The role of privately protected areas in achieving biodiversity representation within a national protected area network

As the world nears the target of protecting 17% of terrestrial environments, there remain significant gaps in achieving the ecologically representative system of protected areas (PAs) called for under the Convention on Biological Diversity. There is increasing recognition of the contribution that privately protected areas (PPAs) can make to protecting biodiversity, offering the potential to increase existing levels of protection for species and ecosystems, but also to protect biodiversity that currently lies outside existing PA networks. Despite the potential importance of PPAs for achieving an ecologically representative PA network, the complementarity between public and private PAs has received little attention. Our study provides a detailed assessment of the role PPAs play in protecting biodiversity in a megadiverse country. Using the Australian PA network as an example, we provide a comprehensive assessment of the contribution PPAs make to the representation and level of protection of species, ecosystems, and key biodiversity areas. We found that while making up only 1.3% of Australia's land area, PPAs protect a wide range of biodiversity, contributing to the protection of 85% of vertebrate species, 80% of threatened ecological communities, 95% of major vegetation types, and 40% of key biodiversity areas. However, our findings reveal that PPAs largely protect biodiversity already within the PA network, and because of their small size, they currently make only minor contributions to the adequacy of protection for biodiversity. Nevertheless, we show that conservation on private land will be critical to strengthening the PA network, with between 70 and 90% of currently unprotected or poorly protected biodiversity occurring largely on private land. Analyses such as ours, which examine the importance of both public and private PAs to achieve conservation targets, are essential to ensure effective planning of PA networks into the future.