Reducing Wildlife Damage with Cost-Effective Management Programmes.

Limiting the impact of wildlife damage in a cost effective manner requires an understanding of how control inputs change the occurrence of damage through their effect on animal density. Despite this, there are few studies linking wildlife management (control), with changes in animal abundance and prevailing levels of wildlife damage. We use the impact and management of wild pigs as a case study to demonstrate this linkage. Ground disturbance by wild pigs has become a conservation issue of global concern because of its potential effects on successional changes in vegetation structure and composition, habitat for other species, and functional soil properties. In this study, we used a 3-year pig control programme (ground hunting) undertaken in a temperate rainforest area of northern New Zealand to evaluate effects on pig abundance, and patterns and rates of ground disturbance and ground disturbance recovery and the cost effectiveness of differing control strategies. Control reduced pig densities by over a third of the estimated carrying capacity, but more than halved average prevailing ground disturbance. Rates of new ground disturbance accelerated with increasing pig density, while rates of ground disturbance recovery were not related to prevailing pig density. Stochastic simulation models based on the measured relationships between control, pig density and rate of ground disturbance and recovery indicated that control could reduce ground disturbance substantially. However, the rate at which prevailing ground disturbance was reduced diminished rapidly as more intense, and hence expensive, pig control regimes were simulated. The model produced in this study provides a framework that links conservation of indigenous ecological communities to control inputs through the reduction of wildlife damage and suggests that managers should consider carefully the marginal cost of higher investment in wildlife damage control, relative to its marginal conservation return.

Bioeconomic modeling in conservation pest management: effect of stoat control on extinction risk of an indigenous New Zealand passerine, Mohua ochrocephala.

Pest control is a key activity undertaken to conserve threatened and declining species. Although bioeconomic analysis has been used to contrast the relative efficiency of control strategies where pests affect economic resources, the same approaches have been adopted rarely in conservation settings. The Mohua (Mohoua ochrocephala) is an insectivorous passerine indigenous to beech (Nothofagus spp.) forests in New Zealand's south island. Mohua have undergone a 75% range contraction since stoats (Mustela erminea) (which prey on nests and nesting females) were introduced to the south island in the late 1800s. Mohua nests are particularly vulnerable when stoat abundance increases in response to eruptions in the density of introduced house mice (Mus musculus), which in turn respond to semiperiodic (4-6year) mass seeding (masting) of beech trees. Controlling stoats only when their abundance poses a threat to Mohua fledging success would theoretically maximize the efficiency with which Mohua are protected. To better synchronize stoat control with periods of high stoat density, control could be initiated according to (1) time since last control, (2) beech seedfall, (3) mouse abundance, or (4) stoat abundance. Monitoring the three environmental cues, however, incurs costs that should be taken into account when considering their relative efficiency I derived an empirically based stochastic model that links sequential change in beech seedfall, mouse, and stoat abundance to a simple demographic model for Mohua. I used the model to contrast the relative cost-efficiency of achieving conservation outcomes for Mohua (specified in a quasi-extinction framework), initiating stoat control according to these cues. The use of environmental cues reduced the frequency with which stoat control had to be undertaken to achieve Mohua conservation outcomes by more than 50%. For some cues, however the costs of monitoring outweighed the savings that could be achieved through reduced frequency of stoat control. The interplay between costs of monitoring environmental cues and the frequency with which stoat control had to be undertaken meant that the most efficient strategy was dependent on the conservation outcome specified for Mohua. My results demonstrate the utility of bioeconomic analysis in formulation of pest management strategies to achieve conservation outcomes.

The numerical response: rate of increase and food limitation in herbivores and predators.

Two types of numerical response function have evolved since Solomon first introduced the term to generalize features of Lotka-Volterra predator-prey models: (i) the demographic numerical response, which links change in consumer demographic rates to food availability; and (ii) the isocline numerical response, which links consumer abundance per se to food availability. These numerical responses are interchangeable because both recognize negative feedback loops between consumer and food abundance resulting in population regulation. We review how demographic and isocline numerical responses have been used to enhance our understanding of population regulation of kangaroos and possums, and argue that their utility may be increased by explicitly accounting for non-equilibrium dynamics (due to environmental variability and/or biological interactions) and the existence of multiple limiting factors. Interferential numerical response functions may help bridge three major historical dichotomies in population ecology (equilibrium versus non-equilibrium dynamics, extrinsic versus intrinsic regulation and demographic versus isocline numerical responses).