It was late May in Waterton Lakes National Park and the snow had just melted, the matted grasses still a winter-killed brown. My field crew and I were putting transects into an expansive rolling grassland dotted with aspen stands. My field technician Blake Lowrey was on point that day, doing dead reckoning with a compass and pulling the transect tape due east through a copse of stunted aspens, making good progress. All at once he stopped and said, "What's that smell?" I had cautioned everyone to be careful around carcasses because of bears. At the head of the transect line I found a dead coyote on a well-used game trail. This relatively fresh coyote carcass had been there for maybe one or two days. It lay on its back, limbs outspread and neck outstretched. Its throat had been ripped out and it had been eviscerated. No other flesh had been removed. All around it lay evidence of the perpetrator of this carnage: wolf feces and tracks. The coyote appeared to have been a young adult male in relatively good health that had perished because it had had the misfortune to come upon a wolf. In most systems wolves make it their business to kill coyotes. This particular carcass had been left on a primary game trail as a grisly marker and warning to other coyotes that wolves rule this system—they are the apex predator.
Two weeks later, in Glacier National Park, we found another coyote in the same position, also on a game trail, its throat and guts ripped out, no other flesh missing, wolf scats and tracks all around. By now my crew had become sufficiently accustomed to carcasses to find this fascinating. We took a break and discussed the possible pattern here—the powerful signature wolves were leaving on this landscape.
Wolf-coyote enmity is not new. Wolves recolonized Isle Royale National Park in the early 1950s. Within two years they had wiped out the resident coyote population. When wolves were functionally extirpated from Yellowstone, wildlife biologist Adolph Murie noted a corresponding steep rise in coyote numbers, which began to formlarger packs and hunt deer. Smith considers what happened next to Yellowstone's coyotes one of the best stories to emerge from the mid-1990s wolf reintroduction. After the wolf 's return, coyote numbers dropped by as much as 50 percent overall and by 90 percent in core wolf pack territories. To survive, they formed smaller groups and spent more time in the interstices between wolf territories and nearby roads. There Smith found coyotes killed by wolves in a similar manner as I'd observed. Most of the pre-wolf coyote population had occurred in packs. Since wolves, half the coyote population has consisted of what Smith calls "floaters," unaffiliated coyotes with higher survival rates. Breeding coyotes have the highest mortality because they are easiest for wolves to find and kill, since their behavior is more predictable and they live in territories.
One of the most powerful indirect effects of predation involves mesopredator release. Defined as medium-sized predators, mesopredators are controlled by top predators—often by direct mortality, as we have seen, but also via competition for shared resources. Humans commonly remove keystone species to protect economically valuable big game from predation. For example, upon removal of the wolf fromthe Endangered Species List in the northern Rockies, the state of Idaho proposed to eliminate 40 percent or more of its wolves to help create more elk for humans to hunt. This type of action causes mesopredators, such as coyotes, to increase and puts abnormal pressure on smaller species, such as game birds, which decline and can become extinct.
In the mid-1980s David Wilcove investigated the effects of human land use on songbirds. He studied small woodlots in rural and suburban sites in Maryland and larger forest patches in Tennessee in Great Smoky Mountains National Park. The biggest tract of virgin forest in the eastern United States, this park retained forest-dwelling mammals and birds long extinct in central Maryland. Wilcove wanted to test the effect of mesopredator release on songbird nest predation—what can be thought of as the empty nest hypothesis. To do this he filled experimental wicker nests with Japanese quail (Coturnix japonica) eggs and placed them in forest locations ranging from the midcanopy to the understory, to reflect native birds' nesting habits. One week later he measured the percentage of experimental nests raided by mesopredators. He found higher rates of nest predation in small woodlots near human communities because these areas had higher populations of raccoons (Procyon lotor) and squirrels (Sciurus spp.) and few, if any, large predators, such as cougars and bobcats (Lynx rufus). This led Wilcove to link nest predation to mesopredator release.