Habitat loss is a bigger threat to African elephants than the ivory trade and the conventional wisdom is that declines in elephant populations mirror increases in human populations. But this is not true--rather, elephants persist up to a certain point and then suddenly disappear, say Richard Hoare and Johan Du Toit of the University of Zimbabwe in Harare, Zimbabwe, in the June issue of Conservation Biology.
"Wild elephant populations may initially show considerable tolerance to expanding human settlement but then precipitously decline and fail to recover," says Hoare. Figuring out how African elephants can coexist with people in savannas is essential because 80% of their range is outside protected areas.
Hoare and Du Toit studied how human settlement affects elephants in the nearly 6,000-square mile Sebungwe region of northwestern Zimbabwe. About 40% of the land is protected and the rest is divided into three contiguous districts where people have been clearing the elephant'sdeciduous hardwood habitat for subsistence agriculture since the mid-1950s. A single elephant population of nearly 12,000 occupies the three districts, and conversion to agriculture in the districts varies from limited to widespread.
The researchers found that the relationship between human and elephant populations is more complex than had been previously thought. Their results show that the number of elephants living in the study area is not affected by the number of people living there until the human population reaches about 28 people per square mile. After this threshold, elephants effectively disappear. The sharpness of this decline suggests that elephants can coexist with people up to a certain point but once that threshold is reached they go elsewhere, presumably to less disturbed areas.
"The 'threshold hypothesis' will let land planners distinguish areas where elephants can be conserved from those where they cannot," says Hoare. The threshold for elephant disappearance can be measured either by the density of people or by the amount of land cleared. While the threshold will vary with the ecosystem, in this study the threshold was reached when about half the elephant habitat was lost and the rest fragmented.
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