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Reduced Effective Population Size In An Overexploited Population Of The Nile Crocodile

Reduced Effective Population Size In An Overexploited Population Of The Nile Crocodile (Crocodylus niloticus)

Changes in the genetic properties of a population over time is determined by the effective size of the population, Ne, rather than the actual number of living individuals, N (Wright, 1931; Crow and Kimura, 1970). Ne describes the size of an ideal population that has the same rate of genetic change as that of the population under consideration and in so doing governs both the effects of selection on populations and the rate at which diversity is lost via genetic drift (Wright, 1931, 1938). In so doing, Ne directly influences population viability because the strength of stochastic changes in genetic properties is reduced in populations with larger effective sizes (Frankham et al., 2002; Hedrick, 2005). Since genetic diversity is a fundamental component of adaptive evolution, its loss via drift and/or selection is predicted to decrease the probability of a population persisting through time; accordingly, Ne is one of the most important parameters in evolutionary and conservation biology (Frankham, 2005; Waples, 1990a). Processes that rapidly deplete genetic diversity via reductions in Ne (e.g. population fragmentation and unchecked exploitation) are predicted to increase the chance of short-term inbreeding depression and reduce the ability of a population to respond to novel environmental change (Amos and Balmford, 2001; Frankham, 2005). As a result, declining populations experience increased genetic drift and these effects are magnified through time (Frankham et al., 2002; Hauser et al., 2002; Spielman et al., 2004a), increasing susceptibility to extinction via demographic and environmental stochasticity (e.g. Newman and Pilson, 1997; Saccheri et al., 1998; Westemeier et al., 1998). Notwithstanding these predictions, a number of recent studies propose that the impact of population decline on genetic variation depends considerably on life history traits that affect population growth (Kuo and Janzen, 2004; Hailer et al., 2006; Lippé et al., 2006). Processes driving the loss of genetic diversity may in fact be buffered by intrinsic biological traits, specifically long generation times in age-structured species, and can result in remnant populations that appear genetically diverse despite periods of substantial decline.

We investigate how extant genetic variation of a large, longlived vertebrate population, occupying an important African wetland, might persist in light of a history characterized by overexploitation and population collapse. The Nile crocodile (Crocodylus niloticus Laurenti) is a top aquatic predator broadly distributed throughout the freshwater and brackish habitats of sub-Saharan Africa. Overexploitation through the mid to late 20th century caused the decline and extinction of Nile crocodile populations in many parts of its range (de Smet, 1998), and studies indicate that demographic stability in crocodile populations is sensitive even to relatively small perturbations (Games, 1992; Graham et al., 1992). Today, wild populations continue to experience harvesting pressure; the species remains one of the top commercially utilized species of crocodilian worldwide (Ross, 1998). The population of the Okavango Delta, Botswana exemplifies the effects of uncontrolled exploitation; from the mid 20th century onwards the explicit targeting of adult individuals by hide hunters resulted in repeated cycles of population collapse. For example, records estimate that up to 48,000 adult individuals were removed from the Okavango Delta during the period 1957–1968 alone (Pooley, 1982). Following a Convention on International Trade in Endangered Species (CITES) ban in 1973 on the trade of products from wild C. niloticus individuals, and together with a decade-long moratorium on the exploitation of the population, commercial ranching was introduced in 1983 as a way of sustainably meeting continued international demands for hides. To populate ranches, farmers removed 1050 live adults and 14,000 eggs from the Okavango Delta between 1983 and 1988; nest surveys conducted in 1987 indicated that this activity had already led to a 50% reduction in the number of active nests (Simbotwe and Matlhare, 1987). Today the Okavango supports a partially recovered but much depleted population of Nile crocodiles; while the quota-based removal of breeding adults and egg clutches for commercial ranching is ongoing (Dzoma et al., 2008).

In this study we assess whether life history traits inherent to Nile crocodiles have buffered the negative genetic effects expected of repeated population collapse in the Okavango Delta.

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