By John Emerick
Those of us who are passionate about our native plant species and ecosystems, and who have spent considerable time on Colorado’s public lands, have undoubtedly observed widespread degradation to these communities by wildlife and livestock grazing. In places of heavy grazing, plant community diversity is low and the composition of introduced plant species is often high. Add the potential impacts of climate change and the degradation is liable to worsen. A fair question, then, is whether wolf reintroduction might have a beneficial effect on our native ecosystems.
Wolves are a keystone species. Their activities as ungulate predators produce trophic cascades affecting plant survival, pollinators, birds, mesopredators such as foxes and weasels, and smaller herbivores, such as rabbits and various rodents. It is difficult to estimate what the eradication of wolves from Colorado during the first half of the 20th century has meant to the structure of our native ecosystems.
Elk are the primary prey of Rocky Mountain populations of wolves, and in Colorado, there are over 280,000 elk, the largest population of any state. There are also 430,000 mule deer, mostly concentrated in western Colorado. However, there are also approximately 500,000 cattle and 175,000 sheep that also are grazed on public lands west of Interstate 25. Grazing by these ungulates has a significant impact on our plant communities.
The most noticeable damage occurs in aspen groves and in riparian willow carrs, particularly if elk use these resources year-round or if unmanaged livestock overgraze these systems. When healthy, both ecosystems support a high diversity of sub-dominant plant species, as well as diverse animal communities including mammals, birds, and invertebrates. This diversity declines with prolonged heavy grazing. In Rocky Mountain National Park, excessive grazing of alpine tundra plants by elk may have contributed to the decline of ptarmigan numbers.
Aspen groves typically are clones in which the trees are interconnected by a common root system, which produces shoots to expand the grove or to replace ageing or diseased trees. When the shoots are heavily browsed by elk, cattle, and other herbivores, they fail to mature. Elk also gnaw the bark of aspen trees during late winter and early spring, and that can lead to infection of the tree by various diseases. Both situations in concert can eventually lead to the death of the entire clone.
(Figure 1. Aspen bark severely damaged by elk: this can lead to disease and death of the grove, as well as the loss of the other plant and animal species that depend on the ecosystem. The inset shows severely browsed aspen suckers.)
Riparian willow shrublands, particularly those in broader valley bottoms, are commonly occupied by beavers. Willows and beavers are codependent. Beavers use the willows for food and building materials for their dams and lodges. Beaver dams raise the water table, providing shallow ground water that willows need. When willows are browsed heavily by elk or cattle, beavers leave due to over-competition, dams are no longer maintained, and the streams and rivers begin to run straight and fast. This results in downcutting of the channel, a drop in the water table, and further demise of the willows. Sedges, grasses, and smaller shrubs that help to maintain channel stability are also affected by heavy elk and cattle browsing.
(Figure 2. Once a lush riparian willow shrubland in Rocky Mountain National Park, heavy competition between elk and beaver forced the beaver to leave, resulting in a drop in the water table and ultimate death of the willows. What was once home to a thriving community of neotropical migrant birds such as MacGillivrays and Wilsons Warblers and plants such as the wood lily is no longer existent.)
The reintroduction of wolves to Colorado is likely to have mostly subtle and indirect effects on our native plant communities, and probably not until wolf populations increase to ecologically effective numbers and stabilize. Some over-browsed ecosystems might benefit through a wolf-elk-plant trophic cascade as has been documented in Yellowstone and Banff National Parks. However, Colorado has experienced almost a century without a full suite of large predators at a time when elk and livestock populations have been climbing on our public lands. The effects of overgrazing on some of these lands will not be reversed by wolves alone; that will also require a public commitment for effective wildlife and grazing management.
Perhaps the reintroduction of wolves to Colorado, if it occurs, will be a catalyst for a larger, more holistic movement in which Coloradoans will press for: better scientifically based wildlife management, including non-lethal predator control; for more responsible livestock management, including predator coexistence strategies and cessation of over-grazing on public lands; and for programs to restore plant and animal diversity to our public lands where it has been lost due to excessive livestock grazing and large elk populations. Our native ecosystems matter for ecological sustainability, for aesthetics, and for maintaining the integrity of the natural world. In the face of changing climate, we need to do as much as possible to conserve and restore these ecosystems. Wolves may be part of that equation.
John Emerick, Ph.D., is an ecologist, and is on the emeritus faculty of the Department of Environmental Science and Engineering, Colorado School of Mines. Nowadays, he spends part of his summers conducting field surveys for the Colorado Natural Heritage Program.