A drop in the bucket – a massive pile of bison skulls about to be ground into fertilizer, photographed circa 1870. From Wikipedia.
From almost the very start, wolves were not welcome in Yellowstone. When the national park was established by the United States government in 1872 the bison population had crashed – a victim of westward expansion, the fur trade, and the desire to deprive native people of an animal important to their existence – leaving the area’s wolves little recourse but to begin preying upon local livestock. This did nothing to help their reputation. Already seen as nature’s villains because of carried-over European folklore, wolves were now viewed as vermin which had to be eliminated if farmer’s livelihoods and desirable prey species were to be preserved. Bounties were offered for the canids, and eventually their destruction became institutionalized by the U.S. Biological Survey (today called the Fish and Wildlife Service) and U.S. Park Service. By 1926, the wolf had been entirely eradicated from the Greater Yellowstone Ecosystem.
Yellowstone did not benefit from the eradication of the wolves. The elk population exploded, causing overgrazing problems and requiring Yellowstone’s naturalists to begin culling the ungulates to keep their numbers under control. The true extent of the damage, however, was not apparent until the controversial reintroduction of wolves to the park in 1994. For decades elk had so heavily grazed young willow trees that relatively few made it to adulthood, and these trees provided essential building materials for beavers. The ability of the beavers to build their dams was hindered, in turn reducing the amount of shady, swampy habitat preferred by moose, certain fish species, and birds. When the wolves came back they began to reverse this trend through the initiation of an ecological cascade. Willows grew in open habitats in which elk were vulnerable, and so wolves created a “landscape of fear” which kept elk away from places where they could be ambushed. As a result, the willows began to grow tall again, allowing for the natural restoration of habitats which had dwindled during most of the 20th century.
The existence of wolves in the American west remains controversial. Many still consider them to be vermin – pests which destroy livestock and compete with human hunters for desirable prey. Yet, despite the narrow-minded opposition to wolf reintroductions, habitats from which predators have been extirpated are not healthy ones. Apex predators not only regulate prey populations, but they also can trigger trophic cascades which benefit other species, particularly through the control of smaller predators which can often do much more damage than large ones.
A young lynx (Lynx lynx). From Johnson 2010.
In the boreal forests of southern Finland, the mountain hare (Lepus timidus) is prey for both the lynx (Lynx lynx) and the red fox (Vulpes vulpes). For most of the 20th century, however, the lynx was almost entirely absent from this area as it had been hunted to near-extinction by the 1950’s. It might be thought that the eradication of one predator would benefit the prey species, but the opposite turned out to be true. As the number of lynx dwindled, so did hare populations – the foxes proliferated and took far more hare than the relatively small number of lynx did. In technical terms the foxes were mesopredators, or generalized carnivores which are often very successful when their top-tier competitors are eliminated.
Then, starting in the 1990’s, the lynx population began to rebound after the species received protected status. Their gradual reestablishment triggered a trophic cascade much like the reintroduction of the Yellowstone wolves had, but for different reasons. Rather than ignore the foxes, lynx regularly killed them, causing about 14% mortality to fox populations each year. (Even in places where lynx and red foxes are not in competition for the same prey, lynx still control red fox numbers.) The lynx were slowly reducing the fox population, easing the pressure on the hare, and since an individual lynx requires a larger area to live in than an individual fox, there ended up being fewer predators in a given area than when foxes were allowed to reproduce unchecked. What this may mean is that the abundance of mesopredators may not be regulated by the availability of prey, but by the presence of top predators.
As described in a new Journal of Animal Ecology paper, ecologist B. Elmhagen and colleagues recorded these shifts through the long-term study of tracks left in transects. What they found was consistent with the Mesopredator Release Hypothesis in which the removal of a top predator results in the “release” of secondary predators and the subsequent decline of prey numbers. Even though the overall population of hare did not increase in the sample – possibly the result of several mild winters during the past two decades – hare populations were not so low in areas where lynx were present. Oddly enough, it seemed that the presence of lynx was still more beneficial given these other environmental pressures. The trend still held.
The mesopredator release phenomenon has been recorded elsewhere, as well. Off the coast of New Zealand on Little Barrier Island, the elimination of feral cats actually harmed populations of native seabirds. With the cats gone, rats (mesopredators) proliferated and killed far more young seabirds than when both cats and rats were present. It was better to have both predators on the island than just rats alone, and so even in ecosystems disturbed by the introduction of feral species the existence of apex predators can be (relatively) beneficial.
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During my elementary school education, I was often presented with the classic, simplified diagrams of predator/prey interactions. One, a “food pyramid”, showed how there were far more plants than herbivores and far more herbivores than carnivores. The other used the same basic shape but included more detail, with arrows connecting the eaters to the eaten. In a very broad sense these diagrams are still true – ecosystems can support far more herbivores than carnivores, for example – but through our elimination and reintroduction of top predators we have learned that these interactions are far more complex. It is not simply a matter of productivity. As far as three-level systems of apex predators, mesopredators, and prey are concerned, the presence or absence of the top predator can have a major influence on the population dynamics of both its competitors and its prey. Applying this to conservation, it appears that one of the best ways to jumpstart ecosystem restoration is to reintroduce once-absent apex predators, but, given our fear of these animals, such efforts will be bound to remain controversial.
Elmhagen, B., Ludwig, G., Rushton, S., Helle, P., & Lind√©n, H. (2010). Top predators, mesopredators and their prey: interference ecosystems along bioclimatic productivity gradients Journal of Animal Ecology DOI: 10.1111/j.1365-2656.2010.01678.x
Johnson, C. (2010). Red in tooth and claw: how top predators shape terrestrial ecosystems Journal of Animal Ecology, 79 (4), 723-725 DOI: 10.1111/j.1365-2656.2010.01706.x