Livestock advocates often state that cattle and sheep have merely “replaced” the native herbivores. And since plants are “adapted” to herbivory from native grazers, then “obviously” livestock grazing is compatible with ecosystem preservation. Some even go so far to claim that plants “need” to be grazed.
There are several things wrong with these assumptions.
First, plants do not “need” to be grazed anymore than most of us “need” our skin to be exposed to large amounts of sunshine. Yes, we have adaptations such as tanning that offer us some protection from harmful sun rays, but most doctors would tell you that the less unprotected sun exposure you have, the better your skin. Too much sun exposure eventually leads to skin cancer.
Most plants react to herbivory as we do to sun exposure—they have adaptations that allow them to “tolerate” and defend themselves against the worse damage. Most plants expend vast amounts of energy, protecting themselves from herbivory. Thorns, prickles, chemical inhibitors, thick bark, and other adaptations are all mechanisms used to cope with herbivory. That is, there is a “cost” to herbivory for plants.
The energy they are expending on chemical inhibitors, thorns, or other protections means less left for basis plant function and survival, whether that is the development of a greater root system or more seeds.
Plants exposed to a long history of herbivory pressure, like some grasses on the Great Plains, have evolved adaptive strategies that permit them to survive herbivory pressure. Still, it is a stretch to claim they “need” to be grazed, except to say that they have a competitive advantage in the face of adversity just as darker-skinned people can tolerate greater sun exposure before they suffer skin damage.
If a plant is grazed during the growing season it reacts something like we do when we are in shock or suffering from hypothermia. It translocates resources from other parts of the plant to maintain vital organs—in the plant’s case—green photosynthetic material—i.e., leaves but with a potential loss of function in other body parts.
But constructing leaves means there is less energy left for other plant functions like building roots or seed production. So over time, a plant that is repeatedly cropped during the growing season is at a competitive disadvantage with other plants because it is sacrificing root development or seed production. So let’s say there’s a drought, the cropped plant may die due to water stress. That is how overgrazing occurs—one bite at a time.
Secondly, livestock uses the landscape differently than native herbivores. Native herbivores tend to be widely distributed over the landscape during the growing season. The chances that a plant cropped by an elk or bison will be regrazed again in any year, much less for years, is remote. Hence individual plants suffer no long-term harm from native herbivores. Herbivores are concentrated in winter by snow and weather, however, plants are dormant and are less impacted by herbivory. Plus regulatory processes like starvation, predation, etc. that helps keep native wildlife numbers in balance with available food sources.
Livestock tends to graze plants repeatedly often during the growing season for several reasons. Unlike native herbivores, livestock are usually concentrated by fences or herding and are often forced to regraze plants. Livestock prefers green plants that are higher in nutrients and protein. Many native grass plants take up to 10 years to fully recover from one grazing event, and there are few pastures rested for that kind of period.
Livestock are also just less mobile than wildlife, so they cannot utilize steeper terrain and other areas that are available to native herbivores, which helps native wildlife reduce pressure on any one plant.
Furthermore, native herbivores all seek out different plants. Bison tend to graze the coarser grasses, elk the regrown grass that follows bison, antelope seek out the forbs (flowers), and deer tend to eat shrubs. When you have a full suite of native herbivores, no one group of plants is overly affected by herbivory.
And let us not forget there are many other herbivores out there, from ground squirrels and prairie dogs to grasshoppers and sage grouse. All of these animals have different influences on native plants, and their collective effects are within the ecosystem’s tolerances.
Livestock, on the other hand, tends to focus on one group of plants. If grazed by cattle, the grasses are removed first, and there are more “forbs.” Sheep ignore the grasses and focus on forbs. But that means one kind of plant group tends to dominate sites grazed repeatedly by one livestock group.
Livestock (cattle), which evolved in moist woodlands in Eurasia, seek out habitat similar to their evolutionary past—i.e., they congregate in riparian areas. Riparian areas are critically important to the West’s wildlife since 75-80 percent of all species rely on riparian areas for food or shelter.
Riparian areas are also giant sponges that slow flooding and help maintain stream flows in low water times of late summer. Cattle destroy riparian areas by trampling plants, breaking down banks, compacting soils, and eating vegetation. Native herbivores like bison tend to avoid riparian areas or, like elk and deer, utilize them primarily in winter when plants are dormant, and the damage is negligible.
Finally, wildlife numbers are determined by natural factors from drought, harsh winters, predation, disease, and other factors. These tend to confine herbivores to certain population levels dictated by available forage and water. On the other hand, livestock numbers are subsidized by additional feed, water, predator protection, and other measures designed to ensure livestock survival even in the face of adverse natural conditions. As a consequence, livestock are often maintained at excessive numbers that can damage rangeland ecosystems.
For these and other reasons, the idea that livestock merely “replaced” native wildlife is an exaggeration.