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The interaction of large carnivores with small and medium sized carnivores was explored in a wildlife population study conducted by Penn State University researchers (Penn State News, August 8, 2018). When a large predator (like a cougar or a wolf) is removed from an ecosystem (an occurrence that has been observed in most human-occupied habitats) there are a number of expected and unexpected consequences.
Expected consequences include the unregulated growth of primary prey populations that the large carnivore preyed upon. Here in Western Pennsylvania the exponential growth of white-tailed deer populations in our wolf-less and cougar-less biotic communities is an excellent example.
Less anticipated consequences of the loss of a large predator, but still quite logical, are the increases in numbers of small to medium-sized predators (like foxes, bobcats and coyotes) whose numbers had been kept in check by the direct or indirect actions of the larger carnivore. The unexpected consequences of these increases in these predators is the increasingly intense competition between them and possibly outright predation of the larger species on the smaller. Coyotes, for example, are extremely intolerant of red foxes and will kill any red fox that they come across. Shift of the predator profile away from the small predators to the medium sized predators can lead to explosive, uncontrolled growth of the small predator’s small prey species. Lack of red foxes, for example, could lead to large increases in mice populations which could, in turn, negatively affect an ecosystem’s plant community and possibly contribute to increases in diseases and parasites carried by the mice. Possibly the explosive growth of white-footed mice here in Western Pennsylvania (and the associated rise in black-legged ticks and Lyme Disease transmission) is a consequence of declining red fox numbers due to the local increases in eastern coyotes.
Similar observations on these consequences of large predator removal are being made in Europe. Wolves were effectively extirpated from the European continent by intensive hunting and poisoning campaigns. The removal of this large predator has allowed the golden jackal (Canus aureus) a nearby, medium-sized predator to spread across Europe (see New York Times, January 11, 2019).
The golden jackal, a native of the Middle East and countries all across southern Asia, is a 15 to 30 pound canid (slightly smaller than a coyote) with omnivorous feeding behaviors that range from active carnivory to detrital and carcass scavenging. Golden jackals were first reported in Europe back in the 1800’s but have, since the middle of the 20th Century become very widespread. In the first two decades of the 21st Century, their numbers and extent of distribution have increased explosively.
Golden jackals are well adapted to not being seen. They live in small family groups typically of four to six individuals with one breeding pair, they tend to hunt alone, they are very furtive and they are nocturnal. They do have a tendency to howl, though, and this howling can be stimulated by other jackals or by other sources of noise (like church bells, for example). Golden jackals especially favor lowland habitats ideally near water (like a river, lake, canal, or the seashore). They also tolerate dry habitats, though, almost up to an extreme desert. They are not well adapted to snow, however, and must travel in the tracks of other animals when moving across a snow covered landscape. For this reason, many ecologists feel that the reduced snow fall and warmer temperatures in a climate changed world will favor the further proliferation and expansion of golden jackal populations.
And, finally, looking at some predators closer to home, it was reported in Science this past fall (Sept. 21, 2018) that the wolves of Isle Royale are going to be “rebooted” (their term, not mine!).
Isle Royale is a 206 square mile island in the northwestern corner of Lake Superior. Technically, the island is part of the state of Michigan, but functionally the main island and the hundreds of smaller islands around it make up Isle Royale National Park. Isle Royale is a patchwork of complex habitats and is the home for populations of moose and gray wolves that were each introduced to the island back in the early years of the 20th Century.
For the past 60 years the Isle Royale moose and wolves have been closely studied. The simplicity of their predator/prey dynamic and the isolated nature of their island habitat have enabled researchers to very precisely observe the population interactions and their ebbs and flows.
The ideal wolf to moose population ratio for an island the size of Isle Royale was calculated to be 25 wolves to 1500 moose. Over the six decades of study, though, this ratio was never achieved and population stability was never observed. Moose numbers fluctuated wildly from a low of 540 individuals to a high of 2450 individuals. Wolf numbers also careened up and down from an historic low of 14 wolves to a maximum of 50 wolves. This summer, though, there were only two wolves left on Isle Royale: an aging mated pair of closely related individuals (the female was the daughter and the half sister of the male). Although over the years new wolves have arrived on Isle Royale (either after a long, cold swim or an icy trot across the winter lake ice!), the wolf population on the island had succumbed to the insidious effects of severe inbreeding.
The consequences of this decline in the wolves has been very predictable. The moose population has grown extensively, and grazing by this large number of moose throughout the park has caused a decline in habitat quality of the island. The island is not in sustainable state!
After a long debate, the National Park Service has decided to “reboot” Isle Royale’s wolf population by introducing new wolves to the island. Some of these wolves will come from Michigan and others will come from Ontario, Canada. The 20 to 30 wolves that will be added to the park will have, then, a broad and diverse gene pool which will, hopefully, avoid the inbreeding problems.
None of these added wolves, though, will have had any experience hunting moose! It is expected, though, that they will quickly acquire moose hunting skills in this very prey limited ecosystem. As one researcher put it, “wolves are wonderful observational learners, and hunger is a strong motivator to test any potential prey.”
I will keep you posted!