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Testing the Relative Influence of Intrinsic and Extrinsic Variation in Food Availability on Feral Pig Populations in Australia's Rangelands

David Choquenot
Journal of Animal Ecology
Vol. 67, No. 6 (Nov., 1998), pp. 887-907
Stable URL: http://www.jstor.org/stable/2647419
Page Count: 21
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Testing the Relative Influence of Intrinsic and Extrinsic Variation in Food Availability on Feral Pig Populations in Australia's Rangelands
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Abstract

1. Intrinsic variation in the availability of food to animal populations reflects the influence of foraging by the animals themselves. Intrinsic variation in food availability provides a link between population density, subsequent food availability and variation in the rate of population increase (r), operating through density-dependent food shortage. In contrast, extrinsic variation in food availability is caused by environmental influences on food or animal abundance, which are independent of animal foraging. Extrinsic variation in food availability is random relative to that arising through intrinsic shortage. Intrinsic and extrinsic variation in food availability can influence animal populations simultaneously. Intrinsic variation will impart a tendency towards an equilibrium between animal and food abundance, which will be progressively obscured by density-independent variation as the influence of extrinsic factors increases. 2. This study used a large-scale field experiment, in which the density of food-limited feral pig (Sus scrofa L.) populations was manipulated on six sites, to assess the relative influence of intrinsic and extrinsic variation in food availability. The experiment evaluated the influence of pig population density on r and the abundance of food resources measured as pasture biomass. It was predicted that if intrinsic shortages dominated variation in food availability, pasture biomass and r would decline with increasing pig density. If extrinsic factors dominated variation in food availability, pig density would have no systematic effect on either pasture biomass or r. If intrinsic and extrinsic sources simultaneously affected variation in food availability, higher pig densities would have no systematic effect on r, but would reduce pasture biomass. The simultaneous model predicts reduced pasture biomass because, in the absence of compensatory changes in other sources of variation, the effects of intrinsic and extrinsic factors will be additive. 3. To examine further the degree of interdependence in pig and pasture abundance, a series of stochastic models of the grazing system were estimated and the feedback loop comprising the functional and numerical responses of feral pigs to variation in pasture biomass was manipulated. 4. In the large-scale experiment, neither pasture biomass nor r declined with increasing pig density, suggesting that food availability was dominated by extrinsic factors. However, limitations of the experiment meant that a minor decline in pasture biomass may have gone undetected. Comparison of simulation models, which included and omitted pasture offtake by pigs, indicated that because they were less efficient grazers and persisted at lower average densities relative to other large herbivores, pigs had little influence on variation in pasture biomass. The monor influence pigs appear to exert on pasture biomass suggests that trophic processes, typically invoked to explain herbivore population persistence, have, at best, limited consequences for pig populations. Despite this, simulation models indicate that pigs are able to persist indefinitely under rangelands conditions, because their high intrinsic capacity for increase (rm =0.69) means that there are able to exploit periods when pasture is plentiful more effectively than other large herbivores. 6. The study concludes that intrinsic and extrinsic sources of variation in food availability represent two extremes of a continuum in the strength of interaction between herbivores and their food resources. The position of a grazing system along this hypothetical continuum depends on the relative influence of stochastic, density- independent environmental processes (extrinsic variation) and the efficiency of its vegetation-herbivore feedback loop (intrinsic variation). Feral pig populations in the rangelands appear to occupy a position towards the extrinsic extreme of this continuum, where their dynamics are more influenced by extrinsic than intrinsic sources of variation in pasture availability.

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