|Diversity, Globalization, and the Ways of Nature (IDRC, 1995, 234 p.)|
Savannas are extensive tropical grassland areas that cover more than 10 million square kilometres of the Earth. The largest expanses are found in Africa (nearly 4 million square kilometres), South America, and, to a lesser degree, Asia and Australia. The dominant vegetation in savannas is usually grasses, intercalated with shrubs and trees. In some cases, shrubs and trees dominate, and savannas evolve into other ecosystems, such as:
· cerrado - a type of forest with a lower canopy and a bushier, grassier undergrowth (see Chapter 4);
· caatinga - a low-density, xerophytic ecosystem of bushes and shrubs with some trees characteristic of northeastern Brazil;
· chaco - a more xerophytic forest of low trees and bushes in extensive flatlands of Paraguay and northern Argentina; or
· sudan - herbaceous ecosystems with intercalated trees extending from the valley of the Nile to the Atlantic Ocean along the low tropical latitudes north of the equator.
The main characteristic of present-day savannas is their periodic (usually annual) water deficit. Both vegetation and fauna have adapted to it. Plants, for example, have developed deciduous aerial systems (leaves) and an annual cycle of growth.
Not all savannas are primary ecosystems (which evolve with little or no human intervention). Many, perhaps most, are secondary ecosystems (which result from human action). Frequently, shrubby, bushy, or forest areas have been and are being burned to make way for agriculture, raising cattle, or hunting. It is difficult to know to what extent todays savannas are the result of a transformation of other preexisting ecosystems rather than naturally occurring. In Africa, the savannization process started very early. Setting forest fires is a common hunting strategy in many societies and Africa was the home of hunters for several hundred thousand years (if not millions).
It is likely that fragile, nongrassy ecosystems gave rise to savanna or even steppe environments perhaps as early as the mid-Pleistocene. Obviously, climate also changed during this geological epoch. During humid periods, savannization of deserts and encroachment of forests into savannas took place. However, the effects of geological changes during the last few hundred thousand years have been somewhat obscured by the impacts of continued human action.
After domestication of herbivorous animals and the spread of agriculture (starting about 10 thousand years ago, but intensifying during the last 3 thousand years), the processes of environmental savannization progressed further. Savannization of the Sudanese region is probably related to domestication of sorghum and millet and to the adaptation of previously domesticated animals to local environments - mainly cattle, goats, and sheep, but later also dromedaries in the Sahelian periphery. In the late 19th century and first half of the 20th century, after widespread European colonization, new commercial crops were introduced (such as peanuts), population growth accelerated, and new forested areas were cleared, transforming most of intertropical Africa into savannas. The remaining forests are retreating at a rapid rate.
In South America, the process of savannization is recent. When the Europeans arrived in the 16th century, there were few, if any, typical savannas. Most South American intertropical ecosystems are in subhumid climates with seasonal rains. Apparently during the humid subperiod of the mid-Quatemary, these areas were covered by trees or bushy vegetation (cerrado and chaco). In more arid areas, a less-dense, shrubby or bushy steppe vegetation developed (caatinga). Grasslands were restricted to temperate areas, particularly in the southern cone - the pampas and to the northern Ilanos.
The temperate grasslands were planted with crops such as wheat and barley very early in history. Farming was widespread on all primary prairies of the Eurasian continent, especially in the Ukraine and southern Russia. Similar activities took place later in other regions of the world: the South American pampas, the North American prairies, the South African rangelands, and southeastern Australia. The remaining grasslands are often secondary (regrowth after cessation of agriculture); they are associated with intensive cattle-farming activities (such as dairy farming); or their soils are inappropriate for farming (too stony, rocky, or sandy, or grassy wetland).
In some countries, grasslands with agricultural potential can remain relatively unaffected for various reasons, such as inadequate access to markets or cultural history. This situation exists in South America and, to a lesser degree, in Australia, Texas, and the Transvaal of South Africa.
A typical surviving prairie ecosystem is found in the South American pampas. The pampas are net or undulating landscapes extending for about a million square kilometres between 28° and 40°S latitude and east of the 500-millimette isohyet. This isohyet (a line on a map joining points of similar precipitation) is the approximate boundary between the semi-arid and subhumid areas of temperate South America.
The Buenos Aires pampas are flat and largely used for farming, particularly in the more humid zones (with more than 750 millimetres of rainfall per year). East of the Parana River, the flatlands gradually become more undulating and, in Uruguay and Rio Grande do Sul, Brazil, undulating and flat areas are interspersed with hilly landscapes. It is in these areas that grasslands have remained less disturbed by farming.
It is important to note that the South American pampas are grasslands not because trees cannot grow, but because they cannot compete with the grasses. Human intervention can radically change this situation through the deliberate planting of trees, which are typically exotic but occasionally indigenous. However, artificial forests in the pampas remained a limited phenomenon until very recently.
Globalization of the economy and encroachment of land-intensive and labour-intensive agriculture on some savanna lands is producing some important environmental changes. Undulating and hilly savannas are being eroded by water, reducing the thickness of their soils and, consequently, their agricultural potential. In flatter areas, soils are less affected by water, but may be seriously degraded by aeolian (wind) action. Irrigation may also have long-term consequences, such as waterlogging, salinization, or alkalinization.
In all cases, repeated cultivation of the same plots brings about a gradual loss of nutrients, reducing fertility and having a negative effect on the biological potential of the exploited areas. When fertilizers are used to compensate for decreasing fertility, other side effects can occur. Not all soil nutrients are replaced by the application of fertilizers; some, especially micronutrients, may not be added, and some impoverishment takes place in any case. Second, some nutrients may be applied in excessive volumes with potential deleterious effects, not only on the soil, but also on the natural water systems, giving rise to algal growth and, in some cases, to eutrophication of lakes and reservoirs.
Pesticides can have an even greater impact because of their natural toxicity (obviously, the main purpose of pesticides is to kill pests). They may find their way not only into the leaves, fruit, and vegetative tissues of crops, but also into surface water bodies and groundwater. Some long-lasting pesticides may appear a few years after their application in the water supply of nearby towns or rural communities.
One of the land uses of savannas, particularly in Africa, is for raising livestock. In drier savannas or in steppes, overgrazing can produce extensive desertification patterns. This can be seen in many African countries (particularly in the Sahelian region), where overgrazing has reduced vegetation density and diversity, promoting soil erosion, decay, and loss of productivity.
An important factor associated with overgrazing is the indiscriminate drilling of wells in the countryside. In pastoral societies, cattle ownership is frequently a sign of prestige. Raising animals can be an important social function, allowing a man to find a wife or to obtain more power. The main factor limiting the size of a herd has always been the availability of water. Areas around the main water holes are heavily overgrazed, but peripheral areas may be almost intact. When many new wells are drilled, people increase their herds and move them into the new locations. After a few years, the whole landscape is degraded beyond recognition and irreversible desertification takes place. Examples of this situation can be found throughout the African continent - northern Senegal (Kerkhof 1992, pp. 105-112), northern Kenya, Sudan, Mali, and Niger.
In South America, a number of savanna ecosystems are being used for agricultural purposes. Those of Brazil are typical. The savanna areas of southern and central Brazil have developed as a result of the removal of forests. In Mato Grosso, the tropical rain forest, which was slightly less dense than the Amazon jungle, was gradually burned and logged to make way for various crops, particularly rice and soybeans.
In dry rice farming, fertilizers are usually not applied. Thus, the nutrients in the soil, which, like most tropical soils, has low fertility, are further reduced and the land quickly becomes inappropriate for further cultivation - without heavy applications of costly fertilizers. Normally, after 3 or 4 years of dry rice cultivation, the land is no longer productive and is converted to cattle ranching. (The secondary savanna in Mato Grosso is a low-productivity rangeland for cattle.)
Soybean cultivation is carried out on a large scale, for commercial purposes, on large farms using a similar nonsustainable approach. Fertilizers and pesticides are applied with little concern for the environment, seriously affecting the aquatic ecosystems in nearby streams. Many indigenous communities of South America that depend on fish for their existence have seen their livelihood and incomes curtailed because of aggressive agricultural practices in these secondary savanna lands (see also Chapter 4).
In southern Brazil, from Sao Paulo to Rio de Janeiro, the original vegetation was subtropical rain forest and an almost monospecific ecosystem of Araucaria (Brazilian pine). Sao Paulos forest was gradually eliminated, at the end of the 19th century and during the first half of the 20th century, to make way for coffee plantations and other crops. In many places, a savanna-type vegetation has replaced the forest. More recently, sugarcane (for alcohol fuel production) has been planted throughout the Sao Paulo region, often supplanting coffee plantations. The Araucaria forest suffered a similar fate. From the 1950s through the 1970s, almost all of the forest was logged; ranching and cultivation of crops such as soybean now occur on the secondary savannas that developed. In some cases, the Araucaria forests were replaced with artificial forests of exotic trees.
Modification of natural ecosystems may have unexpected effects on neighbouring farming systems. Planting some types of trees can alter the nesting habits of some birds and favour their reproduction. For example, if a birds natural predators cannot reach the nest, the species may reproduce without constraint. Many crops, including sunflowers, corn, and fruit trees, can be severely affected by increases in the number of budgies and pigeons. Some of these birds may have wide ranges (more than 50 kilometres) and, therefore, may affect crops some distance from their nesting area. Exotic trees (such as pine trees in South American prairies) may also provide a feeding area for birds or insects. Indiscriminate planting of such trees in grasslands and associated farming areas can have a significant destabilizing effect on both the natural ecosystems and the crops.
The eucalyptus tree, which is native to Australia, has a very high growth rate. Because of this, they are planted throughout the world for many purposes. Eucalyptus is also a good nesting tree. It grows to 30 metres or more and, therefore, is difficult for grassland predators (mammals and other vertebrates) to climb. Frequently, farmland close to eucalyptus forests is plagued by an overabundance of birds that nest on the upper branches of the eucalyptus trees, where they thrive in the absence of predators that otherwise could prevent their multiplication (see box 3).
In addition, eucalyptus has been pinpointed as a strong aridifier. Poore and Fries (1987) found that, by its third year, Eucalyptus grandis reduced stream flow (measured as equivalent millimetres of rainfall) by between 300 and 380 millimetres per year. In Nigeria, Sharda et al. (1988) found that eucalyptus trees reduced the amount of water flowing from a basin by about 23%. Similar studies in India revealed a decrease in runoff of 28% (Poore and Fries 1987).
Eucalyptus also effects soil fertility. A decrease in soil nutrients, such as phosphorus, has been recorded in many cases. However, the main effects on fertility come from logging, which removes large volumes of key elements from the ecosystem, reducing its potential for other uses. After eucalyptus trees have been harvested two to four times - over about 30 years - the land is rendered useless for any other productive purpose. Not only do the soils become infertile and dry, but they are also crisscrossed by the remains of many root systems. Farming is impossible without major, expensive corrective measures, which are not cost-effective for most agricultural activities. For all practical purposes, eucalyptus soils are no longer usable for farming.
In a number of African projects, described in Agroforesny in Africa (Kerkhof 1992), eucalyptus trees were introduced to traditional African societies without much consideration of their suitability to local ecosystems or cultures. As in Uruguay, the effects have frequently been deleterious to the local environment or quality of life. In spite of this result, strong pressure and funding is coming from many sources to expand this nonsustainable type of forestation. Uruguay, for example, received over $50 million from international banks for such forestation projects.
3. Planting trees in grassland ecosystems
Lately, as a result of globalization and the transfer of labour- and land-intensive activities to developing countries, large-scale forestation has reached the pampas. The trees used are mainly eucalyptus and pine: Eucalyptus globulus, E. grandis, E. rostrata, Pinus pinaster, and P. maritimus. Large tracts of former grasslands have been dedicated to monospecific plantations of these trees, which are grown for lumber, fuel, or paper pulp, often for export, but also for consumption in the pampas countries.
The consequences of this widespread planting of exotic trees are becoming apparent. Indigenous trees are not very tall and are sparsely distributed. The new, exotic trees offer an interesting nesting niche for many birds that have spread beyond their natural habitat.
The prairie predators of birds - raccoons, foxes, opossums, wildcats - are unable to climb the tall trees, resulting in an population explosion of some species of birds, such as budgies and wild pigeons. These birds plague nearby crops, reducing their quality or destroying them completely.
Although the eucalyptus plantations are the main nesting habitat, they do not provide a feeding ground for birds; there is little or no undergrowth in the forests and the seeds of eucalyptus trees cannot be eaten by most birds. As a result, birds make feeding forays into nearby agricultural crops. These crops supply food for only a few months of the year, usually late spring to fall. Artificial pine plantations provide food for the birds during winter. The frequent association of eucalyptus and pine trees in agricultural areas can be a recipe for disaster. Eucalyptus trees provide a nesting place, pine trees the winter food, and crops the main staple for the rest of the year. A new artificial ecosystem is formed, but it is not in the best interests of farmers nor does it contribute to local biodiversity (Gutierrez et al. 1993).
The balance of the effects (actual and potential) of globalization in the grasslands is unfavourable. Most savannas are the result of the degradation of forests. Although their soils are fertile, the fertility is often inherited from the former ecosystem and is substantially lower than it would be for a climatic savanna. Continuous farming on savannas gradually reduces fertility and, unless fertilizers are applied, agriculture becomes unproductive. In many cases, the use of fertilizers makes farming uneconomical and savanna lands are abandoned for other uses, usually raising cattle or planting with exotic trees, further unbalancing the ecosystem. Where slopes are steeper, the soft, highly friable savanna soils are easily eroded, further reducing their productivity.
These ecological changes bring about important geomorphologic and hydrologic changes. Gullies and ravines are formed by erosion, and stream-flow patterns become irregular (more severe droughts, increased frequency and intensity of floods). Navigation canals may be blocked by sediments, dams and reservoirs may become useless (or their life span may be considerably reduced), water intakes may became clogged, water-treatment plants become more difficult and costly to operate, and so on.
Envionmental degradation of prairies is also frequently the result of increased runoff, soil erosion, and related geodynamic phenomena. In addition, there is a growing risk of soil deterioration as a result of care less forestation.
The belief that planting trees is good does not necessarily hold in grassland areas, where forestation may bring about an ecological imbalance that may end in a substantial reduction not only in productivity, but also in the systems biodiversity. As in other ecosystems, the sustainability of grasslands depends on recognizing the value of their diversity. It is through its resource base that the system can maintain the flexibility it needs and the potential for sustained use in the future.