Agricultural Systems: Ecology

Transcription

1 Agricultural Systems: Ecology Clive A Edwards, Ohio State University, Columbus, Ohio, USA Agricultural systems have four main inputs, cultivations, nutrients, crops and pest management, all of which are interlinked and influenced by each other. They are affected by land and water and energy availability and soil erosion. To achieve long-term agricultural sustainability, the need for fossil-fuel-based inputs should be minimized and the biological and ecological inputs maximized.. Introduction Secondary article Article Contents. Land Area of the World Utilized by Agriculture. Freshwater Resources Utilized by Agriculture. Primary Chemical Hazards due to Agriculture. Soil Erosion and the Environmental and Economic Consequences. Fossil Energy Use in Agriculture; Quantities and Major Uses; Renewable Sources. Procedures to Make Agriculture More Sustainable Introduction Agriculture has been successful in increasing human per capita food productivity; between 1950 and 1984 cereal grain production increased from about 700 million tonnes to over 1800 million tonnes per year, an annual growth rate of about 2.7%, but it has been declining since 1984 (Brown, 1994). These increases were mainly in North America and Europe, although they also involved some of the more fertile, irrigated lands in developing countries, particularly in Asia (Green Revolution). The bases of these greatly increased yields were the introduction of new seed varieties, greatly increased use of inorganic fertilizers (9- fold increase since 1950), and extensive use of pesticides (32-fold increase since 1950), and increased irrigation (Pimentel and Lehman, 1993). By comparison, yield increases in resource-poor countries with fragile soils and little access to chemicals and irrigation were very small. (see History of scientific agriculture.)(see Agricultural production.) This industrialized pattern of agriculture, very dependent on fossil-fuel-based inputs, soon began to produce environmental consequences. These include: progressively lower productivity as the soil quality declines due to intensive soil cultivation and overuse of chemical fertilizers and pesticides; increased pest and disease problems due to destruction of natural enemies by pesticides; decreases in crop rotations; pollution of groundwater with nitrates and pesticides; progressive loss of available fertile cropland; serious soil erosion; and decreases in amounts of available water for irrigation. (see Energy use in agriculture.) (see Soil resource conservation.) (see Phosphorus budgets.) (see Nitrogen budgets.) The current human world population reached 6 billion in October Based on the present growth rate of 1.4% per year, the population has been predicted to double in approximately 50 years or could even increase to as much as 14 billion by However, unless we can slow the rates of loss of nonrenewable natural resources and find adequate sources of renewable energy, neither food production nor population growth can continue indefinitely at such rates, and society will be faced with a choice of either controlling its numbers voluntarily or letting natural forces such as disease, malnutrition, and other disasters limit human numbers. Although food production tripled from the 1950s to the 1980s, by the 1990s grain yield increases slowed to barely 0.5% per annum, less than onethird of the rate of population increase (Edwards and Pimentel, 2000). (see Malnutrition.) (see Conservation v human survival.)(see Human disease: eects of economic development.) Current global food shortages are critical, with more than 3 billion humans malnourished worldwide. Rapidly increasing human populations, especially in urban areas, and increasing food, water, air and soil pollution by pathogenic organisms and chemicals, are causing concomitant increases in the prevalence of diseases and the numbers of human deaths. Clearly, even to maintain the current human populations in the long term, agriculture must provide more food per capita. However, even with significant reversals of the rates of loss of critical natural resources, such as fertile land and soils, to inefficient agriculture, urbanization and water shortages, there is a finite limit to how many people the Earth can support, based on currently available technology and renewable energy sources, once fossil-fuel-based sources of energy are exhausted in the foreseeable future. Land Area of the World Utilized by Agriculture About one and a half billion hectares (ha) of land have been used for agriculture, but by 1990, a third of this had been lost or damaged by mismanagement or soil erosion. Land area devoted to grain peaked at about 750 million ha in 1985 and since then has been declining progressively. Although as much as 3 billion ha of land could potentially be used for agriculture, two-thirds of this area has extremely low productivity potential and less than half a million ha have relatively high productivity potentials. Against these figures, it has been estimated that about 16 million ha of productive land are lost annually, 10 million ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / 1

2 ha to soil erosion, 2million ha to salinization and waterlogging and 4 million ha to deforestation (Gardner, 1996). Another 20 million ha of land are degraded annually, through overproduction, and short rest periods between cropping, to the point of becoming unprofitable. It has been estimated that more than one-third of the world s agricultural lands are over-cultivated. Globally, the loss of land to urbanization and highways ranges from 10 to 35 million ha (approximately 1%) per year, with half of this lost land being cropland. (see Soil resource conservation.) (see Plant salt stress.) (see Forestry management and production.) In 1960, when the world population numbered about 3 billion, approximately 0.5 ha of cropland was available per capita worldwide. This 0.5 ha of cropland per capita is the area needed to provide a diverse, healthy, nutritious diet of plant and animal products similar to the typical diet in the United States and Europe. The average per capita cropland currently available worldwide is only 0.27 ha, or about half the amount needed to meet industrial nation food standards globally. This increasing shortage of productive cropland, and its uneven regional distribution, is the main underlying cause of the current worldwide food shortages and poverty. More than 38% of the global cultivated land area has already been damaged by agricultural mismanagement. Currently, a total of 1481 kg per year, per capita, of agricultural products is produced to feed Americans, whereas the Chinese have already reached or exceeded the limits of their agricultural system. The current Chinese reliance on large inputs of fossil-fuel-based fertilizers, as well as other regionally limited inputs, to compensate for shortages of arable land and severely eroded soils, indicates severe problems for their future. The Chinese already import large amounts of grain from the United States and other nations, and are increasing these imports rapidly. (see Energy use in agriculture.) The world s tropical forests are being lost extremely rapidly. They are cleared for a variety of purposes, including production of pasture for animal farming and harvesting wood for fuel. Many of these uses are theoretically related to increased agricultural productivity, after the forest has been cleared. The clearing of tropical forests to grow crops is a traditional method of food production in most developing countries. This practice involves the cutting and clearing areas of forest, usually by burning, and then growing crops for one to three years until the nutrients are exhausted. Although nutrients are released from the organic matter by burning, much of the nitrogen is volatilized and lost in the process and is unavailable for crops. (see Forest ecosystems.) After cropping for several seasons, the forest is left to regenerate. The time taken for the land to regenerate is variable, but the longer the fallow period the more fertile the land when next used for crops. However, in some tropical soils cleared for cropping, fallowing for even 15 years is insufficient for phosphorus levels to recover. Deforestation for crop production is unlikely to make any major long-term contribution to agricultural productivity, since most tropical forest soils are low in nutrients and a large proportion of the above ground nutrients are in the tree biomass that is hauled away for timber or burned. With growing population pressures, the fallow regenerative periods in shifting agriculture are decreasing in length, so that fertility is decreasing progressively, since forests do not have sufficient time to regenerate the needed nutrients. The side effects of deforestation include major contributions to increased drought, eutrophication of lakes, increases in soil erosion, and nutrient runoff as well as possible effects on global warming through increased carbon dioxide emissions. Most serious is the effect on soil erosion, which may increase many times after forests are cleared. For instance, soil erosion rates increased 200- to more than 5000-fold when African forest land was cleared. (see Phosphorus budgets.) (see Eutrophication of lakes and rivers.) Freshwater Resources Utilized by Agriculture The present and future availability of adequate supplies of freshwater for human and agricultural needs is already critical in many regions, such as the Middle East. Rapid human population growth and increased total water consumption are rapidly depleting the overall availability of water worldwide and this is made more serious by periodic regional droughts. Between 1960 and 1997, the per capita availability of freshwater worldwide declined by 60% and a further 50% decrease in per capita water supply is projected by the year 2025 (Pimentel and Edwards, 2000). All vegetation requires and transpires massive amounts of water during the growing season. Agriculture requires more water than any other human activity on the planet. Currently, 93% of the water removed from all sources worldwide is used solely for crop irrigation. Of this amount, about two-thirds is consumed by plant life (nonrecoverable). Irrigation currently supplies a disproportionate amount of food: the 237 million ha of irrigated land is 16% of the total global cropland, but supplies more than a third of the global food supply. A major problem is the uneven regional distribution of water supplies. Water demand already far exceeds supplies in nearly 80 nations of the world. In China, more than 300 cities suffer from inadequate water supplies and the problem is intensifying as populations increase. In arid regions, such as the Middle East and parts of North Africa, shortages of water are already critical. Where yearly rainfall is low and irrigation is expensive, the future of agricultural production is grim and becoming even more so as human populations continue to grow. Political conflicts over water in some areas, particularly the Middle East, have even strained 2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group /

3 international relations between severely water-starved nations. (see Energy, radiation and temperature regulation in plants.)(see China: life science organizations.) Even where water supplies are adequate, pollution by chemicals and sewage decreases the amount of usable water significantly. Worldwide, about half of the water available is polluted and, in developing countries, even more is unavailable as drinking water. Primary Chemical Hazards due to Agriculture Enormous losses of food are due to pest attacks both in crop production and in storage: global losses to pests are as much as 40% of crop production and regional losses as high as 75% have been reported. Since World War II, thousands of synthetic chemical insecticides, nematicides, acaricides, fungicides and herbicides have been developed, and many of these are used extensively and in quantities up to 6 kg a.i. (active ingredient) per hectare, to control pests. It has been forecast that currently nearly 2.5 million tonnes of pesticides are used worldwide with a value of nearly 30 billion dollars. Of these 45% are herbicides, 30% are insecticides, 19% are fungicides, and 6% are other pesticides (Edwards, 1994). Until the 1980s, the global use of pesticides continued to increase, particularly in developed countries. It was forecast in 1986 that the use of pesticides would continue to increase in an almost exponential manner globally. However, since 1992the global use of pesticides has begun to level off internationally, and particularly in developed countries, although use has continued to increase in some developing countries. Pesticides differ in their relative toxicity to humans and wildlife, but they are all biocides with the potential to kill or cause chronic illness in animals and humans. It has been estimated that there are about 26.5 million human pesticide poisonings each year with about deaths. These figures may be very much higher, since many human pesticide poisonings are not diagnosed and there are still major questions on the role of pesticides in causing cancer or endocrine malfunctions. Pesticides can also have drastic effects on wildlife, particularly on fish and birds in the higher trophic levels. Clearly pesticides cannot offer longterm solutions to pest problems. Many pests develop crosslinked resistance to pesticides and it has been forecast that the fossil fuels needed to produce organic pesticides will be exhausted in 50 years. (see Human disease: eects of economic development.) Fertilizers can also cause environmental hazards mainly through nitrates percolating down into the groundwater and passing ultimately into drinking water. It has been suggested that contaminants such as nitrates may cause increases in the incidence of cancer in humans, and there are other potential hazards from such exposure. Soil Erosion and the Environmental and Economic Consequences Erosion of soils by wind and water is the most serious cause of loss of fertile land and soil degradation. Current soil erosion rates are becoming progressively greater than those previously recorded. Soil erosion on cropland ranges from an average of 18 tonnes per hectare per year in the United States to 40 tonnes per hectare per year in China (Gardner, 1996). Worldwide, soil erosion averages approximately 30 tons per hectare per year, or about 30 times faster than the soil formation replacement rate. During the past thirty years, the rates of soil loss in Africa have increased 20-fold. More than 11% of the world s cropland was identified in 1989 as severely eroded. Wind erosion in China is so serious that Chinese soil particles have been detected in the Hawaiian atmosphere during the spring planting period. Similarly, soil eroded by wind in Africa has been detected in Florida and Brazil. (see Soil resource conservation.) Soil erosion affects crop productivity adversely by reducing the water-holding capacity of the soil, lowering the water availability, decreasing nutrient levels, and amounts of organic matter, as well as lessening the depth of soils. There are estimates that agricultural land degradation alone can be expected to depress world food production between 15% and 30% by the year Such estimates emphasize the urgent need to implement conservation practices such as including live and dead mulches, adoption of no-till, ridge-till, terracing, grass strips, crop rotations and intercropping, and combinations of all these. All these techniques basically require keeping the land protected from wind and rainfall effects, using some form of vegetated cover. The current high rates of soil erosion throughout the world are of great concern because of the slow rate of topsoil renewal: it takes approximately 500 years for 2.5 cm (1 inch) of topsoil to form under agricultural conditions. Approximately three thousand years would be needed for the natural reformation of topsoil to the 150 mm depth needed for satisfactory crop production. The fertility of nutrient-poor soils can be improved by large inputs of fossil-fuel-based fertilizers. However, this practice increases dependency on the limited fossil fuel resources necessary to produce these fertilizers. Even with fertilizer use, soil erosion still remains a critical problem in current agricultural production. Moreover, excessive inorganic fertilizer use on poor soils, with little organic matter, results in greater leaching of nitrates into groundwater. (see Energy use in agriculture.) ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / 3

4 The area of arable land currently used for crop production already includes a considerable amount of marginal land that is highly susceptible to erosion. Whenever soil degradation occurs, the requirement for fossil energy inputs in the form of fertilizers, pesticides and irrigation may be increased, to offset the losses, thereby creating nonsustainable agricultural systems in the long term. Fossil Energy Use in Agriculture; Quantities and Major Uses; Renewable Sources Over time, people have relied on various sources of power for food production and other purposes. These sources have ranged from human, animal, wind, tidal and water energy, to wood, coal, gas, oil and nuclear sources for fuel and power. Currently, fossil fuel energy permits nations to feed an increasing number of humans, as well as improving their general quality of life. About 395 quads (1 quad BTU or joules) from all energy sources are used worldwide per year (Edwards and Pimentel, 2000). (see Energy use in agriculture.) Energy use has been growing even faster than world human population growth. From 1970 to 1995, energy use was increasing at a rate of 2.5% annually (doubling every thirty years) whereas the worldwide population grew at only 1.7% annually. From 1995 to 2015, energy use was projected to increase at an annual rate of 2.2% compared with an annual population growth rate of 1.5%. Although about 50% of the solar energy captured by photosynthesis worldwide is used by humans, it is still inadequate to meet all of the planet s needs for food worldwide. To make up this shortfall, about 345 quads of fossil energy (oil, gas and coal) are utilized worldwide each year, and this is increasing annually. Developed nations consume annually about 70% of the fossil energy used worldwide, while the developing nations, which have about 75% of the world population, use only 30%. The United States, with only 4% of the world s population, consumes about 22% of the world s fossil energy output. Fossil energy use in the different US economic sectors has increased 20- to 1000-fold in the past three to four decades, attesting to the USA s heavy reliance on this finite energy resource to support their affluent lifestyle. Several developing nations that have high rates of population growth are rapidly increasing their use of fossil fuel to augment the agricultural production of food and fibre. For instance, in China, there has been a 100-fold increase in fossil energy use in agriculture for fertilizers, pesticides and irrigation since However, overall fertilizer production, which is fossilfuel-based, has declined by more than 23% since 1988 from a peak of nearly 150 million tonnes, especially in the developing countries, due to fossil fuel shortages, currency problems and high prices. In addition, the overall projections of the availability of fossil energy resources for fertilizers and other uses, are decreasing progressively because of the limited global resources of these fossil fuels. The world supply of oil is projected to last approximately 50 years at current production rates. Worldwide, the natural gas supply is adequate for about 50 years, and that of coal for about 100 years. These estimates, however, are based on current consumption rates and population numbers. If all the people in the world enjoyed a standard of living and energy consumption rate similar to that of the average American, and the world population continued to grow at a rate of 1.5% annually, the world s fossil fuel reserves would last only about 15 years (Pimentel and Edwards, 2000). Using all of the currently available renewable energy technologies, such as biomass and wind power, an estimated 200 quads of renewable energy could be produced worldwide but would need 20% to 26% of the land area. A self-sustaining renewable energy system producing 20 quads of energy per year would provide each person with 5000 litres of oil equivalents per year or half of current US consumption per year, but this would represent an increase for most people of the world. Moreover, the use of more than 20% of the land area for renewable energy production would limit land for production further and hence the resilience of the vital ecosystem that humanity depends upon for its life support system. Procedures to Make Agriculture More Sustainable Crop production depends upon four major inputs, cultivations, nutrient supply, cropping patterns and pest management, all of which impact on one another (Figure 1) (Edwards et al., 1993). These inputs can consume large amounts of fossil fuel, particularly in terms of cultivations, inorganic fertilizers and pesticides, particularly in developed countries that depend upon high agricultural productivity for cheap food. Since 1980, there has been an increasing trend to depend less upon practices that consume fossil fuels. Cultivations have moved progressively from deep ploughing, through shallow time ploughing, conservation tillage, all the way to no till or no cultivation at all. Fertilizers, which are still relatively inexpensive in developed countries, have been used on a routine crop basis with little attention to the soil type, nutrient status or needs of the crop, but are now being substituted with biological nutrient sources. There has been a significant move in the USA and Europe to apply fertilizers only in the amounts that are essential. 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group /

5 Land Cultivations Nutrient supply Water Figure 1 Climate Crop Pest management Food supply Main components of a crop production system. The use of pesticides has increased 20-fold since their introduction in the 1950s, although, in recent years, amounts used have begun to fall off. This is in response to the increasing awareness of farmers and the public that pesticides are dangerous, can cause groundwater pollution, can kill pollinating insects, have many undesirable side effects on wildlife, and result in the progressive development of resistance to pesticides by a broad range of pests. (see Biological control.) (see Biological control by microorganisms.) Moreover, the extensive use of pesticides in crop production eliminates many of the natural enemies of pests, thereby minimizing the natural control mechanisms and increasing overall losses to pests. Approximately 99% of potential pests could be controlled by diverse, natural enemy species, as well as through the use of pest resistance in host plants, derived from wild plants in natural ecosystems. A more ecological approach to pest control, maximizing biodiversity and encouraging the maintenance and build-up of natural control agents, could quickly reverse the current trends away from such maintenance of ecological integrity. As agroecological approaches to pest control have been adopted progressively, and biodiversity emphasized, it has become increasingly clear that the use of pesticides can be decreased greatly or even eliminated with no significant losses in yields (Dover and Talbot, 1987). Since the 1980s, in the USA and Europe, the concept of sustainable agriculture has developed. There are many definitions of sustainable agriculture, but most incorporate the following characteristics: long-term maintenance of natural resources and agricultural productivity, minimal environmental impact, adequate economic returns to farmers, optimal production with minimal chemical inputs, and provision for the food and social needs of farm families and communities. All definitions explicitly promote environmental, economic and social goals, and the need for an interdisciplinary systems approach to agriculture. (see Environmental impact assessment.) A definition that has received broad acceptance is: Sustainable agriculture involves integrated systems of agricultural productions, with minimum dependence upon high inputs of energy, in the form of synthetic chemicals and cultivation, that substitute cultural and biological techniques for these inputs. They should maintain, or only slightly decrease, overall productivity and maintain or increase the net income for the farmer on the sustainable basis. They must protect the environment in terms of soil and food contamination and maintain ecological diversity in the long-term structure, fertility and productivity of soils. Finally, they must meet the social needs of farmers and their families and strengthen rural communities in a sustainable manner. Many farmers and agricultural scientists view the various practices they use to produce crops as completely independent of each other. They rarely consider how the amounts of fertilizer they use affect pests, diseases or weeds. Neither is the impact of cultivations on pest disease and weed problems a factor in deciding the type of tillage that a farmer uses. Even with integrated pest-management systems, it is rare for any account to be taken of the impact of herbicides on pests and diseases, of insecticides on plant diseases, or of fungicides on insect pests. A farming system is not just a simple sum of all its inputs and components, but rather is a complex system with intricate interactions (Figure 1). In conventional higherinput farming, large yields can often be obtained without any appreciable attention to interactions between inputs. For instance, if heavy fertilizer use renders a crop much more susceptible to pests and diseases, through production of lush, soft growth, this can be compensated by increased pesticide usage. The decreases in natural pest and disease control caused by herbicides, through loss of foliar and habitat diversity, is compensated by increased use of insecticides and fungicides. Any effect of pesticides on earthworms and other soil organisms that promote organic-matter turnover, nutrient cycling and soil fertility can be covered by using more inorganic fertilizers. When chemical inputs are lowered in sustainable agriculture, we need to understand how the main inputs impact upon each other in much more detail, so that the beneficial effects of these interactions can be maximized. This can be summarized more readily as identifying ways of adopting an integrated ecological systems approach to farming which bases the disease and pest management upon sound ecological principles and maximizes the contribution of biological inputs to crop production. There is an urgent need to expand our understanding of agro-ecology and the ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / 5

6 ecological functions of farming systems, and to apply this knowledge to food production. Only in this way can food productivity be increased in a sustainable, long-term way. References Brown LR (1994) Facing food insecurity. In: Brown LR et al. (eds) State of the World. World Watch Institute Report on Progress Toward a Sustainable Society 10, pp New York. Dover M and Talbot LM (1987) To Feed the Earth: Agro-Ecology for Sustainable Development. Washington DC: World Resources Institute Report, 88. Edwards CA (1994) Pesticides as environmental pollutants. In: Ekstro m G (ed.) World Directory of Pesticide Control Organizations, 2nd edn, pp Cambridge, UK: Royal Society of Chemistry. Edwards CA and Pimentel D (2000) Global aspects of agricultural sustainability. In: Burdyuzha V (ed.). The Future of the Universe and the Future of Civilization, pp Paris, France: UNESCO. Edwards CA, Grove TL, Harwood RR and Colfer CJP (1993) The role of agroecology and integrated farming systems in agricultural sustainability. In: Edwards CA, Wali MK, Horn DJ and Miller F (eds) Agriculture and the Environment, pp Amsterdam: Elsevier. Gardner G (1996) Preserving agricultural resources. State of the World 1996, pp New York: World Watch Institute. Pimentel D and Lehman H (eds.) (1993) The Pesticide Question: Environment, Economics and Ethics. New York and London: Chapman and Hall. Pimentel D and Edwards CA (2000) Agriculture, food, populations, natural resources and ecological integrity. In: Crabbe D and Westra L (eds) Implementing Ecological Integrity 25, pp Brussels: NATO. Further Reading Carroll CR, Vandermeer JH and Rosset PM (1990) Agroecology: Biological Resource Management Series, 641. Edwards CA, Lal R, Madden P, Miller RH and Haise G (1990) Sustainable Agricultural Systems. Ankary, IA: Soil and Water Conservation Society. Edwards CA, Wali MK, Horn DJ and Miller F (1993) Agriculture and the Environment. Amsterdam: Elsevier. Gleissman SR (ed.) (2000) (Edwards CA (Series ed.)) Agroecosystem Sustainability: Developing Practical Strategies. Boca Raton. FL: CRC Press. National Research Council (1989) Alternative Agriculture. Washington DC: National Academy Press. National Research Council (1992) Sustainable Agriculture and the Environment in Humid Tropics. Washington DC: National Academy Press. 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group /