The compression overabundance hypothesis

The decline in woody vegetation is traced directly to an overabundance of elephants, especially in protected areas. The overpopulation of elephants is caused by a compression of their range due to expanding human settlements, combined perhaps with a release from traditional hunting in habitats that are now protected as wildlife reserves. This hypothesis has been stated in various forms by Buechner and Dawkins in their study at Murchison Falls; Lamprey and colleagues, based on their Serengeti study; and Laws, who synthesized a variety of examples in a key 1970 publication. Laws even speculated that elephants may have been responsible for the creation of deserts in Africa.

Implicit in the hypothesis is the assumption that elephants and woody vegetation can be at equilibrium, or nearly so, when the former do not exceed the "carrying capacity" of the habitat under natural conditions. The problem of elephant overabundance may be the result of one of several factors. Given its positive rate of population growth, an elephant population would quickly exceed the carrying capacity unless regulated through expansion of its range or dispersal to other habitats. The boundaries of elephant population ranges, however, were becoming fixed or even contracting in the face of human expansion. The end result was the same in both instances—too many elephants, due either to a positive population growth rate or their being squeezed into smaller and smaller habitat areas even if population sizes were not increasing. A combination of these two factors was also possible. The creation of protected areas added to this problem by reducing or eliminating mortality from hunting. In times past, the mortalities from hunting would have helped to maintain the growth rate at zero.

Evidence for loss of habitat, compression of populations, and hence, increase in elephant densities, coincident to loss of woody vegetation, had been presented for several East African habitats. Graeme Caughley, however, argued in a 1976 article that no significant loss of habitat or compression of elephants could be demonstrated for the Luangwa Valley in Zambia, a region that faced a similar situation, with its declining woodlands seemingly under pressure from elephants. He further stated that compression may not be a "logical necessity" to explain this problem, but that "the crux lies in whether elephants would have changed habitats in much the same way if compression had not occurred" (p. 275). The Luangwa experience suggested otherwise. A decade later, Dale Lewis suggested that some compression had indeed taken place along the eastern boundary of the South Luangwa National Park due to human disturbance, such as hunting, resulting in redistribution of elephants. While this functional "compression" may have imposed a localized pressure on woodlands, this does not seem to be a general explanation for the earlier woodland loss in the Luangwa.

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