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Pests are often viewed as having consistently strong and negative effects on biodiversity values and agricultural productivity, especially if they have been introduced from elsewhere. However, pests do not necessarily have just negative impacts, and there is evidence that in some situations their effects can be beneficial. The positive effects of pests arise when they become deeply embedded in ecological communities and are involved in webs of direct and indirect interactions with other species. In this paper, I first outline the concepts of direct and indirect interactions, and then describe two case studies that illustrate how these interactions develop between pests and native animals. In the first case study, house mice (Mus domesticus) introduced to Boullanger Island in Western Australia have direct effects on a small dasyurid marsupial (Sminthopsis griseoventer boullangerensis), but also exert indirect negative and indirect positive effects on four species of insular skinks. In the second case study, high levels of activity of domestic house cats (Felis catus) in suburban bushland in Sydney are associated with reduced richness of bird species. However, high cat activity also appears to depress the rate of egg predation in above-ground bird nests, apparently by suppressing the activities of small egg predators such as rats. In these and other examples, control of the putative pest would have unintended negative consequences on some native species, making decisions about management more difficult. I propose a preliminary framework to predict the likelihood of indirect interactions occurring at different times after a pest has been introduced, at different places, and at different pest densities, and use this to suggest options for management of the pest species.

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