Though considered icons of our region, elk are not native to northern Arizona. Before European colonization, elk were rare here, perhaps even absent. A likely reason for this absence was the lack of water. With the exception of Mormon and Stoneman lakes, and a handful of small springs, few sources of water exist in northern Arizona. Adult elk rarely venture farther than one mile from water and the distance is even shorter for pregnant and nursing females.
Historically, the lack of water likely prevented elk from populating much of Arizona. But that changed dramatically with the importation of cattle. Soon after Spanish explorers arrived, cattle ranching began and the number of cattle exploded from an estimated 5,000 in all of the Arizona Territory in 1870 to as many as 1,500,000 in 1891. Ranchers built hundreds of artificial ponds to provide drinking water for their cattle. These ponds provided water for wild animals as well and allowed for the successful introduction of elk into northern Arizona.
Starting in the 1920s and continuing through the 1950s, Rocky Mountain elk (Cervus canadensis nelsoni) were brought from Yellowstone and released into parts of northern Arizona and New Mexico. With their importation, combined with the new and abundant sources of water, elk flourished.
Elk quickly became a common sight in northern Arizona. In fact, their growing numbers soon constituted a new pressure on plants. Studies elsewhere have shown that introduction and/or increased numbers of elk can be detrimental to plant populations. For example, in Yellowstone and other ecosystems, the removal of wolves and consequent increases in elk populations caused dramatic decreases in the numbers of young aspens, cottonwoods, willows and other trees.
Another plant that elk eat is Solidago velutina, commonly known as three-nerve goldenrod, which is commonly found in the understory of ponderosa pine forests.
Here in northern Arizona, recent studies by myself and others have shown that goldenrod is quickly evolving in the presence of elk. Goldenrod plants from sites exposed to elk grazing have evolved to flower nearly three weeks earlier than plants from sites where elk do not graze.
Two experimental techniques allowed researchers to draw these conclusions. The first is a technique called a common garden, which isolates the effects of genetics on plants traits. Using a common garden, the researchers showed that the different flowering times were caused by changes in plant genetics (i.e. caused by evolution), rather than by external factors.
Second, the researchers collected plants from inside and outside of elk exclosures. Inside fences that exclude the otherwise abundant elk, goldenrods show no evolutionary pressure to flower earlier. This earlier flowering time is believed to provide a reproductive advantage in the presence of elk. Specifically, it tends to improve the plants' chances of not being eaten before reproducing.
A further complication though is that the difference in flowering time affects the numbers and types of insects that interact with goldenrod flowers. Flies, which can act as important pollinators, were twice as abundant on plants that originated from sites with elk. Moreover spiders, which are important predators of pollinating insects, are nearly absent on plants originating from sites with elk.
In summary, the elk that have only been in northern Arizona for about 80 years have caused plants to rapidly evolve different flowering times and consequently a different community of interacting insects. This is a great example of what John Muir meant when he said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.”
Here, when we “picked out” elk and introduced them into the ecosystem, we found that their presence influenced the evolution of a native plant, as well as altering the food web of 45 species of insects, including herbivores, predators and parasites associated with goldenrod flowers. Numerous studies show that such complex linkages are extremely common, meaning that changes in one species can cascade to affect many other species.