| Summary: | Conservation managers have long considered parasites and pathogens to constitute promising tools for the control of invasive species with the potential to generate tremendous ecological and economic benefits. However, outside a very few high-profile systems, little is known about the utility of this approach for applied conservation. We review here three well-characterized case studies of vertebrate biocontrol (rabbits in Australia and New Zealand, cats on Marion Island, and European carp in Australia), and discuss some of the conservation insights that have emerged from these systems. At a minimum, managers should be able to demonstrate that a proposed agent is: (i) safe in terms of its effects on nontarget species; and (ii) effective in regard to impacts on survival or reproduction of the focal species. Actual effects on the targeted population will likely depend on a complex interplay between environmental conditions, natural selection, and immunity, and need to be investigated carefully before any releases. Furthermore, the presence of other pathogens already circulating in the target population has the potential to either amplify or diminish impacts on the focal population. Biocontrol agents are best used to rapidly reduce target species populations rather than achieve complete host extinction, unless they are released into a small, well-circumscribed area. For a biocontrol agent to have a strong and long-term effect, it typically needs to be followed up by other control measures that capitalize on the population reduction brought on by the pathogen. In summary, pathogen biocontrol efforts are most successful if they are preceded by careful planning and if ongoing evaluation and adaptive management techniques are applied.
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