Rainforests have always been subject to destructive natural forces -storms, landslides, floods, mud flows, volcanic eruptions, high winds, fire (mainly from lightning), drought, and climate change. These factors have recently been overwhelmed by anthropogenic (human) forces, however, as rainforests are being cut down everywhere at a very high rate. Approximately 50% of land which could support tropical rainforests now lacks it because of human activities. Deforestation is not a recent human activity only, as rainforests have been inhabited by humans for thousands of year. Charcoal deposits, indicative of human activity, have been found in the sediments of many tropical rainforests which we regard as virgin forests. Many patches of rainforest also show peculiar distributions of plant species some attribute to human “enrichment” of the forest with useful species. Granules of various starchy root vegetables have been detected on 5000 – 7000-year-old stone grinding tools found in Panama, indicating that people were raising crops in forests a very long time ago (Piperno, et al., 2000). However, at that time, human populations were low, and the forests regenerated.
Originally tropical rainforests covered 15-18 million km2 of land surface, but by 1989 this area had been reduced by human activity to less than eight million km2. The total area remaining as tropical rainforest is even smaller now, as rainforests are being removed at a rate of 100,000 to 200,000 km2 per year, with approximately the same area being greatly disturbed (Skole and Tucker, 1993; Katzman and Cale, 1990); some say, several times as much (Pimm, et al., 2001). Sponsel, Bailey and Headland (1996) estimate the deforestation rate as 142,000 km2 annually. Potter (1999) estimated that 13.7 million hectares are deforested annually in developing countries, equivalent to 137,000 square km, but down from 15.5 million hectares (155,000 km2) per year in the 1980′s. Achard, et al., (2002) give figures of 5.8 million hectares (58,000 km2) annually, as well as 2.3 million hectares (23,000 km2) degraded in the humid tropics. Meadows, Meadows and Randers (1992) estimate that 17 million hectares (170,000 km2) per year of rainforest (2.1% of total area) were cut in 1990. The discrepancies and uncertainties are due to the fact that the main means of measuring deforestation – satellite imaging – is only approximate, and may underestimate the actual area cut. These rates, however approximate, indicate that about 1.8% (in Brazil, probably 2.6%) of the remaining tropical rain forest is disappearing annually, twice the rate of 1979. One Florida per year is being destroyed; one football field is cut per second. While the highest rate of deforestation is occurring in Southeast Asia, about 70% of the area deforested (approximately two million hectares per year) is in the Brazilian Amazon, followed by Indonesia and Zaire (Laurance, 2001; Skole and Tucker, 1993). The Amazon has lost about 14% of its rainforest, while 40% has been damaged by fragmentation. In the countries of Benin, Ivory Coast, Ghana, Nigeria, and Togo in Africa; western Ecuador, El Salvador, Atlantic Brazil, and Haiti in the Neotropics, the rainforests are virtually gone. In The Philippines, dipterocarp forests were reduced from 16 million hectares in 1960 to less than one million hectares twenty years later (Repetto, 1990). Thirty per cent of Ethiopia was forested in the 1960′s; by the late 1980′s, this had been reduced to perhaps 1% (MacNeill, 1989). In 1957, 57% of Myanmar (then Burma) was forested; by 1990, perhaps 25-40% of the country was still covered by forest. There is even less now, as Thai and Malaysian logging companies move in to exploit timber which is no longer available in their countries. If this rate were to continue, all rainforests will disappear in less than 50 years; if the rate were to increase exponentially, at the same rate that the human population is growing in tropical countries (about 2.3% annually), then the forests will disappear within 30 years. Projections of the rate of deforestation are complex because future demand is unknown, future human population size is unknown, and the remaining forest is in more remote areas and in terrain difficult for logging. Environmental groups are putting political pressure on governments to save rainforests, which may mitigate some of the pressures facilitating rainforest destruction.
Nevertheless, it is possible that by the year 2010 little will be left of this biome in a primary state, except in the central basin of Zaire and in the western Amazon, and a few other small areas, such as the Guyanas. Even these forests will not last beyond the mid-21st century. Lowland forests, because of their accessibility and many useful products – timber, rattan, fruits – are the most threatened, since they are also prime terrain for cultivation of oil palm, rubber and other crops, and livestock, mainly cattle. Despite these dire predictions, relatively little rainforest is under government protection, (in the Amazon, about 3-4% is protected) and many of these protected areas exist only on paper. Many protected areas are small and fragmented, which is detrimental to their continued existence and to the preservation of biodiversity. The areas which will be preserved are those which are in inaccessible or inhospitable areas, such as mountainsides, rocky or steep terrain, areas which are too dry, too wet, have very poor soils or are otherwise incompatible with agriculture.
Some areas are being reforested (about 10%) but these hardly begin to compensate for losses, and they are nothing like the original forest. Rainforests are not as amenable to reforestation as temperate forests, partly due to the poverty of many tropical soils, the fertility of which is further reduced by logging, and to the extensive erosion which follows logging activity. In the tropics in general, one tree is planted for each ten which are cut; in Africa, this proportion is one to 29 (MacNeill, 1989). And the resulting “forests” are often simply tree plantations without any of the diversity or richness of the original forest. In 1982 Lanly estimated that 11.3 million hectares of mature forest were being deforested annually, of which 5.1 million hectares were left to reforest (become secondary forest) and about one million hectares were forested by human activities. However, the gains in forest area from regrowth cannot begin to compensate for the ongoing deforestation by logging and burning.
We do not know the exact relationship between deforestation rates and loss of biodiversity. Many factors are involved in this complex relationship, since even the removal of a few trees or a few species from a forest may cause a cascade of species losses due to changes in microenvironment, the removal of “keystone” species or disruption of the highly complex interrelationships among species. It has been estimated that on islands where 90% of the forests have been disrupted, 50% of species will be lost (For a discussion of biodiversity loss, see above, Part II, G.) Of course extinction rates depend on many factors – type of forest, soil type, intensity of disruption due to human activities, degree of endemism, extent of land degradation, and so forth. Some species will also survive deforestation – those which can adapt to disturbed forest or exist in human-dominated landscapes. Even in the areas of rainforest which survive, however, the plants and animals will be subject to “edge” effects – heat, light and predation (see above). As forests become ever more fragmented and isolated, the organisms in them are finding it increasingly difficult to survive. Small populations will be unlikely to persevere as their environments change and they are exposed to increasing hunting/gathering pressures and stress. As mentioned above, there is an enormous number of species in these forests, but most are composed of relatively few individuals, randomly distributed throughout the geographical range of that species, or located in one small area only (endemism). In addition, many rainforest species require large, contiguous areas of undisturbed forest; most cannot thrive in degraded habitats. Thus forests which have been fragmented do not contain the full range of forest species. Perhaps 50% of the world’s species are threatened with extinction because of deforestation, and some estimates of the current extinction rate place it at one thousand times the natural one (see Part II, G1).