Content uploaded by Oscar Venter
Author content
All content in this area was uploaded by Oscar Venter on Nov 16, 2015
Content may be subject to copyright.
LETTER doi:10.1038/nature13717
A global strategy for road building
William F. Laurance
1
, Gopalasamy Reuben Clements
1,2
, Sean Sloan
1
, Christine S. O’Connell
3
, Nathan D. Mueller
4
, Miriam Goosem
1
,
Oscar Venter
1
, David P. Edwards
5
, Ben Phalan
6
, Andrew Balmford
6
, Rodney Van Der Ree
7
& Irene Burgues Arrea
8
The number and extent of roads will expand dramatically this century
1
.
Globally, at least 25 million kilometres of new roads are anticipated
by 2050; a 60% increase in the totallength of roads over that in 2010.
Nine-tenths of all road construction is expected to occur in develop-
ing nations
1
, including many regions that sustain exceptional biodi-
versity and vital ecosystem services. Roads penetrating into wilderness
or frontier areas are a major proximate driver of habitat loss and frag-
mentation, wildfires, overhunting and otherenvironmental degrada-
tion, oftenwith irreversible impacts on ecosystems
2–5
. Unfortunately,
much road proliferation is chaotic or poorly planned
3,4,6
, and the rate
of expansion is so great that it oftenoverwhelms the capacity of envi-
ronmental planners and managers
2–7
.Herewepresentaglobalscheme
for prioritizing road building. This large-scale zoning plan seeks to
limit the environmental costs of road expansion while maximizing
its benefits for human development, by helping to increase agricul-
tural production, which is an urgent priority given that global food
demand could double by mid-century
8,9
. Our analysisidentifies areas
with high environmental values where future road building should
be avoided if possible, areas where strategic road improvements could
promote agricultural development with relatively modest environ-
mental costs, and ‘conflict areas’ whereroad building could have size-
able benefits for agriculture but with serious environmental damage.
Our plan provides a template for proactively zoning and prioritizing
roads during the most explosive era of road expansion in human history.
A multitude of factors is promoting rapid road expansion globally,
includinga quest for valuable resources such astimber, minerals, oiland
arable land, and initiatives to increase regional trade, transportation and
energy infrastructure
4,7
. Yet, while new roads can promote social and
economic development
10,11
, they also can open a Pandora’s box of envi-
ronmental problems
2–7
. This is especially the case in pristine or frontier
regions, wherenew roads often dramaticallyincrease land colonization,
habitat disruption, and overexploitation of wildlife and natural resources
2–6
.
It is broadly understood that the best strategy for maintaining the integ-
rity of wilderness areas is by ‘avoiding the first cut’—keeping them road-
free
4
—because deforestation is highly contagious spatially
12
and because
new roads tend to spawn networks of secondary and tertiary roads that
greatly increase the extent of environmental damage
4
. Unfortunately,
new roads are now penetrating into many of the world’s last surviving
wildernesses, including the Amazon
2,5,6,10
, New Guinea
13
, Siberia
14
and
the Congo Basin
3,11,15
.
However, some roads generate substantial social and economic ben-
efits with only modest environmental costs. Particularly in developing
nations, vast expanses of land havebeen settled buthave low agricultural
productivity because of poor access to fertilizers and modern farming
technologies
11,16
. In such contexts, new roads—or road improvements
such as paving—could increase access to agricultural supplies and markets,
facilitating production increases and lowering post-harvest crop losses
13,17
.
As such accessible areas tend to sustain more prosperous rural livelihoods,
they may also act as ‘magnets’, attracting colonistsaway from environ-
mentally vulnerable frontier areas, such as the margins of forests
17,18
.In
this way, improving transportation in suitable areas could help to con-
centrate and improve agricultural production, raising farm yields
11,13
while
potentially promoting land sparing for nature conservation
19
.
Despite the pivotal role that roads have in human land-use, efforts
to plan and zone roads are extremely inadequate. First, although roads
increasingly dominate muchof Earth’s land surface (Fig. 1),many roads
are unmapped, especially in developing nations; in the Brazilian Amazon,
for example, the total length of unofficial or illegal roads is nearly triple
that of official roads
20
. Second,environmental-impact assessmentsoften
place the burden of proof on road opponents
21,22
, who rarely have suf-
ficient information on rare species, biological resources and ecosystem
services
23
needed to determine the actual environmental costs of roads.
Third, many road assessments are limited in scope
4,22
, focusing only on
the direct effects of road building while ignoring its critical indirect effects,
such as promoting deforestation, fires, poaching and land speculation.
Finally, because there is no strategic, proactive system for zoning roads
globally, road projects must be assessed with little information on their
broader context (see the 2013 report on high-risk road development by
the ConservationStrategy Fund; http://conservation-strategy.org/sites/
default/files/field-file/CSFPolicyBrief_14_english_1.pdf). This increases
the burden on road planners and evaluators,who are being swamped by
the unprecedented pace of contemporary road expansion
2–7,11,15,20
.
For these reasons, we devised a ‘global roadmap’ to identify areas in
which roadsor road improvementsare likely to have major costs or ben-
efits. The map has two components: an environmental-values layer that
estimates the natural importance of ecosystems, and a road-benefits layer
that estimates the potential for increased agricultural production, in part
via new or improved roads. Combining these two layers allows us to
identify areas where roads or road upgrades could have large potential
benefits, areas where road building should be avoided wherever possible,
and conflict areas where their potential costs and benefits are both sizeable.
We created the environmental-values layer (Fig. 2a) by integrating
global data sets on three classes of parameters: biodiversity (number of
threatened terrestrial-vertebrate species, estimated number of plant spe-
cies per ecoregion); key wilderness habitats (G200 terrestrial ecoregions,
important bird areas and endemic bir d areas, biodiversity hotspots, fron-
tier forests, high-biodiversity wilderness areas); and carbon storageand
climate-regulation services ofthe local ecosystem (seeMethods and Sup-
plementary Figs 1–11). Values for each class were equally weighted, rescaled
(range: 0–1) and then averaged to produce the environmental-values
layer. Regions that scored highly on this layer include wet and humid
tropical and subtropical forests, Mediterranean ecosystems, wildlife-rich
savanna woodlands in South America and Africa, many islands, certain
mountain ranges, and some higher-latitude forests, among others.
The road-benefits layer(Fig. 2b) identifies areas where new roads or
road improvements could potentially help to improve agricultural pro-
duction.Like the environmental-values layer, it is a relative index(range:
0–1). In general terms, areas that score highly on this layer have been
largely converted to agriculture (and thus have little native vegetation
remaining), arerelatively low-yielding despite having soils and climates
1
Centre for Tropical Environmental and Sustainability Science, and College of Marine and Environmental Sciences, James Cook University, Cairns, Queensland 4878, Australia.
2
Kenyir Research Institute,
Universiti Malaya Terengganu, 21030 Kuala Terengganu, Malaysia.
3
Institute on the Environment, and Department of Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, Minnesota
55108, USA.
4
Center for the Environment, Harvard University, Cambridge, Massachusetts 02138, USA.
5
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
6
Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
7
Australian Research Centre for Urban Ecology, and School of Botany, University of Melbourne, Melbourne, Victoria 3010,
Australia.
8
Conservation Strategy Fund, 663-2300 Curridabat, San Jose
´, Costa Rica.
11 SEPTEMBER 2014 | VOL 513 | NATURE | 229
Macmillan Publishers Limited. All rights reserved
©2014
broadly suitable for agriculture, are not so distant from urban markets
that crop-transportation costs would be prohibitive even with new or
improvedroads, and are expected to see large future increases in agricul-
tural production to meet projected food or export demands (see Methods
and Supplementary Figs 12–16 for details of how these data sets were
integrated). All continents have regions that score highly, including parts
of south Asia, east and southeast Asia, West and East Africa, central Eur-
asia, west-central North America, Central America and Mexico, and the
Atlantic region of South America.
We classified each of the environmental-values (Fig. 2a) and road-
benefits (Fig. 2b) layers into deciles and then cross-tabulated them to
generate 100 unique colour combinations (see Supplementary Infor-
mation for details). In this scheme, green-shaded areas are where road
building would have relatively high environmental costs and only modest
potential benefits for agriculture. Red-shaded areas are the opposite, with
high potential to increase agricultural production and lower scores on the
environmental-values axis. Black and dark-shaded areas are ‘conflict
zones’ with high values on both axes, whereas white and light-shaded
areas are lower priorities for both environment and agriculture.
On top of this scheme we overlaid polygons for 177,857 protected areas
(Supplementary Fig. 17) globally, using available data from the World
Database on Protected Areas (http://www.wdpa.org). Protected areas
Figure 1
|
The distribution of major roads globally. Roads are indicated
in black; white areas lack mapped roads. The quality of road maps varies
greatly among nations, with many smaller and unofficial roads remaining
unmapped. We generated this map using data from the integrated gROADS
database (http://sedac.ciesin.columbia.edu/data/set/groads-global-roads-
open-access-v1; accessed 7 June 2014); Center for International Earth Science
Information Network - CIESIN - Columbia University, and Information
Technology Outreach Services - ITOS - University of Georgia. 2013. Global
Roads Open Access Data Set, Version 1 (gROADSv1). Palisades, NY: NASA
Socioeconomic Data and Applications Center (SEDAC). http://dx.doi.org/
10.7927/H4VD6WCT.
a
High : 1
Low : 0
b
High : 1
Low : 0
Figure 2
|
The environmental-values and road-benefits layers. a,b, The
environmental-values layer (a) integrates data on terrestrial biodiversity, key
habitats, wilderness, and environmental services. The road-benefits layer
(b) shows areas broadly suitable for agricultural intensification, where new
roads or road improvements could potentially promote increased production.
See Supplementary Information for data sources.
RESEARCH LETTER
230 | NATURE | VOL 513 | 11 SEPTEMBER 2014
Macmillan Publishers Limited. All rights reserved
©2014
were zoned fully green because we judged that they should be free of
new roads wherever possible, given that roads can facilitate illegal acti-
vities such as poaching, encroachment, and vehicle-related road-kill of
wildlife
2–4
that are contrary to the goals of protected-area management
24,25
.
The resulting global roadmap (Fig. 3)attempts to portray key relative
risks and rewards of road building for each 1-km
2
pixel on Earth’s land
surface. In broad terms, our map illustrates the enormous potential for
environmental loss and degradation as a result of contemporary road
expansion (Table 1 and Supplementary Fig. 18). Roads are currently pro-
liferating or planned in manyareas categorized as having high environ-
mental values but only modest agricultural potential, such as the Amazon
Basin, parts of the Asia-Pacific region, and higher-latitude forests in the
Northern Hemisphere.
The roadmap also reveals extensive conflict areas (Fig. 3), where environ-
mentaland agricultural values are both high, particularly inSub-Saharan
Africa,Madagascar, Central America, the Mediterranean, southeast and
south-central Asia, the Andes, and theAtlantic regionof South America.
Conflict zones often occur in regions withrapid population growth,high
speciesendemism, or both.In total, 1.97 billionhectares (15.3%of global
land area) fall into conflict areas (Table 1). Land-use pressures in such
regions are mounting rapidly; it has been estimated that, unless current
agricultural yields markedly improve, approximately 1 billion hectares
of additional farming and grazing land will be needed by 2050 to meet
projected food demands
9
, with extensive additional lands converted for
production of biofuels
26
.
However, our road-planning scheme also suggests that many areas
could be targeted for agricultural production increases with relatively
modest environmental costs.Such areas include expanses of the Indian
subcontinent, centralEurasia, the Irano-Anatolian region, and African
Sahel, amongothers (Fig. 3). In total, 1.46billion hectares of land(11.4%
of global land area) is zoned red (Table 1), suggesting that there is con-
siderable potential on every continent to increase agricultural produc-
tion, by raising yields on existing farming and grazing land.
Although improved roads or other transportation can facilitate agricul-
tural yield increases
11,13,17,18
, additional measures—such as investments in
improved farming methods, fertilizers and, where appropriate, irrigation—
will also be essential. A particular challenge will be devising strategies
to help developing nations with exceptional environmental values, such
as Madagascar and Indonesia (Fig. 2a), to meet pressing economic and
food-production needs while limiting the environmental costs of rapid
road development. For such nations, international payments forecosys-
tem services, ecotourism, and sustainable harvesting of native production
forests could potentially help to balance economic and environmental
priorities
27
. A further priority when planning road and agricultural invest-
ments is to considerhow factors such as inter-annual weather variability
or projected future climate change could impact on crop yields
28
.
The global roadmap we created underscores the potential benefits and
need for strategic road planning, but actual road planning will be under-
taken at smaller national or regional scales. For this, we created more
detailed maps that show finer-scale features (for example, Extended Data
Fig. 1). These maps and their components are freely available (http://
global-roadmap.org) andcan be combined with additional data, such as
more detailed information on topography, soils, existing croplands and
local road networks, to facilitate road planning.
Integrating local information is important because the drivers and
environmental impacts of road construction will vary in different con-
texts. For example, in arable, largely road-freeareas of East Africa (Fig. 4a),
new roads driven by a burgeoning mining boom
11,29
could provoke major
land-use changes andhabitat loss. Yetexpanding roads from timber and
miningoperations could also have large impacts in Siberia (Fig.4b), even
No data
Environmental values →
← Agricultural potential
Figure 3
|
A global roadmap. Shown are priority road-free areas (green
shades), priority agricultural areas (red shades), conflict areas (dark shades),
and lower-priority areas(light shades). Values of the environmental-values and
road-benefits layers are each divided into deciles, yielding 100 unique colour
combinations. See Supplementary Information for details and data sources.
Table 1
|
Percentages of seven geographical regions that fall into four broad categories on the global roadmap
Zone Africa Asia Australia Europe North and Central America South America Oceania Global
Conserve 29.03 45.69 34.21 26.44 47.39 66.28 95.29 42.96
Agriculture 7.93 12.44 3.63 32.92 11.35 6.83 0.23 11.40
Conflict 24.75 14.87 7.01 9.10 8.70 15.74 0.58 15.34
Low-tension 38.30 27.00 55.15 31.54 32.55 11.14 3.89 30.30
Total area 29,805 44,174 7,693 9,670 23,395 17,662 412 132,811
Data on the total areas of each region are given in km
2
x10
3
. ‘Conserve’ zones are where road building would have relatively high environmental costs (above-median environmental values; Fig. 2a) and modest
potential agricultural benefits (below-median road-benefits values; Fig. 2b). ‘Agriculture’ zones have the opposite attributes (above-median road-benefits values and below-median environmental values).
‘Conflict’ zones have both above-median environmental values and above-median road-benefits values, whereas ‘low-tension’ zones are lower priorities for both environment and agriculture (with below-median
environmental and road-benefits values). See Supplementary Fig. 18 for a map of these zones.
LETTER RESEARCH
11 SEPTEMBER 2014 | VOL 513 | NATURE | 231
Macmillan Publishers Limited. All rights reserved
©2014
though agricultural potential is limited, by promoting forest fires and
clearing
14
. In general, we expect road impacts to be lowest in unproduc-
tive, arid regions, moderate in carbon-rich ecosystems such as higher-
latitude forests, and most damaging in species- and carbon-rich ecosystems
such as tropical forests, particularly where few roads currently exist.
We see our global road-mapping scheme as a working model—an
important first step towards strategic road planning to reduce environ-
mental damage—that can be downscaled and tailored for particularcir-
cumstances. We believe such proactive planning should be a central
element of any discussion about road expansion and associated land-
use zoning
13,30
. Given that the total length of new roads anticipated by
mid-century
1
would encircle the Earth more than 600 times, there is
little time to lose.
Online Content Methods, along with any additional Extended Data display items
and SourceData, are available in theonline version of the paper;references unique
to these sections appear only in the online paper.
Received 19 May; accepted 28 July 2014.
Published online 27 August 2014.
1. Dulac, J. Global Land Transport Infrastructure Requirements: Estimating Road and
Railway Infrastructure Capacity and Costs to 2050 (International Energy Agency,
2013).
2. Laurance, W. F. et al. The future of the Brazilian Amazon. Science 291, 438–439
(2001).
3. Blake, S. et al. Forest elephant crisis in the CongoBasin. PLoS Biol. 5, e111 (2007).
4. Laurance,W. F., Goosem, M. & Laurance, S. G. Impactsof roads and linear clearings
on tropical forests. Trends Ecol. Evol. 24, 659–669 (2009).
5. Adeney, J. M.,Christensen, N. & Pimm, S. L. Reserves protect against deforestation
fires in the Amazon. PLoS ONE 4, e5014 (2009).
6. Fearnside,P. M. & Graça, P. BR-319: Brazil’sManaus-Porto Velho Highwayand the
potential impact of linking the arc of deforestation to central Amazonia. Environ.
Manage. 38, 705–716 (2006).
7. Forman, R. T. T. et al. Road Ecology: Science and Solutions (Island Press, 2003).
8. Tilman, D., Cassman, K. G., Matson, P. A., Naylor, R. & Polasky, S. Agricultural
sustainability and intensive production practices. Nature 418, 671–677 (2002).
9. Tilman, D. et al. Forecasting agriculturally driven global environmental change.
Science 292, 281–284 (2001).
10. Perz, S. G. et al. Regional integration and local change: road paving, community
connectivityand social-ecological resiliencein a tri-national frontier, southwestern
Amazonia. Reg. Environ. Change 12, 35–53 (2012).
11. Weng, L. et al. Mineral industries, growth corridorsand agricultural development in
Africa. Glob. Food Security 2, 195–202 (2013).
12. Boakes, E. H., Mace, G. M., McGowan, P. J. K. & Fuller, R. A. Extreme contagion in
global habitat clearance. Proc. R. Soc. Lond. B 277, 1081–1085 (2010).
13. Laurance, W. F. & Balmford, A. A global map for road building. Nature 495,
308–309 (2013).
14. Bradshaw, C. J. A., Warkentin, I. G. & Sodhi, N. S. Urgent preservation of boreal
carbon stocks and biodiversity. Trends Ecol. Evol. 24, 541–548 (2009).
15. Laporte, N. T., Stabach, J. A., Grosch, R., Lin, T. S. & Goetz, S. J. Expansion of
industrial logging in central Africa. Science 316, 1451 (2007).
16. Mueller, N. D. et al. Closing yield gaps through nutrient and water management.
Nature 490, 254–257 (2012).
17. Weinhold, D. & Reis, E. Transportation costsand the spatial distributionof land use
in the Brazilian Amazon. Glob. Environ. Change 18, 54–68 (2008).
18. Rudel, T. K., DeFries, R., Asner, G. P. & Laurance, W. F. Changing drivers of
deforestation and new opportunities for conservation. Conserv. Biol. 23,
1396–1405 (2009).
19. Phalan, B., Onial, M., Balmford, A. & Green, R. E. Reconciling food production and
biodiversityconservation: Land sharing and land sparing compared.Science 333,
1289–1291 (2011).
20. Barber, C. P., Cochrane, M. A., Souza, C. M. Jr & Laurance, W. F. Roads,
deforestation, and the mitigating effect of protected areas in the Amazon.
Biol. Conserv. 177, 203–209 (2014).
21. Gullett, W. Environmental impact assessment and the precautionary principle:
Legislating caution in environmental protection. Australas. J. Environ. Manage. 5,
146–158 (1998).
22. Laurance, W. F. Forest destruction: the road to ruin. New Sci. 194, 25 (2007)
http://www.newscientist.com/article/mg19426075.600-forest-destruction-the-
road-to-ruin.html.
23. Lawrence, D. P. Environmental Impact Assessment: Practical Solutions to Recurrent
Problems (John Wiley & Sons, 2003).
24. Laurance, W. F. et al. Averting biodiversity collapse in tropical forest protected
areas. Nature 489, 290–294 (2012).
25. Caro, T., Dobson, A., Marshall, A. J. & Peres, C. A. Compromise solutions between
conservation and road building in the tropics. Curr. Biol. 24, R722–R725 (2014).
26. Warner, E. et al. Modeling biofuel expansion effects on land use change dynamics.
Environ. Res. Lett. 8, 015003 (2013).
27. Campbell, W. B. & Lo
´pez Ortı
´z, S. (eds) Integrating Agriculture, Ecotourism, and
Conservation: Examples from the Field (Springer, 2011).
28. Challinor, A. J. et al. A meta-analysis of crop yield under climate change and
adaptation. Nature Clim. Chang. 4, 287–291 (2014).
29. Edwards, D. P. et al. Miningand the African environment. Conserv. Lett. 7, 302–311
(2014).
30. Balmford, A., Green, R. & Phalan, B. What conservationists need to know about
farming. Proc. R. Soc. Lond. B 279, 2714–2724 (2012).
Supplementary Information is available in the online version of the paper.
Acknowledgements We thank T. Brooks, S. Butchart, J. Geldmann, S. Goosem,
C. Mendenhall,N. Pares, S. Pimm, U. Srinivasan,N. Velho, and two anonymous referees
for comments and feedback. The Australian Research Council provided support.
Author Contributions W.F.L. and A.B. initially conceived the study, and W.F.L.
coordinated its design, analysis, and manuscript preparation. G.R.C. and S.S.
conducted the spatial analyses; C.S.O., N.D.M., O.V., G.R.C., S.S. and B.P. generated or
collated key datasets; and M.G., D.P.E., R.V.D.R. and I.B.A. provided ideas and critical
feedback.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Readers are welcome to comment on theonline version of the paper. Correspondence
and requests for materials should be addressed to W.F.L. (bill.laurance@jcu.edu.au).
ab
Figure 4
|
Mapped roads overlaid onto the roads-benefits layer. a,b,In
eastern Africa (a) and Siberia (b), roads are rapidly expanding into relatively
road-free areas, but for different reasons. Narrow black lines indicate mapped
roads. In both regions, areas with darker-red colours have greater agricultural
potential than those with lighter colours. See Supplementary Information for
data sources.
RESEARCH LETTER
232 | NATURE | VOL 513 | 11 SEPTEMBER 2014
Macmillan Publishers Limited. All rights reserved
©2014
METHODS
We used ArcGIS 10.1 and IDRISI Selva to integrate spatial data relevant to our
global roadmap. Analyses were conducted using Goode’s homolosine equal-area
projection and a 1-km
2
pixel size, yielding ,132.8 million pixels for Earth’s ter-
restrial surface (excluding Antarctica). Larger freshwater bodies (.50 km
2
) were
removed before analysisbut land areas under ice or permafrost were not excluded.
A small fraction (2.21%)of all pixels lacked data (mostly in Greenland) and so were
excluded from the analysis.
We created the environmental-values layer (Fig. 2a) by integrating global data
sets on biodiversity (numberof threatened terrestrial-vertebrate species,estimated
number of plant species per ecoregion); key wilderness habitats (G200 terrestrial
ecoregions, important bird areas and endemic bird areas, biodiversity hotspots,
frontier forests, high-biodiversity wilderness areas); andcarbon storage and climate-
regulation services of the local ecosystem (Supplementary Figs 1–11). Areas that scored
highly on the road-benefits layer (Fig. 2b) were defined by having: a high propor-
tion of land already under farming or grazing; soils and climates that are broadly
suitable for agriculture; large agricultural yield gaps; large projected increases i nf uture
agricultural production; and the potential to access urban markets with improved
transportation (Supplementary Figs 12–16). The globaldata sets that comprise the
environmental-values and road-benefits layers, and the methods by which they
were integrated, are described in detail in the Supplementary Information.
LETTER RESEARCH
Macmillan Publishers Limited. All rights reserved
©2014
Extended Data Figure 1
|
Roadmaps for northern South America and
Sub-Saharan Africa. Magnified images such as these could be integrated
with local-scale data to facilitate actual road planning. Values of the
environmental-values and road-benefits layers are each divided into deciles,
yielding 100 unique colour combinations. See Supplementary Information for
data sources.
RESEARCH LETTER
Macmillan Publishers Limited. All rights reserved
©2014
CORRECTIONS & AMENDMENTS
CORRIGENDUM
doi:10.1038/nature13876
Corrigendum: A global strategy for
road building
William F. Laurance, Gopalasamy Reuben Clements,
Sean Sloan, Christine S. O’Connell, Nathan D. Mueller,
Miriam Goosem, Oscar Venter, David P. Edwards, Ben Phalan,
Andrew Balmford, Rodney Van Der Ree & Irene Burgues Arrea
Nature 513, 229–232 (2014); doi:10.1038/nature13717
In this Letter, as a result of an inadvertent spreadsheet error, four values
presented in Table 1 were slightly inflated. These relate to the propor-
tions of Earth’s total land surface located within the ‘conserve’, ‘agri-
culture’, ‘conflict’ and ‘low-tension’ zones. The correct percentage values
for these four zones under the ‘global’ heading are 42.96, 11.40, 15.34
and 30.30, respectively. Two of these values (the global percentages for
conflict and agriculture zones) were also mentioned in the main text.
We apologise for these errors, which have now been corrected in the
online versions of the Letter, and do not affect the interpretation of our
analyses.
262|NATURE|VOL514|9OCTOBER2014
Macmillan Publishers Limited. All rights reserved
©2014
