneasy, but I couldn't put words to it.
We finished just as the blizzard hit. When I looked back toward where we began the transect, I encountered a sight so unexpected, so shocking, that initially I thought I was imagining it. For there, conspicuous even through a scrim of falling snow, lay a fresh bull elk carcass. The animal looked impossibly huge. The wind carried clouds of steam from its gaping belly. I was amazed that we hadn't heard the takedown, even over the moaning wind. One minute the dead elk wasn't there and the next minute it was, having met death in an area I had moments earlier characterized as having very high predation risk.