ArticlePDF AvailableLiterature Review

The exceptional value of intact forest ecosystems

February 2018
Nature Ecology & Evolution 2(4)

DOI:10.1038/s41559-018-0490-x

Authors:

James E. M. Watson

University of Queensland, Brisbane, Australia

Tom D Evans

Wildlife Conservation Society

Oscar Venter

University of Queensland

Brooke Williams

Queensland University of Technology

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As the terrestrial human footprint continues to expand, the amount of native forest that is free from significant damaging human activities is in precipitous decline. There is emerging evidence that the remaining intact forest supports an exceptional confluence of globally significant environmental values relative to degraded forests, including imperilled biodiversity, carbon sequestration and storage, water provision, indigenous culture and the maintenance of human health. Here we argue that maintaining and, where possible, restoring the integrity of dwindling intact forests is an urgent priority for current global efforts to halt the ongoing biodiversity crisis, slow rapid climate change and achieve sustainability goals. Retaining the integrity of intact forest ecosystems should be a central component of proactive global and national environmental strategies, alongside current efforts aimed at halting deforestation and promoting reforestation.

| Forest degradation and carbon loss. Examples of published case studies that have examined the effects of forest degradation on carbon loss 23,117,176-191. Supplementary Table 1 provides in-depth summaries of each of the 15 case studies.

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https://doi.org/10.1038/s41559-018-0490-x

1School of Earth and Environmental Sciences, The University of Queensland, St. Lucia, Brisbane, Queensland, Australia. 2Wildlife Conservation Society,

Global Conservation Program, Bronx, New York, NY, USA. 3Natural Resources & Environmental Studies Institute, University of Northern British Columbia,

Prince George, British Columbia, Canada. 4Canadian Forest Service, Sault Ste, Marie, Ontario, Canada. 5Wildlife Conservation Society Canada, Toronto,

Ontario, Canada. 6The Grantham Institute - Climate Change and the Environment and Department of Infectious Disease Epidemiology, Imperial College

London, London, UK. 7University of Maryland, College Park, MD, USA. 8Division of Ornithology, American Museum of Natural History, New York, NY, USA.

9Centre for Tropical Environmental and Sustainability Science (TESS) and College of Science and Engineering, James Cook University, Cairns, Queensland,

Australia. 10Woods Hole Research Center, Falmouth, MA, USA. 11Swedish University of Agricultural Sciences, Umeå, Sweden. 12Forest Trends Association,

Washington DC, USA. 13Global Programme on Nature for Development, United Nations Development Programme, New York, NY, USA. 14Fenner School of

Environment and Society, The Australian National University, Canberra, Australian Capital Territory, Australia. 15These authors contributed equally: James

E. M. Watson and Tom Evans. *e-mail: jwatson@wcs.org

Although Earth has lost at least 35% of its pre-agricultural for-

est cover over the past three centuries1, forests are still widely

distributed, covering a total of 40 million km2 (~25%) of

Earth’s terrestrial surface2. Of the remaining forests, as much as 82%

is now degraded to some extent as a result of direct human actions

such as industrial logging, urbanization, agriculture and infrastruc-

ture3,4. This figure is probably an underestimate of the true level

of anthropogenic impact as it does not incorporate other, more

cryptic forms of degradation, such as over-hunting5. As the human

footprint continues to expand4, remaining forest free of significant

anthropogenic degradation is in rapid decline (Fig. 1).

Over the past decade, there has been increasing international

concern around the loss of forest and the impact this has on climate

change, the loss of biodiversity and the provision of ecosystem ser-

vices1. The 2015 Paris Agreement, together with earlier agreements

under the United Nations Framework Convention on Climate

Change (UNFCCC), acknowledges the importance of forests for

limiting a future temperature increase to well below 2 °C above pre-

industrial levels6. The United Nations’ Sustainable Development

Goals (adopted in 2016) have the ambitious aim of fully halting

deforestation by 20207. However, while these targets are clearly war-

ranted, they fall short of specifically prioritizing the crucial quali-

ties of a forest that contribute most to achieving each convention’s

specific goals1. For example, indicators tracking progress towards

the 2015 New York Declaration on Forests — among the most sig-

nificant global forest conservation targets to date — focus on forest

extent and make almost no acknowledgement of forest condition8.

In this Perspective, we argue that to achieve the goals of global

international environmental accords it is insufficient to treat all for-

ests as equal regardless of their condition. Instead, forest that is free

of significant anthropogenic degradation (which we term ‘intact

forest’) should be identified and accorded special consideration in

policymaking, planning and implementation. Anthropogenic deg-

radation here includes all human actions that are known to cause

physical changes in a forest that lead to declines in ecological func-

tion9,10. Well-studied examples include forest fragmentation, stand-

level damage due to logging, over-harvesting of particular species

(such as over-hunting) and changes in fire or flooding regimes.

We first summarize published evidence that intact forests sup-

port an exceptional confluence of globally significant environmen-

tal values relative to forests that have experienced those damaging

human actions. We show that intact forests are indispensable not

only for addressing rapid anthropogenic climate change, but also for

confronting the planet’s biodiversity crisis, providing critical ecosys-

tem services and supporting the maintenance of human health. We

then show that the relative value of intact forests is likely to become

magnified as already-degraded forests experience further intensi-

fied pressures (including anthropogenic climate change). While it is

beyond the scope of this paper to set thresholds for acceptable for-

est fragment size and configuration, logging intensity or any other

measure of damage, we provide evidence that human activity that

exceeds the natural range of variation in a forested system reduces

key ecological functions, and the greater the level of alteration, the

greater the reduction in function. Here we outline the significant,

The exceptional value of intact forest ecosystems

James E. M. Watson 1,2,15*, Tom Evans2,15, Oscar Venter3, Brooke Williams1,2, Ayesha Tulloch 1,2,

Claire Stewart1, Ian Thompson4, Justina C. Ray5, Kris Murray6, Alvaro Salazar1, Clive McAlpine1,

Peter Potapov7, Joe Walston2, John G Robinson2, Michael Painter2, David Wilkie2,

Christopher Filardi8, William F. Laurance9, Richard A. Houghton 10, Sean Maxwell1,

Hedley Grantham1,2, Cristián Samper2, Stephanie Wang2, Lars Laestadius11, Rebecca K. Runting1,

Gustavo A. Silva-Chávez12, Jamison Ervin13 and David Lindenmayer 14

As the terrestrial human footprint continues to expand, the amount of native forest that is free from significant damaging

human activities is in precipitous decline. There is emerging evidence that the remaining intact forest supports an exceptional

confluence of globally significant environmental values relative to degraded forests, including imperilled biodiversity, carbon

sequestration and storage, water provision, indigenous culture and the maintenance of human health. Here we argue that main-

taining and, where possible, restoring the integrity of dwindling intact forests is an urgent priority for current global efforts to

halt the ongoing biodiversity crisis, slow rapid climate change and achieve sustainability goals. Retaining the integrity of intact

forest ecosystems should be a central component of proactive global and national environmental strategies, alongside current

efforts aimed at halting deforestation and promoting reforestation.

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and probably intensifying, threats to intact forests and argue that

action is required to halt and reverse their loss. Such action requires

explicit consideration on global, national and sub-national scales,

and we conclude by identifying specific policy mechanisms where

intact forests should be addressed.

Our call for an increased emphasis on intact forests does not

imply that other forms of forest are unimportant. Given the scale

of the environmental challenges facing humanity, there is also an

undoubted need to cease deforestation and degradation at forest

frontiers11, and to promote large-scale reforestation12. We believe

that coherent environmental policy should give due weight to

intact forests, clearance frontiers and restoration opportunities,

because all three have crucial and complementary roles to play.

The primary reasons why we focus on intact forests are twofold.

First, they are overlooked in international policy. Second, intact

forest protection can typically secure very high environmental

values with often relatively low implementation and opportunity

costs13, which serves to reinforce the need for their direct inclusion

in global environmental accords.

Evidence for the exceptional values of intact forest

ecosystems

There has been rapid growth in our understanding of the link between

anthropogenic pressures on forest and impacts on ecosystem ser-

vice values across a range of forest types (Box 1). Anthropogenic

pressures, especially at industrial intensities and large spatial scales,

have been shown to alter forest characteristics, including physical

structure, species composition, diversity, abundance and functional

organization compared with their natural state, and as a result, to

reduce a wide range of environmental values14–17. These pressures

also interact with natural disturbance regimes such as fire and pests

to perturb forests beyond their capacity to regenerate18. The follow-

ing sections show how the loss of forest intactness leads to declines

or changes in these key environmental values: global and regional

scale climate regulation; local climate and watershed regulation;

biodiversity conservation; indigenous cultures; and human health.

Climate mitigation. Climate change is causing pervasive and

potentially irreversible impacts on ecosystems and people19. Of the

anthropogenic contribution to atmospheric CO2 since 1870, 26%

is due to emissions from deforestation and forest degradation20. It

is now accepted that actions that avoid emissions from the land

sector, especially forests, and maximize removals of greenhouse

gases are critical if the goals of the UNFCCC Paris Agreement are

to be achieved12,21.

Degradation typically causes fewer emissions per hectare than

deforestation, but is much more widespread3,4,9. In the tropics,

where most net forest emissions occur, degradation may account

for 10–40% of total emissions of aboveground carbon22. Industrial-

scale logging (that is, large-scale market-orientated logging using

heavy machinery, with offtakes that exceed natural rates of tree

mortality) directly reduces carbon stocks through a combination of

tree removal, collateral damage to non-target trees, decomposition

of logging waste and wood fibre products23, and the depletion of

soil and peatland carbon stocks24,25. Industrial logging creates for-

ested systems dominated by regenerating stands of younger, smaller

trees, and although some regrowth does occur during each logging

cycle, the cyclical peaks in biomass typically do not return to pre-

logging levels, and the time-averaged carbon stocks can be expected

to decline progressively over subsequent cutting cycles in many

cases26. Reported carbon losses through industrial logging vary

widely across forest types and due to the different types of logging

undertaken (Fig. 2).

As forest patches are fragmented by agriculture and infrastruc-

ture, the area exposed to edge effects increases disproportionately;

already 70% of the world’s forests lie within 1 km of a forest edge

and this proportion is rising27. Globally, locations up to 500 m

from a forest edge average 25% less biomass carbon than locations

Degree of human footprintGlobal forest cover

High NoneLowForest Intact forest

Tundra

Tropical and subtropical moist broadleaf forests

Tropical and subtropical grasslands, savannas and shrublands

Tropical and subtropical dry broadleaf forests

Tropical and subtropical coniferous forests

Temperate grasslands, savannas and shrublands

Temperate coniferous forests

Temperate broadleaf and mixed forests

Montane grasslands and shrublands

Mediterranean forests, woodlands and scrub

Flooded grasslands and savannas

Boreal forests/taiga

0 10 15 200 1015

Million km

Fig. 1 | The global extent of intact forest. a–c, There are many ways to map intact forest: the first example is mapped as defined by Intact Forest

Landscape methodology3 (a), the second example by the global Human Footprint methodology138 (b) and, for both measures, by biome (c). The definition

of overall forest estate was based on ref. 136, with forests defined as > 75% tree coverage.

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remote from forest edges, and even locations up to 5 km from an

edge can have > 10% less biomass carbon28. These edge effects are

mediated by a wide range of ecological changes, including increased

windthrow and evaporation, and increased access for people, fire

and invasive species27. Another form of degradation is loss of fauna

through over-hunting, which can significantly disturb vegetation

composition and the long-term carbon storage potential of tropical

forests by depriving key, high-carbon tree species of their seed dis-

persal agents, and through other ecological disruptions29,30 (see Box

2). Such effects can extend over vast areas (for example, at least 36%

of the Amazon31) because over-hunting is pervasive where human

access is facilitated by new infrastructure, and can also occur even

in very remote areas32,33.

Degradation reduces the capacity of forests to function as major

net carbon sinks, actively sequestering carbon into soils and living

biomass34,35. The global residual terrestrial sink, much of which is

considered to take place in intact forests, removes an extraordinary

25% (2.4 Pg C yr−1) of anthropogenic emissions from all sources,

and hence greatly slows the pace of climate change36,37. This aspect

of global carbon dynamics is often under-emphasized in climate

policy because it is seen as part of the background of natural fluxes.

However, the large-scale degradation of intact forests would result

in a major anthropogenic reduction in this critical ecosystem ser-

vice38. The intact forest sink is distinct from the sink resulting from

reforestation and forest recovery following cessation of degradation.

Both are large and both are likely to be indispensable in efforts to

meet global climate targets36,39.

Regulating local climate regimes and providing watershed ser-

vices. There is increasing evidence that forests are a key factor

in the regulation of local and regional climate regimes through

the exchange of radiation, moisture and wind energy between the

Box 1 | Evidence of the exceptional values intact forest ecosystems have when compared with degraded ecosystems

Climate change mitigation

More above- and belowground carbon stored. Intact forests

store more carbon than logged, degraded or planted forests

in ecologically comparable locations. Industrial logging and

conversion of forest to cropland causes heavy erosion and

contributes to the loss of belowground carbon21,22,144 (see Fig. 2 and

Supplementary Table 1).

More faunal complexity, which helps carbon storage and

sequestration. Defaunation can signicantly erode the long-term

carbon storage potential of forests by depriving key, high-carbon

tree species of seed-dispersal agents, and through other ecological

disruptions such as reduced vegetation diversity and composition

or increased herbivory by non-hunted species (see Box 2)29,31.

Major carbon sequestration. Intact forests continue to function

as major net carbon sinks, actively sequestering carbon into soils

and living biomass12,34,37.

Regulating local and regional weather regimes

Eects on weather. Local and regional weather patterns are

partly a function of the amount of intact forest cover and its

condition40,42,167.

Generation of rain and reduced risk of drought. When intact forests

are cleared or degraded, there is a reduction in cloud cover and rainfall.

Degradation and loss of intact forest can increase the number of dry

and hot days, decrease daily rainfall intensity and wet day rainfall, and

increase drought duration during El Niño years41,168,169.

Ensuring hydrological services are maintained

Eects on water runo availability. Intact forests have a positive

eect on the redistribution of runo, stabilize water table levels and

retain soil moisture by altering soil permeability. ese processes

interact with physiography to regulate the ow distribution of

energy and materials across the land surface and help stabilize

slopes, prevent water and wind erosion, and regulate the transport

of nutrients and sediments48,50.

Buer human settlements against negative eects of extreme

climatic events. Non-degraded forests diminish the impact of

heavy rain events by decreasing runo and reducing the negative

consequences of climate extremes50,170.

Conserving biodiversity

Consistently higher numbers of forest-dependent species. More

forest-dependent species are found in intact ecosystems than

degraded ones. In some regions, the loss of large tracts of forest has

meant wide-ranging forest-dependent species have either retreated

to the last remaining intact forest systems or gone extinct14,68,171.

More eectively sustain important large-scale ecological

processes. Key functions supported by intact forests include

natural disturbance regimes that sustain habitat resources,

constitute selective forces to which species are adapted, or

otherwise inuence community composition17,172,173.

Intact forests have higher functional diversity. Degrading

activities such as selective logging lead to trait shis in communities

that can aect ecosystem functioning, in addition to taxonomic

diversity5,33,173 (see also Box 2).

Higher intra-species genetic diversity. e larger populations of

forest-dependent species that inhabit intact forests provide greater

options for local adaptation and phenotypic plasticity, which will

facilitate species’ potential for evolutionary and plastic responses

to the rapidly changing environmental conditions69,126,128.

Higher ability for species to undertake dispersal or retreat to

refugia. e connectivity provided by large, contiguous areas

spanning environmental gradients, such as latitude, altitude,

rainfall or temperature, maximize the potential for key processes

such as gene ow and genetic adaptation to play out, while also

allowing species to track shiing climates131,152.

Refuge for forest species from increased re frequencies in

degraded landscapes under changing climates. Intact forests

act as re refuges in landscapes where non-intact forests burn too

frequently to support persistence of plant and animal communities

dependent on long time intervals between burning100,124.

Increased likelihood of providing key pollination and dispersal

processes. Direct logging and secondary eects of degradation

such as loss of vertebrate seed dispersers or pollinators leads

to reduced ecosystem functions, such as seed dispersal and

pollination services, for example, reduced fruit set due to reduced

pollinations in fragmented forests31,174.

Indigenous cultures

Increased basis for the material and spiritual aspects of

traditional indigenous cultures to function. Long-established

cultural norms intricately linked to the ecology of intact areas and

vulnerable to damaging change80,91,92.

Human health benets

Reduced health impacts of wildres. Fires attributed to forest

degradation activities such as burning for land clearing result in

premature deaths due to generation of haze. Lower burning rates

in intact forests mean that health eects of wildres are lower than

in degraded landscapes with larger, more frequent res99.

Reduced infectious disease risks. e emergence of novel

diseases from forests and the increase of endemic disease impacts

in forested landscapes are thought to be related to encroachment

and degradation arising from increasing human presence in these

habitats96,97,175.

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land and atmosphere. Local and regional weather patterns are

therefore a function of not just the amount of forest cover but also

its state and condition40.

Intact tropical forests are critical for rain generation because

air that passes over these forests produces at least twice as much

rain as air that passes over degraded or non-forest areas41. When

intact forests are degraded, there is a resulting reduction in con-

vective cloud cover and rainfall42. The influence of intact forests

on precipitation, temperature and surface hydrology is particu-

larly relevant in reducing the risks of drought imposed by cli-

mate extremes42. In Australia, the degradation and loss of intact

forest can increase the number of dry and hot days, decrease

daily rainfall intensity, and increase drought duration during El

Niño years43. The last pattern also has been shown in Amazonia,

where deforestation and forest degradation produce warmer and

drier conditions that favour more frequent and intense droughts

than in the past44. Importantly, the local climate benefits of

tropical and sub-tropical forests occur primarily during the dry

season and in regions with low rainfall, and during heat

waves where the temperature is buffered by the cooling effects

of evapotranspiration45.

Intact forests also have a direct influence on water availability

through the redistribution of runoff, water table levels and soil

moisture by altering soil permeability46. These processes interact

with physiography to regulate the flow distribution of energy and

materials across the land surface and help stabilize slopes, prevent

water and wind erosion, and regulate the transport of nutrients

and sediments46. Several studies have shown that when forests are

degraded, the soil infiltration rates and water infiltration capacity

are decreased because of changes in soil structure and aggregation

by organic matter and plant litter production47. For example, intact

mountain ash (Eucalyptus regnans) forested ecosystems of south-

ern Australia have been shown to produce > 12 Ml ha−1 yr−1 more

water than equivalent forested ecosystems that have been degraded

through logging48. In many cases, intact forests also buffer the nega-

tive effects of heavy rainfall events by reducing peak discharge and

regulating runoff, and by diminishing the negative consequences of

climate extremes49,50.

Conservation of biodiversity. The global biodiversity crisis is

heavily driven by anthropogenic threats to forests51, as forested

ecosystems support the majority of global terrestrial biodiversity52.

Biodiversity has intrinsic value and there is also increasing evidence

that diverse, intact species assemblages underpin ecosystem func-

tions such as tree productivity, nutrient cycling, seed dispersal,

pollination, water uptake and pest resistance that are critical for

human well-being53.

Intact forests have particular value for the conservation of bio-

diversity54. Beyond outright forest clearance (which is the great-

est threat facing biodiversity51), forest degradation from logging is

the most pervasive threat facing species inhabiting intact forests3.

Many species are sensitive to logging, and studies across many taxo-

nomic groups have shown impacts increasing with the intensity of

logging and with the number of times a forest has been logged17,55.

Fragmentation of intact forest blocks (and associated edge effects)

is also a severe threat to forest-dependent species, especially those

requiring large areas to maintain viable populations (for example,

wide-ranging predators and tree species that occur naturally at very

low densities)27,56. In temperate, boreal and tropical forest regions,

the loss of large contiguous tracts of forest has meant wide-ranging

forest-dependent species have either retreated to the last remaining

intact forest systems or are extinct57–60. Furthermore, there is evi-

dence that — even for some forest species that may persist for a time

in degraded fragments — intact forests are necessary to ensure their

persistence over the long term18,61,62.

Defaunation resulting from commercial and subsistence hunting

is a critical threat for large-bodied forest vertebrates, especially in

the tropics5,63. Many large carnivores and ungulates that play impor-

tant roles as ecosystem engineers (for example, Sumatran serow

(Capricornis sumatraensis), gaur (Bos gaurus) and forest elephant

(Loxodonta cyclotis)) are now found only as remnant populations

in the remaining intact tropical forests33,64. The synergistic interac-

tion of stand damage, fragmentation and hunting is an increasingly

significant challenge for biodiversity conservation65,66 as it is well

known that forest fragmentation increases access for hunters67, and

logging damage has more severe impacts when combined with frag-

mentation17. Forest biodiversity is best conserved by minimizing the

6. Papua New Guinea. A

decline of 31% was measured

in a medium-crowned

rainforest within four years of

logging

181,182

1. Canada. A decline of

12% was modelled over

250 years within a

boreal forest

176

15. United States.

A decline of 50% was

modelled over 57 years

in a temperate

coniferous forest

23,191

1. Canada. A decline of

10–51% was modelled

over 250 years within

coastal forest ecosystems

in British Columbia

176

1. Canada. A decline of

7–25% was modelled

over 250 years within

forest ecosystems in the

interior of British Columbia

176

7. Australia. A 55%

decline was measured in a

montane ash forest

repeatedly logged since

before the 1930s

. Australia. A 50%

decline over 100 years

was modelled in a

Tasmanian wet eucalypt

forest

23,183,184

11. Brazil. A decline of

24% was measured in

Paragominas. Time

since last disturbance

was two years

187

13. Brazil. A decline of

35–57% was measured in

Santarem. Time since last

disturbance unknown

189

14. French Guiana.

A decline of more than

50% was measured in

a lowland tropical rainforest

immediately post-logging

190

12. Brazil. A decline of

37% was measured within

various areas of the

Amazon. Disturbance

ages varied

182,188

2. Malaysia. A decline

of 53% was measured

at a maximum of 19

years since disturbance

in a dipterocarp forest

117

4. Indonesia. A decline

of 15% was measured

after various years of

disturbance in a lowland

tropical forest

178,179

5. Papua New Guinea. A

decline of 24–37% was

measured over various

lowland tropical forest

within a year after logging

180

3. Philippines. A decline of

50% was measured in a

dipterocarp forest. Measurements

were taken in a using a

chronosequence of 1–21 years

177

9. Republic of Congo.

A decline of 3% was

measured after one

year since logging

within a rainforest

182,185

10. Gabon. A decline

of 6% was measured

after logging within a

dense humid evergreen

rainforest

182,186

Fig. 2 | Forest degradation and carbon loss. Examples of published case studies that have examined the effects of forest degradation on carbon loss23,117,1 76–191.

Supplementary Table 1 provides in-depth summaries of each of the 15 case studies.

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encroachment of productive activities that promote forest loss and

fragmentation because the initial intrusion leads to rapid degrada-

tion of intact forests, via not only the direct effects of habitat loss,

but also the coinciding effects of wildfires, overhunting, selective

logging and biological invasions, alongside other stressors65,68. For

example, a recent global analysis of nearly 20,000 vertebrate spe-

cies showed that even minimal initial deforestation within an intact

landscape had severe consequences for vertebrate biodiversity in a

given region, emphasizing the special value of intact forests in mini-

mizing extinction risk68. Moreover, those forest ecosystems that are

more affected by humans support less genetic diversity than those

systems that are still intact, which has potentially significant ramifi-

cations for evolutionary change69.

Indigenous peoples. At least 250 million people70 live in forests, and

for many of them, their cultural identities are deeply rooted in the

plant and animal species found there71. Archaeological and ethno-

graphical evidence indicate that forests have been inhabited by peo-

ple for millennia: in Latin America, records go back 13,000 years72;

in Asia, some 40,000 years73; and in Central Africa, more than

250,000 years74. Forest-dwelling indigenous peoples have tended to

do so at very low population densities distributed in dispersed set-

tlements75. Today, tropical forest societies that depend almost exclu-

sively on the direct use of natural resources to meet their basic needs

seldom exceed population densities of 1–2 people km−2 (ref. 76), and

tend to change location from time to time to ensure that their taking

of food and other products will not permanently deplete an area of

key resources. Through their selection and management of useful

plants and animals, these communities have significant and long-

lasting impacts on the structure and composition of the forests in

which they live77,78.

Industrial-scale degradation of intact forest erodes the mate-

rial basis for the livelihoods of indigenous forest peoples, depleting

wildlife and other resources79. It also renders traditional resource

management strategies ineffective, and undermines the value of tra-

ditional knowledge and authority80. Fragmentation and degradation

of the forest makes a traditional life style no longer tenable, push-

ing indigenous peoples off their land81, and driving people to adopt

Box 2 | The eect of defaunation on carbon storage and sequestration in intact forests

Even where forests have not been cleared, many are not func-

tioning as they once were166. Species such as the Asian and South

American tapirs (Tapirus spp.), forest elephant (L. cyclotis) and the

great apes have disappeared across much of their ranges. Habitat

degradation and fragmentation are major causes of this defauna-

tion, as many large-bodied species depend on great expanses of

high-quality forest to sustain viable populations5,192. Increased hu-

man accessibility to forests is another, with unsustainable hunting

now aecting greater areas of tropical forest than the combined

extent of deforestation, selective logging and wildres193. Wildlife

species are not equally aected by hunting, with stronger impacts

of hunting pressure on larger-bodied primates and ungulates

compared with smaller-bodied vertebrates such as birds and ro-

dents31,75,194.

Defaunation signicantly erodes key ecosystem services and

functions through direct and indirect cascading eects on species

diversity and trophic webs195–197. ere is evidence for negative

eects on pollination, seed dispersal, pest control, nutrient cycling,

decomposition, water quality and soil erosion192,198. Studies

across the African and Atlantic tropical forests indicate that the

disappearance of large frugivores and subsequent loss of seed

dispersal reduces recruitment and natural regeneration of large-

seeded hardwood plant species, which are key contributors to

carbon storage199–201. By simulating the local extinction of trees that

depend on large frugivores in 31 Atlantic forest communities, one

study29 found that defaunation has the potential to signicantly

erode carbon storage even when only a small proportion of large-

seeded trees are extirpated. is is because of strong functional

relationships between seed diameter, wood density and tree height,

which are traits related to carbon storage202. Similar results have

been shown for the Amazon31 and other parts of the tropics203.

ere is also likely to be another link between defaunation and

lowered carbon storage in tropical forests; lower herbivory rates

in defaunated forests allow fast-growing herbivore-sensitive plants

to outcompete slower-growing animal-dispersed trees that have

better defence mechanisms against hunted frugivores31,204,205. In

defaunated forests, carbon storage is potentially reduced when

these fast-growing carbon-poor plants replace an equal basal area

of carbon-rich animal-dispersed trees206 — a process that may be

irreversible once the seed stock is lost.

Degree of defaunation

Schematic representation of the transition (from left to right) of a non-hunted, faunally intact tropical forest to an overhunted, defaunated forest.

Shown is the degree to which large arboreal or terrestrial forest frugivores such as elephants and apes decline in abundance and, with these declines,

the associated replacement of large-fruited high-biomass trees by smaller-fruited and wind-dispersed trees that have lower biomass and carbon

storage. Credit: Blake Alexander Simmons.

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production systems that are incompatible with the maintenance

of intact forests82–85. As traditional forest peoples become increas-

ingly sedentary and connected to urban markets, gender roles,

diets and cultural values also change86–88. These changes in the life

styles of indigenous and traditional peoples create greater depen-

dence on urban markets for provisioning, which can lead to effects

that erode their cultural identities89. Indeed, for many indigenous

forest peoples their cultural sense of self is inextricably linked to

intact forests80.

Forcible alienation from their territories has even more severe

impacts, with the forest homes of many indigenous and traditional

peoples being taken from them, often by force, by more powerful

state, corporate and private actors, whose interests often involve for-

est conversion for cattle pasture, agricultural fields, oil-palm planta-

tions90 and mining concessions91–93. This can have serious impacts

on the health of these peoples as they are often exposed to new

disease vectors and hostile settlers and ranchers. As many indig-

enous and traditional peoples are motivated to conserve their for-

ests (because they are the foundation of their economic and cultural

well-being), there is now mounting evidence (which we discuss

below) that strengthening the land tenure of indigenous peoples is a

powerful way to protect intact forests94,95.

Human health. Forested ecosystems are major sources of many

medicinal compounds that supply millions of people with medi-

cines worldwide96,97. Degradation and outright forest loss com-

promise the supply of these benefits as medically relevant species

decline or are lost98. Degradation can also cause substantial nega-

tive health impacts. For example, during the 2015 human-caused

forest fires in Indonesia, the haze generated after 261,000 ha of

degraded forest and peatland was burned caused more than 100,000

premature deaths across Indonesia, Malaysia and Singapore99.

Fragmented forests experience more numerous and intense edge-

related wildfires in comparison with intact forests100, which severely

exacerbates the extent of health impacts of both intentional and

unintentional burning.

Forest degradation may also lead to infectious disease impacts.

Against a backdrop of declining overall burden of infectious dis-

eases on a global scale101, an increasing rate of novel disease emer-

gence and an increase in the incidence of some endemic diseases in

forested landscapes have been, at least in part, attributed to increas-

ing human presence in, and degradation of, these habitats102,103. For

example, deforestation and resultant environmental changes are

considered key drivers of zoonotic malaria in Malaysian Borneo104.

Although wildlife and arthropod vector species within forests are

natural sources of potential human infections105, increasing human

presence and anthropogenic land-use changes often promote oppor-

tunities for disease transmission, as human-reservoir/vector contact

rates increase or as impacts on host or vector distributions or com-

munity composition perturb natural disease dynamics106. Numerous

infectious diseases associated with forests, including Ebola virus103,

dengue fever107, Zika virus108, several hantaviruses109, yellow fever110

and malaria111, are undergoing changes in risk to humans due to

deforestation, forest degradation and human encroachment.

The increasing significance of intact forests

The differences in important environmental and social values of

intact forests relative to degraded forests are likely to become mag-

nified in the future due to two negative processes in degraded areas:

progressive anthropogenic damage and reduced resilience to envi-

ronmental change.

Vulnerability of degraded forests to further degradation. Once

initiated, forest degradation often intensifies over time112. This is

mediated by: (1) increased levels of human accessibility; (2) suc-

cessive cycles of logging of often progressively lower value trees113;

(3) increased hunting pressure5; (4) forest clearance and fragmenta-

tion due to colonization by farmers and loggers facilitated by new

roads114; and (5) the entry of new extractive development projects

such as mining55. For example, in the Brazilian Amazon, 16% of

logged areas are cleared for agriculture in the first year following

logging, with further losses of more than 5% per year for the next

four years115. This cycle is exacerbated if conversion becomes more

politically acceptable once a forest has been labelled ‘degraded’116.

Once identified as ‘lower value’ for conservation, degraded forests

can mistakenly be considered to have ‘no value’ by some stakehold-

ers, despite extensive evidence to the contrary17,117.

Degraded forests also have increased risk of, and susceptibility

to, natural disturbances such as fire, as forests are drier along their

edges118. There is clear evidence that forests that are logged are at

high risk of burning at uncharacteristically high severity119, with

an elevated fire proneness lasting for decades120. Degraded forests

are also at higher risk from invasion by exotic invasive species18

when compared with non-degraded forests. With fire frequency in

many forest areas predicted to increase under climate change sce-

narios121–123, intact forests might become refuges from fire in many

landscapes where degraded forests burn too frequently to support

the persistence of plant and animal communities dependent on old

forests. This cascade of damage, referred to as a ‘landscape trap’124,

is becoming more common and many forests are now subject to

repeated disturbances that lock them in early successional states.

Loss of resilience following forest degradation. In addition to

present direct anthropogenic threats, forested ecosystems also have

to adapt to large-scale environmental changes, including changes

in climate19, which interact with the myriad of current threats that

they already face125. Intact forest ecosystems have greater capability

to overcome these regional and global stressors than degraded ones,

as they have inherent properties that enable them to maximize their

adaptive capacity126. For example, intact forested ecosystems often

house important populations of forest-dependent species and high

intraspecific genetic diversity, which both provide options for the

local adaptation and phenotypic plasticity127 that facilitates species’

ability to survive changing environmental conditions128. Large, con-

nected and functionally intact forest ecosystems also enable species

to undertake adaptive responses such as dispersal or retreating to

refugia129, which will be critical as the climate changes and species

react130. Moreover, the connectivity provided by large, contiguous

areas spanning multiple environmental gradients, such as altitude,

latitude, rainfall or temperature, will maximize the potential for

key processes such as gene flow and genetic adaptation to play out

naturally, while also allowing species to track shifting climates in

space131,132. Intact forests have been shown to be more resilient in

response to short-term climatic anomalies (for example, droughts

and wildfires during drought) than degraded forests133.

Intact forest ecosystems sustain large-scale ecological processes,

such as natural disturbance regimes, which maintain disturbance-

adapted species that influence native community composition18,127.

For example, the biodiversity of boreal and temperate forests

includes evolutionary lineages that are uniquely adapted to survive

major seasonal temperature changes and landscape-level distur-

bances over time, such as large fires and insect infestations134.

The future of intact forests

The capacity to map human pressures on the environment on

global scales is rapidly improving135 and published results so far

show that not only has loss of global forest cover accelerated since

the 1990s8,136,137 but also that there are higher levels of degradation

within the shrinking forest estate. The recently updated global

Human Footprint138, a composite index of eight human pressures

that is believed to be a good proxy for overall intactness, found that

in 2009, 18% of forests had no detectable human pressure, a 35%

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decline since 1993 (Fig. 1b). According to a related but distinct met-

ric, Intact Forest Landscapes covered 24% of the world’s forests in

2013, a decline of 7.2% since 20003. Recent mapping of roadless for-

est139 and hinterland forest140 shows similar declines using alterna-

tive data sources.

These assessments underestimate the total loss of intactness as

they do not fully take into account other forms of forest degrada-

tion, including invasive species, some forms of logging, over-hunt-

ing, and altered fire and flood regimes, nor do they address the

impacts of climate change. For example, vast areas of Central Africa

that are mapped as ‘intact’ by both satellite imagery and the Human

Footprint have lost their forest elephant (L. cyclotis) populations in

the past 20 years due to poaching. This causes dramatic long-term

ecological changes, given the role of this species as an ‘ecosystem

engineer’ though seed dispersal, trampling and herbivory33.

These figures suggest that even if existing global targets to halt

deforestation are achieved, much of what is saved will be no lon-

ger intact. Outright deforestation is currently concentrated in the

tropics and sub-tropics136, but the loss of intactness is a pervasive

global forest phenomenon3. It seems likely that this rapid decline

in forest intactness will accelerate in line with the underlying driv-

ers of change (including human economic demands, which are

growing rapidly as a result of rising population and even more

quickly rising per-capita consumption141). One stark forecast is

that 25 million km of new roads will be built globally by 2050142,

threatening many intact areas.

Focal mechanisms for action on intact forests. It is clear that many

intact forests are under severe and rising pressure, and there is an

urgent need for greater conservation efforts3. Below, we offer some

potential avenues for enhanced action, while acknowledging that

the scale of the challenge is very significant, and will achieve long-

term success only if nations turn away from ‘business as usual’ activ-

ities that extract natural resources without appropriately valuing the

cost of lost natural capital. An essential first step towards greater

success is achieving widespread recognition that rapid loss of for-

est intactness represents a major threat to sustainable development

and human well-being. Policymakers need to understand the chal-

lenge that the loss of forest intactness represents for achieving stra-

tegic goals outlined in key multilateral environmental agreements,

including the Convention of Biological Diversity, the UNFCCC and

the UN Sustainable Development Goals139,143, and this recognition

needs to be translated into meaningful changes on the ground.

A fundamental constraint to progress is the fact that interna-

tional definitions of forests have not differentiated among types of

forest and, in most policy settings, they treat all forests, regardless

of their condition, as equivalent1,144. As such, international policy

processes seldom acknowledge the special qualities and benefits

that flow from intact ecosystems as compared with those that are

degraded. The consequence is that few policy processes (or partici-

pating nations) clearly articulate conservation goals for intactness,

forest quality or integrity143. There is an emerging, critical role for

the science community to develop policy-relevant metrics of for-

est intactness that account for the different forms and levels of for-

est degradation, and assess how they impact on different globally

important social and environmental values. The lack of recognition

of the varying qualities and condition among forest types has impli-

cations for targeting by international funding programmes such as

the Global Environment Facility, Green Climate Fund and Critical

Ecosystems Partnership Fund, which are distributing billions of

dollars annually to help developing countries achieve the goals of

multilateral environmental agreements. All three of these mecha-

nisms could adjust their criteria for funding so as to explicitly rec-

ognize the value of investments that protect intact forests.

A number of emerging policy opportunities for the global com-

munity to recognize the special values that intact forests preserve,

when compared with degraded ones, are within the UNFCCC.

Because the scientific community has not worked out a practi-

cable definition for emissions from land use, land use-change and

forestry (LULUCF) that would separate direct human-induced

effects from indirect human-induced and natural effects, parties

to the UNFCCC in reporting on LULUCF in their greenhouse gas

inventories may choose to apply the managed land proxy145. Under

the managed land proxy, land where human practices have been

applied is considered ‘managed’ and included in reporting under

the UNFCCC. However, by definition, many intact forest land-

scapes are located on ‘unmanaged lands’ and therefore their contri-

bution to meeting mitigation goals is not quantified or understood.

Increased attention to unmanaged lands, and to transitions between

the managed and unmanaged lands categories, through key venues

such as the Intergovernmental Panel on Climate Change Special

Reports and the Global Stocktake and Facilitative Dialogue will not

just improve understanding of the climate mitigation role of intact

forests but also support nations in articulating interventions, targets

and funding needs for protecting these forests in formulating and

implementing their nationally determined contributions.

Further policy enhancements could be identified in existing

frameworks and programmes for financing for tropical intact for-

est conservation, such as the UNFCCC REDD+ process (reducing

emissions from deforestation and forest degradation and the role of

conservation, sustainable management of forests and enhancement

of forest carbon stocks in developing countries), the Green Climate

Fund and the Forest Carbon Partnership Facility. To date, these pro-

cesses have been focused on rewarding countries and jurisdictions

with performance-based payments for reducing near-term threats

of deforestation and (to a much lesser extent) degradation, based

on a historical emissions baseline. Given this goal of achieving

near-term climate mitigation results (that is, typically within five

to ten years), programme rules often directly limit the eligibility or

amount of support for conservation of intact forests that have, by

definition, low historical emissions from deforestation and degra-

dation, and that may be under threat over one or more decades.

For example, so-called ‘high forest, low deforestation’ nations have

relied on projections that implicitly or explicitly assume higher rates

of emissions in the future. A more straightforward approach would

focus on existing stocks and reservoirs of forest carbon, which could

be elaborated within the ‘+ ’ in REDD+ (the role of conservation,

sustainable management of forests and enhancement of forest car-

bon stocks in developing countries). Such an approach may require

new incentives that differ from and are complementary to existing

results-based payment approaches; instead, they would reward the

long-term maintenance of existing carbon stocks and the other ‘+ ’

activities, and bypass rules stipulating that this financing must

target areas with high historical (‘baseline’) levels of emissions146.

Additional climate-related policy approaches are also clearly needed

for temperate and boreal intact forests, especially those in devel-

oped countries that would not expect to receive finance support

under the Paris Agreement and related UNFCCC mechanisms.

There are current efforts underway to generate new 2030 global

biodiversity targets, and operationalizing a clear, mandated target on

preserving ecosystem intactness is critical to this143. The first steps

are underway, with the International Union for the Conservation

of Nature recently adopting a new key biodiversity area criterion

(criterion C) covering those sites that contribute significantly to

the global persistence of biodiversity because they are exceptional

examples of ecological integrity and naturalness147. If the key biodi-

versity area standard becomes formally recognized within the 2030

strategic plan for biodiversity, this would be a very positive step in

proactively conserving intact forests.

Change in policy at the global level should be reflected in the

design and implementation of effective national and sub-national

policies, and forest management plans that recognize the value of

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PersPective Nature ecology & evolutioN

intact forests to the host nation and specify policies for their pro-

tection and restoration. National and sub-national policies can be

supported by longer-term planning that is incentivized by climate

funding streams (for example, conditional targets in nationally

determined contributions, the Green Climate Fund) that recog-

nize the mitigation contribution of intact forest landscapes. These

policies will vary based on the specific context of different nations,

but there is a clear need to focus on halting degrading activities,

including limiting road expansion142, reducing negative impacts of

hunting through legal controls coupled with sustainable resource

use strategies5, preventing large-scale developments such as mining,

forestry and agriculture in intact forests51, and investing in restora-

tion activities. One obvious intervention that nations can prioritize

is the creation of large protected areas, including transboundary

areas. When well designed, financed and enforced, protected areas

have been shown to be effective in slowing the impacts of industrial

logging3, land clearance148 and over-hunting33,148.

A range of other designations exists beyond protected areas

that can prevent the loss of intactness or promote its restoration.

There is evidence that the designation of ‘roadless areas’ in the

USA, for example, has led to an effective expansion in the degree

of ecoregional representation under protection and increases in the

number of areas big enough to provide refugia for species needing

large tracts relatively undisturbed by people149. There is a need for

mechanisms relating to the private sector that prioritize the protec-

tion and restoration of intact forest, including specific investment

and performance standards for lenders and investors (for example,

the World Bank, International Finance Corporation and regional

development banks) and increasing the effectiveness of existing for-

est and extractive industry certification standards. Recent initiatives

to make supply chains deforestation-free need to be strengthened,

and to include measures to protect intact forests. While there are

some signs of success (for example, the Brazil Soy Moratorium150),

implementation is lagging well behind pledges and it is too early to

demonstrate lasting impacts151.

One emerging strategy that can be effective in slowing the

degradation of intact forests is enabling indigenous communi-

ties to establish title and management over their traditional lands.

Although comprehensive global analyses are lacking, some regional

data reveal the remarkable contribution of stewardship by forest

peoples to sustaining high-integrity forest systems, often in the

face of substantial pressures to liquidate forest timber or mineral

resources. For instance, the creation and management of indig-

enous territories has reduced (although, as with protected areas, not

halted) deforestation across the Amazon Basin152–154. It is believed

over half of the Amazon Basin’s 7 million km2 are under some form

of protection, and nearly 1.8 million km2 are indigenous lands155. In

the boreal north of Canada, First Nations peoples have been able to

sign formal agreements with the government and the private sector

to ensure that national economic development policies and prac-

tices respect their rights and commit to conserving their lands and

waters. For example, the Final Recommended Peel Regional Land

Use Plan, co-developed by the government of Yukon and four First

Nation governments, has an explicit goal of “managing develop-

ment at a pace and scale that maintains ecological integrity”, and has

placed 81% of the 67,000 km2 area under protection156. These exam-

ples are drawn mostly from regions where indigenous peoples live

at very low densities and have made cultural choices not to exploit

the territories they own for timber or minerals; where population

densities are higher, or where communities make different cultural

choices, levels of forest degradation associated with subsistence and

income-generating activities will also tend to be proportionately

higher, as with non-indigenous communities.

Funding for protection and restoration of intact forests could also

be used to establish payments for ecosystem services. The approach

has many challenges, but there are some encouraging examples

where these types of activities are being undertaken. For example,

in Brazil, the Amazon Regional Protected Areas programme, partly

funded by international performance-based payments under a pro-

totype REDD+ framework, supports the creation and management

of protected areas and sustainable natural resource use157. This is

being accomplished in collaboration with local peoples with the

overarching aim to maintain forest carbon stocks and protect large-

scale ecological processes158.

There is also a need for increased efforts to restore the intact-

ness of degraded systems. This should not be seen as a substitute for

conserving fully intact systems in their current state, as forest deg-

radation can often only be partially reversed over reasonable tim-

escales112, and it is generally more cost-effective to conserve at-risk

intact forests than to protect or restore fragmented and degraded

ones. If the goal of restoration is to achieve sustainably managed

production forests, this may serve to alleviate pressure on intact for-

ests, while also providing some biodiversity and ecosystem service

benefits159. Further intensifying production systems in previously

degraded land may allow even more intact forests to be spared.

Such a ‘land sparing’ approach has been shown to achieve biodi-

versity benefits in agricultural landscapes relative to ‘land sharing’

(integrating biodiversity and production objectives on the same

land)160, and emerging evidence suggests the same is true in timber

production landscapes161. In both cases, it is imperative that strong

regulation and governance systems are in place to ensure intact

forests are actually spared in practice; otherwise, the higher eco-

nomic returns that come from intensifying production may create

incentives for further forest degradation162. Nonetheless, in already-

degraded systems, partial restoration will clearly bring significant

environmental benefits in many cases112. Important efforts are being

undertaken worldwide, for example through UN-REDD and the

Bonn Challenge, ranging from enabling natural regeneration, active

replanting of native forests, removal of invasive exotic species163, fire

management164, reconnecting landscapes through the establishment

of corridors165, and ‘rewilding’ initiatives to re-establish top preda-

tors and large-scale ecosystem processes in regenerating forests166.

Conclusion

There are still significant tracts of forest that are free from the dam-

aging impacts of large-scale human activities. These intact forests

typically provide more environmental and social values than forests

that have been degraded by human activities. Despite these values,

it is possible to envisage, within the current century, a world with

few or no significant remaining intact forests. Humanity may be left

with only degraded, damaged forests, in need of costly and some-

times unfeasible restoration, open to a cascade of further threats

and lacking the resilience needed to weather the stresses of climate

change. The practical tools required to address this challenge are

generally well understood and include well-located and managed

protected areas, indigenous territories that exemplify sound stew-

ardship, regulatory controls and responsible behaviour by logging,

mining, and agricultural companies and consumers, and targeted

restoration. Currently these tools are insufficiently applied, and

inadequately supported by governance, policy and financial arrange-

ments designed to incentivize conservation. Losing the remaining

intact forests would exacerbate climate change effects through huge

carbon emissions and the decline of a crucial, under-appreciated

carbon sink. It would also result in the extinction of many species,

harm communities worldwide by disrupting regional weather and

hydrology, and devastate the cultures of many indigenous commu-

nities. Increased awareness of the scale and urgency of this problem

is a necessary pre-condition for more effective conservation efforts

across a wide range of spatial scales.

Received: 14 July 2017; Accepted: 30 January 2018;

Published: xx xx xxxx

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Acknowledgements

We thank the John D. and Catherine T. MacArthur Foundation for funding this research,

and C. Holtz, A. Rosenthal, B. Mackey, D. DellaSalla, C. Kormos, J. Funk, J. Feidler,

S. Lewis, B. Mercer, S. Rumsey, P. Dargusch and E. Sanderson for conversations around

different ideas that have been presented within this manuscript. A special thank you to

B. Simmons for creating the figure in Box 2.

Author contributions

J.E.M.W. and T.E. conceived the study. The remaining authors provided ideas and

critical feedback.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/

s41559-018-0490-x.

Reprints and permissions information is available at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to J.E.M.W.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in

published maps and institutional affiliations.

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