Several weeks ago, a ‘trophic cascades’ buzz surged briefly through the media, with a couple of articles – one in Yale Environment360 – talking about the ‘discovery’ that predators are important to ecosystem function, particularly in regulating biodiversity. This is actually not news to the ecology community – studies of trophic cascades from the 1960’s now rank among classic ecological papers – but perhaps it’s taken longer to reach the mainstream than I’d realized.
I’ve been meaning to write about this because people frequently end up on this blog through queries about the wolverine’s role in the ecosystem. This question comes in several forms – “How does the wolverine affect its ecosystem?” “What would happen to the ecosystem if the wolverine went extinct?” “How does the wolverine help the ecosystem?” Unfortunately for the people who are asking these questions, we still have a lot of research to do before we can provide an answer. But looking at the role of predators within ecosystems gives us a place to begin hypothesizing.
What is a trophic cascade anyway? First of all, you need to be familiar with a basic food chain, which is divided into trophic levels – producers (plants), which take energy from the sun; primary consumers, (herbivores), which take energy from (read: eat) the producers; and secondary consumers (carnivores), which take energy from the primary consumers. Sticking to this very simple structure, it’s easy to imagine that those in the top tier of the food chain – the predators – are having an effect on those in the next tier down – the herbivores. To put this into even simpler terms, otters eat sea urchins, therefore otters have an effect on the sea urchin population.
The idea of a trophic cascade takes things beyond the obvious by suggesting that the otters also have an effect on the plants that the sea urchins are eating. This is less intuitive but likewise pretty simple when you think about it: if there are fewer sea urchins, the plants that they eat are likely to experience some benefit. This, in the simplest form possible, is a trophic cascade: a species affecting other species in non-adjacent trophic levels. These effects have been observed most strongly in aquatic ecosystems, where, for example, the extirpation of sea otters led to the elimination of kelp beds as the sea urchin population exploded. In terrestrial systems, trophic cascades occur at both the small (spiders, grasshoppers, and goldenrod) and large (wolves, elk, aspen) scales.
Of course, things aren’t always this straightforward. There’s no such thing as a simple food chain in nature; energy and nutrients cycle through complex networks of production, consumption, and decomposition, and frequently these webs include tertiary consumers (predators that eat other predators) as well as secondary consumers. And conceivably, the same individual could be both a secondary and a tertiary consumer (a heron, for example, might eat an herbivorous rodent or a carnivorous fish.) Direct numerical effects on population aren’t the only consequence of having carnivores around; research on trophic cascades has provided evidence that there is also an ‘ecology of fear,’ whereby the presence of predators changes the behavior of herbivores in ways that can also contribute to trophic cascades.
To complicate things even further, some secondary consumers – generally referred to as ‘apex predators’ – don’t like other secondary consumers – generally referred to as mesocarnivores or mesopredators – and kill them to eliminate competition, which can have an effect on the preferred prey of the mesopredator. (Wolves don’t like coyotes, and kill them when they can; coyotes prey extensively on pronghorn fawns. Since the wolf reintroduction to Yellowstone, coyote numbers have dropped and pronghorn fawn survival rates have risen. A number of other examples of this phenomenon in reverse – eg, removing apex predators from ecosystems, which is sometimes referred to as ‘mesocarnivore release’ – can be found here.) If this doesn’t give you enough to mull over in contemplating the complicated relationships among species, consider that there’s also evidence that wolves in Yellowstone, by reducing elk numbers and thereby encouraging the growth of willows and aspens, have allowed the return of certain rare songbirds by providing nesting habitat. As to the effects that those songbirds are undoubtedly having on their food networks, well….you can imagine that the repercussions go on and on.
For a long time, ecologists thought that ecosystem structure was regulated from the bottom up, by resource availability. Only with the advent of work on trophic cascades did people begin to appreciate that top-down regulation and structuring were also occurring. The realization was significant for conservation advocates because it gave a much more quantifiable value to predators, which were traditionally reviled for depleting supplies of game. Traditional Western wildlife management, from its inception, revolved around the removal of predators to ‘benefit’ the ecosystem as a whole. Evidence of top-down ecosystem regulation finally provided some rationale for keeping predators on the landscape (aside from “These animals are cool and inspiring,” which, unfortunately but understandably, has never been enough for some people.)
How does this relate to wolverines? Wolverines straddle the line between being a top predator and being a mesocarnivore; they hunt, they scavenge, they can scare grizzlies or wolves off a kill, but they are also killed by larger predators. Among the high peaks of the Lower 48, where there isn’t much apparent overlap with bears and wolves, they are likely the top predator on bighorn sheep and mountain goats, although they also apparently rely on sheep and goats that fall off cliffs. They consume a wide array of small mammals and may be consuming some plants (chemical compounds derived from vegetation are found in wolverine musk) although obviously not enough to have a major effect on plant communities. The point is, they interact with a number of different species because they are, within their habitat, generalists. One could speculate that a female wolverine denning in an area near pika colonies might have an effect on those pika colonies, which in turn could effect the plants on which the pikas graze. It could also regulate disease transmission among pika colonies by thinning out the population and reducing the density of vectors; conversely, it might have a negative effect on dispersal and the founding of new pika colonies. We could also hypothesize that wolverines might preferentially prey on lambs or kids (the goat variety, not the human….), and that this in turn would restructure the plant communities and maybe the distribution of goat or sheep herds in their high-altitude pastures. Finally – a question that the scientists who work on trophic cascades haven’t, as far as I know, asked – we could begin to test some hypotheses about whether mid-level carnivores or scavengers can affect ecosystems as a whole – for example, if wolverines are preferentially hunting mountain goat kids, do they reduce the mountain goat population enough to exclude some other predator? Do wolverines compete with raptors (by burying carrion, for example) in such a way that the raptors and/or the raptors’ preferred prey is affected? At the northern extent of their range, is wolverine predation on caribou calves high enough to have an impact on the caribou and if so, how does that affect the plants on which caribou graze? Are wolverines on the landscape playing any role in rates of wolf predation by pushing wolves off kills and forcing a pack to hunt more frequently as a result? Honestly, it seems fairly ridiculous to imagine that this might be true – but who knows?
Everything that I’ve written in the paragraph above is entirely speculative, and the alpine tundra ecosystems in which the wolverine lives at the southern edge of its range aren’t necessarily well understood as systems. The point is that we don’t know exactly how wolverines affect ecosystems in either the northern or the southern parts of their range, but it’s likely that they do in some way, and there are a million possibilities to explore. For people who are looking for an answer to how wolverines affect the ecosystem, hopefully the idea of trophic cascades gives you a place to start thinking about the many connections that exist among species and the many possibilities for asking targeted questions on the topic.