My interest in wolves started early. In middle school, I remember being assigned a project in which all I had to do was pick a topic I was excited about, do some research it, and present my findings with a poster and a paper. I don’t remember how I came to this decision, but I chose wolves as my topic. The research I did for that project kicked off a lifelong fascination and respect for the Canis lupus, so naturally I was excited when I found a paper about how wolves are at the top of a trophic cascade in Yellowstone National Park. Even more exciting was the discovery that the reintroduction of wolves into Yellowstone in the 1990s has been mitigating the effects of climate change* on the entire park.
What is a tropic cascade, you ask? A trophic cascade is when the introduction of predator into a food web decreases the abundance (and/or behavior of their prey), which in turn can cause the prey’s food to increase in abundance. The reverse of this process can also take place: the removal of a top predator from a food chain can cause an increase in the abundance of their prey, which then decreases the abundance of the prey’s food (1). In Yellowstone, grey wolves are at the top of their food web. Beneath the Canis lupus are the Cervus Canadensis, or elk. Elk behavior and abundance affects a number of other species in the region, as they graze on plant life (like young aspen trees, for example), and their bodies provide food for scavengers after the wolves take their share (2). This cascading effect of grey wolf abundance onto other species in their food web is what qualifies the interaction as a trophic cascade.
Instrumental to the discovery of this trophic cascade is the fact that there haven’t always been wolves in Yellowstone. In fact, wolves were locally extinct (i.e. absent) from Yellowstone from 1927 until 1995, when they were controversially reintroduced to the area (3). Reintroduction has not only proved to be healthy for the local ecosystem, it also has been incredibly useful to scientists who can now compare data from the two distinct time periods in the park—before and after wolf reintroduction.
This before-and-after data has also allowed scientists to investigate the effect of wolf reintroduction on the symptoms of climate change in the park. Specifically, wolf reintroduction is causing a change in carrion timing and quality, which affects scavenger species like ravens, eagles, coyotes, and bears, who sustain themselves on the carrion leftover from wolves during the harsh winter months (4). This is important because due to the warming effects of climate change on Yellowstone, winters are becoming increasingly shorter and snow is melting earlier. Without wolves, elk were living through the shorter winters with relative ease, causing food shortages for the scavengers. Since reintroduction in 1995, wolves kill elk at a steady rate throughout the winter, which mitigates the negative effects of climate change on the scavenger species (4).
Wolf reintroduction into Yellowstone has proved an interesting example of a time when human interference in a complex ecosystem has had positive effects, though scientists are still working to quantify the full effects. If anything, learning about the complex cause-and-effect of wolves in Yellowstone has shown us how little we know about inter-species relationships and what role humans play in the complicated web of nature.
*A quick disclaimer about climate change: it’s real. Don’t believe me? If you’re comfortable reading primary scientific sources, read these: A Globally Coherent Fingerprint of Climate Change Impacts Across Natural Systems; Evidence and Implications of Recent Climate Change in Northern Alaska and other Arctic Regions (5, 6). If you’re not comfortable with the primary literature, instead reference this: National Geographic on Climate Change (7). These sources are by no means the only evidence for climate change, but they’re a good place to start if you’re curious.
- trophic cascade | ecology | Britannica.com, (available at https://www.britannica.com/science/trophic-cascade).
- D. Fortin et al., Wolves Influence Elk Movements: Behavior Shapes a Trophic Cascade in Yellowstone National Park. Ecology. 86, 1320–1330 (2005).
- W. J. Ripple, R. L. Beschta, Wolf reintroduction, predation risk, and cottonwood recovery in Yellowstone National Park. For. Ecol. Manag. 184, 299–313 (2003).
- C. C. Wilmers, W. M. Getz, Gray wolves as climate change buffers in Yellowstone. PLoS Biol. 3, e92 (2005).
- C. Parmesan, G. Yohe, A globally coherent fingerprint of climate change impacts across natural systems. Nature. 421, 37 (2003).
- L. D. Hinzman et al., Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions. Clim. Change. 72, 251–298 (2005).
- Climate Change. Natl. Geogr., (available at http://www.nationalgeographic.com/environment/climate-change/).