Wolves, Foxes, and Food

Large scary predators are, by their nature, rare. There simply can't be too many of them, or they would run out of food to eat, something typified by the trophic pyramid often seen in biology textbooks: plants are eaten by herbivores are eaten by carnivores are eaten by bigger carnivores, with each step necessarily having a smaller total mass. The reality is, of course, somewhat more complex than this simple chain would suggest, most obviously because it ignores omnivores and decomposition.

But the basic idea holds, and apex predators - those that are large enough that nothing else normally eats them - exist in much smaller numbers than herbivores or smaller carnivores. This means that, relative to their numbers, they have a disproportionate effect on the ecosystem within which they live. Take away the apex predators and, even though there weren't very many of them to begin with, you will radically change the local ecology.

And, because they are relatively few in number, apex predators tend to be especially vulnerable to being wiped out. That's even assuming that humans don't focus on them deliberately out of fear, whether for their own lives or for the good of their livestock. Globally, apex predators are declining. (On this blog, we're mostly interested in mammals, but consider, for example, that at least eight of the 23 species of crocodile/alligator are currently thought to be endangered).

Against this, however, we have the fact that modern conservation efforts often include the reintroduction of animals to areas where they have previously been eliminated. Determining whether or not this has worked in the straightforward sense of the introduced animal not dying out again is relatively straightforward, but it's also going to change the local ecology, and that's less often studied. The assumption, perhaps, is that it's going to change things for the better, back to how they were before humans interfered... but even if that's true it doesn't mean that we don't need to check that's actually happening, or what the details of the change are.

In the case of reintroduced apex predators, there have been several studies looking at the effect this has on the various animals that the predators eat. For example, the reintroduction of wolves to Yellowstone Park in the 1990s caused a decline in the local elk population. That may not have been good for the elk, but the overall effect is probably beneficial, because with fewer elk around, trees such as aspen prospered, increasing the general forest cover and restoring some vanished woodland. Which, in turn, was good news for beavers and bison, both of which increased in number following the return of the wolves.

This shows how the reintroduction of a predator may have knock-on effects that you would not necessarily predict; in this instance, having more wolves around also increased the number of bison. But predators can also have effects on each other. The most obvious way in which this happens is through direct competition. Two apex predators in the same environment are likely to eat similar prey, especially if they are of similar size. And, because predators have specifically evolved to kill things, if they come into conflict, things can easily turn violent. In which case, whichever of the two is larger is, on average, likely to come out on top.

But the same doesn't necessarily apply if the predators are of radically different size. This is because apex predators may eat medium-sized carnivores that in turn eat really small ones - in which case, the very small carnivores, which are too small for the apex predators to bother with themselves, gain a boost... which can affect the herbivorous insects (or whatever) that they eat, which affects the plants that those are pests on. And so on.

Isle Royale lies in Lake Superior, just on the American side of the border with Canada. At 535 km² (205 square miles) it is the fourth-largest island in the world to lie inside a lake rather than the sea, and has been a National Park since 1940. Although there are some (barely used) camping sites, walking trails, and an automated lighthouse, there has been no permanent human settlement on the island for decades. 

There had been plans to introduce timber wolves (Canis lupus lycaon) to the island since the National Park was created, but before they could be carried out, a wolf pack spontaneously crossed over the frozen lake during a particularly cold winter in 1949 and saved humans the bother. Studies of how the isolated populations of wolves and moose on the island interacted started in 1958 and continued for decades.

An outbreak of canine parvovirus on the island in the early 1980s triggered a population collapse among the wolves, to the point that the survivors became too inbred to reproduce successfully. Although the parvovirus itself ceased to be a problem within a few years, the damage had been done. By 2018, the population was down to just one... and the National Park Service decided to bring them back.

This gives us an unusual opportunity to study the reintroduction of predators to an ecosystem because there is so little else on the island. We know that there is sufficient to support a wolf population because it happened before - there were around 80 wolves on the island before the viral outbreak and their ancestors had been there for four decades. But with so few others animals present, studying the interactions between those that are present becomes relatively straightforward and it is this that aided the wolf/moose studies starting in the '50s.

In fact, apart from bats, there are known to be just six species of terrestrial mammal living on the island - although there are also some snakes. Of those, only two are predators that even come close to competing with the wolves: red foxes and American martens - the latter being recent arrivals themselves. Were either of these affected by the return of wolves in significant numbers?

Researchers examined over 600 dung samples from foxes and martens, collected both before and after the return of the wolves in 2019, to see if there was any evidence that their diet had changed. As an additional source of information, they examined hairs caught in non-invasive traps near trails known to be used by the animals. (Essentially, you put something tasty in a mesh container so that the animal can reach it, but include a spring that snags some of its hair when it backs out). 

While dung can provide detailed information about what has been eaten, hair analysis is obviously more limited. Nonetheless, with some adjustments, isotope analysis of hair can be used to determine the general types of food that have been eaten - plants, small herbivores, or browsing animals such as moose.

The results of this study showed that the martens were largely unaffected by the return of the wolves. Foxes, on the other hand, ate more moose meat once the wolves were on the island. 

For instance, smaller carnivores can benefit from the presence of larger ones if the latter leave carrion around for them to eat. This might seem surprising, especially given that it's hard to envisage a fox chasing down even a baby moose in order to kill and eat it. What's probably happening here is that the wolves are leaving dead moose carcasses around that they couldn't quite all of, and the foxes are feeding on the resulting carrion. Something similar has been seen in Argentina, where condors rely heavily on pumas to provide them with food and one can imagine that the presence of apex predators would be beneficial to other scavengers, too.

There was also a smaller effect where some individual foxes apparently increased their consumption of human leftovers from campsites and ranger stations, although this wasn't a wider trend among the fox population as a whole. Perhaps for them, the risk of competing with wolves, even if it was only trying to eat a carcass that the wolf had hopefully finished with, was more worrying than the risk of humans...

[Photo by Henrique Pacheko, from Wikimedia Commons.]