Making endangered species safe: the case of the kokako of North Island, New Zealand

The theory of population regulation predicts that threatened species are safest at high population numbers, partly because density-dependent compensatory mechanisms counteract unpredictable disturbances. We illustrate this principle using data from the endemic kokako (Callaeas cinerea wilsoni) populations in the North Island, New Zealand. First we calculate the fledging rate per female (production) necessary to stabilise the population and thereby the residual numbers of nest predators, namely ship rats and possums, which have to be achieved to reach this production. Both predator species must be reduced to low densities to exceed this threshold production. At these low predator densities kokako numbers increase rapidly, and we predict that the population will at some stage experience density-dependent negative feedback producing a declining rate of increase. We review evidence for such feedback at Mapara. More research is required to verify and understand these density-dependent causes of loss at high density, particularly the role of territoriality and intra-specific and inter-specific competition, and to generalise across species. In addition to the increased resilience of a threatened species when at high numbers. the degree of predator removal can be relaxed. Consequently, the cost of management will also decline.