- 1Factors that have been considered when deciding how to invest resources in conservation of species include the efficacy and cost of management, the importance of the species, the level of threat, and the timeframe over which results are to be achieved. However, it is unclear how each of these different factors should be weighted and combined when making a decision.
- 2We examine how the probabilities of species changing in IUCN Red List categories are influenced by expenditure of resources. We use these relationships to determine optimal investment strategies, using Australian birds as a case study.
- 3The optimal level of investment in different species depends critically on whether managers wish to minimize the number of extinct species or a weighted average of all threatened species, and on the available budget. The level of investment should not necessarily reflect the level of threat. In our case study, the timeframe of management had little influence on the investment decision.
- 4Our results show that extinctions of Australian birds can be largely avoided over the next 80 years given current expenditure, but greater investment in conservation is required to reduce the number of threatened species.
- 5Synthesis and applications. The most efficient allocation of resources to conserve species is difficult to determine intuitively; therefore, this decision demands the use of formal decision theory. The influence of the particular management objective on the optimal decision means that this feature needs careful consideration. Our approach can be used to determine the level of investment that is required to reduce the number of threatened species.