Agriculture, water, and ecosystems: avoiding the costs of going too far
such as nitrogen uptake from the atmosphere and pollination of crops. Agricultural systems are thus viewed as ecosystems that are modified, at times highl , by activities designed to en- sure or increase food production (box 6.1). ese ecosystems are often referred to as agroeco- systems; the difference between an agroecosystem and other ecosystems is considered to be largely conceptual, related to the extent of human intervention or management.
Disruption of the processes that maintain the structure and functioning of an eco- system, such as water flow, energy transfer, and growth and production, can have dire con- sequences, including soil erosion and loss of soil structure and fertility. Severe disruption can result in the degradation or loss of the agroecosystem itself or other linked ecosystems and the ecosystem services that it supplies (see chapter 15 on land). e degradation of the Aral Sea is a dramatic example of human intervention having gone too far (box 6.2).
Agriculture makes landscape modifications unavoidable
There are many land and water manipulations that can increase the productivity of agricultural land in order to meet increasing demands for more food. All have consequences for ecosystems. The key message is that agriculture makes landscape modication unavoidable, although smarter application of technology and more emphasis on ecosystemwide sustainability could reduce adverse impacts. These land and water manipulations include:
Shifting the distribution of plants and animals. Most apparent are the clearing of native vegetation and its replacement with seasonally or annually sown crops, and the replacement of wild animals with domestic livestock.
Coping with climate variability to secure water for crops. As water is a key material for photo- synthesis, crop productivity depends intimately on securing water to ensure growth. Three differ- ent time scales need to be taken into account when considering water security: seasonal shortfalls in water availability that can be met by irrigation so that the growing season is extended and extra crops can be added; dry spells during the wet season that can be met by specic watering that can be secured, even in small-scale farming, if based on locally harvested rain; and recurrent drought that has traditionally been met by saving grain from good years to rely on during dry years.
Maintaining soil fertility. The conventional way to secure enough air in the root zone is by drainage and ditching through plowing to ensure that rain water can inltrate. However, this also leads to erosion and the removal of fertile soil by strong winds and heavy rain. These side effects can be limited by focusing on soil conservation actions, such as minimum tillage practices.
Coping with crop nutrient needs. The nutrient supply of agricultural soils is often replenished through the application of manure or chemical fertilizers. Ideally, the amount added should bal- ance the amount consumed by the crop, to limit the water-soluble surplus in the ground that may be carried to rivers and lakes.
Maintaining landscape-scale interactions. When natural ecosystems are converted to agricultural systems, some ecological processes (such as species mobility and subsurface water ows) that connect parts of the landscape can be interrupted. This can have implications for agricultural systems as it can affect pest cycles, pollination, nutrient cycling, and water logging and saliniza- tion. Managing landscapes across larger scales thus becomes important; an increasing number of studies illustrate how to design landscapes to increase the productivity of agriculture while also generating other ecosystem services (Lansing 1991; Cumming and Spiesman 2006; Anderies 2005; McNeely and Scherr 2003).