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.

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ECOLOGY
How to protect half of Earth to ensure it protects
sufficient biodiversity
Stuart L. Pimm1*, Clinton N. Jenkins2, Binbin V. Li3
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 cap-
tured. 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 pro-
tect 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 con-
serving biodiversity. It is far from sufficient. (Protecting large wild places for reasons other than biodiversity pro-
tection, 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 pro-
tected 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.
INTRODUCTION
Human impacts dominate Earth (1), eliminating many species from
large areas of the land (2) and ocean (3) and driving the current rate
of species extinction to 1000 times its natural rate (4). The principal
solution for stemming biodiversity loss is the establishment of pro-
tected areas. On land, they have grown from a few million square
kilometers in the 1960s to >20 million km2 today (5–7)—between
13 and 15% of the Earth’s land surface, depending on whether one
considers ice-covered areas like Antarctica and how one defines
“protected.” The Convention on Biological Diversity’s Aichi Biodi-
versity Targets include the protection of at least 17% of terrestrial
areas, conserving them with ecologically representative and well-
connected systems of protected areas, and halving the rate of current
habitat loss (6). The Global Strategy for Plant Conservation hopes
to protect 60% of plant species (8).
Beyond these near-term targets are aspirational ones such as
Wilson’s “Half Earth” (9) and Dinerstein et al.’s considerations of
what it would take to do that on an ecoregion-by-ecoregion basis (10).
Wilson’s heuristic suggests that protecting half of Earth might protect
85% of its species and be a “safe limit” for human impacts. These
efforts expand on a tradition going back at least to Odum and Odum
(11), the Wildlands Project in the United States (12), and others who
assert that much larger areas must be protected than at present.
Half Earth aspirations are composed of two interrelated goals—
protecting 50% of the surface of the planet and 85% of its species.
Now, there are important reasons to protect large, contiguous areas
without a necessary focus on the numbers of species they contain.
These areas likely retain their natural ecosystem processes and the
services they provide humanity. For example, unconstrained Amazon
deforestation might have catastrophic consequences to climate (13),
somewhat separate from the issue of how many species might be
lost. Some species—large predators are obvious examples—must roam
over large areas for their populations to be viable, and even small-
bodied species may thrive in large areas but suffer in small ones.
Here, we count all species as being equal and consider the fractions
of species that protected areas encompass. We return to the caveats
in this approach at this paper’s end. There, we will also consider why
this species counting approach has practical utility.
We are concerned that simply protecting half of Earth without
strict attention to the specific places protected and the species they
contain will not achieve the larger aspirational goals and indeed
might harm them. To address this concern, we must answer several
questions.
1) Is it theoretically possible to protect large fractions of species
in relatively small regions? At least for plants, the answer is known
to be yes; 85% of species occur entirely within just over a third of the
land surface, if carefully optimized to maximize the species captured
(8). Well-known vertebrate taxa show similar patterns (14, 15).
Simply, protecting half of Earth is not necessary to protect 85% of
species, but would it be sufficient if protected areas were not “care-
fully optimized”?
2) How well has the existing network of protected areas captured
biodiversity, particularly those species with small geographical ranges
that suffer disproportionate threats of extinction?
The predilection of national governments is to protect areas that are
“wild,” that is, typically remote, cold, or arid (16, 17). Unfortunately,
those areas often hold relatively few species. Wild places also afford
the easier opportunities for the future expansion of protected areas.
The expansion of protection into human-dominated landscapes will
surely be the greater challenge. This leads to our final question.
1Nicholas School of the Environment, Duke University, Box 90328, Durham, NC 27708 ,
USA. 2IPÊ–Instituto de Pesquisas Ecológicas, Nazaré Paulista, São Paulo 12960-000,
Brazil. 3Environmental Research Centre, Duke Kunshan University, Kunshan, Jiangsu
215316, China.
*Corresponding author. Email: stuartpimm@me.com
Copyright © 2018
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).

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3) Were we to expand protection to the wilder half of the Earth’s
ice-free land, what fraction of species live in such places and so might
contribute to the Half Earth goals?
Here, we identify what fractions of species ranges are already
within protected areas. Furthermore, we consider what fractions are
in wild regions that are not currently protected. Other things being
equal, these places might afford the best chances for expanding the
global network of protected areas and further increases in the frac-
tions of species protected. Lewis et al. (7) underscore the urgency of
this. They show that extensive degazetting of protected areas over
the past decade has offset much of the newly protected area and led
to only modest increases in the total area protected. Our paper’s key
objective is to assess how successful the strategy of protecting addi-
tional wild places might be in conserving biodiversity.
We will show that expanding protection to the wilder half of
Earth is far from sufficient to protect biodiversity. Unexpectedly, we
will also show that, despite the substantial bias in establishing pro-
tected areas in wild places to date, enough protected areas are in
biodiverse places that they partially protect substantial fractions of
species. So, while a preoccupation with protecting large areas for the
sake of getting half of Earth might achieve relatively little for biodi-
versity, there is more progress in protecting high-biodiversity areas
than currently appreciated. Quality, not just size, matters.
Defining protected areas and wilderness
We start with Wilson’s definition of wilderness—“regions with small
human populations, and particularly indigenous ones.” There are
numerous efforts to define these places, including McCloskey and
Spalding (18) who estimated 37 million km2, Hannah et al. (19)
who estimated 36 million km2, Sanderson et al. (20) who estimated
55 million km2, and Watson et al. (21) who estimated 28 million km2.
These estimates typically exclude Antarctica, Greenland, and other
areas of mostly rock and ice and differ because of their different
assumptions as to what constitutes wilderness.
A first set of problems for wilderness is conceptual. These may
remain insoluble, because there is no indisputable boundary between
human-dominated and wilderness. The answer depends on the species
of interest and the time span over which one poses the question.
For example, one might define wilderness as only those places
with a full complement of a region’s original species. If so, one would
include only 3.4 million km2 of the ~10 million km2 of savannah
Africa that has lions. That would exclude areas with low densities of
people and several national parks (22). Requiring natural densities
of savannah elephants as a criterion for wilderness would exclude
many more national parks (23). Yet, large areas of savannah Africa
have extensive areas with few people or livestock, and they afford
substantial opportunities for conserving biodiversity.
For a second example, consider the difficulty in interpreting whether
fires are the result of natural or human-planned ignitions. The ques-
tion might be unanswerable, although fire data are global, frequent,
and spatially highly resolved (24). Fires have likely been a feature of
Africa’s savannahs perhaps since human origins. Should one exclude
extensively and frequently burned areas of drylands, considering that
a sufficiently low human density is within any practical definition of
wilderness? Indigenous people also burn huge swaths of Australia.
Its initial use coincided with a massive loss of native species following
human colonization 50,000 years ago (25), but how much was caused
by direct burning? The colonists also removed most large mammals in-
cluding herbivores, leading to marked changes in the vegetation (26).
A second set of problems in defining wilderness is that the data
to define it vary substantially in quality and reliability. Forest cover
data are highly resolved spatially (27), but there is substantial dis-
agreement about what constitutes a forest (28). Distinguishing a
naturally sparse forest from one where human actions have thinned
the forest is not always possible. Moreover, remote sensing does not
readily distinguish forests of tree plantations, such as rubber, teak,
Eucalyptus, pines, and oil palm from natural forests. Data sets for
other land covers have similar issues.
Both human population data (29) and livestock data (30) are
much less spatially resolved. They often show obvious differences at
administrative boundaries, often essentially “counties,” that can cover
large areas. This is especially true of livestock data, which assume
constant densities within these areas. Moreover, we are most inter-
ested in areas with low human and livestock densities, because
we might putatively consider these areas to be wilderness. A priori,
we expect data in these sparsely populated areas to be less reli-
able than in higher-density areas. Here, those high densities also
associate with smaller and better-documented administrative
areas.
Recognizing these and many other caveats, a consensus on what
constitutes wilderness is unlikely. We chose to go with the human
footprint index because it is readily accessible and recent, and the
rules that establish it are transparent (31). To select half of Earth, we
started with the areas that are already protected (13.3% of the ice-free
land) and then considered additional areas with the lowest human
footprint to constitute close to 51.9% of the land surface. This was as
close to half as we could get, given the discrete values of the human
footprint index (Fig.1). Hereafter, we call this 51.9% of the land
“protected plus wilderness.”
RESULTS
Figure1 maps the distribution of areas that we deem to be wilder-
ness (human footprint index ≤ 3.3) or protected under various
categories (see definition in Materials and Methods). A Venn dia-
gram summarizes their relationships. The figure juxtaposes what
has broadly been noted before—the distribution of protected areas
is highly nonrandom with respect to different kinds of ecosystems
(16)—with explicit mapping of the areas with the least human foot-
print. Protected areas are predominantly in wilderness—often cold,
dry, or otherwise remote from centers of human population and im-
pact. Approximately 65% of the protected area has a human foot-
print of ≤3.3, and protected areas overall have a median human
footprint of 1.3. We consider all protected areas, regardless of their
footprint index value, as contributing toward Half Earth.
We next consider how many species of birds, mammals, and
amphibians that protected areas and wilderness encompass. These
data consist of 5311 terrestrial mammal species, 10,079 terres-
trial bird species, and 6397 amphibian species. An example is
the red-faced warbler, Abroscopus albogularis. Its total range is
~1.7 million km2, and this includes ~6100 km2, 10,500 km2, 11 km2,
24,000 km2, 200 km2, and 25,800 km2 of the six International Union
for Conservation of Nature (IUCN) protected area categories, re-
spectively, from most (I) to least (VI) restrictive, 9500 km2 with no
IUCN category, plus 26,000 and 18,900 km2 of the two China
nature reserve categories (national and local). Some 65,100 km2 of
the range is in wilderness. The remainder is in areas with higher
human impact.

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What should we expect?
If the protected areas were random with respect to species ranges,
one would expect 13.3% of each species range to fall within them—
the same proportion as the terrestrial Earth as a whole. For the war-
bler, this would be 228,171 km2. Similarly, 51.9% of each species range
should be in protected areas and wilderness. The corresponding ex-
pected value for the warbler is 888,468 km2.
To summarize these data for all species, we calculate the log10 of
the size of each species’ geographical range. We grouped the results
into half log10 bins, labeling each with the floor of the area size interval.
For example, areas between 100,000 km2 (log10 = 5) and 316,228 km2
(log10 = 5.5) are in the figures and tables as in the 100,000 km2 bin.
We added 1 km2 to all areas.
Figure2 summarizes the data for mammals, and tables S1 to S6
contain the tabulated data for mammals, birds, and amphibians. In
Fig.2 (top), the black bars are the number of species against the bin
of geographical range sizes. For all three taxa, the distributions of
their range sizes are broadly lognormal but with a long tail of small
values (4). The modes are range sizes between 100,000 and 316,228 km2
for mammals, 1,000,000 and 3,162,278 km2 for birds, and 3162 and
10,000 km2 for amphibians.
At the mid-left of Fig.2 (and tables S1 and S2, third column), the
gray bars plot the numbers of species that we would expect in these
bins if protected areas encompassed 13.3% of each species range. At
the bottom left, we do likewise for species in protected areas plus
wilderness, where the proportion is 51.9%.
What we observe
In the example of this warbler, the total area of its range in protected
areas, 121,723 km2, is less than one would expect, and the total area
within both protected areas and wilderness, 181,864 km2, is substan-
tially less than the expected. At the right of Fig.2 (in color), we show
the actual distribution of the species in protected areas (mid-right)
and in protected areas and wilderness (bottom right). To facilitate
comparison to the expected distributions, we repeat those distribu-
tions at the right in light gray.
The salient features are the numbers of species that have <1 km2
of their ranges outside of protected areas or protected areas plus
wilderness. So, for example, the mid-right shows, in various colors,
the 319 species (6%) of mammals having <1 km2 of their ranges
within protected areas. This compares to the 36 species expected if
ranges protected were at random, as shown in gray in the mid-left
figure. The comparable numbers for birds are 314 (3%) compared
to 51 expected, and those for amphibians are 1176 (18%) compared
to 437 expected.
Across all three taxa, these fractions of species with essentially no
protection are substantially higher than one would expect if pro-
tected areas were distributed randomly across species distributions
(Fig.2). Nonetheless, most species do have a portion of their ranges in
protected areas. Only a small fraction of bird species lie completely
outside of protected areas. There are much larger fractions of am-
phibian and mammal species (tables S1 to S6).
At the lower right of Fig.2 (and tables S1 and S2, fourth column),
we repeat these calculations for mammal species within protected
plus wilderness. Some 165 species (3%) have <1 km2 of their ranges
within protected plus wild areas, compared to 11 expected. The com-
parable figures for birds are 186 (2%) compared to 20 expected, and
those for amphibians are 814 (13%) compared to 108 expected. Fi-
nally, Fig.2 and tables S1 and S2 (columns 5 to 20) break down the
distributions of ranges within protected areas (top right) and within
protected plus wilderness areas (bottom right) by the original range
size of the species involved.
Figure3 summarizes a subset of these data in an alternative format.
It shows the average fractions of ranges protected (blue circles), and
in protected plus wild areas (green circles), as a function of the species
original range size. For example, on average, about 40% of the ranges
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%).

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of mammal species with ranges <10 km2 are protected, while for am-
phibians, it is about 35% for these small-ranged species, and for birds,
it is between 45 and 80% of the species. Overall, protected areas do
consistently better at protecting species than one would expect, es-
pecially so for species with range sizes of <10,000 km2. The dashed
blue line shows the expected values. In contrast, adding in wild areas
makes only modest contributions to increasing the fractions of species
ranges included, and they perform poorer than expected (shown by
the dashed green line).
What do these results tell us about the goals of Half Earth, spe-
cifically the objective of protecting 85% of species? To answer this
question, we must decide on how well covered a species’ range
should be for us to consider it “sufficiently protected.” Table1
shows the percentages of mammal species captured when consider-
ing two criteria: a minimum amount of area protected within the
species range and a minimum percentage of the range protected.
One could imagine other metrics: There is nothing special about the
specific thresholds used here. Results for birds and amphibians are
in tables S7 and S8.
What Table1 shows is that existing protected areas do not reach
the 85% goal except when using the lowest of thresholds (1% of the
range and 100 km2 or less). For example, if a species must have at
least 1 km2 and 1% of its range protected, we capture 92.3% of
mammal species in existing protected areas. Increasing the require-
ment to 100 km2 and 10% of its range, they capture only 62.2% of
mammals. Even when expanding coverage to wilderness, the num-
bers are not encouraging (10% and 100 km2 or less to capture 85%
of species), considering that half the planet is now included.
Given that current protected areas are still far from the goal of cover-
ing 85% of species, and further expansion to wild areas is an inefficient
strategy, where should the focus be? At www.biodiversitymapping.org,
we map the richness of species of mammals, birds, and amphibians,
plus maps of the halves of those species that have smaller than the
median range size. For convenience, we repeat the latter maps in
Fig.4A and figs. S1 and S2. The distribution of species with small
ranges closely approximates Myers et al.’s definition of biodiversity
hotspots (14, 15) as well as a large fraction of species threatened with
extinction. These concentrations of small-ranged species include not
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).

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only half of all species found entirely within them but also a consid-
erable number of other more wide-ranging species.
Figure4B also shows where these species live in areas that are
outside of protected areas but in wilderness—the putatively easier
targets for strategically expanding protected areas. Few such places
are extensive, but examples are in the western Andes and in insular
tropical Asia and New Guinea. These wild but diverse areas should
certainly be among global priorities for conservation.
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.)
Table 1. Percent of terrestrial mammal species with a given minimum area and percentage of their range covered by protected or protected plus wild
areas. Combinations of criteria, where more than 85% of species are included, are bolded. For species whose original entire ranges are smaller than the
minimum area requirement, we include them once 100% of their range is covered.
Mammals (% of 5311 species) Percent range protected
≥1% ≥5% ≥10% ≥50%
Area protected ≥1 km292.3% 82.6% 63.9% 8.2%
≥100 km288.9% 80.6% 62.2% 7.4%
≥10,000 km261.6% 58.2% 45.3% 2.8%
≥1,000,000 km27.3% 7.3% 6.8% 0.8%
Mammals (% of 5311 species) Percent range protected + wild
≥1% ≥5% ≥10% ≥50%
Area protected + wild ≥1 km296.5% 92.8% 87.6% 31.4%
≥100 km293.0% 90.5% 85.8% 30.6%
≥10,000 km268.7% 67.8% 65.8% 23.9%
≥1,000,000 km215.9% 15.9% 15.9% 9.2%

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DISCUSSION
Caveats and limitations
The data we present are for birds, mammals, and amphibians.
Combined, these species represent about 1% of the 2 million species
that taxonomists have described. In turn, these may be only 10% of
the very uncertain numbers of all species (32). Why is there no anal-
ysis for insects that constitute perhaps 90% of all species? Simply,
there are no suitable data. What few data we have on insect distribu-
tions likely are about relatively widespread species, because tax-
onimists describe these species first (4). The problem with the plant
data is that, while only an estimated 15% more species are likely to
be as yet unknown (33), the distributional data for known species
are still far too coarse to estimate overlaps with protected areas and
wilderness (8).
Might protecting large, wild areas better encompass the great
majority of species for which we do not have geographical data? We
think this is unlikely. The coarse data on plant distributions suggest
that concentrations of endemic species match those for the taxa we
describe here (8). Moreover, models of species discovery rates sug-
gest that these concentrations are where taxonomists will eventually
describe the currently unknown taxa (33).
Some conservation organizations prioritize large-bodied charis-
matic species. For instance, the website of the World Wildlife Fund
lists giant pandas, tigers, elephants, gorillas, polar bears, and rhinos as
its terrestrial species of concern. All have very large geographical ranges.
These six mammals, while entirely deserving of protection, are an even
smaller fraction of the world’s total biodiversity. Certainly, in pro-
tecting the giant panda, Chinese authorities have protected substantial
fractions of their endemic vertebrates (34) and done so in areas rich in
endemic plants (8). There is little compelling reason though that these
six species, or other similar ones, might protect biodiversity more
generally than the ~20,000 species we consider here.
Finally, we recognize that, while protected areas encompass large
fractions of especially small-ranged species, they may not do so ade-
quately. This is a concern that applies a fortiori to wilderness into which
human activities can expand unfettered. (New Guinea is an obvious ex-
ample.) Evaluating adequate protection is beyond the scope of this pa-
per, but a first step is to identify which species have no protection at all.
Conclusions
We have presented several results expected from previous studies and
some new and unexpected ones. We can conclude the following.
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.

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1) Achieving the goal of protecting large fractions of biodiversity
is theoretically possible in relatively small fractions of the Earth’s sur-
face, because small-ranged species are geographically concentrated.
With their protection comes the inclusion of more wide-ranging and
generally much less threatened species.
2) The aspirational target of protecting half of the Earth’s land
surface may be broadly attainable. There are large areas that still have
low human impacts. They are most extensive in the Arctic and bo-
real regions, deserts, and the few remaining large blocks of tropical
moist forest (31).
3) Most current protected area is in these wild places (Fig.1).
The key question is whether it is practical to protect large fractions
of species with area targets such as those of Half Earth (9, 10). Obvi-
ously, it depends on which areas are selected. Protecting large land-
scapes has many benefits, not least for the ecosystem services that
they provide. Our concerns, however, are with the implication that
protecting half of Earth will also protect substantial fractions of species.
In general, only wilderness of tropical moist forests supports locally
high diversity. This begs our question of how much biodiversity will
we protect if the trend to protect wild places continues.
4) Wild places do less than expected by chance at protecting spe-
cies (Fig.3). Moreover, very few wild places house concentrations of
small-ranged species not already protected by existing protected areas
(Fig.4B and figs. S1B and S2B). Simply, achieving conservation goals
by creating more protected areas in current wilderness might locally
be helpful, but it is not sufficient to protect biodiversity at large.
5) On a species-by-species basis, the efficacy of existing protected
areas is nuanced. Existing protected areas completely fail to encom-
pass from 3% (birds) to 18% (amphibians) of species. That birds do
so much better may well be due to deliberate efforts to establish pro-
tected areas that house small-ranged, threatened bird species (for
example, BirdLife’s Important Bird and Biodiversity Areas).
6) In contrast, for the most vulnerable species—those with small
geographical ranges—protected areas are surprisingly inclusive, with
much greater fractions of species’ ranges covered than expected. This
is particularly striking for species with ranges of <10,000 km2. The
explanation for this apparent contradiction is surely that there are
many relatively small protected areas that strongly focus on areas
with concentrations of small-ranged species.
7) For species with larger range sizes—that is, 100,000 km2—the
fractions of species’ ranges currently protected are close to what one
would expect by chance if protected areas were placed randomly (Fi g.3).
Three large, species-rich countries—Brazil (8.5 million km2),
China (9.6 million km2), and the United States (9.6 million km2)—
exemplify the concerns our results prompt. All three have substantial
wild areas and globally important concentrations of small-ranged
species in their southeast (Brazil and United States) or southwest
(China and plant species in California). With the exception of south-
west China, these concentrations are also where many people live.
Much of the Amazon basin lies within Brazil. It contains the
largest species-rich wilderness on Earth with much of it protected,
inter alia, by indigenous reserves. They are effective at protecting
forest cover at least (35, 36) but, based on current knowledge, hold
relatively few small-ranged species. These species are concentrated
in Brazil’s coastal forests along with a large fraction of the country’s
threatened species (37), albeit often in small, isolated protected areas.
The fate of these species will likely depend on the ability to enforce
that protection and to reconnect habitats in this massively fragmented
landscape (38).
China has extensive protected areas in the sparsely populated,
high, and arid west. Its endemic species concentrate in the southwest
where, fortuitously, they extensively overlap with the iconic giant
panda (34). Future improvements in protecting China’s biodiversity
will require a targeted expansion of the current protected areas and
a recognition that other species besides pandas can act as umbrella
species for conservation.
The United States has extensive protected areas, also in its sparsely
populated, high, and arid west. There are very few large protected
areas in the southeast where important concentrations of endemic
species live. The United States also has an extensive network of pri-
vate land conservancies, especially in the east. These tend to protect
green spaces in wealthy counties in the northeast, rather than eco-
nomically poorer, but species-rich areas in the southeast (39).
In sum, the preoccupation with summarizing the total area pro-
tected can be misleading—quality, not quantity, matters. Protected
areas, to date, have been more effective at including species than
commonly thought. Whether we can achieve large-scale protection
of biodiversity will depend much on the details of how countries set
priorities for protection.
MATERIALS AND METHODS
For protected areas, we follow (6) except for China, where we used
data in (34) and 2014–2017 updates from the China Ministry of En-
vironmental Protection (now the Ministry of Ecology and Environ-
ment) website. From the World Database on Protected Areas, only
areas listed as designated and national or regional were included. In
the case of overlaps, we ranked the area as the strictest of the over-
lapping categories. We did not consider protected areas having only
point data as we are interested in precisely where land is protected
relative to species ranges. Consequently, our estimate of total area
protected is slightly lower than if including the areas represented
only as points.
The Human Footprint Index (31) scales from 0 to 50. We defined
wild areas as those with values ≤3.3 for doing so meant that the area
encompassed combined with protected areas came as close to half
of the land surface as practical, not including Antarctica. Had we
chosen to ignore the protected areas, values of ≤3.8 would have en-
compassed half of the land surface.
Species range data were from (40, 41). We took the ranges of all
extant birds, mammals, and amphibians and, for each, calculated how
much of their range was in protected areas of IUCN categories I to
VI, protected but lacking an IUCN category, or unprotected but wild.
We considered only the extant ranges of species. We used ArcGIS
10.5 and the Eckert IV equal-area projection for analyses.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/4/8/eaat2616/DC1
Fig. S1. Map of small-ranged bird species and unprotected small-ranged bird species.
Fig. S2. Map of small-ranged amphibian species and unprotected small-ranged amphibian species.
Table S1. Mammals in protected areas.
Table S2. Mammals in protected areas plus wilderness.
Table S3. Birds in protected areas.
Table S4. Birds in protected areas plus wilderness.
Table S5. Amphibians in protected areas.
Table S6. Amphibians in protected areas plus wilderness.
Table S7. Percent of terrestrial bird species with a given minimum area and percentage of their
range covered by protected or protected plus wilderness.

Pimm et al., Sci. Adv. 2018; 4 : eaat2616 29 August 2018
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Table S8. Percent of terrestrial amphibian species with a given minimum area and percentage
of their range covered by protected or protected plus wilderness.
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Acknowledgments: Original data sources are publicly available as described in the text.
We thank E. Dinerstein, A. Jacobson, J. Riggio, E. Wilson, and two anonymous reviewers for
input on earlier drafts of the paper. Funding: The authors acknowledge that they received no
funding in support of this research. Author contributions: All authors designed and
performed the analyses and wrote and revised the manuscript. Competing interests: S.L.P.
and C.N.J. are president and vice president, respectively, of SavingSpecies, a nonprofit
conservation organization that relies on scientific analysis to set priorities for conservation
actions. The authors declare that they have no other competing interests. Data and
materials availability: All data needed to evaluate the conclusions in the paper are present
in the paper and/or the Supplementary Materials. Additional data related to this paper may
be requested from the authors.
Submitted 8 February 2018
Accepted 20 July 2018
Published 29 August 2018
10.1126/sciadv.aat2616
Citation: S. L. Pimm, C. N. Jenkins, B. V. Li, How to protect half of Earth to ensure it protects
sufficient biodiversity. Sci. Adv. 4, eaat2616 (2018).