Testing the relative influence of intrinsic and extrinsic variation in food availability on fetal pig populations in Australia's rangelands

1. Intrinsic variation in the availability of food to animal populations re flects the influence of foraging by the animals themselves. Intrinsic varia tion in food availability provides a link between population density, subse quent food availability and variation in the rate of population increase (r ), operating through density-dependent food shortage. In contrast, extrinsi c variation in food availability is caused by environmental influences on f ood or animal abundance, which are independent of animal foraging. Extrinsi c 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 t he 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 s ites, to assess the relative influence of intrinsic and extrinsic variation in food availability. The experiment evaluated the influence of pig popula tion density on r and the abundance of food resources measured as pasture b iomass. 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, pi g density would have no systematic effect on either pasture biomass or r. I f intrinsic and extrinsic sources simultaneously affected variation in food availability, higher pig densities would have no systematic effect on r, b ut would reduce pasture biomass. The simultaneous model predicts reduced pa sture biomass because, in the absence of compensatory changes in other sour ces of variation, the effects of intrinsic and extrinsic factors will be ad ditive. 3. To examine further the degree of interdependence in pig and pasture abun dance, a series of stochastic models of the grazing system were estimated a nd the feedback loop comprising the functional and numerical responses of f eral pigs to variation in pasture biomass was manipulated. 4. In the large-scale experiment, neither pasture biomass nor r declined wi th increasing pig density, suggesting that food availability was dominated by extrinsic factors. However, limitations of the experiment meant that a m inor decline in pasture biomass may have gone undetected. Comparison of sim ulation models, which included and omitted pasture offtake by pigs, indicat ed that because they were less efficient grazers and persisted at lower ave rage densities relative to other large herbivores, pigs had little influenc e on variation in pasture biomass. 5. The minor influence pigs appear to exert on pasture biomass suggests tha t trophic processes, typically invoked to explain herbivore population pers istence, have, at best, limited consequences for pig populations. Despite t his, simulation models indicate that pigs are able to persist indefinitely under rangelands conditions, because their high intrinsic capacity for incr ease (r(m) = 0.69) means that they 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 or a grazing system along this hypothetical continuum depends on the relative i nfluence of stochastic, density-independent environmental processes (extrin sic variation) and the efficiency of its vegetation-herbivore feedback loop (intrinsic variation). Feral pig populations in the rangelands appear to o ccupy a position towards the extrinsic extreme of this continuum. where the ir dynamics are more influenced by extrinsic than intrinsic sources of vari ation in pasture availability.