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International and national initiatives aim to conserve at least 30% of lands and waters by 2030. To safeguard biodiversity, conservation actions must be distributed in places that represent ecosystem and species diversity. Various methods of prioritizing sites for conservation have been used in local and global assessments. However, the performance and consequences of alternative methods are usually unknown. Such comparisons are needed to confidently implement national and international conservation initiatives. Here, we compared four widely-used methods of prioritizing sites in the contiguous United States for conserving species of mammals, birds, amphibians, and reptiles. Specifically, we calculated and mapped species richness, rarity-weighted richness, and two complementarity-based prioritizations (additive benefit function [ABF] and core area zonation [CAZ] in the software Zonation). We compared maps derived from these alternatives with respect to spatial locations and overlap, patch size distributions of the top-30% priorities, and existing ownership and protected-area status. We used species-accumulation curves across ranked priorities to evaluate performance of methods and compared results at 30% total area. Mapped locations and patch sizes of the highest priorities varied by taxonomic class and method of prioritization. Complementarity-based methods (ABF and CAZ) more efficiently represented species than methods based on richness or rarity-weighted richness, especially for taxa with higher beta diversity (amphibians). ABF and CAZ methods also resulted in greater conservation opportunity for the top 30% of priorities compared to maps of richness. Area-based conservation targets, such as the “30 by 30” initiative, must distribute limited resources in ways that safeguard all species. Our results show that spatial locations and configuration, performance, and conservation opportunity vary among prioritization methods and taxonomic classes.

Although they are the foundations of most efforts to conserve biodiversity, protected areas in the United States have, historically, not always been located in the most important areas to accomplish this goal. We investigated the overlap between suitable habitat for wildlife species of conservation concern and the location of public and private highly protected areas to assess the degree to which current highly protected areas are providing fine-scale habitat for wildlife species of conservation concern, to explore the relationship between the size of total suitable habitat of a species and its vulnerability to extinction, and to identify the species that are poorly represented in highly protected areas and determine where their habitats overlap—i.e., where future protected areas may best be located. We found that nearly one third of terrestrial wildlife species in the contiguous U.S. are vulnerable to extinction and that even though there is a relationship between total suitable habitat area and vulnerability, we find that synthetic indices of endemism and rarity-weighted richness are not necessarily good indicators of whether a species will be of conservation concern. Of all 537 wildlife species of conservation concern, only 62 (11%) are well represented in highly protected areas. To increase representation of habitats of wildlife species of conservation concern, scientists and managers should look to preserve lands where there is the greatest overlap among habitats for species of concern that are currently poorly represented in the reserve system, with special focus on species with small ranges on public lands.

Protected areas, such as wilderness, form the foundation of most strategies to conserve biological diversity. However, the success of protected areas in achieving conservation goals depends partly on how well ecological diversity is represented in a network of designated lands. We examined how well the world’s largest highly-protected conservation network—the U.S. National Wilderness Preservation System (NWPS)—currently represents ecological systems found on federal lands in the contiguous United States and how ecological system representation has accumulated over the 50-year tenure of the Wilderness Act (passed in 1964 and giving the U.S. Congress authority to establish wilderness areas). Although the total area of NWPS has risen fairly steadily since 1964, the diversity of ecological systems accumulated in wilderness areas (436 ecological systems) reached an asymptote 30 years ago that is well below the total pool of ecological systems available (553) on federal lands. Thus, NWPS currently under-represents ecological system diversity. Additionally, only 113 ecological systems are represented at more than 20% of federal land area. As the designation of new wilderness areas becomes more difficult, it is important to increase the ecological representation of those areas to achieve greater protection of biological diversity. Over the next 50 years of the Wilderness Act, federal land-management agencies and the U.S. Congress could increase the ecological diversity of wilderness areas by prioritizing under-represented ecological systems in new wilderness legislation.