1. What are we farming? Geographic patterns of major crop types
2. How are we farming? Changing agricultural management
3. Agriculture as a force of global environmental change
It is fair to say that our planet’s most precious resource is land. Land is the source of the vast majority of our food and fresh water, nearly all of our fiber and raw materials, and many other important goods and services. It is also our home. But our relationship to the land has been dramatically changing over the history of our species, mainly through the invention and evolution of agriculture. Today, with the emergence of modern agricultural practices, coupled with the population growth and technological developments of recent centuries, we have transformed a staggering amount of the Earth’s surface into highly managed landscapes. Even more startling: the widespread use of irrigation and chemical fertilizers has fundamentally altered the flows of water and nutrients across large regions of the globe. These modifications to the land have driven fundamental changes to the ecology of our planet. Even the effects of future climate change may not have such a major, transformative effect on the environment and on human society as agriculture. However, despite the importance of agriculture in the global environment, we still know relatively little about how it affects ecological systems across local, regional, and global scales.
cropland. Land used for growing crops. The UN Food and Agriculture Organization defines this as the sum of arable lands and permanent crops. Arable land is defined by FAO as including “land under temporary crops (double-cropped areas are counted only once), temporary meadows for mowing or pasture, land under market and kitchen gardens, and land temporarily fallow (less than 5 years). The abandoned land resulting from shifting cultivation is not included in this category. Data for arable land are not meant to indicate the amount of land that is potentially cultivable.” Permanent crops are defined as “land cultivated with crops that occupy the land for long periods and need not be replanted after each harvest, such as cocoa, coffee, and rubber; this category includes land under flowering shrubs, fruit trees, nut trees and vines, but excludes land under trees grown for wood or timber.”
extensification. The practice of increasing the amount of agricultural land that is under cultivation.
human appropriation of net primary production (HANPP). How much of the biological productivity of a given location is used, consumed, or co-opted by human activities.
intensification. The practice of stimulating more agricultural production per unit area, mainly through increasing use of agricultural chemicals, irrigation water, high-yielding plant varieties, and machinery.
land cover. Describing the physical state of the land surface, such as “rainforest,” “cropland,” or “desert.”
land use. The practices employed on a particular piece of land, such as rotating grazing or intensive maize cultivation.
net primary production (NPP). The biological productivity of the landscape, that is, the rate of conversion of physical energy (sunlight) into biological energy (through photosynthesis) in a given location.
pasture. Agricultural land used for animal grazing. The UN FAO defines this category as “land used permanently (5 years or more) for herbaceous forage crops, either cultivated or growing wild (wild prairie or grazing land). The dividing line between this category and the category ‘Forests and woodland’ is rather indefinite, especially in the case of shrubs, savannah, etc., which may have been reported under either of these two categories.”
Since the dawn of agriculture, some 9000 years ago, humans have progressively transformed the landscapes of our planet.
Over time, agricultural land use steadily spread across the globe, reaching into nearly every region, setting the stage for an explosion of agricultural activity after the rise of the Industrial Revolution. Equipped with new technologies, and rapidly increasing population and income levels, agriculture quickly expanded to meet increased food demand over the last 300 years (see plate 23). But this global expansion of farmland was not uniform. Instead, it has traced a path determined largely by the history of European economic and political control. In particular, the direct impact of European settlement was seen in the rapid expansion of agricultural land through North America, Argentina, South Africa, and Australia/New Zealand. The rest of the world also experienced significant cropland expansion as regions became connected to European markets and spheres of influence.
Understanding the agricultural expansion over the last three centuries is especially critical because of the tremendous growth in global cropland area (an increase of ~12 million square kilometers, or ~466%) that happened during this time. During the 1700s and 1800s, croplands expanded most rapidly in Europe, one of the most economically developed regions of the world at that time. After the mid-1800s, the newly developing regions of North America and what would become the Soviet Union witnessed rapid cropland expansion. Cultivation in tropical developing nations expanded only gradually between 1700 and 1850 but has experienced exponential growth rates since that time. Since the 1950s, cropland areas in North America, Europe, and China have stabilized and even decreased somewhat in Europe and China.
Today, the most productive landscapes of the planet —those with the best climate and soil conditions—are already used for cropland agriculture. “Breadbaskets” of cultivated land are found largely in the temperate and subtropical zones, especially in regions of rich soils and adequate rainfall, such as the midwestern United States, the Prairie Provinces of Canada, the Argentinian Pampa, through Europe and the former Soviet Union, the major river basins of India and China, and Australia. New frontiers of cultivation are found in Southeast Asia and southeastern Brazil. Cultivation is also prevalent in West Africa, the Ethiopian highlands, the Rift Valley, and southern Africa, and at lower intensities surrounding the major growing areas. Pastures, or “Meat Baskets,” are predominantly found in the grasslands and savannas of the world, in drier regions compared to croplands. The greatest extent of pastures are seen in the western United States and Canada, Patagonia, southeastern Brazil, Sa-helian and southern Africa, through much of central Asia, and Australia (plate 24).
Altogether, nearly 15 million square kilometers of land (roughly the size of South America) is now used as croplands on the planet, and another ~30 million square kilometers of land (roughly the size of Africa) is now used for pastures; together, croplands and pastures occupy ~35% of the ice-free land surface of the planet (Ramankutty et al., 2008).
Because most of the fertile lands of the world are already under cultivation, there are only relatively few opportunities for further agricultural expansion. Today, only the rainforest regions of Latin America, Africa, and Indonesia offer any significant remaining cultivable lands, especially for crops such as soybeans and oil palm. However, the expansion of agricultural lands would come at the expense of ecologically valuable rainforests, posing a serious dilemma of balancing the needs of economic development and ecological sustainability.
Although agriculture has come to shape landscapes across the entire planet, it assumes many different forms. Mediterranean olive groves, short-season North American maize, and the perennial plantain rhizomes of tropical Africa all fall under the rubric of agriculture, yet they hold radically different implications for people and the environment. Fortunately, new data sets illustrating the geographic patterns of individual crop types (Mon-freda et al., 2008) have begun to fill in the contours of global cropland maps to answer a question crucial to understanding how agriculture has transformed the planet: Which crops grow where? And why?
History, culture, climate, and economics have shaped the complex patterns of crop production seen today. Globally, three major crop groups make up the majority of the 13.4 million square kilometers of crops harvested every year: cereals (6.6 million square kilometers), oil crops (1.8 million square kilometers), and fodder crops (1.4 million square kilometers). Cereal crops are the only crop group to occur in every growing region of the world. Three cereal crops—wheat, rice, and maize—are by far the most widespread of all crops and together make up 38% of the global crop area. Of these, wheat occupies the greatest area, extending across the fertile soils of the Gangetic Plain, the Canadian Prairie Provinces, the Former Soviet Union, northern China, and parts of Australia and Argentina. Maize is spread across the greatest array of climates, whereas rice is overwhelmingly concentrated in the densely populated lowlands stretching from eastern India to the lower reaches of the Yellow River in China. Soybeans, rapeseed, groundnuts, and sunflower are the most widely grown oil crops, covering about 11% of all crop area. Although oil crops do not cover as wide an area as cereals, they follow a similar geographic distribution and often grow in rotation with cereals. By contrast, fodder crops, which are distinct from pastureland and include alfalfa and other hay crops, are for the most part confined to the wealthy, high-latitude countries that have a largely animal-based diet.
The three major crop groups—cereals, oil crops, and fodder crops—often appear in combination with minor crops, which may be very important locally but occupy a lesser extent worldwide. These minor groups include pulses (0.67 million square kilometers), roots and tubers (0.50 million square kilometers), fruits (0.48 million square kilometers), vegetables (0.44 million square kilometers), fiber crops (0.35 million square kilometers), and sugar crops (0.26 million square kilometers).
The major and minor crop groups occur in various combinations to form a patchwork of farming systems across the planet. Just one or two crops dominate the least agriculturally diverse crop belts, which occur in areas that are dry, major grain exporters, or both. These low-diversity crop belts include the maize-soybean rotations of the U.S. Midwest and the enormous soybean monocultures expanding into the Brazilian Amazon. Extensive low-diversity croplands in dry regions include the wheat-barley fields of southern Australia, the wheat fields of the western United States, and the maize-millet-sorghum belt of the Sahel. By contrast to those regions that grow just one or two crops, the regions of the greatest crop diversity cultivate a high proportion of noncereal crops. These areas include the intensive lowland rice-vegetable-oil crop systems of East Asia and maize-rice-potato belt of the Peruvian Andes. Exceptional agricultural diversity also occurs in the Mediterranean region, which grows substantial amounts of grapes, olives, sunflowers, and other noncereal crops in near equal proportions with maize, barley, and wheat.
Developing a good picture of these farming systems is key to understanding the effect of agriculture on the global environment for at least two reasons. First, different crops are more or less suited to the climate and soils of different regions. Understanding how well crops grow across different regions indicates where and how much agricultural expansion may need to occur to meet growing demand for food, fiber, and biofuel. Second, different crops require different kinds of farming practices, including the use of fertilizer or irrigation. The geographic distribution of farming practices has major implications for the environment, including freshwater resources, the carbon cycle, biodiversity, and human health. Getting a better handle on the diversity of crops and farming practices across the planet is therefore an important part of grasping the key questions facing agriculture and the global environment.
Although changes in the geographic extent of our agricultural lands have been considerable, they do not provide the entire story.
Although significant agricultural expansion has occurred in the past few decades, the intensification of agricultural practices—under the aegis of the “Green Revolution”—has dramatically increased, completely changing the relationship among humans, agriculture, and environmental systems across the world. Specifically, since the 1950s, there has been a major shift toward agricultural intensification, in lieu of expansion, enabled by the widespread development of irrigation systems, the invention of inorganic fertilizers in the early 1900s, and the development of crops that better exploit water and nutrients and are more resistant to pests and diseases.
Simply put, the world’s existing agricultural lands are being used much more intensively as opportunities for agricultural expansion are being exhausted elsewhere.
In the last 40 years, global agricultural production has more than doubled—although global cropland area has increased by only *12%—mainly through the use of high-yielding varieties of grain, increased reliance on irrigation, massive increases in chemical fertilization, and increased mechanization (Foley et al., 2005). As a result, the growing demand for food in the past few decades has been increasingly met through higher yields on existing croplands rather than through agricultural expansion.
Indeed, in the past 40 years, there has been an ~700% increase in global fertilizer use and an ~70% increase in irrigated cropland area (Foley et al., 2005). These represent significant changes to the planet’s hydrologic and biogeochemical cycles: for example, we now apply more nitrogen fertilizer than is naturally fixed in the biosphere. And the diversion of freshwater flows for irrigation alone exceeds the changes in water availability expected from future climate change. So although these modern agricultural practices have successfully increased food production, they have caused extensive environmental damage across many portions of the planet (Foley et al., 2005).
The expansion and intensification of agriculture have become major forces in shaping the human impact on the global environment. Whether it is through clearing tropical rainforests, practicing subsistence agriculture on marginal lands, or intensifying industrialized farmland production in temperate croplands, modern agricultural practices are changing the world’s landscapes in many ways. Although the precise character of agricultural land use varies greatly across the world, the ultimate outcome is generally the same: the production of new agricultural goods for human needs, often at the expense of degrading environmental conditions.
Agricultural practices can have dramatic effects on many environmental processes, ranging across local, regional, and global scales. At these different scales, it may be useful to define “first-order” responses of the environment to agricultural land-use change as well as “second-order” environmental effects.
“First-order” environmental effects of agricultural land use are those that directly, and immediately, affect the environment. For example, the expansion of agricultural land immediately and directly diminishes the extent and increases the fragmentation of natural ecosystems, often degrading critical habitats and diminishing biological diversity. Furthermore, agricultural land use can directly impact the ecological functioning of the landscape, especially in terms of water, carbon, and nutrient cycling. These changes in ecosystem processes can directly affect the amount and availability of freshwater flows, the fixation and sequestration of carbon, and the flow of nutrients in an ecosystem.
But the impacts of our land-use practices go far beyond their immediate surroundings, generating “second-order” environmental effects. For example, clearing forests for croplands or pastures turns the carbon in living trees into carbon dioxide—a greenhouse gas in the atmosphere that is contributing to global warming. Changes in land cover also have profound impacts on climate through altering the flows of energy and water from the surface to the atmosphere, thereby changing the atmospheric circulation and the climate system. Furthermore, changes in the hydrologic and nutrient balance of watersheds (a first-order effect) resulting from intensive agricultural practices can lead to downstream problems of water quality and degradation in streams, lakes, wetlands, and coastal areas. Other examples include human health consequences brought about by changes in the ecology of disease organisms and their vectors that may accompany land-cover change.
Below, we briefly describe some of the many effects agricultural land-use practices have on the global environment.
Our civilization’s growing demand for land comes at the expense of natural ecosystems. Croplands and pastures are established in lands that used to be covered with forests, savannas, and grasslands—so there is a clear, direct relationship between agricultural expansion and ecological degradation.
In terms of biodiversity, it has been shown that agricultural land-use practices have caused significant losses of species, mainly through habitat loss, modification, and fragmentation (Sala et al., 1995). Agricultural land use can also degrade biodiversity through changes in the quality and supply of freshwater resources, the degradation of soil, or the introduction of nonnative species (Sala et al., 1995).
Beyond biodiversity, agricultural land-use practices can strongly influence the structure and functioning of terrestrial ecosystems. One key aspect of how human actions are altering ecosystems is through changes in productivity. In a groundbreaking study, Peter Vitousek and colleagues (Vitousek et al., 1986) asked how human practices, including agriculture, affect the terrestrial biosphere by estimating the “human appropriation of net primary productivity” (HANPP). HANPP is defined as the share of the world’s biological productivity that is used, managed, or co-opted by human actions. From their analysis, Vitousek et al. estimated that roughly 30% of the planet’s terrestrial net primary productivity is appropriated by human actions, largely through agriculture and forestry. This surprising fact—that something like 30% of terrestrial biological production ends up in human hands—is one of the most quoted facts in modern ecological science.
One of the key problems in understanding the human impact on global productivity is first determining how productivity has changed from land-use practices— including productivity decreases from landscape degradation (e.g., soil erosion, deforestation) or possible productivity increases from agricultural technology (e.g., plant breeding, inputs of fertilizer and irrigation water). On the basis of an analysis of global cropland productivity and patterns of natural ecosystem productivity (Foley et al., 2007), we see major changes in global productivity in many parts of the world. Although much of the terrestrial biosphere experiences a significant decrease in NPP from land-use practices, several regions see a substantial increase in productivity—especially regions that are heavily influenced by irrigation, fertilizer inputs, and tree crop plantations, such as the western United States, the upper midwestern United States, western Europe, northwestern India, northeastern China, and large parts of Indonesia and Malaysia.
It is also interesting to consider the fate of this human-appropriated production—in other words, how are we using ecosystem production, where are the products going, and who is consuming them? Using global trade and agricultural statistics, we can examine the how global crop production is allocated to different human uses, such as food and nonfood uses. Furthermore, we can document the different economic roles of cropland products, including the percentage of crop production used for domestic consumption versus international exports.
Agricultural land use has also caused significant changes to the quantity and quality of freshwater resources around the world through their impacts on hydrology and nutrient cycles.
To begin, agricultural land-cover change (e.g., converting a landscape from natural vegetation to an agricultural system) can have a major impact on hydrology by altering the amount (and seasonal timing) of key hydrologic processes, including evapotranspiration from the surface, soil moisture storage, water yield into surface and groundwater flows, and the discharge of streams and rivers. Numerous studies have shown how agricultural land use can significantly affect the water balance and freshwater flows of large watersheds across the world.
In addition to these changes in the water balance, many watersheds and aquifers have been heavily affected to withdraw water for irrigation and other agricultural uses. As a result, many large rivers, especially those in semiarid regions, have greatly reduced flows, and some routinely dry up before reaching the ocean. Altogether, agriculture accounts for ~85% of global consumptive water use; and it has been estimated that nearly 50% of the available renewable freshwater supply is currently withdrawn by human activity (Postel et al., 1996).
Furthermore, agricultural land-use practices— especially agricultural chemical use—can dramatically affect freshwater quality over large regions. In particular, nutrient inputs from agriculture—mainly from chemical fertilizers and livestock wastes—now exceed the natural sources to the biosphere and have widespread effects on water quality and coastal and freshwater ecosystems (Matson et al., 1997; Bennet et al., 2001). The resulting degradation of inland waters and coastal ecosystems causes oxygen depletion, fish kills, increased blooms of toxic cyanobacteria, and increased episodes of water-borne disease.
Agricultural land-use practices have also played a critical role in changing the greenhouse-gas composition of the atmosphere and, therefore, the global climate system. In particular, it is estimated that roughly 35% of the world’s cumulative CO2 emissions since 1850 resulted directly from land use, and ~15% of current-day anthropogenic CH4 emissions come from flooded rice fields (Prentice et al., 2001). In addition, a large portion of global N2O (another important greenhouse gas) emissions comes from heavily fertilized agricultural fields. The fact that land-use practices constitute a significant portion of our greenhouse gas emissions is often unrecognized. In the future, changing agricultural land-use practices will likely play a significant role in the mitigation of greenhouse gas emissions.
Agricultural land use can also affect the physical climate system directly through changes in surface energy and water balance: changes in land cover strongly affect the physical properties of the land surface and how it interacts with the atmosphere. Replacing forest cover with pasture, for example, reduces the amount of water evaporated back into the atmosphere, leaving less water and energy available to fuel weather systems, large-scale convection, and atmospheric circulation.
Numerous computer modeling studies have shown that changes in land cover can produce large changes in climate over several areas of the world—sometimes larger than the changes in climate expected from greenhouse gas emissions, at least on a regional scale (Foley et al., 2003). It is therefore critical to consider the effects of agricultural land-use change on climate when considering the future behavior of our climate system.
During the course of recent history, agricultural land-use practices have had many positive impacts on human health, largely by enhancing access to nutrition and medicinal products. Nevertheless, land-use practices have also led to many serious, unintended health consequences.
In particular, habitat modification, changing hydrologic conditions, and increased proximity of people and livestock can all modify the transmission of infectious agents and can lead to serious disease outbreaks (Patz et al., 2004). For example, several recent studies have shown that increasing tropical deforestation coincides with an upsurge of malaria and/or its vectors in Africa, Asia, and Latin America (Vittor et al., 2006). Disturbing wildlife habitat for agricultural land use is of particular concern because ~75% of human diseases have links to wildlife or domestic animals. And irrigation in tropical areas often increases the habitat and breeding sites for disease vectors and infectious agents, including schistosomiasis, Japanese encephalitis, and malaria (Patz et al., 2005).
The loss of biological diversity from agricultural land use can also increase the risk of infectious disease. For instance, many agricultural practices promote rodent populations—important reservoirs and vectors of many diseases—by decimating their natural predators and supplementing their food supply. In addition, forest fragmentation in the eastern United States alters biodiversity and predator abundance in ways that promote deer populations, with a subsequent rise in the density of ticks that can carry Lyme disease. Moreover, biodiversity loss favors expansion of mouse populations; because the white-footed mouse is the most competent reservoir host for the bacterium that causes Lyme disease, such land-use change can ultimately increase the risk of contracting Lyme disease (Ostfeld and Keesing, 2000).
Furthermore, the combined effects of agricultural land-use practices and extreme weather events can also have serious impacts, both on direct health outcomes (e.g., injuries and fatalities from storms) and on infectious diseases. For example, Hurricane Mitch, a devastating storm that hit Central America in 1998, demonstrates the combined effects of land use and extreme weather: thousands of people perished, widespread illness from water- and vector-borne diseases ensued, and an estimated 1 million people were left homeless. It has been widely reported that areas with extensive deforestation and poor agricultural practices experienced far greater morbidity and mortality. The southern part of Lempira Province, Honduras, however, escaped Hurricane Mitch with only minor damage and no loss of life, even though it endured some of the most intense rainfall and winds of the hurricane. The practice of sparing large shade trees, and planting crops interspersed underneath, was reported as the key protective factor compared with other regions that experienced disastrous mud slides (Patz et al., 2004).
Throughout history, agriculture has played a crucial role in sustaining the health, nourishment, and economy of the world’s population. This is especially true today, as human population and consumption continue to grow, increasing our reliance on secure food, fiber, and biofuel supplies. At the same time, many agricultural practices can disturb the environment in ways that affect the quality of ecosystem services and natural resources—including ecosystems, soils, waterways, climate, and even the air we breathe.
Agricultural land-use practices have been, and will continue to be, a major driver of global environmental change, especially in terms of changing key ecosystem services, natural resources, and human health. In fact, it is possible that the environmental effects of land-use practices could rival, and even exceed, the effects of global warming. (Naturally, this does not diminish the importance of global warming as a serious scientific and policy issue. Rather, we argue that land-use practices and global warming are both crucial global environmental issues and that a balanced science and policy framework will consider the combined, synergistic effects of land use, greenhouse gas emissions, and other major drivers of global environmental change. Fortunately, such frameworks are already emerging from the International Geosphere-Biosphere Programme, the Millennium Ecosystem Assessment, and from many national research programs.)
Tackling the widespread environmental challenges of agricultural land use requires decisionmaking and policy actions that reduce the negative impacts of land-use practices while maintaining the positive societal and economic benefits. Fortunately, there are numerous opportunities for simultaneously improving the environmental and economic benefits of agricultural land use. Examples include precision agricultural techniques that increase production per unit land area, per unit fertilizer applied, and per unit water consumed; land-use practices designed to maintain water flows and water quality, such as the use of buffer strips near sensitive streams and rivers; and agroforestry practices that provide food and fiber but also maintain critical habitats for threatened species. Land-use policies should also aim to enhance the resilience of these critical systems, making them more robust to outside disturbances (such as a new disease, invasive species, or pest) and environmental “surprises” (including sudden changes in climate or water availability).
Developing new land-use strategies that balance immediate human needs with those required for long-term environmental sustainability will be a critical challenge to ecological science in the coming decades. Ultimately, it will require a much tighter collaboration between scientists, policy makers, corporations, and real-world practitioners than we often see today. However, such collaborative ventures offer tremendous potential for better managing our landscapes—and the air, water, and biological diversity on which all life depends—sustainably into the future.
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