Increasing Human Dominance of Tropical Forests

REVIEW farming and enrichment planting of tree crops led to tropical forests being “cultural parklands” and thus whether current “primary” forests are actually very old secondary forest and forest gar- Increasing human dominance dens (17). Archaeological remains indicate some intensively cultivated areas, including anthropo- of tropical forests genic soil creation in Africa and Amazonia (18), as well as extensively cultivated areas associated Simon L. Lewis,1,2* David P. Edwards,3 David Galbraith2 with ancient empires (Maya, Khmer), forest king- doms (West Africa), concentrated resources [South- Tropical forests house over half of Earth’s biodiversity and are an important influence ern Amazonia near rivers (17)], and technological on the climate system. These forests are experiencing escalating human influence, altering innovation [western Congo basin, 2500 to 1400 yr their health and the provision of important ecosystem functions and services. Impacts B.P. (19)]. These were always a small fraction of started with hunting and millennia-old megafaunal extinctions (phase I), continuing via total forest area. Even when farming collapsed low-intensity shifting cultivation (phase II), to today’s global integration, dominated after the 1492 arrival of Europeans in the Amer- by intensive permanent agriculture, industrial logging, and attendant fires and icas,when~90%ofindigenousAmericansdied, fragmentation (phase III). Such ongoing pressures, together with an intensification of pre-Colombian cultivated land likely represented global environmental change, may severely degrade forests in the future (phase IV, <10% of Neotropical forest extent (13). Additionally, global simplification) unless new “development without destruction” pathways are thetendencytocomparecontemporarychanges established alongside climate change–resilient landscape designs. only to the recent past, known as shifting baselines, gives pervasive underestimates of wildlife abun- he functioning of Earth is dominated by Africa depauperate in tree species (9), while Indo- dance before European arrival in the tropics. For the redistribution of incoming solar radi- Malayan forests are often dominated by one example, 24 million Amazonian turtle eggs were ation through fluxes of both energy and family of trees, the Dipterocarpaceae (10, 11). Differ- harvested in 1719 alone, producing 100,000 liters T matter, within which life plays a pivotal ences in structure occur: Closed-canopy African of lighting oil (20). Overall, although pre-Colonial role. Tropical forests are critical to this func- forestshavefewertreesperhectare(mean,426 human activity altered parts of the tropical forest tioning as a major regulator of global climate, via stems >10-cm diameter ha−1)thanforestsinAma- landscape, low population densities and shifting on August 21, 2015 water transpiration, cloud formation, and atmo- zonia or Borneo (both mean, ~600 stems ha−1) cultivation systems maintained forest health. spheric circulation (1–3). Overall, they exchange (12), while Amazonian forests have shorter trees more water and carbon with the atmosphere for a given diameter (10) and, on average, contain Recent changes in forest function than any other biome: Changes in the balance one-third lower aboveground biomass (AGB) than and health of photosynthesis and respiration in tropical Three major trends dominate tropical forest func- vegetation dominate interannual variability in tion and health in the recent past: conversion to ’ Earth satmosphericCO2 concentration (4). Fur- “Beyond national networks nonforest, mostly for farmland (21, 22) and min- thermore, over half of Earth’s 5 to 20 million ing (23); degradation of remaining forest, via species reside in tropical forests (5, 6). of well-protected forested hunting (24), selective logging (11), fire (25), and Some 1.2 to 1.5 billion people directly rely on landscapes and formal fragmentation and associated edge effects (26); www.sciencemag.org tropical forests for food,timber,medicines,and collective tenure of forest and regeneration of secondary forest (8). Logging other ecosystem services (7), including both closed- is a frequent gateway to degradation and conver- canopy and more open seasonal systems (Fig. 1). lands, large-scale landscape sion, although other routes occur (Fig. 2A). These This multiplicity of forest functions and services planning will be required trends are driven by socioeconomic factors that are underpinned by their diverse resident spe- ” scale from local use to international markets and cies, such that diverse forests are healthy forests to maintain forest health. that occur legally and illegally, making their (8). Here we consider threats to tropical forests management and mitigation complex. and their impacts on forest health and the eco- African or Bornean forests (12). Thus, a priori it is The extent of these changes is large: ~100 mil- Downloaded from system services they supply in three parts: first, expected that different regions may respond dif- lion ha of tropical forest were converted to farm- historical changes since humans began living in ferently to environmental changes. land between 1980 and 2012, a rate of ~0.4% year−1, the tropical forest biome, because impacts can Humans began living in African tropical for- commonly for soybean or oil palm production last millennia; second, the much greater changes ests ~60,000 years before the present (yr B.P.) (21, 22). Selective logging affected ~20% of trop- over recent decades; and third, the future of trop- and have since colonized all tropical forests (since ical forests between 2000 and 2005 (27). Only ical forests, given the twin pressures of further ~50,000 yr B.P. in Indo-Malayan and Australasian a minority remain as Intact Forest Landscapes: agricultural expansion and rapid global environ- tropics, ~10,000 yr B.P. in Neotropics, and ~2000 yr i.e., areas >500 km2 and >10 km wide with no mental change. B.P. in Madagascar). The first impact was hunting, settlements or industrial logging (Fig. 1) (28). with greater fractions of the megafauna becom- Across the world’s extant tropical forests a recent Historical human impacts on forest health ing extinct in more recently colonized biogeo- estimate suggests, 24% are intact, 46% frag- There are five major biogeographic regions in the graphic regions. Thus, whereas only 18% of African mented, and 30% otherwise degraded (29). Be- moist tropics—Neo- (tropical America), Afro-, Indo- megafauna were lost, some 83% disappeared in cause even structurally intact forests are hunted, Malayan, and Australasian (largely New Guinea) South America (13). Regardless of the exact contri- including in protected areas (30), threats are global. tropics, plus Madagascar—each an evolutionary butions of hunting relative to glacial-to-interglacial descendant following the breakup of Pangea climate change, these extinctions likely altered Global carbon and water cycle impacts ~200 million years ago (Fig. 1). Rainforest area plant and animal species composition, nutrient Changes in forest extent alter biogeochemical contractions during glacial periods have left cycling, and forest structure (12, 14, 15). The lower cycles and the biophysical properties of Earth’s AGB of Amazonia may reflect long-term cascad- surface. Net tropical land carbon flux estimates 1Department of Geography, University College London, ing impacts of megafauna loss (12). have high uncertainty, with studies giving net 2 London, UK. School of Geography, University of Leeds, Tropical agriculture began ~6000 yr B.P., with zero exchange or a modest source over recent Leeds, UK. 3Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK. the area affected slowly increasing over millennia decades (31, 32). Net values mask large and un- *Corresponding author. E-mail: [email protected] (16).Thereisdebatearoundtheextenttowhich certain opposing gross fluxes: to the atmosphere SCIENCE sciencemag.org 21 AUGUST 2015 • VOL 349 ISSUE 6250 827 FOREST HEALTH from deforestation and degradation (2.0 to 2.8 Pg sive forests contain few to no large-bodied ver- snakes (Boiga irregularis) led to the extinction C year−1); and from the atmosphere in intact (0.5 tebrates (20). Increasing rarity raises prices and of all forest bird and bat frugivores, stopping to 1.0 Pg C year−1) and regenerating forest [1.4 makes it economically viable to seek out even seed dispersal services, including to secondary to 1.7 Pg C year−1; the three pairs of figures are the last individuals of a species (36). For exam- forest areas dominated by an invasive tree spe- central estimates, the first from (33)andthesec- ple, the last Javan rhino (Rhinoceros sondaicus cies, thereby arresting its carbon stock recovery ond from (34), over the early 2000s]. Thus, intact annamiticus)inmainlandSoutheastAsiawas (44). Away from islands, an invasive fungus has forest provides a valuable service, avoidable emis- shot in 2010 for its horn, commanding a higher contributed to the extinction of several main- sions from deforestation and degradation are price than gold (Fig. 2B) (37). land Neotropical amphibian species (45). globally significant, and substantial carbon seques- Large-bodied vertebrates, which disperse large- tration via permanent forest restoration is pos- seeded trees, are vital to healthy tropical forests. Directly degrading tropical forest health sible (compared to 7.8 Pg C year−1 emitted from In their absence, seed dispersal becomes more Over 400 million ha of tropical forest are within fossil fuels

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