REPORT How large do reserve networks need to be? Ana S. L. Rodrigues and Kevin J. Gaston Biodiversity and Macroecology Group, Department of Animal & Plant Sciences, University of Shef®eld, Alfred Denny Building, Western Bank, Shef®eld S10 2TN, U.K. Correspondence: Ana Rodrigues. E-mail: ana.rodrigues@shef®eld.ac.uk Abstract Reserve networks are essential for the long-term persistence of biodiversity. To ful®l this goal, they need not only to represent all species to be conserved but also to be suf®ciently large to ensure species' persistence over time. An extensive literature exists on the required size of individual reserves, but to date there has been little investigation regarding the appropriate size of entire networks. The IUCN's proposal that 10% of each nation be reserved is often presented as a desirable target, but concerns have been raised that this is insuf®cient and is dictated primarily by considerations of feasibility and politics. We found that the minimum percentage of area needed to represent all species within a region increases with the number of targeted species, the size of selection units, and the level of species' endemism. This has important implications for conservation planning. First, no single universal target is appropriate, as ecosystems or nations with higher diversity and/or higher levels of endemism require substantially larger fractions of their areas to be protected. Second, a minimum conservation network suf®cient to capture the diversity of vertebrates is not expected to be effective for biodiversity in general. Third, the 10% target proposed by the IUCN is likely to be wholly insuf®cient, and much larger fractions of area are estimated to be needed, especially in tropical regions. Keywords Biodiversity, complementarity, conservation targets, macroecology, protected areas. Ecology Letters (2001) 4: 602±609 INTRODUCTION Article 8 of the Convention on Biological Diversity (http:// www.biodiv.org/) obliges contracting parties to establish networks of protected areas for conservation. As these areas cannot be expected to protect what they do not contain in the ®rst place, the initial minimum requirement of such networks is that they represent all the species that are to be conserved. IUCN ± The World Conservation Union advocates that at least 10% of the land area of each nation be set aside for this purpose (IUCN 1993). Although achieving this target would require nearly doubling the currently protected land area (Hobbs & Lleras 1995), recent concerns have been raised that even this is woefully insuf®cient and dictated more by considerations of feasibility and politics than of biology (Soule  & Sanjayan 1998). However, to date there has been little investigation of what would be an appropriate target from a biological perspective. Complementarity-based methods provide a way to integrate political and biological considerations in the selection of networks of protected areas. They have been proposed in acknowledgement of the fact that resources available for conservation purposes are limited and should therefore be employed in ef®cient ways that maximize the diversity of biological features bene®ted (Pressey & Nicholls 1989; Pressey et al. 1993). When data on the distribution of all the species within a region are available, this is achieved by selecting areas that complement one another to the fullest possible extent in terms of their species composition. In the most widespread type of analyses applying these methods, minimum sets (i.e. sets of sites with minimum total area) are obtained which represent all of the target species at least once. These minimum networks are unlikely to be suf®cient for ensuring the long-term persistence of the species represented (Rodrigues et al. 2000a,b), but they provide a lower bound to the size of an adequate network for conserving those species. These methods are therefore particularly suitable tools for determining the minimum percentage of a given region that needs to be reserved in order to ensure the representation of its species diversity. In this study, complementary-based methods are used as a tool to explore the issue of how large reserve networks need to be. First, we examine patterns of variation in the sizes of minimum networks for a variety of assemblages. Ecology Letters, (2001) 4: 602±609 Ó2001 Blackwell Science Ltd/CNRS