Desertification and its causes
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Concepts and Definitions
Desertification (also called desert creep, desert encroachment, desert formation, etc.) comes from the French meaning "making deserts." There are two schools of thought regarding its cause: (1) long-term (in the geologic sense) climatic change and (2) the destructive consequences of man's activities. The possibility of climatic change being responsible has prompted as many theories as controversies.
Instrumental climatology, the precise, modern science based upon actual measured observations, dates only from the middle of the nineteenth century, which is hardly enough time to detect climatic changes in geological terms. Desertification is occurring at a rapid rate in certain situations. Lamprey (1975), by comparing reconnaissance surveys in 1958 and 1975 in the Sudan, asserted that the desert's southern boundary had shifted south by an average of 90-100 kilometers during the 17-year period. This represents a rate of between 5.3 and 5.9 kilometers per year. Such a rapid rate of vegetation change would require a climatic change of huge magnitude and one that certainly would be measured by meteorological instruments.
Desertification symptoms have been observed to occur in patches only a few kilometers apart or even on the other side of fences. This is illustrated in Figures 2, 3 and 4. Figure 2 is a view of two different ranches in the Sertâo region of northeast Brazil. Vegetation on the ranch on the left is approaching the last stages of the desertification process. Desirable and environmental stabilizing vegetation dominates the ranch on the right. This is due to management.
Figure 3 is a view of an undisturbed range site in northwest Mexico which has not been grazed, at least heavily, because of the lack of livestock drinking water. There is no evidence of desertification symptoms. However, the same kind of range site, as shown in Figure 4, has been decertified because of prolonged destructive grazing pressure. These areas are near each other and the differences in vegetation cover could not be due to different climates.
Climatic change should not be confused with weather fluctuations or with micro-climatic changes. Climate or macroclimate is a long-term average of weathers. Weather is the day to day, month to month and year to year occurrence of climatic features: precipitation, temperature, wind, humidity and radiation. Weather fluctuates and cold spells, heat waves, wet spells, dry spells and droughts are normal. The media and general public often refer to "normal" rainfall, "normal" temperatures, " etc. The only thing "normal" about weather is its variability. However, patterns of recurrent droughts, possibly cyclic in nature, appear to be "normal" for the arid and semiarid regions. This is important regarding desertification and its control.
Macroclimate is the average of weathers as measured by meteorological instruments, all of which are located above ground and some of which are located a meter or more above the soil surface. Microclimate is the climate in which plants actually live including conditions both above and below ground. For example, if only 200 mm of 400 mm of annual rainfall measured in a gauge is available for plant growth because of runoff and evaporation, the plants are actually living in a 200 mm rainfall habitat, where only xerophytes can survive.
Microclimates can change, for better or worse, depending on man's interventions. Man has no control over weather and macroclimate, but he does have control over microclimate. This is fundamental to the understanding of what desertification really is and how it can be controlled and rehabilitated.
There is a possibility that there is an irreversible warming trend because of an increasing greenhouse effect due to an artificial elevation of atmospheric gases such as carbon dioxide, nitrous oxides and methane. The release of extra CO2 into the atmosphere is due to man's prodigious use of fossil fuels. Some claim that this is largely due to the energy required by livestock production systems, especially beef production in the U.S.A. Yet, Byers (1990) calculated that agriculture uses less than 3% of the fossil energy used in the U.S.A. and beef cattle production directly uses only 5% of this 3%. It is also claimed that the burning of grasslands and forests releases tons of carbon into the atmosphere. Yet the pyric factor is one of the factors involved in the formation of pristine vegetation. Grassland fires likely occurred in greater frequency and intensity in the past than at present (Billings, 1966). The consultant does not subscribe to the widespread destruction and burning of forests in Latin America.
Ruminants generate methane, but all ruminants are not domesticated. There are millions of head of ruminant wildlife and their numbers in the past, in some situations, were greater than the current number of domestic ruminants. The American bison existed in greater numbers than the current number of cattle in the Great Plains of the United States. Claims that domestic ruminants contribute significantly to global methane production and global warming is highly questionable. Obviously, any warming trend is due to man rather than animals. Man has caused and is causing desertification and only man can undo what he has done and is doing.
Figure 5. Schematic illustration of desertification process.
Table 2. Classification of the world's climates according to average annual precipitation (Willie, 1962)
|Average annual precipitation (mm)||Classification||% of Land Surface|
|Less than 500||Arid and Semiarid||55|
|500 to 1,000||Subhumid||20|
|1,000 to 1,500||Humid||10|
|More than 1,500||Very Humid||15|
The end product or the last stage of the deterioration process is called either "desertized" or "desertified." Figure 5 is an attempt to schematically demonstrate the process. The symptoms and manifestations of desertification are highlighted in the Figure. The degree of departure of these from an environmentally stabilized plant community represents stages of desertification. The stages can be measured or at least estimated on rangeland ecosystems and perhaps other ecosystems as well.
The rate of desertification as measured in terms of years depends on a multitude of factors beginning with the character of the original plant community and the kind and intensity of misuse. A desertified ecosystem is irreversible regarding natural regeneration. It appears that it takes a very long time for rangelands to become desertified due to overgrazing. This is evidenced by the rangelands in the Near and Middle East which have the longest history of domestic livestock grazing in the world. These lands have been grazed for centuries and heavily overgrazed for the last 50 or more years. Yet, many have not reached the decertified stage (See Figure 1).
Reversibility or natural renewal is essential for desertification control and rehabilitation of the Region's rangelands. Most of the stages of degradation of the desertification process as shown in Figure 5 are reversible. Planners and policy makers need to be aware of this and implement appropriate actions.
Slightly more than one-half of the world's climates are classified as arid and semiarid (Table 2). Further sub-division of the arid and semiarid areas is loosely defined. Generally speaking, deserts receive 100 mm or less of rainfall, arid lands receive between 100 and 300 mm and semiarid
Areas with limited options generally confined to native pastures and extensive animal production. Irrigation possible in some situations.
Slightly more options than for arid areas. Primarily limited to native pastures and livestock production, but food crop and fodder production are feasible in specific areas. Irrigation also possible in some situations.
Although all climatic areas are subject to desertification the arid and semiarid areas in general and the arid areas in particular, are highly susceptible because they are only a few millimeters of rain away from being true deserts. These areas also represent the heart of the Region's livestock industry.
Arid and semiarid areas have harsh climates made more harsh by weather fluctuations. There are normally many more years of below average rainfall than average or above. Prolonged droughts occur frequently. Droughts occur so frequently in northeast Brazil, for example, that the area is called the drought polygon. Weather variation, per se, does not cause desertification, but it is an accelerator to desertification, especially when man continues land uses supportable only by average or above average rainfall. Failure to plan for the "bad" years and modify land use accordingly accelerates the desertification process.
Around 75% of the Region's total land area is considered to be arable (FAO, 1991). These lands are capable of sustaining cultivation. Unfortunately, attempts to meet the demands of a rising population have resulted in cultivation of rangelands and, in some cases, forest. These lands are incapable of sustaining cultivation for various reasons.
The common practice of cultivating steep slopes causes degradation. Fortunately, in some cases the system slash-burn-plant-harvest-and-shift to another spot does provide some environmental protection and rehabilitation in the intervals between plantings. The system does provide the rural inhabitants with some food and perhaps some income. The question is: "What could these people do if shifting agriculture were not available to them?" Shifting agriculture is not only a land-use problem, it is also a socioeconomic problem which must be solved. The development of conservation farming systems applicable to this situation is badly needed.
Mechanized farming of rangelands greatly accelerates the desertification process. The tractor and plough can destroy the living components and greatly damage the non-living component of an ecosystem in minutes. Continued cultivation until abandonment becomes necessary leaves a decertified condition unfit for man and beast. This practice also contributes to overgrazing because the area for grazing is reduced and only the most productive range sites are cultivated. This results in less and less land for grazing with lower and lower grazing capacities for more and more livestock.
Many forest soils, especially tropical forest soils, cannot sustain agriculture. While this is connected with deforestation, attempts to convert forest lands into arable lands can also create a condition unfit for man and beast. Establishment of cultivated perennial pastures is better than cultivation because of some environmental stabilization. Nevertheless, desertification has taken place.
Deforestation should not be confused with brush control. As previously stated, many of the Region's rangelands have been invaded by undesirable and high water-requiring woody species. This invasion is a desertification symptom because, if for no other reason, it upsets water regimes (See Figure 5). Brush control, provided it is followed with proper management, is a desertification control measure.
Deforestation of true forests in Latin America and the Caribbean is a serious problem. Table 3 describes current rates of forest destruction in selected countries, and the destruction is likely to continue (Belk, et al., 1990A).
Livestock grazing does very little damage to mature forests, although it can interfere with natural reforestation. Deforestation is due to man's decisions to expand cultivation and ranching to meet an increasing demand for land forced by an ever increasing population (Table 4). The establishment of cultivated perennial pastures does create a new ecosystem which provides some environmental protection, provided the pastures are properly managed afterwards. Regardless, many of the desirable attributes of the original forest ecosystem are lost. As a famous ecologist, E.J. Dyksterhuis, said, "a grassland invaded by woody species is not a 'woodland' and a forest converted to pasture is not a 'pastureland'. "
There is no evidence that tropical forest defoliation in the Region is due to an American (U.S.A.) hamburger diet. Belk, et al. (1990), concluded; "Central America and Brazil combined contributed only 9.5% of total U.S. beef and veal imports. This figure can be further defined to show that less than 0.6% of the total beef consumed in the U.S. was from Central America or Brazil. It is important to point out that the contributions by Australia and New Zealand to total U.S. beef and veal imports were almost entirely made up of frozen, boneless beef for grinding ( > 90% lean) thus, a complete boycott of fast-food hamburgers would affect only Oceanic imports and would not resolve the rainforest destruction problem. "
Moreover, the U.S. does not accept meat from countries with endemic or sporadic foot-and-mouth disease unless the meat has been previously cooked. Brazil exports only canned and frozen cooked meats to the U.S. which are not used in the fast-food hamburger industries.
Table 3. Rates of deforestation in selected tropical Latin American countries, 1980-85. (Adapted from Belk, et al., 1990B and World Resources Institute, 1986 as cited by Belk, et al., 1990A.)
|Country||Closed Forest Area, 1980 (1,000 acres)||Annual Rate of Deforestation, 1981-85 (%)||Area Deforested Annually (1,000 acres)||Categorya|
a I = Higher than averages rates of deforestation and large areas deforested, II = Relatively low rates of deforestation but large areas deforested, m = High rates of deforestation and small areas of remaining forest, and IV = Low or moderate rates of deforestation and small areas affected.
Table 4. Population growth from 1975 to 1991 in selected Latin American countries! (FAO, 1991).
|Country||Population in Thousands 1975||% Increase 1991||(Rounded)|
1 Selected to correspond with those countries in Table 3 undergoing vast deforestation.
Ever Increasing Population
Table 4 is an analysis of the population growth from 1975 to 1992 in selected countries. These were selected to correspond with those countries in Table 3 where tropical forest deforestation is largely taking place. Except for Cuba and El Salvador, the population increased forty or more percent in all countries during this sixteen year period. The percent of increase ranged from 40% in Colombia up to 72% in Honduras. The average was 51.9%. Increases in Cuba and El Salvador were 15 and 32%, respectively.
This analysis indicates that the population is overcoming or has already overcome the capacity of the land. Since the people have no other recourse, they turn to the forest and the rangelands for the cultivation of their food crops. Compulsory abandonment after a few years owing to the collapse of soil structure and fertility leaves decertified land.
This very serious problem made by man is in dire need of solution. The establishment of biosphere reserves, forest reserves and national parks to protect the environment is commendable, but this does very little to help the people who have no other place to go. The question is, "What can and should be done?"
The number of small trees and shrubs that are either cut down or uprooted for fuel is enormous. In Jordan, it was estimated that Bedouins annually uproot 182 million fodder shrubs just for cooking and in the Sudan, 548 million acacia shrubs are annually destroyed for the same purpose (FAO, 1976). The uprooting of fodder shrubs and trees puts man and his livestock in direct competition which contributes even funkier to desertification especially with overgrazing.
Data regarding the extent of fuel gathering in Latin America and the Caribbean are limited. Stuart (1992) reported that each family in Guatemala's Petén region consumes 10 to 20 cords of wood a year, requiring hundreds of trees. Similar consumption rates are likely applicable in many other countries where fossil fuels are either not available or too expensive. Fuel gathering is an important factor contributing to Haiti's serious desertification problem. As in the Near East, people and livestock in Haiti are too often in direct competition with each other. This, coupled with destructive grazing, and other land misuses, is resulting in the country's environmental destruction.
Continued Destructive Grazing
An understanding of this subject requires some knowledge of certain definitions. Stocking rate is the number of specific kinds and classes of animals grazing or utilizing a unit of land for a specified time period. The widely publicized term "carrying capacity" is an original wildlife term. In its true sense, it means the maximum number of individual animals that can survive the greatest period of stress each year on a given land area (ASRM, 1964). In other words, where the stocking rate greatly exceeds the feed supply, some animals die, some survive and a balance is restored without permanent damage to the habitat.
Grazing capacity refers to domestic animals. It is the maximum stocking rate possible which is consistent with maintaining or improving vegetation or related resources. It may vary from year to year on the same area due to fluctuating forage production owing to fluctuating weather. Grazing and carrying capacities have become synonymous over the years. The term "grazing capacity" is used in this document in the context described above. Carrying capacity is used only where applicable as previously defined.
A stocking rate that exceeds grazing capacity, even slightly or for a short period of time, is commonly called "overgrazing." A stocking rate less than grazing capacity is commonly called "undergrazing. " Range scientists have more specific descriptive terms which are discussed in more detail in a later section.
Overgrazing is categorically blamed for worldwide desertification, which is partly true and partly false, depending on the situation. There are various overgrazing intensities such as slight, heavy, very heavy and destructive. There are also overgrazing durations which can be measured in months, years, decades or even centuries.
Apart from prolonged droughts, acceptable and fixed stocking rates will cause temporary overgrazing in some years because of grazing capacity fluctuations. Damage to the vegetation, if any, is usually repaired by natural processes. Designed temporary overgrazing is useful in desertification control and reversal of the process. Many range improvement technologies such as rotation, deferred rotation and rest rotation grazing systems include temporary overgrazing. Sheep and goat overgrazing can be used to control undesirable and noxious weeds and brush without damage to the environment. To the contrary, the microenvironment or microclimate is improved.
In some cases, overgrazing has led to the creation of new ecosystems which are just as productive, if not more, and as environmentally stable as the original one. The Edwards Plateau of Texas is an example. The original vegetation of this area was suitable mainly for cattle and a few deer, turkey and other wildlife species. It has been transformed into one of the most productive livestock and wildlife areas in Texas. It is now suitable for cattle, sheep and goat rearing. Around 6% of Texas' cattle, 77% of its Angora goats and 73% of its sheep are raised on the Edwards Plateau. The area now has one of the highest white-tail deer populations in the U.S. and the highest Rio Grande wild turkey population. The Edwards Plateau is prized for its watershed and recreational values. It also has suitable habitats for various endangered species. These habitats probably did not exist in the beginning.
This is not intended to say that range abuse is not taking place on some ranches, because it is. Neither is it intended to say that the current overall ecosystem cannot be improved, because it can and should be. There is evidence that the desertification process has taken place which has resulted in some undesirable water regime and vegetation changes. Decertified land is practically non-existent.
Continual undergrazing or no plant defoliation at all can result in poor vegetation health and declining productivity. This is often seen in exclosure studies whereas the vegetation outside the exclosure with proper grazing is better than that within it. Pristine vegetation did not develop in the absences of defoliation either by wildlife or fire or both. Thus, the establishment and maintenance of a desirable and protective vegetation cover requires some grazing and defoliation and the participation of domestic livestock is plausible.
Stocking rates and managerial systems that result in continual destructive grazing are a major cause of desertification on rangelands. The desertification process is accelerated when these practices are maintained during droughts and certain seasons where plants are highly vulnerable to abuse. While it takes a long time, this will eventually result in decertified rangelands.
Destructive grazing also causes poor livestock performance and many private landowners have learned this lesson the hard way. It is, therefore, not as common on private lands as it is on public or community grazing lands where central control is lacking. In spite of this, overgrazing at intensities lighter than destructive is a major problem in the Region. This not only causes gradual deterioration, it also adversely affects animal production.
Lack of Land use Policies
The above identification of desertification causes is not intended to be a comprehensive review of the subject. These were selected because they are related in one way or another to livestock production and rangeland desertification. Tropical forest deforestation is highlighted because it is, in part, due to attempts to expand the livestock industry. For this reason, it could be indirectly related to the rangeland desertification problem.
Considering that rangelands are producing only a fraction of their potential due to degradation, it is very likely that they are not meeting national demands and needs for whatever reason. If so, this reinforces the urgent need for land use policies aimed at desertification control on rangelands and the implementation of sustained and improved livestock production systems. This would result in greater production of animal products which in turn might reduce the need for expanding the livestock industry onto forest lands. Why jeopardize yet another valuable natural resource when the other can be rehabilitated?
Many desertification symptoms or manifestations shown in Figure 5 are also occurring on the Region's arable lands because of poor agriculture practices. These can be reversed and crop production increased with implementation of improved cultural practices. These technologies are generally known in the Region, but they are not being widely applied. Concerted implementation efforts might reduce the pressure for expanding cultivation onto range and forest lands.
The fundamental cause of desertification is the use of land beyond its capabilities. Generally speaking, the countries have not made land capability surveys and developed land use policies