It is fascinating to consider the effects which agriculture has had on this planet over the thousands of years since it originated. The conversion of forest to agricultural land has had numerous repercussions on the physical and biological environment. Conversion increases albedo levels (the proportion of light energy which is reflected from the land surface), increases heat transfer to the atmosphere, reduces evapotranspiration from plants and trees, compacts soil (which increases rainfall runoff), increases erosion, and affects air turbulence (and therefore air movements and winds). From a biological point of view, conversion has led to a loss of biodiversity, movements of species around the world, shifts in local plant and animal populations, the destruction of ecosystems, and the invasion of exotic organisms and diseases into areas where they are not endemic.
Agricultural land differs in almost every respect from the original forested land. The removal of the vegetation cover and its alteration during the conversion of forested land to agricultural land lead to:
a. Chemical, physical and biological alterations in soil: After forest conversion, the soil environment is seriously perturbed. The soil structure often becomes compacted, chemical processes in the soil are disrupted, and the diversity and quantity of soil microbes declines.
i) Erosion: the removal of the vegetational cover for agricultural purposes reduces the proportion of rainfall absorbed by the soil, leading to runoff and erosion. The silting of streams, rivers, lakes and estuaries results, reducing water supplies, lowering fish survival, and inhibiting photosynthesis in water plants. Erosion additionally depletes the nutrients in the soil, which means that it cannot sustain as much plant growth as formerly. In Malaysian rainforests, erosion removes approximately 24.5 m3 of soil per square kilometer per year; in tea plantations which have replaced rainforest, the erosion rate is 488 m3 per kilometer per year (Jacobs, 1988).
ii) Alterations in soil microorganisms: Soil microorganisms are closely adapted to their environments and the plants that inhabit it, and they regulate decomposition and nutrient availability in the soil. They are vital also for the cycling of organic compounds from soil to vegetation and back again. Studies have shown that the variety of species and the abundance of organisms in the soil in tropical agricultural lands is less than 50% of that of the primary forest originally on that land (Matson, et al., 1997).
iii) Loss of organic material in soil: During land conversion to agriculture, organic matter is lost from the soil. Much of the vegetation which provides organic material to the soil is removed, so there is less humus, the loss of which leads to alterations in soil structure, lessened water retention, and lowered fertility. In tropical soils converted to agricultural purposes, soil carbon can drop more than 50% within five years (Matson, et al., 1997).
iv) Soil compaction: When tropical soils are cultivated, they become substantially compacted. This is due to the use of heavy machinery, trampling by livestock and so on. The porosity of the upper layers is reduced, particularly when the land is used for pasturage. When porosity declines, drainage is poor and gas diffusion reduced, which profoundly alters the composition of the soil flora and fauna by reducing their abundance and biodiversity by more than two-thirds. These changes may act to compact the soil even further because compacting and decompacting forces become unbalanced after deforestation. Many South American pastures have been invaded by exotic grasses and by a type of earthworm which can constitute more than 90% of the soil invertebrates. The “casts” from these earthworms cover the soil surface and lead to substantial compaction, and they impede gas exchange and encourage methane production (Chauvel, 1999).
b. Reductions in biodiversity: One of the more obvious effects of the conversion of forest land to agricultural uses is the loss of animal and plant diversity. In many cases, the very complex ecosystems of the forest are reduced to a simple system of only one or a few crops – cattle, oil palm, or rubber. Many, if not most, rainforest animals require either undisturbed forest or well-grown secondary forest. Many cannot survive in small fragments, as their ranges are too large, or their distribution (especially in the case of trees) is too sparse for adequate reproduction. Again, the presence of large open areas discourages the growth of shade-loving seedlings, and encourages only those tolerant of high temperatures and hot sun. Many animals cannot cross even moderate-sized open spaces and thus are trapped in forests fragmented by agricultural plots. Uhl and Parker (communication to Dobson, 1995, p. 234) have calculated that a hectare (1 ha = 10,000 m2) of rainforest can support about 800,000 kilograms of animal and plant mass. Used for pasture, 6 m2 of land (1/1666 of a hectare) can provide the meat for a quarter pound hamburger, (and, by extrapolation, one hectare of land could theoretically provide 1666 small hamburgers, weighing approximately 189 kilograms, compared to the abovementioned 800,000 kilograms of biomass). Even the tiny area of 6 m2, however, as forest, could support one sixty-foot tree, 50 saplings, the seedlings of twenty to thirty species, which vegetation could sustain many birds, thousands of arthropods, and many transient amphibia, reptiles, and mammals (Dobson, 1995). Or, as Prance put it, a “cow lives on an area [1 1/2 ha] that could have had over 700 individual trees of about 200 species, and many other plants and animals as well – an enormous natural biomass with much greater productivity and value than that offered by the skinny, malnourished cow that now wanders around the weed-infested pastures that have replaced the forest.” (Prance, 1986, p. 84)
c. Depletion of forest ecosystems because of the spread of pathogens and the incursion of exotic species: Because of the prevalence of monocultures and the importation of exotics, agriculture is an inviting feast for pathogens, because there are large stands of uniform hosts. Epidemics in agricultural areas can spread to nearby forests, particularly when they are fragmented. An unexpected effect is that forests may be cut in an attempt to find areas which are not contaminated with the pathogen. This happens particularly in large-scale agricultural operations, such as occurred in Central America when banana plantations were ravaged by the fungus Fusarium oxysporum. New plantations then had to be cultivated by cutting virgin forest. Banana companies now own great tracts of land so that, if this scenario should be repeated, they will have pristine land in which to make new plantations. Recently it has been suggested that the malaria parasite evolved from a common ancestral population around the time of the development of agriculture. Agriculture, by enabling a great increase in human population, and with its large areas of standing water promoting the breeding of mosquitoes, apparently provides an excellent milieu for the spread of the malarial parasite Plasmodium (Pennisi, 2001).
d. Chemical contamination of soil and water and alterations of natural mineral cycles (carbon, nitrogen, phosphorus): In a natural tropical rainforest system, the input of gases and chemicals from the environment is approximately equal to the outgo, but these connections to the outside environment are small compared to the internal cycling of chemicals from vegetation/animals to soil and back again. This cycling is severely altered in agricultural systems since the quantity of vegetation is much reduced and the crop is removed from the system, thus depleting it of essential organic matter. Because of this, nutrients must be added in the form of fertilizer (mainly nitrogenous). The use of fertilizer adds another dimension to this equation, as it substantially alters the global nitrogen cycle. Only half of the nitrogen and phosphorus from fertilizer is utilized by the crops; the other half remains in the ground and enters the groundwater. Both phosphorus and nitrogen cause eutrophication of waterways – nitrogen of estuaries and coastal waters, phosphorus of lakes and streams. Eutrophication frequently leads to toxic blooms, the loss of biodiversity, and changes in species composition in aquatic ecosystems. Nitrous oxides and ammonia (a nitrogen compound, NH3) enter the air and change atmospheric chemistry; they contribute to the greenhouse gas load, they contribute to acid rain and are important components of smog. All in all, increases in nitrogen and phosphorus levels can cause great losses in biodiversity and radical alterations of both aquatic and terrestrial ecosystems. Pesticides used in agriculture are toxic, and can damage adjacent forests. Some of them mimic natural animal and plant hormones and others are immunosuppressants, further damaging the survival of plants and animals. Tilman, et al.(2001b), estimate that pesticide production will increase to 2.7 times present levels by 2050 (mean projection), with the expected consequences of declining human (and animal) health and continued environmental degradation.
e. Detrimental alterations in water supplies and in waterways: Irrigation of converted lands leads to salinization (salt deposits), water logging of soil, high nutrient levels in waterways in the vicinity of agricultural areas and water depletion in streams, rivers and other waterways. Meanwhile, agriculture consumes approximately 70% of the fresh water used by humans – 35% of the total available fresh water (Vitousek, et al., 1997; Johnson, Revenga, & Echeverria, 2001).
f. Displacement of native species and disruption of ecosystems by the introduction of exotic species: Many forest species are threatened by the invasion of exotic species introduced either deliberately as crops and livestock or inadvertently. Many of these have no natural enemies in forest systems and are able to invade the habitats of native species, driving them to population declines or to local extinction. Others act as pathogens, parasites and predators of local species. These biological “invasions” are very extensive and many are irreversible. They at the least disrupt local ecosystems and drive losses in the biodiversity of native species and populations.
g. Soil depletion and loss of productivity: Many farms are established by small-scale cultivators who follow logging roads into the forest. Once roads have penetrated the forest, access becomes easy, and people who are fleeing the poverty of cities or worn-out farms (often rain forest land which has been degraded by agricultural activities) follow and establish small agricultural or ranching operations. When the nutrient level of the land decreases sufficiently, they abandon these farms and penetrate farther into the virgin forest, leaving degraded fields behind. Often this deserted land is unable to regenerate forest and becomes scrub or wasteland. This has occurred in northern Vietnam (among many other places) where Hmong farmers kept cleared areas under cultivation to the point of soil exhaustion. These areas have become permanent grasslands, no longer able to sustain a forest (Fox, et al., 2000). In Malesia, huge areas of “lalang” (Imperata cylindrica), a tough, aggressive, and virtually inedible large grass, have replaced former forests converted to farms. In Indonesia 8.6 million hectares [86,000 km2] are already covered with Imperata, and 2000 km2 more are lost to this grass yearly (Jacobs, 1988). It forms a monoculture with little diversity and supports few animal species, particularly in comparison with the original tropical forest. Imperata is extremely difficult to eradicate because herbicides are prohibitively expensive, as is manual removal. Farmers who burn the grass find that this activity reduces soil nutrients and only enhances the regeneration of the grass – a “double negative” score.
h. Increase in surface albedo and decrease in surface roughness, both leading to temperature increases and decreases in precipitation.
All of these consequences are related. As mentioned above in numerous places, most tropical soils are not very suitable for agriculture and their fertility is transient when the vegetation is removed. The ecosystems which replace rainforests are less productive than they are and are not as valuable economically. Often, agricultural practices and crops planted on deforested land are unsuitable for local conditions. Also, when many tropical soils are farmed or grazed, they rapidly become infertile. If the forest is not allowed to regenerate, the soil will be permanently damaged. Then, yet more forest needs to be cut. Despite the patent unsuitability of many tropical forest areas to support agriculture, governments in tropical countries frequently encourage and subsidize the migration of people into forests. The government often stipulates that one can take title to land only by “improving” it, i.e., by cutting down the forest and cultivating the land or by building a house on it.