deep groundwater extraction-the rich tend to have the biggest impact (Watson et al., 1998).

     Agriculture is sustainable if the productive resource base is maintained at a level that can sustain the benefits obtained from it. These benefits are physical, economic and social. Ecological sustainability thus needs to be defined in relation to the sustainable use of natural resources, i.e., maintaining the productive capacity of an ecosystem.

Pressures on ecosystems have important consequences for agricultural production. In turn, agriculture has ecological impacts on ecosystems, and on the services provided by ecosystems.

     The IAASTD recognizes that in agriculture, there is most often a continuum between a farming system and a natural ecosystem, as the term agroecosystem indicates. Farmers have a pivotal role as managers of these systems, and as stewards of their resource base. Their role includes for example the conservation of soil properties and water availability, the development and maintenance of crop species and the pursuit of multipurpose production objectives. Issues relating to NRM management are often framed as specific problems such as soil degradation, water pollution, biodiversity loss. We should also frame agriculture's contribution to NRM positively: farmers create and enhance resources such as arable soil, agrobiodiversity, productive forest stands. Working with the natural resource base, they often enrich and enhance it.

     Drivers of natural resource degradation and depletion. As with other ecosystems, a range of direct and indirect drivers influence changes in natural resources in agricultural ecosystems. These drivers can act directly or indirectly to cause change. They may range from well defined drivers to those involving complex interactions. Among the key drivers assessed here is the role of decision makers and identification of those drivers that influence their decisions. Also important are the specific temporal, spatial and organizational scale dependencies as well as linkages and interactions between these drivers. The approach adopted also assumes that decisions are made at local, regional and international levels. Many globally recognized drivers are likely to influence natural resources in the context of agriculture, including demographic, economic, sociopolitical, science and technology, cultural and religious, and physical, biological and chemical drivers (see Figure 1-7).

Definition of natural resources

No unanimously accepted definition of natural resources exists. Natural resources can be defined as "factors of production provided by nature. This includes land suitable for agriculture, mineral deposits, and water resources useful for power generation, transport and irrigation. It also includes sea resources, including fish and offshore minerals" (Black, 2003). Natural resources may also be more broadly referred to as resources that "include all functions of nature that are directly or indirectly significant to humankind, i.e., economic functions as well as cultural and ecological functions that are not taken into account in economic models or which are not entirely known" (CDE, 2002). Climate can also be considered as a natural resource.

     In these broader definitions, resources such as timber or fish are part of ecosystems that are living environments containing forests, rivers, wetlands and drylands as well agroecosystems embedded in broader ecosystems that make use of selected resources within the ecosystem (WRI, 2005). From here, it is a short step to integrating natural resources in the "ecosystem services" concept (MA, 2005a), i.e., to describe natural resources as system elements that ensure human well-being through a range of interdependent regulating, supporting, provisioning and sociocultural functions.

Availability of natural resources. The Millennium Ecosystem Assessment concluded that the global availability of natural resources is shrinking. "Over the past 50 years, humans have changed ecosystems more rapidly than in any comparable period of time in human history, largely to meet rapidly growing demand for [natural resources]. This has resulted in a substantial and largely irreversible loss in the diversity of life on earth" (MA, 2005a). Ecosystem change means that availability of natural resources should not be expressed exclusively in terms of physical availability. Their functional availability needs to be indicated as well.

Natural resource dynamics. As a result of intensifying global interactions, spatial and temporal effects become more interlinked and these are related to the weak recognition of the multifunctional nature of agroecosystems at all hierarchical levels. Resource degradation in one location may lead to pollution in another location. High discount rates for agricultural investments, in particular in developing countries, have been an incentive for short-term decision making, with the effect that farmers undervalue both future benefits and the costs of their present resource use. However, hunger may influence a household's view of the agricultural discount rate. Thus, while many households are aware that their decision-making is short term, the severe cost of hunger makes long-term considerations of benefits of natural resources irrelevant to them. Both poverty-induced expansion of agricultural activities into fragile and vulnerable lands (Bonfiglioli, 2004), and capital-intensive extraction of resources such as groundwater can contribute to increased vulnerability of natural resources.

     The functionality of ecosystems and the temporal effects of system alterations are insufficiently understood. For example, understanding and using ecosystem functions in agriculture could result in enormous ecological savings while at the same time contributing to sustainable production of food (e.g., Costanza et al., 1997). There is an increased risk of non-linear changes as a result of system alteration (MA, 2005a). Therefore, the understanding of spatial and temporal effects of natural resource use for agricultural production is an increasingly important issue for science and technology in agricultural development.

Vulnerability and resilience of natural resources. The loss of ecosystems such as wetlands and mangroves has reduced natural protection of resources by destroying all or part of the inherent system functionality (MA, 2005a). The differences between damage caused by the December 2004 tsunami on shores protected by functional coral reefs and shores