How to protect half of Earth to ensure it protects sufficient biodiversity

Abstract

It is theoretically possible to protect large fractions of species in relatively small regions. For plants, 85% of species occur entirely within just over a third of the Earth's land surface, carefully optimized to maximize the species captured. Well-known vertebrate taxa show similar patterns. Protecting half of Earth might not be necessary, but would it be sufficient given the current trends of protection? The predilection of national governments is to protect areas that are "wild," that is, typically remote, cold, or arid. Unfortunately, those areas often hold relatively few species. Wild places likely afford the easier opportunities for the future expansion of protected areas, with the expansion into human-dominated landscapes the greater challenge. We identify regions that are not currently protected, but that are wild, and consider which of them hold substantial numbers of especially small-ranged vertebrate species. We assess how successful the strategy of protecting the wilder half of Earth might be in conserving biodiversity. It is far from sufficient. (Protecting large wild places for reasons other than biodiversity protection, such as carbon sequestration and other ecosystem services, might still have importance.) Unexpectedly, we also show that, despite the bias in establishing large protected areas in wild places to date, numerous small protected areas are in biodiverse places. They at least partially protect significant fractions of especially small-ranged species. So, while a preoccupation with protecting large areas for the sake of getting half of Earth might achieve little for biodiversity, there is more progress in protecting high-biodiversity areas than currently appreciated. Continuing to prioritize the right parts of Earth, not just the total area protected, is what matters for biodiversity.

Fig. 1. Protected areas (green) plus the areas having the lowest human footprint index (≤3.3), which we call “wilderness” (buff), up to a combined extent that is as close to half of the Earth’s land surface as we could make it (51.9%) given the discrete nature of the index.

In the Venn diagram, protected areas are composed mostly of wilderness (65%) but also include some more heavily affected areas (35%).

Fig. 2. Distribution of geographical range sizes of mammals, the expected areas within the 13.3% of the land surface (mid-left), and, in Half Earth, the 51.9% of the land surface.

(Right) Actual fractions of the species within these two classes. We color-code the actual areas by the species’ original range size. So, for example, we expect only 36 species to have <1 km² of their ranges in protected areas (gray bars, mid-left), yet 319 species do so. Most of these have geographical ranges of <10,000 km² (colored yellow to red). Some have large ranges (colored green or blue). The bins are such that the value shown for (say) 10 km² encompasses ranges from that value (log₁₀ = 1) up to 31.62 km² (log₁₀ = 1.5).

Fig. 3. The average percentages of ranges protected (blue) and in protected plus wild areas (green) as a function of the original range size for mammals, birds, and amphibians.

The horizontal lines show the expected values of 13.3% for protected areas and 51.9% for protected plus wild areas. (We do not display the confidence intervals on these fractions because they are very broad, with some species completely protected and others having no protection at all—as Fig. 2 shows.)

Fig. 4. Map of small-ranged mammal species and unprotected small-ranged mammal species.

Numbers of terrestrial mammal species with less than the median range size (A) and those that are in unprotected wilderness (B). Right: Details for parts of South America and Southeast Asia. Figures for birds and amphibians are shown in the Supplementary Materials.