Modeling the dynamics of exploited populations

Elephants in both continents have been exploited through historical times for several purposes. The capture of Asian elephants in large numbers over the past four millennia for taming has not necessarily been a random process, but often was selective in targeting males bearing tusks. The tusked males have also been selectively targeted for their ivory. The exploitation of African elephants is ancient and has been largely for ivory. While both sexes have been hunted for ivory, there has also been selection for large-tusked individuals, which are usually the older males and old females. Thus, human exploitation of elephant populations could be expected to have influenced their dynamics even more significantly perhaps than natural environmental factors. Added to the direct impact on elephant numbers and population structures is the complication introduced by the compression of elephant populations with the spread of human populations.

When the debate on the role of elephant "overabundance" in damage to woody vegetation in African savanna-woodlands was raging, Clive Spinage wrote that, "We need look no further for an explanation as to why elephant numbers may have fluctuated, than the exploitation of elephants for ivory" (1973, p. 281). He was referring to the extensive historical evidence for ivory exploitation that resulted in dramatic declines of elephants, especially since the early seventeenth century (chapter 8).

The exploitation of elephant populations has several demographic and genetic consequences. Many African elephant populations have remained depressed at a young age structure because of disproportionate removal of older individuals. Not only do age structures distort, but also sex ratios skew as males suffer higher rates of exploitation. Demographic changes also include changes in birth rates of the population. Thus, I found that a sex ratio extremely biased in favor of female elephants at Periyar in southern India had clearly depressed the birth rate. Similarly, Richard Barnes and E. B. Kapela recorded a decline in recruitment among the elephants at Ruaha in Tanzania that had been impacted by ivory poaching. The population genetic consequences include a reduction in the frequencies of genes for tusks, thus favoring the increase of tusklessness in the population. Among Asian elephants, the selective hunting of males is usually the most significant determinant of demography and population genetics.

The modeling of human-impacted elephant populations is thus even more important and relevant than modeling "natural" population dynamics. It is not an easy task, however, to obtain reliable field information on exploitation. Modelers therefore have relied on a variety of indirect evidence, such as the sizes and volumes of tusks in the trade, carcass ratios in the field, and standing age structure to simulate the dynamics of individual populations or those at a regional or subcontinental scale. I consider some of these models and what they have revealed about the nature of exploitation or its consequences for the dynamics and conservation of elephant populations.

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