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.