restricted access VI.4 Human-Dominated Systems: Agroecosystems
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VI.4 Human-Dominated Systems: Agroecosystems Alison G. Power, Megan O’Rourke, and Laurie E. Drinkwater OUTLINE 1. Introduction 2. Supporting ecosystem services 3. Regulating ecosystem services 4. Supporting biodiversity 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 agroecosystems 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. GLOSSARY 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 bene ficial 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 1. INTRODUCTION 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. 2. SUPPORTING ECOSYSTEM SERVICES Regulation of Agricultural Pests Agricultural crops are inevitably attacked by insect pests and pathogens that reduce the quantity and quality of the products that humans derive from agroecosystems . 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...