Abstract

We assess progress toward the protection of 50% of the terrestrial biosphere to address the species-extinction crisis and conserve a global ecological heritage for future generations. Using a map of Earth's 846 terrestrial ecoregions, we show that 98 ecoregions (12%) exceed Half Protected; 313 ecoregions (37%) fall short of Half Protected but have sufficient unaltered habitat remaining to reach the target; and 207 ecoregions (24%) are in peril, where an average of only 4% of natural habitat remains. We propose a Global Deal for Nature—a companion to the Paris Climate Deal—to promote increased habitat protection and restoration, national- and ecoregion-scale conservation strategies, and the empowerment of indigenous peoples to protect their sovereign lands. The goal of such an accord would be to protect half the terrestrial realm by 2050 to halt the extinction crisis while sustaining human livelihoods.

Evaluating protected area networks using ecoregions

The 2001 map of the terrestrial ecoregions of the world (Olson et al.2001) facilitated the design of representative networks of protected areas. It has also been used to depict species distributions, to model the ecological impacts of ­climate change, to develop landscape-scale conservation plans, and to report on progress toward international targets. The revised map, named Ecoregions2017^©Resolve, that is the basis for this scheme is unchanged for large sections of the seven biogeographical realms but differs from the original map in four regions: the Arabian Peninsula, some of the desert and drier ecoregions of the African continent, Antarctica, and the southeastern United States (figure1). Further details and justification for changes are presented in supplemental appendix S1.

Calculating the extent of protection by ecoregion and biome provides a scorecard to measure progress toward Half Protected (table1, figure2). Summing across all 14 biomes and their constituent 846 ecoregions, 98 ecoregions (12%) have already achieved Half Protected. The largest category is Nature Could Reach Half, with 313 ecoregions (37%), followed by the 228 ecoregions classified as Nature Could Recover (27%). Half Protected remains a reasonable goal in these regions. Within Nature Could Reach Half, 119 (38%) ecoregions have greater than 20% of their land area protected; the remaining 194 ecoregions (62%) have limited coverage of protected areas but retain considerable intact natural habitat. To achieve Half Protected, these 313 regions require only an expansion of their protected area network. The remaining 207 ecoregions (24%) classified as Nature Imperiled have little natural habitat and will require intensive efforts to achieve Half Protected or even to conserve the fraction that remains.

Analyses conducted at a global scale inevitably involve error. Here, we were unable to differentiate “paper parks”—designated protected areas that remain unprotected because of lack of enforcement—from those that are well managed. Protected areas subjected to severe bushmeat-hunting pressures or overgrazing by domestic livestock are also ignored at this scale, although these are major threats. There are also protected areas where activities (e.g., industrial extraction) have been expressly allowed by governments even though these activities are plainly inconsistent with conservation objectives. We elucidate the major sources of error, including the assessments of tree-cover change and land-cover classes, in supplemental appendix S2.

Forested ecoregions and biomes. The 476 forested ecoregions are distributed unevenly among each of the four categories of protection: 40 (8%) achieve Half Protected; 198 and 130 fall into Nature Could Have Half and Nature Could Recover categories, respectively; and 108 are classified as Nature Imperiled.

The tropical and subtropical moist broadleaf forests biome has more species and ecoregions than any other on Earth. Covering only 14% of the Earth's land area, this biome supports at least 50% of the world's species (table1), many of which have likely yet to be discovered (Mora et al.2011). Fortunately, over half (61%; 140) of the ecoregions within this species-rich biome (n = 230) fall into the Half Protected or Nature Could Reach Half category: 24 (10%) ecoregions have achieved Half Protected (table1, ­supplemental appendix S2), and 116 (50%) have achieved Nature Could Reach Half (many of which already exceed Aichi target 11). Of the best-protected ecoregions, the majority (15) occur in the Neotropics, followed by the Indomalayan realm (11; figure2).

In contrast to the moist forests, the tropical and subtropical dry broadleaf forest is the most endangered biome on Earth; only 2 ecoregions (among 56) are Half Protected, 20 are Nature Could Recover, and 26 are Nature Imperiled. The temperate broadleaf and mixed forests biome has the second largest number of ecoregions (83) but shows a distribution of protection categories skewed toward those needing restoration: Nature Could Recover and Nature Imperiled. The boreal forest ecoregions are among the largest and have the greatest potential to reach Half Protected because of their vast remaining intact forest blocks.

The majority of mangrove ecoregions fall into the categories of Half Protected or Nature Could Reach Half. The remaining mangrove ecoregions are degraded but can recover through restoration (table1, supplemental appendix S2).

The Nature Imperiled category includes 108 (23%) ­forest ecoregions (n = 476; table1; supplemental appendix S2, supplemental table S1a, S1b). Assessing recent trends in tree cover, of the 16 forest ecoregions with the greatest extent of tree loss between 2000 and 2014 (ranging from 20% to 86%), 9 are in the Afrotropics, and 4 are in the Indo-Malayan realm of India. Deforestation was greatest in the Nigerian lowland forests and the Cross-Niger transition forests.

Nonforested ecoregions and biomes. The protected area ­network is far less extensive in nonforested biomes. The tundra biome is best protected among the seven nonforested biomes: 26 of the 51 tundra ecoregions (51%) fall under Half Protected, and another 24 ecoregions (47%) are in Nature Could Reach Half. Desert and xeric shrubland ecoregions also have expansive networks of protected areas and large swaths of natural habitat remaining: over half fall into Half Protected or Nature Could Reach Half (figure2). Ecoregions in the remaining nonforested biomes have been more heavily degraded: 99 (27%) nonforested ecoregions were categorized as Nature Imperiled.

Human impact and revisiting the most endangered biomes on Earth. Land-use change as a result of human activities is a dominant feature in the large majority of ecoregions, as has also been shown by Venter and colleagues (2016). In the 207 Nature Imperiled ecoregions, an average of 96% of natural habitat has been converted to an anthropogenic land use. Many of the fragments in these ecoregions are of disproportionately high biodiversity value. Here, protecting Key Biodiversity Areas (KBAs) will be ­crucial, and the goal of NNH remains aspirational and of secondary concern to protecting what remains (Eken et al.2004).

Forested and nonforested biomes are evenly represented in the Nature Imperiled category (table1). Hoekstra and colleagues (2005) described the temperate grasslands, savannas, and shrublands biome as the most endangered in the world. However, our results show that the most critically endangered biome—as is determined by the proportion of Nature Imperiled ecoregions that constitute each—is the tropical dry forests, whereas two nonforested biomes are nearly as endangered: (1) tropical and subtropical grasslands, savannas, and shrublands and (2) Mediterranean forests, ­woodlands, and scrub.

Without considering fine-scale endemism and beta-diversity (turnover of species with distance or along gradients), simple metrics of habitat loss and percent protection may underestimate the conservation crisis among biomes. Biodiversity loss would therefore be much greater and more sensitive to habitat conversion in tropical and subtropical grasslands, savannas, and shrublands; in Mediterranean forests, woodlands, and scrub; and in tropical moist and tropical dry forests. These four biomes support higher endemism and greater beta-diversity levels than those found in other biomes.

Beyond Aichi targets: Toward Half Protected

The need to go beyond Aichi protection targets was approved by delegates at the 2014 IUCN World Parks Congress. They further decided that the total area of protected areas and connectivity lands needs to be far higher than current conceptions and agreed on the importance of setting ambitious targets (IUCN 2014). Results from our global assessment suggest that the ambitious target of protecting half of terrestrial nature is attainable for many of the Earth's more intact ecoregions. Among the 846 ecoregions, 98 (12%) occupy the Half Protected category. Although these ecoregions are largely concentrated in two biomes—tropical and subtropical moist forest and tundra—there is at least one ecoregion achieving this status in 12 of the 14 biomes. Within Nature Could Reach Half (n = 313), 26 ecoregions (8%) are at least 40% protected and therefore require modest additional protection to reach Half Protected in each. These and the other 287 ecoregions constituting the Nature Could Reach Half category provide the greatest conservation opportunity, because adequate habitat remains to reach Half Protected. These ecoregions are found within every biome and should rank high in the formulation of the next Aichi target 11 post-2020.

Because Aichi target 11 requires protected area networks to be ecologically representative, an ecoregion assessment provides an indispensable tool for meeting the new targets to be set in 2020. Greater effort is needed to complete these ecoregion strategies. For example, only 94 of the 846 terrestrial ecoregions (11%) have published plans that address all four biodiversity conservation goals (figure3; see supplemental appendix S3 for methods). Formal conservation strategies that address three-fourths of the biodiversity conservation goals were published for 22% of ecoregions globally. Most of these strategies focus on identifying priority areas for protection and on conserving species of conservation concern (figure3). Notably, a high percentage of ecoregions in the Nature Imperiled category have plans that address all four conservation goals. This is because biodiversity hotspots—biologically rich areas containing less than 30% of the original habitat—are explicitly targeted by Critical Ecosystem Partnership Fund (CEPF) profiles (Myers et al.2000, Olson2010). Of great concern are the 337 ecoregions that lack biodiversity plans (­supplemental appendix S3).

Robust ecoregion strategies must be followed by effective implementation to realize biodiversity conservation goals formulated at a national scale. Three countries advancing to or already surpassing Half Protected—Namibia, Nepal, and Bhutan—are worth singling out as compelling examples of where effective implementation embodies key principles of biodiversity conservation. They also refute some of the criticisms raised over the NNH approach that (a) it could displace rather than empower indigenous communities, (b) it is a paradigm only suitable for wealthy countries, and (c) it can only succeed in sparsely populated, remote ecoregions.

Namibia's conservation strategy includes conservation areas managed by local communities alongside government-run strict nature reserves across all its ecoregions. These communities are awarded autonomy to manage vast tracts of land for wildlife conservation and income generation, in large part by allowing communities to own the wildlife. Now widely touted as a success story in global conservation, these lands were largely defaunated through poaching only 25 years ago. Community-managed lands, calledcommunal conservancies, now contribute to Namibia's national protected area network, which covers 47% of the country. Communal conservancies range in size from 43 square kilometers (km²) to 9120 km² (the mean being 1953 km²). In fact, many conservancies function as vital corridors connecting other protected areas and allowing dispersal, movement, and range recovery of large mammals, including elephants, lions, and others that are in steep decline elsewhere in sub-Saharan Africa (figure 4a; Naidoo et al.2016).

In Nepal, ecoregion conservation strategies that involve local communities are the rule and complement the country's strictly protected areas. In the lowlands and midlands, community forestry and agroforestry in designated landscapes yield economic returns while strategically extending habitat and connectivity among reserves (figure4b, table2; Wikramanayake et al.2010). Community-managed forest parcels are small—some are as little as 20 hectares in size—but abundant and interspersed among larger protected areas, often facilitating population recovery of endangered large mammals (Wikramanayake et al.2010). Community forests, linked together to form corridors, play a pivotal role in landscape conservation. Handing over forest management to communities, which then receive 50% of the revenue generated by wildlife parks in designated buffer zones, led to a 61% increase in tigers and a 31% increase in rhinos over a 5-year period (2008–2013). No rhinos, tigers, or elephants have been poached in Nepal in several years (Dhakal et al.2014).

In the Himalayan and trans-Himalayan ecoregions overlapping Nepal, conservation areas managed by local communities exceed in area the land under national-park status and some, such as the Annapurna Conservation Area, return large sums of tourism-generated revenues annually to local funds. These are sparsely populated ecoregions. In contrast, the protected areas and community forests of the Terai-Duar savannas ecoregion in Nepal are intermingled with some of the highest rural population densities on Earth. In this densely settled, productive ecoregion situated on alluvial soils, there is room for intensive rice production and park protection (Dinerstein et al.1999), the latter of which returns more than $1 million annually to local development funds in demarcated buffer zones.

Bhutan protects 51% of its land through national parks and corridors connecting reserves (figure4c, table2). In a novel policy framework, Bhutan's constitution requires that at least 60% of the country remains forested (currently, forest cover is estimated at 72%). Mid-elevation temperate broadleaf forests, which are so heavily converted elsewhere, are particularly well protected. Bhutan, as with Nepal, ranks among the nations with the lowestper capita GDP but protects enough habitat to conserve biodiversity (Dinerstein2013).

All three examples stress core protected areas, buffer zones, and connectivity—all key components of ecoregion conservation strategies and securing biodiversity. The first two examples illustrate how extensive areas can be put under conservation management by engaging local communities. The example of Bhutan offers a different mechanism through constitutional decree. Both approaches work.

Strengths and weaknesses of the Nature Needs Half approach to conserving half the terrestrial realm

NNH, like any paradigm, has strengths and weaknesses. NNH offers a simple, inspirational, and science-based message that can be easily understood by the general public. It also provides the conservation movement with a unifying goal. Incremental gains in global protection targets have proved insufficient in response to the magnitude of the biodiversity crisis. Conservation efforts have often been mired in process or targets that do not track onto an ultimate conservation goal or vision statement (Wilson2016). NNH provides an endgame: Achieving Half Protected will help realize the outcomes and objectives of maintaining a living biosphere, avoiding mass extinction, and preserving ecological processes that benefit all human societies. NNH also provides a goal and a planning framework under which all conservation efforts can fit.

Importantly, 50% avoids setting targets too low and being surpassed by the synergistic effect of threats to nature from climate change and mass extinction. The recent Paris Agreement under the United Nations Framework Convention on Climate Change provides targets for stabilizing atmospheric greenhouse gas concentrations at a level that prevents “dangerous anthropogenic interference with the climate system.” We contend that for the climate deal to succeed, we need a Global Deal for Nature (box1). NNH provides a baseline from which we can monitor progress as the environmental data sets are increasingly dynamic, annually updated, and freely available and serve as a scorecard to underpin a Global Deal for Nature and assist the CBD in measuring progress. Finally, NNH could help provide government, lenders, citizens, and industry guidance about where to site extractive industries and develop large infrastructure projects.

Providing clear implementation guidelines can help address weaknesses associated with NNH. For example, insisting that NNH be empirically derived for each of the world's ecoregions is important. However, in trying to erect a simple, science-based target that nonscientists can understand—50% protected by 2050—the approach runs the risk of giving the misimpression that 50% is the “right” target for each ecoregion. In fact, the amount of habitat that needs to be conserved in each region will vary. This guideline will help avoid pitfalls, such as a case in which governments could assign large areas to be protected just to reach the 50% target (e.g., high-elevation rock and ice, barren desert, contaminated areas, unproductive soils, or lands of low economic value) without consideration of the design, through ecoregion strategies, of representative networks to capture unique patterns of biodiversity. One clear guideline is that site selection is as important as total area protected in achieving conservation objectives (Margules and Pressey2000). Tools such as ecoregion conservation planning, CEPF hotspot profiles, Key Biodiversity Areas, and systematic conservation planning that focus on the quality or irreplaceability of areas considered for protection will be most useful to avoid this danger (Margules and Pressey2000, Myers et al.2000, Eken et al.2004, IUCN 2016).

A potential pitfall is that policymakers not well versed in ecosystem function might view NNH as license to clear the other 50%. This would be a disaster in some ecoregions, such as those in the Amazon and Congo Basins, that perform vital ecological roles only if contiguous forest cover is maintained. Conservation planning will need to underpin the implementation of NNH to avoid these abuses.

Another concern is that the NNH approach risks overlooking, however unintentionally, those 207 ecoregions determined by our analysis to average only 4% of remaining natural habitat outside protected areas that fall into the Nature Imperiled category. Where ecoregions contain global centers of endemism but with only fragments of natural habitat remaining replete with irreplaceable sites, a concern is that the global importance of these sites of rarity could be downplayed. Donors and agencies might concentrate on those less biodiverse ecoregions but those likely to come closer to achieving the 50% target. In most of these ecoregions, Key Biodiversity Areas, if properly conserved will protect the biodiversity that remains (Eken et al.2004). CEPF profiles should include all possible options for restoration (Butchart et al.2015).

A possible concern expressed by critics of Wilson (2016) and of the NNH approach is that protecting half the terrestrial realm adversely affects humans in remote regions (e.g., Büscher et al.2016). In contrast, implementing NNH is an opportunity to empower indigenous peoples and local communities. Many indigenous reserves in Latin America, Asia, Africa, and Australasia are an essential part of the formal protection network, but the decisionmaking is in the hands of those within the reserves. Several indigenous communities are also advocating for half their lands to be protected. The Dehcho Dene in northern Canada, for example, has articulated an explicit 50% protected goal for their own territory (Norwegian2005). For many groups, such as the Dehcho Dene, protecting half is an approach derived from their traditional ecological knowledge. Conservation should be achieved through careful planning while respecting rights, improving livelihoods, and sharing decisionmaking.

Achieving Half Protected hinges on a reduction of human disturbance, sparing nature

Fortunately, two schools of thinking—how to save half for nature and how to feed and fuel advancing societies— are in growing concordance. As societies urbanize and develop, there is a well-documented trend toward “decoupling”: an increasingly efficient use of land and resources that reduces environmental degradation (Ausubel2000, Fischer-Kowalski and Swilling2011, Tilman et al.2011, Ausubel et al.2012). These trends have already produced major recoveries of woodland and other vegetation in many regions (Ellis et al.2013, Blomqvist et al.2015). The prospects for feeding growing human populations while recovering natural habitat are not only aspirational but also achievable as long as these aspirations are put to work guiding land-use policy and commodity-chain interventions (box1; Lambin et al.2014).

The global phenomenon of growing urbanization, accentuated in some ecoregions, sets the stage for reaching Half Protected. In remote areas in many parts of the world, depopulation due to socioeconomic changes such as increasing wages and career opportunities have resulted in rural populations moving to population centers; by 2050, 70% of people will live in cities. This phenomenon, driven by economics, could lead to expansion of the protected area network and restoration of disturbed or abandoned lands (Ellis et al.2013).

Nature Needs Half is an ambitious goal that will allow humanity to maintain a world with space for all life and the continuance of critical ecosystem services. Our findings show that a large number of ecoregions are Half Protected and that NNH is achievable in the vast majority of remaining ecoregions. However, achieving NNH requires further research into the desirability, feasibility, and progress toward the goal at ecoregional and national scales. Here, we provide tools and information to chart progress toward NNH and call on advocates and leaders around the world to set new global protected area targets: 50% of the terrestrial realm by 2050. Doing so through carefully balanced ecoregion plans that promote economic development while sustaining nature will also make the planet more livable for humanity (Mulligan2014, 2015).

Supplemental material

Supplementary data are available atBIOSCI online.

Acknowledgments

We dedicate this article to the inspirational efforts of Edward O. Wilson and his lifelong efforts to save biodiversity. We thank Linus Blomqvist, Julia John, Naomi Kingston, James McNamara, Robin Naidoo, Stuart Pimm, Taylor Robb-McCord, John Robinson, and Chris Stadler for their comments on the manuscript. We acknowledge Tom Allnutt, Nicola Bergh, Phil Clarke, Jon Fisher, Jane Glavan, Glen Griffith, Stephen Holness, Larry Kopold, Peter Stranger, Abdul Wali Al Khulaidi, James Omernik, Jon Oetting, Kate Parr, William J Platt, Amanda Recino, Mathieu Rouget, Michael Rutherford, Leonardo Sotomayor, Aleks Terauds, Joseph W Veldman, and Crammy Wanyama for their review of ecoregion maps and mapping. Abu Dhabi Global Environmental Data Initiative and Saudi Wildlife Authority provided ­biogeographic data.

References cited

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