Emerging Threats to Tropical Forests

The ultimate and proximate drivers of deforestation are changing. In the 1980s and 1990s, an expanding human populace, as manifested by hundreds of millions of small-scale farmers and rural residents living in tropical frontier regions, was often seen as the primary driver of forest loss (e.g., Myers 1993). More recently, industrial drivers of deforestation—such as large-scale agriculture, plantations, and ranching—have risen sharply in importance (Butler and Laurance 2008; Rudel et al. 2009). Industrial activities such as selective logging, infrastructure expansion, and oil, gas, and mineral projects are also fueling a proliferation of roads in frontier regions, which also promote forest loss (see below).

The rising importance of industrial drivers is effecting several important changes in the tropics. First, it is accelerating the per capita rate of forest loss in some regions (Wright and Muller-Landau 2006) because forests are increasingly being felled by bulldozers and other heavy equipment, rather than by small-scale landowners armed only with machetes and chainsaws. Second, it may weaken the historically strong relationship between a country’s population density and its remaining forest cover (Laurance 2007a). A country like Gabon or Cameroon, for instance, does not need a high population density to effect considerable deforestation when virtually every cut log the country produces is being exported to China. Industrialization will surely continue to grow globally as scores of developing nations—such as China, India, Brazil, South Africa, and many others—continue to expand economically. The Millennium Ecosystem Assessment projects a 300–500 % increase in global industrial activity by the year 2050 (MEA 2005). In the future, industrial drivers will continue to be a major driver of forest loss.

We live in an era of unprecedented road and highway expansion. Many tropical regions that were remote and largely inaccessible just a decade ago have now been penetrated by networks of roads (Laurance et al. 2009). The problem with such roads is that their impacts often extend far beyond the road surface itself, unleashing a Pandora’s Box of environmental problems that are difficult or impossible for governments to control. These include illegal land colonization, rampant land speculation, and influxes of illegal loggers, hunters, and gold miners. Fires and deforestation tend to increase dramatically near roads (Laurance et al. 2002; Kirby et al. 2006; Adeney et al. 2009). Such effects are especially severe and extensive around paved highways, which provide year-round access to forest regions and tend to spawn large networks of secondary and tertiary roads (Laurance et al. 2002).

Road expansion is one of the most important factors determining the magnitude and rate of forest destruction. For instance, new roads are projected to have far-reaching effects on future forest disruption in the Amazon (Fig. 5.1) (Laurance et al. 2001). Fortunately, road expansion is highly amenable to policy modification. Carbon-offset funds, for instance, might be used to promote the rerouting or closure of frontier roads in vulnerable regions, in order to reduce deforestation and atmospheric carbon emissions (Laurance et al. 2009).

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In the tropics, one of the most profound changes in the last decade is the stunning rise of timber consumption by China, which has transformed tropical timber markets. China is now overwhelmingly the biggest global consumer of tropical timber, importing around 40–45 million m³ of timber annually (Laurance 2008a). More than half of all timber being shipped anywhere in the world today is destined for China (Globaltimber 2011).

China is being criticized because it has been remarkably aggressive in pursuing timber supplies while being little concerned with social or environmental equity. It has also focused mostly on buying raw timber (round logs) from timber-producing countries, which provides little employment or value adding for timber-producing nations, with most of the profits being realized by foreign loggers, log shippers, and wood-products manufacturers. Finally, it has been a poor global citizen in attempting to combat the scourge of illegal logging (Stark and Cheung 2006; Laurance 2008a), which is an enormous problem in many tropical nations. Illegal logging is slowly diminishing globally but this is despite, rather than because of, China’s influence (Lawson and MacFaul 2010). For such reasons, China is vulnerable to a major boycott of its wood products, two-thirds of which are exported internationally (Laurance 2007b).

As petroleum costs rise, biofuels are emerging as the most likely near-term alternative (Roberts 2005). Most expansion of biofuel production will occur in the tropics. It is where plants, which are the main feedstocks for biofuels, grow the fastest and where land is typically the cheapest. At least 35 million hectares of land—an area the size of Germany—is likely to be devoted to biofuel production by the year 2030, mostly in tropical countries (Sunderlin et al. 2008).

The proliferation of tropical biofuels will promote the conversion of forests and other native ecosystems for biofuel or food production. It will also drive up competition for land and elevating land prices, thereby increasing opportunity costs for conservation (Laurance 2008b). Under such circumstances, carbon trading will become less viable as a means to reduce deforestation and is likely to be competitive only for remote areas or those with poor, unproductive soils (Scharlemann and Laurance 2008). Unfortunately, such areas are under the least under threat of forest conversion.

The United Nations (2011) projects that the global population will peak at 10.1 billion people near the end of this century, from its current total of seven billion. Most of the additional three billion people on Earth will be added to the rosters of tropical nations (PAI 2006). Population growth is one of the most pervasive, ultimate drivers of deforestation and environmental degradation. Population pressures manifest themselves both locally and globally. For instance, growing international demand for wood products, paper pulp, and foodstuffs is helping to spur deforestation across much of the tropics. Population densities and population growth rates both tend to be very high in biodiversity hotspots (Cincotta et al. 2000), which contain large concentrations of species that are both locally endemic and imperiled by extinction. Among hotspot nations, population growth rates are a strong and positive predictor of deforestation rates (Jha and Bawa 2006), whereas population density is a strong predictor of endangered bird and mammal species among continental nations (McKee et al. 2003). Growing human numbers will worsen pressures on tropical forests in many different ways.

We live in an era of astonishing international mobility—one in which people and their associated goods and species are in a constant state of flux. As a result, natural ecosystems are being bombarded by potential new pathogens. The resulting “pathogen pollution” could be a far more serious threat for biodiversity than many appreciate (Cunningham et al. 2003).

Some of the most dramatic wildlife mortality events ever witnessed were caused by exotic pathogens or new disease vectors (Daszak et al. 2000). Examples include the mass die-off of African ungulates from rinderpest, the global extinctions or widespread extirpations of endemic Hawaiian birds from avian malaria, and the near extirpation of Diadema sea urchins in the Caribbean Sea. Today, ebola hemorrhagic fever is burning its way across the Congo Basin, causing mass mortality of gorillas and chimpanzees. Perhaps the most devastating of all contemporary wildlife pathogens is the chytrid fungus currently driving global amphibian declines (Laurance et al. 1996; Berger et al. 1998). Around 200 frog species, mostly from tropical and subtropical regions, have been driven to global extinction or been severely reduced in numbers by the fungus (Skerratt et al. 2007).

Such wildlife pathogens may be merely the tip of the iceberg. As molecular genetics and other techniques to detect foreign pathogens improve, we may discover that emerging diseases are among the most serious threats to biodiversity.

Climate change could affect tropical forests in a variety of ways. The biotas of tropical mountains, which contain many local endemics, seem especially vulnerable to global warming. Warming could shrink and fragment the geographic ranges of montane species adapted to cool, wet conditions, increasing the likelihood of extinction (Williams et al. 2003; Thomas et al. 2004). Warming will also tend to elevate the cloud base on tropical mountains, reducing moisture inputs and increasing thermal radiation and desiccation stress (Pounds et al. 1999). Montane species may also have to contend with the arrival of lower-elevation competitors, predators, and pathogens as climates warm (Epstein 2001; Harvell et al. 2002).

Another key concern is that, in many parts of the tropics, even small reductions in dry-season rainfall could greatly increase the vulnerability of forests to droughts and fires (Cochrane and Laurance 2008). Unfortunately, our capacity to project future precipitation and temperature—and particularly to downscale projections for specific locations—is generally poor (Vera et al. 2006).