The Danger of Being a Wide-Ranging Predator
In the last three decades wild dogs, Lycaon pictus, have vanished from twenty-five of the thirty-nine African countries in which they once lived. Today six countries support populations greater than one hundred animals, and few of those populations are stable or increasing. The total number of wild dogs left in all of Africa is approximately 3,000 individuals, and their rapid decline makes this species one of the most endangered. The major reasons for the decline of wild dogs are loss of habitat and prey, and human persecution.
Wild dogs live in packs of eight to ten adults with their offspring, and hunt savannah animals such as impalas, kudu, wildebeests, and warthogs. Wild dogs weigh about 50 pounds, while their competitors, hyenas and lions, weigh in at 100 and 300 pounds respectively.
Rosie Woodroffe and Joshua Ginsberg observed that small populations of African wild dogs in protected areas were vulnerable to extinction because although the protected areas set aside for them were quite large, whenever the dogs crossed the border, they were almost always killed.
Conservation biologists know that small populations of species are more vulnerable to extinction than larger populations. Random events like a hard winter, or disease might not kill a big population but they can wipe out a small one. However, Woodroffe and Ginsberg in the course of their field work observed that wild dogs were suffering significant mortality when they got close to or crossed the boundaries of their refuges. They call this increased mortality near the border an edge effect.
To see if this is important to the survival of other endangered species, they compiled extinction data on ten different wide ranging carnivores: African wild dogs; gray wolves in Canada; dhole in India(a kind of wild dog); lions in Africa; tigers in India; snow leopard in India, Nepal, Pakistan; jaguar in Central America; spotted hyenas in East Africa; the California black bear; and grizzly bears in Canada and the Northwestern United States. The collected data show that animals that range widely, large predatory animals at the top of the food web, are most exposed to threats on reserve boarders, regardless of their population size. The grizzly bear and the African wild dog both require over 3,500 square kilometers (l,400 square miles) of habitat to support a population, while the California black bear has a critical reserve size of only 36 square kilometers (14 square miles).
Conservation biologists spend a lot of time and money managing small populations for inbreeding and disease, and other internal threats, but if animals are killed whenever they cross the border of their reserves, they are not going to survive long enough for internal threats to be a problem. We spoke with the Joshua Ginsberg and Rosie Woodroffe about their work and its implications for conservation biology.