2. Supporting ecosystem services
3. Regulating ecosystem services
5. Managing agricultural systems for ecosystem services
Agricultural ecosystems around the globe differ radically. These systems, designed by diverse cultures under diverse socioeconomic conditions in diverse climatic regions, range from temperate zone monocultural corn production systems to species-rich tropical agroforestry systems to arid-land pastoral systems. This diversity of agricultural systems produces a variety of ecosystem services. Just as the provisioning services and products that derive from these agro-ecosystems vary, the support services, regulating services, and cultural services also vary. In general, agricultural activities are likely to modify or reduce the ecological services provided by unmanaged terrestrial ecosystems (except for provisioning services), but appropriate management of key processes may improve the ability of agroecosystems to provide a range of ecosystem services.
agroecosystem. An ecosystem designed and managed by humans to produce agricultural goods
agroforestry. An agricultural system in which woody perennials are deliberately integrated with crops and/or animals on the same unit of land
biological nitrogen fixation. A process carried out by specific microbes that have the ability to convert atmospheric N2 gas into forms that can be used by plants
decomposition. The breakdown of organic residues carried out by bacteria and fungi resulting in the release of energy, nutrients, and CO2
mineralization. The release of nutrients occurring during decomposition; nutrients such as N and P are converted from organic forms to soluble inorganic ions that can be taken up by plants
natural enemy. A predator, parasite, parasitoid, or pathogen of another organism; often describes beneficial organisms that attack pests in agricultural systems
polyculture. An agricultural system in which multiple crops are grown on the same unit of land at the same time
Agricultural ecosystems cover approximately 40% of the terrestrial surface of the Earth. These highly managed ecosystems are designed by humans to provide food (both plant and animal), forage, fiber, biofuels, and plant chemicals. The primary ecosystem services provided by agriculture are these provisioning services. Influenced by human management, ecosystem processes within agricultural systems provide other services that support the provisioning services, including pollination, pest control, genetic diversity for future agricultural use, regulation of soil fertility and nutrient cycling, and water provisioning.
In addition to these provisioning services, however, agroecosystems can also provide a wide range of regulating and cultural services to human populations. Regulating services from agriculture may include flood control, water flow and quality, carbon storage and climate regulation through greenhouse gas emissions, disease regulation, and waste treatment (e.g., nutrients, pesticides). Cultural services include scenic beauty, education, recreation, and tourism, as well as traditional use. Traditional use may comprise the incorporation of agricultural places or products in traditional rituals and customs that bond human communities. One additional ecosystem service that might be classified as a cultural service is the support of biodiversity. To the extent that appreciation for nature is an explicit human value, the ability of a particular agroecosystem to maintain and enhance biodiversity may be included under cultural services. Biodiversity may, in return, provide a variety of supporting services to agricultural and surrounding systems.
In the discussion below, major ecosystem services from agriculture are described in the context of some alternative management systems. In some cases, agricultural modifications to ecosystems will undoubtedly lead to a decline in the quantity or quality of ecosystem services. Here we identify management practices that prevent or ameliorate potential loss or degradation of services where possible. Clearly, there are instances where there is a trade-off between increasing yields and supporting a broader array of ecosystem services, but some agricultural practices may both enhance yields and support ecosystem services. Not all ecosystem services are addressed in detail; in particular, cultural services are not treated extensively here.
Agricultural crops are inevitably attacked by insect pests and pathogens that reduce the quantity and quality of the products that humans derive from agroecosys-tems. Management systems that emphasize crop diversity through the use of polycultures, cover crops, crop rotations, and agroforestry can reduce the abundance of insect pests that specialize on a particular crop while providing refuge and alternative prey for natural enemies. A variety of organisms, including insect predators and parasitoids, insectivorous birds and bats, and microbial pathogens, can act as natural enemies to agricultural pests and provide biological control services in agroecosystems. These biological control services can reduce populations of pest insects and weeds in agriculture, reducing the need for pesticides.
Conservation biological control, where agricultural habitat and management practices are manipulated to conserve and enhance populations of beneficial organisms already present in a system, can be effective in reducing pest populations and pesticide usage (figure 1). The goal of conservation biological control is to sustain natural enemy populations even when pests are scarce. This allows natural enemies to be abundant at the beginning of pest outbreaks, when they have the greatest chance of controlling pest populations below economically damaging levels. Conservation biological control techniques include planting polycultures where different crop species or genotypes (varieties of the same crop) are interplanted; conserving uncultivated areas throughout a farm; planting refuge strips within fields that are not tilled or sprayed with pesticides; reducing tillage; and reducing applications of broad-spectrum insecticides (i.e., those that target many insects, even beneficial ones).
Figure 1. Components of conservation biological control.
Many conservation biological control techniques are complementary in providing natural enemies with refuge and alternative prey. The microclimate in field crops can often be too dry and hot for natural enemy species during the summer. Conserved areas with trees and shrubs and intentionally planted refuge areas with mixed grasses and flowers can provide cool, moist refuges for natural enemies. Many natural enemies also need an uncultivated space to overwinter, which can also be provided by natural and planted refuge areas. Uncultivated areas within farms also provide refuge for natural enemies from pesticide applications that can be directly toxic. Reducing tillage within fields, which increases plant debris, has been shown to provide refuge for natural enemies. Provisioning of alternative prey when pests are scarce can be provided by polycultures, planted refuges, and conserved natural areas, all of which can enhance the diversity of insect prey and species of nectar-bearing plants.
Even agroecosystems based on monocultures can conserve the diversity of natural enemies if pesticides are not used. For example, paddy rice monocultures managed without pesticides can have a surprisingly high diversity of herbivorous insects, predators, and para-sitoids compared with similar monocultures in which pesticides are used. By refraining from applying pesticides, tropical rice farmers enable natural enemy communities of hundreds of species per hectare, providing good economic pest regulation. Pest-management programs in Southeast Asian paddy rice have taken advantage of this diversity of natural enemies and have drastically reduced pesticide inputs without sacrificing yields. In traditionally managed rice fields, predators are likely to include fish and amphibians, which contribute to pest regulation and also provide additional nutritional resources for farm families.
Approximately 65% of plant species require pollination by animals, and 75% of crop species of global significance rely on animal pollination, primarily by insects. Although much of agriculture relies on the pollination services of domesticated honeybees (Apis mellifera), native bees can enhance pollination rates, fruit size, and seed set of some crops.
There is much concern about reported declines in the abundance and diversity of wild pollinators and about increasing disease problems in domesticated bees. These declines are, in part, a result of the intensification of agricultural systems. Broad-spectrum and systemic insecticides can be directly toxic to pollinators. Broad-leaf herbicides decrease the abundance and diversity of flowering weeds that provide food resources for pollinators in agricultural landscapes. Expanding agricultural acreage decreases the amount of natural areas available to pollinators, areas that provide nesting sites and contain plants with a diversity of flowering times and food resources.
Specific agricultural practices can be adopted to benefit wild pollinators. These include a reduction of pesticide usage. No-till soil management has also been shown to increase the abundance of ground-nesting bees. Conserving natural habitats can increase the amount of nesting areas and food resources available to pollinators. Seminatural areas that contain mixtures of different types of flowering plants can be planted throughout a farm to increase the diversity of pollinators. Crop rotations with mass-flowering crops such as rape, clover, alfalfa, and sunflower can provide important food resources and support higher densities of native pollinators.
Agriculture has profound effects on cycling of nutrients at local, regional, and global scales. Nitrogen and phosphorus are the two most important nutrients limiting biological production in ecosystems, and they are the most extensively applied nutrients in managed terrestrial systems. Use of fertilizers and increased biological nitrogen fixation in agricultural ecosystems accounts for 60% of new biologically active N from anthropogenic sources. The amount of available phosphorus in the biosphere has also increased tremendously in the last 50 years, largely as a result of phosphorus applications to agricultural lands. Phosphorus flux to coastal oceans has nearly tripled. Nutrient enrichment of the environment with N and/or P has a series of complex, often detrimental, consequences for natural ecosystems.
Intensive annual crop production systems are among the most important food production systems and are also the most problematic in terms of their contributions to greenhouse gases, nutrient enrichment, and soil degradation. Annual inputs of nitrogen and phosphorus to agricultural fields consistently exceed the amounts taken out of the system by harvest. This excess, which can be anywhere from 40 to nearly 100% of what was applied, is lost to the environment.
Conventional nutrient management in agriculture is based on developing optimum delivery systems for soluble inorganic fertilizers and managing the crop to create a strong sink for fertilizer by removing all other growth-limiting factors. The problem with this strategy is that soluble inorganic forms of N and P are fast cycling and are subject to multiple pathways of loss (figure 2A). When the pool of soluble inorganic N or P is greatly increased, losses of these added nutrients from the ecosystem also increase, leading to environmental degradation. Although conventional nutrient management has resulted in greater yields, it has also resulted in poor nutrient use efficiency and major losses of fertilizers to the environment. Soil degradation is also a secondary consequence of these intensive, fertilizer-driven cropping systems, mainly because of the use of intensive tillage combined with reduced inputs of crop residues and bare fallows.
Nutrient-management strategies that target a broader range of internal cycling processes and that integrate N, P, and C cycling are more effective at supporting ecosystem services beyond yield. For example, practices such as cover cropping or polyculture enhance plant and microbial uptake of N, promote nitrogen retention in the soil organic matter, and reduce standing pools of nitrate, the form of N that is most susceptible to loss (figure 2B). Other examples of effective management practices include using organic residues or animal manures as nutrient sources, legume intensification for biological N fixation and P-solubilizing properties, replacing soluble P fertilizers with sparingly soluble forms, diversifying rotations using plant species that promote rhizosphere (near root) processes such as soil aggregate formation, and integrating crop and animal production systems.
Figure 2. Contrasting nutrient management practices and their impacts on nitrogen cycling and loss pathways. (A) Conventional management with inorganic fertilizers as the main input and bare soil periods ranging from 2 to 6 months. (B) Ecologically based management relying on diversified nutrient sources and practices such as cover cropping. The size of the box or arrow indicates the size of the pool or flux. (Modified after Drinkwater and Snapp, 2007)
To maintain the full array of ecosystem services, nutrient pools such as soil organic pools, microbial biomass, and sparingly soluble P that can be accessed through plant- and microbially mediated processes, must be intentionally managed. Management practices that build these pools while minimizing processes leading to nutrient losses increase the capacity of the internal cycling processes to supply crops with nutrients. Such practices might also include the strategic use of buffer zones and hedgerows so that nutrients leaving fields are trapped before they reach other, sensitive systems (including rivers and lakes). Agroecosystems that integrate crop and animal production at the scale of either the farm or regional landscapes by, for instance, feeding crop wastes to animals and applying animal wastes as fertilizer also recouple N, P, and C cycling and can support multiple ecosystem services.
Maintenance of water quantity and continuance of water quality are significant ecological services provided by terrestrial ecosystems. Water flow and water storage in the soil are regulated by plant cover, soil organic matter, and the soil biotic community. The plant community plays a central role in regulating water flow by retaining soil, modifying soil structure, and producing litter. Pore structure, soil aggregation, and decomposition of organic matter are also influenced by the activities of bacteria, fungi, and macrofauna such as earthworms, termites, and other invertebrates. Trapping of sediments and erosion are controlled by the architecture of plants at or below the soil surface and the amount and decomposition rate of surface litter. Macrofauna that move between the soil and litter layer influence water movement within soil as well as the relative amounts of infiltration and runoff.
Agriculture modifies the species identity and root structure of the plant community, the production of litter, the extent and timing of plant cover, and the composition of the soil biotic community, all of which influence water infiltration and retention. The intensity of agricultural production and management practices will affect both the quantity and quality of water in an agricultural landscape. Practices that maximize plant cover, such as minimum tillage, polycultures, or agro-forestry systems, are likely to decrease runoff and increase infiltration. Irrigation practices may influence runoff, sedimentation, and groundwater levels in the landscape. Agricultural production systems that involve the application of significant levels of industrial nitrogen fertilizer can increase nitrate leaching and nitrate levels in drinking water, which can cause human health problems, particularly for infants. Applications of pesticides can result in pesticide residues in surface and groundwater. Hence, agricultural systems that rely heavily on agrochemicals can degrade the water-provisioning services provided by agroecosystems.
Globally, agriculture is estimated to be responsible for about 14% of greenhouse gas emissions. Land use change is the second largest global cause of CO2emissions after fossil fuel combustion, and some of this change is driven by conversion to agriculture, largely in developing countries. In developed countries, forest conversion to cropland, pasture, and rangeland were common through the middle of the twentieth century, but current conversions are primarily for suburban development. Approximately half of global annual emissions of methane (CH4) and a third of global annual emissions of nitrous oxide (N2O), both greenhouse gases, are attributed to agriculture.
Agricultural activities contribute to emissions in several ways. N2O emissions occur naturally as a part of the soil nitrogen cycle, but the application of nitrogen to crops can significantly increase the rate of emissions, particularly when more nitrogen is applied than can be taken up by the plants. Nitrogen is added to soils through the use of inorganic fertilizers, application of animal manure, cultivation of nitrogen-fixing plants (e.g., legumes), and retention of crop residues. In addition to direct N2O emissions from fertilizer application, the production of synthetic nitrogen fertilizers is a very energy-intensive process that produces additional greenhouse gases. Flooded rice cultivation also contributes to greenhouse gas emissions through anaerobic decomposition of soil organic matter by CH4-emitting soil microbes. The practice of burning crop residues also contributes to CH4 and N2O production.
Livestock also produce CH4 and N2O. Ruminant livestock such as cattle, sheep, goats, and buffalo emit CH4 as a by-product of their digestive processes (enteric fermentation). Livestock waste can release both CH4, through the biological breakdown of organic compounds, and N2O, through microbial metabolism of nitrogen contained in manure. The magnitude of emissions depends strongly on manure-management practices (e.g., the use of lagoons, field spreading) and to some degree on the type of livestock feed.
An array of agricultural practices can reduce or offset the agricultural greenhouse gas emissions described above. Effective manure management can significantly reduce emissions from animal waste. Increasing the use of biological nitrogen fixation in place of synthetic nitrogen fertilizers can reduce CO2 emissions from agricultural production by half. The restructuring of agroecosystems that accompanies legume intensification also modifies internal cycling processes and increases N use efficiency within agroecosystems via the recoupling mechanisms discussed above. Chronic surplus additions of inorganic N, which are currently commonplace, can be reduced under these scenarios, leading to reductions in NOx and N2O emissions.
Agriculture can offset greenhouse gas emissions by increasing the capacity for carbon uptake and storage in soils, i.e., carbon sequestration. The net flux of CO2 between the land and the atmosphere is a balance between carbon losses from land use conversion and land-management practices and carbon gains from plant growth and sequestration of decomposed plant residues in soils. In particular, conservation tillage and no-till cultivation can conserve soil carbon, and planting of cover crops can reduce the degradation of subsurface carbon. Under most conditions, the increased use of legumes in rotation is also expected to increase soil C storage. Many farmers have already adopted these practices to achieve higher production and lower costs.
Finally, agricultural land can also be used to grow crops for biofuel production. Biofuels have the potential to replace a portion of fossil fuels and may lead to lower greenhouse gas emissions. Although burning fossil fuels adds carbon to the atmosphere, biofuels, if managed correctly, avoid this by recycling carbon. Although carbon is released to the atmosphere when biofuels are burned, carbon is recaptured during plant growth. The replacement of fossil fuel-generated energy with solar energy captured by photosynthesis has the potential to reduce CO2, N2O, and NOx emissions. However, management practices used to grow crops and forages for biofuel production will influence net emissions. Development of appropriate biofuel systems based on perennial plant species that do not require intensive inputs such as tillage, fertilizers, and other agrochemicals have the potential to help offset fossil fuel use in agriculture and possibly in other sectors of the economy; biofuel systems that rely on annual plants such as corn may not be as beneficial.
Agricultural systems may play a role in regulating some infectious diseases of humans, both tropical and temperate. In the tropics, large-scale irrigation systems based on dams, reservoirs, and large canals can increase appropriate habitat for the snails that serve as intermediate hosts for the parasites that cause schistosomiasis, a debilitating disease that affects millions of people in the tropics. Small-scale systems, however, are less likely to lead to large increases in snail populations. In agricultural systems where soil erosion is not well managed, sedimentation and runoff can slow stream flow and decrease water depth, thereby creating excellent mosquito habitats of warm, shallow water with little or no flow. Irrigated rice paddies, in particular, can serve as excellent breeding grounds for the mosquitoes that transmit malaria and other human pathogens. Effective management of water flow and sedimentation, however, can disrupt vector development and reduce disease transmission. Moreover, traditional rice paddies in Asia that are managed without pesticides or fertilizers often harbor significant fish populations that effectively limit mosquito populations.
Changes in temperate agriculture within a subur-banizing landscape may also influence the prevalence of infectious diseases. Forest fragmentation in the northeastern United States, historically driven by agriculture but now by suburban development, has led to increased densities of the white-footed mouse, the principal natural reservoir host of Lyme disease, in the remaining forest patches. Because white-footed mice are the most competent hosts of the spirochete that causes Lyme disease, the presence of alternative hosts serves to dilute the prevalence of disease. Lyme disease risk to humans is thus correlated with the diversity of mammalian hosts, and mammal diversity is correlated with size of forest patches. Larger forest patches contain a higher diversity of mammals that, although hosts of Lyme disease, are less effective at transmitting it to humans than are white-footed mice. Small patches have relatively higher densities of white-footed mice, leading to higher disease risk. The abandonment of small areas of land from agriculture, leading to small patches of secondary forest, may also contribute to higher mouse densities and increased disease risk.
It is well documented that biodiversity provides many ecological services that aid human endeavors, including agriculture. Pest regulation by naturally occurring populations of natural enemies, as described above, is one example. Biodiversity is also a cultural value embraced by most human societies. Despite the value of biodiversity to humans, it is estimated that extinction rates during the last 100 years are 100 to 1000 times higher than the average rates of extinction that preceded large-scale human modification of landscapes. Given the extensive nature of agricultural activities in terrestrial ecosystems, many of the world’s species are affected by agricultural production. Arguably, agricultural production systems are a main driver of increased extinction rates through conversion of natural habitats to agriculture and increasingly intensive management. However, management options do exist to help conserve biodiversity in conjunction with agricultural production. Restructuring the agricultural system by increasing crop and livestock diversity is one approach to enhancing associated biodiversity in agroecosystems.
The spatial and temporal arrangement of domesticated plants and animals that farmers purposely include in the system may include several dimensions of diversity, including genetic diversity, species diversity, structural diversity, and functional diversity. This planned diversity may also include beneficial organisms that are deliberately added to the agroecosystem, such as biological control agents or plant-associated nitrogen-fixing bacteria. Unplanned diversity includes all the other associated organisms that persist in the system after it has been converted to agriculture or that colonize it from the surrounding landscape. As planned diversity increases along any of its dimensions, unplanned diversity also tends to increase.
The unplanned diversity that accompanies planned diversity in agricultural systems can provide many ecological services to agriculture. Uncultivated species, including wild relatives of crops that occur in and around the agroecosystem, are an important source of germplasm for developing new crops and cultivars and can provide habitat for beneficial organisms. Increasing planned crop diversity can augment the resources available to plant pollinators and to natural enemies and result in higher populations of these beneficial organisms. Increasing planned diversity may also foster beneficial soil organisms and the conservation of functional processes such as decomposition and nutrient cycling.
In addition to crop and livestock diversification, other management options to support biodiversity include reduced chemical inputs and the deliberate provision of resources and refuge habitats to wild plants and animals. Pesticides can have both direct and indirect effects on biodiversity in agroecosystems. Broad-spectrum insecticides directly reduce biodiversity by killing many nontarget insects. Loss of arthropod abundance and diversity affects other species that feed on insects including many birds and bats. Herbicides directly reduce the diversity and abundance of herbaceous plants within crop fields. This loss of plant diversity, in turn, also affects species at higher trophic levels such as pollinators and natural enemies. Fertilizer runoff from agricultural fields is linked to aquatic eutrophication and fish kills. Fertilizers may also indirectly affect plant diversity bordering fields by favoring annual plants adapted to high nutrient availability. Management practices that reduce the need for chemical inputs, such as the conservation of natural enemies of pests, crop rotations to help control pests and improve soil fertility, and the use of organic fertilizers, can all help to preserve biodiversity in and around farms.
Agricultural practices that provide a diversity of habitats can also be beneficial to wildlife. Homog-enization of the agricultural landscape reduces available niches and the number of species supported. Mixed cropping systems, where a variety of crops are planted together, directly increase the diversity of plants in fields and the species supported by those plants. Agroforestry systems enhance structural diversity, which also leads to a greater diversity of habitats and resources for associated fauna and flora. On a larger spatial scale, diverse crop rotations increase the diversity of plants present in the landscape at the same time. Conserving natural field edges, planting permanent grassy areas, integrating farm operations to include arable crops and pasture and forestry land, and reducing land drainage, can all help to preserve habitat diversity, which can support biodiversity in agricultural landscapes. Often, although not always, this can be done without significant loss to yields because the productivity of the remaining agricultural area is enhanced by the management of ecosystem services.
Habitat diversity supports the diversity of plants and animals through a variety of mechanisms. Noncropped and low-intensity management areas provide a physical refuge from farm operations. They also provide nesting and overwintering sites. They modify the microclimate to provide refuge that is cool and moist compared to crop fields. Woody field borders can provide cover from predators for foraging mammals. Compared with large, intensively managed monocultures, diverse habitats provide greater temporal stability in the range of food resources for wildlife. Unmanaged field edges can also act as dispersal corridors among larger habitat patches for many species. Dispersal allows recolonization of disturbed habitats and the population mixing that prevents inbreeding, which can compromise the vigor of beneficial organisms.
Agroecological practices to promote biodiversity are not expected to conserve all species equally; effects will be highly dependent on species’ life histories. Effects of management practices may also depend on the surrounding landscape. For example, changes in chemical inputs within a field or farm have been shown to have high impacts on plants with viable seed banks, whereas vertebrates with large home ranges may be influenced more by landscape composition. Furthermore, transition to organic management has been shown to have greater benefits for wildlife when the surrounding landscape is dominated by conventional agriculture than when the surrounding habitats are already diverse.
A time delay may be expected between changes in agricultural management and the effects on biodiversity. For example, a lag between rates of agricultural intensification in Great Britain and the decline of farmland bird populations has been detected. This time lag may be attributable to spatial thresholds of intensification. Below certain thresholds, species may be able to compensate for deteriorating local conditions. Conversely, a shift toward conservation-oriented management may also have delayed effects. For example, dispersal-limited species may take years to colonize newly restored habitats if those habitats are not near source populations. Shifts in crop rotations and organic inputs are likely to have long-term, accumulating effects on soil structure and chemistry. These changes influence soil biota and may have cascading food-web effects on other species.
The particular suite of agricultural practices that will optimize ecosystem services from agroecosystems is site specific and reflects the biological (pests and pathogens, natural enemies, microbial symbionts), physical (climate, soils), and socioeconomic (government regulations, agricultural policies, market structure) environment of the agroecosystem, the crops that are being grown, resources available to the farmer, and the livelihood goals of the household (figure 3). One traditional dilemma is weighing the importance of provisioning services against the value of supporting, regulating, and cultural ecosystem services that are provided by agroecosystems. However, recent analyses have suggested that this may be a false dilemma.
Figure 3. Interplay of constraints affecting farm management decisions, agroecosystem services, and crop yield.
Recent studies have synthesized data on the yield performance from agroecosystems around the world and found that, on average, agroecosystems using the ecological management approaches described here (e.g., conservation tillage, crop diversification, legume intensification, conservation biological control, etc.) perform as well as intensive, high-input systems (Badgley et al., 2007). That is, the provisioning services provided by the agroecosystem are not jeopardized by modifying the system to improve its ability to provide other ecological services. Moreover, the introduction of these types of practices into resource-poor agroecosystems in 57 developing countries resulted in a mean relative yield increase of 79% (Pretty et al., 2006). These synthetic analyses suggest that it may be possible to manage agroecosystems to support a full suite of ecosystem services while still maintaining the provisioning services that agroecosystems were designed to produce.
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