Laelaps

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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.

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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.

* * *

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., & Lindén, 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

Comments

  1. #1 Bill
    June 14, 2010

    Does anyone know of any mesopredator studies where the released mesopredator is of another Class to the controlled predator? i.e a reptile or bird released by control of a mammal?

  2. #2 Daniel J. Andrews
    June 14, 2010

    A great post, Brian, right in my area of interest too. This is one of the areas of study I was involved in the Yukon. The Kluane Project (Kluane Lake, Yukon) attempted to look at the various top-down and bottom-up trophic processes. It was a massive 10-year well funded study involving scientists from many disciplines. 1 km x 1 km electric fences were set up to exclude top herbivores (moose), predator fences were erected, small mammal fences were tried, networks of wires kept out aerial predators in other plots, and other kilometer-sized plots were aerially fertilized with Nitrogen.

    There were plots of all sizes located up and down that section of the Alaska Highway in the 80s to 90s. Even now you can recognize spots where aerial fertilization was done (more grasses).

    The info was summarized in the book Ecosystem Dynamics of the Boreal Forest (abstracts can be found by searching for The Kluane Project–is first hit in Google–>AINA at U Calgary; the book is very expensive but I had to order it anyway for my thesis).

    My thesis tried to answer some of the new questions raised by that big project, and I was lucky to work with, and be advised by, some of the big names on that project, and a few years later, publish a paper with some of them (just had another one published this April, one more to go).
    ——-

    Bill…I’ve been sitting here trying to recall something that is niggling at the edge of my memory that may answer your question. I think it might have something to do with the aerial predator exclusion zones, or maybe the mammal predator exclusion zones and the effect on aerial predators..??..I will have to check the book, which is in storage.

  3. #3 Tsu Dho Nimh
    June 14, 2010

    Wolves will kill coyotes, and that has also altered the predator/prey balance where there are wolves.

    One of the really flaming topics in the Western US is the supposed “annihilation” of elk by wolves: they are far harder for humans to hunt when the elk have wolves to worry about all year long. This leads to a lot of whining by hunters because instead of the fat, dumb and happy elk they remember, they have wary animals you actually have to HUNT with some skill.

  4. #4 jakc
    June 15, 2010

    speaking of the west/plains, what is the impact of the elimination of wolves on prairie dogs? I’m always amazed by the claims for the need to “control” prairie dogs (which usually means posioning) I know that wolves can control coyote numbers – another example of apex predators controlling mesopredators I suppose – just curious if anyone has tried to see how this all plays out

  5. #5 Dan L
    June 15, 2010

    As usual on the topic, Aldo Leopold said it best:

    “I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer. And perhaps with better cause, for while a buck pulled down by wolves can be replaced in two or three years, a range pulled down by too many deer may fail of replacement in as many decades.”

  6. #6 Bill
    June 16, 2010

    @Jakc – I would imagine that the historical distribution of coyotes (i.e in the SW) overlapped with prairie dogs, and they, along with blackfooted ferrets, were the major controller of the prairie dogs. Do wolves take PDs? Ethiopian wolves rely on giant mole rats but they hunt singly, unlike most wolves; though I guess wolves will eat a lot of small rodents too. Control of PDs probably happens in areas (suburban) where even coyotes are far and few between, or, more likely, are doing fine on anthropogenic waste. Or I am completly wrong – I’m not an American.
    @Daniel – thanks. I was thinking of a situation in Australia where cats and foxes are controlled to help small marsupials, but might release suppressed native predators – specifically snakes – that might then wreak havoc on a small mammal population that has not recovered. I’m interested because of the wildly different growth and breeding rate of reptiles vs mammals

  7. #7 William Barkley
    June 16, 2010

    Great article. The lion in Botswana is able to hunt elephant if the pride has 30 adults. I have seen this on “Planet Earth” and on “Be the Creature.” Your article mentioned the wolf hunting the bison in the Old West. I should hope that the conservationists will argue that the wolf population actually has to increase in order to trigger this action, and the bison must be allowed to increase,too. The hunting establishment in America and in Africa would be better served if they should more tolerance to wolf and lion. This would create population ratios throughout these ecosystems that benefit everyone. There will probably always be easy hunts available on hunting ranches, anyway.

  8. #8 Matt Skoglund
    June 18, 2010

    Excellent article, Brian. The ecological importance of top predators needs to be better appreciated by wildlife managers and the public. I will try to spread your article widely. Many thanks.

  9. #9 Klank
    June 20, 2010

    See if they had only left things alone and let the Native Americans be, all might have been so much better today! Go figure!

  10. #10 Aurora
    June 28, 2010

    Thanks for a great article, and even more insights in the comments. I’ll definitely pass this on to friends working in Environmental Education, for whom I think it’s a must-read.

  11. #11 mp3 indir
    July 13, 2011

    Great article. The lion in Botswana is able to hunt elephant if the pride has 30 adults. I have seen this on “Planet Earth” and on “Be the Creature.” Your article mentioned the wolf hunting the bison in the Old West. I should hope that the conservationists will argue that the wolf population actually has to increase in order to trigger this action, and the bison must be allowed to increase,too. The hunting establishment in America and in Africa would be better served if they should more tolerance to wolf and lion. This would create population ratios throughout these ecosystems that benefit everyone. There will probably always be easy hunts available on hunting ranches, anyway.