Prey Dynamics of Middle Earth

Investigating the middle-of-the-food-chain species

For decades, ecologists have debated whether ‘top-down, carnivores’ or ‘bottom-up, plants’ have greater influence on community function and stability; and Yellowstone is no exception. But what about those middle-of- the-food-chain species?

At YERC, we’ve focused on studying the herbivores—or plant-eaters—forming the prey base of mesopredators—carnivores that are not at the apex of the food pyramid. In Yellowstone, voles, squirrels, and pocket gophers make up a large majority of the diet of mid-sized carnivores like foxes, hawks, weasels, and badgers. For the larger carnivores such as wolves, bears, and cougars, as well as dozens of scavenger species, large herbivores like deer, elk, pronghorn, and bison provide critical food resources. This is especially true during the winter season when food is scarce. Overall, herbivores provide a critically important link in the food chain: they consume the carbohydrates that plants produced with photosynthesis, in turn transferring that energy to consumers up the food chain.

The dynamics of herbivores in Yellowstone has been dramatic and at times controversial. In fact the elk of the northern range have been at the center of controversy for 60 years. Now a growing and largely non-migratory bison herd has been making national news for 20 years, overtaking elk in total biomass on the Northern Range. Pocket gophers have increased due possibly to an invasive weed known as Canada thistle, while Uinta ground squirrels have dramatically increased since the great fires of 1988. Beaver and pronghorn antelope populations are on the rise quite possibly due to wolves.

How do these changes in the middle of the trophic pyramid affect species above and below them? The effects that ripple outward in many directions from the ‘middle earth’ could have profound effects on the ecosystem.

YERC has built a foundation of data and research based on extensive and long-term measurements of herbivores. See our research publications on multiple herbivore species: elk, bison, pronghorn, ground squirrels, and small mammal community. We now propose to expand and synthesize our previous work to focus on how two trophic chains have reacted to major deceases in consumer populations. First, how have the small mammal communities responded to declines in coyote populations? And secondly, how have riparian communities responded to major declines in elk numbers? Theory would also predict that dozens of other species would then respond to possible increases in small mammal communities and riparian vegetation. The food pyramid here provides a graphic depiction of the many hypotheses yet to be examined.