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An agricultural ecosystem is an ecosystem managed with a purpose. This purpose usually is to produce crops or animal products. Agricultural ecosystems are designed by humans, and current agroecosystems are products of a long chain of experimental work. These experiments have been performed by individual farmers as well as research institutions, and when results were positive for the purpose, the methods have been adopted.

The purpose has, however, changed with time. In highly productive regions, for example, Western Europe, the emphasis has changed from maximum productivity to environmental considerations, such as reduction of nutrient losses to groundwater and maintaining an open landscape with high biodiversity, etc. In less-productive regions, where resources such as water or fertilizers are scarce and production is too low to properly feed the farmer, environmental considerations have low priority. This is a major global problem, since this leads to land degradation and even lower production, etc. in a downward spiral.

Agroecosystems are conceptually fairly similar to managed forests and grasslands, and whether extensively cattle-grazed natural grasslands should be included under the category of agroecosystems is a matter of choice in the individual case. Arable land is defined as land that is soil cultivated regularly, but also here the boundaries are not sharp (seminatural grasslands, permanent crops, etc.). At the other end, agroecosystems border horticultural systems, that is, vegetable cropping. Alternatively, horticulture can be viewed as a subset of agriculture. Production of cabbage in a field can be considered as agriculture, but hydroponic (soil-less) production of tomatoes in a greenhouse under artificial light can perhaps not be included. However, in many respects even an artificial ecosystem such as this can be considered as an agricultural ecosystem. It is designed for production of a crop and is just managed to a higher extent than an arable field.

According to FAO statistics for 2002, agricultural ecosystems comprise almost 40% (5 Gha) of the total land area of the Earth. About 11 % of the total land area is arable land (cultivated with crops), and approximately

27% of the total land area is under permanent pasture, grazed by cattle, goats, sheep, camels, etc. Clearly, we are actively managing a considerable part of our planet for agricultural purposes, and to this one can add other similar systems, such as intensively managed forest systems (planted and harvested, sometimes fertilized), etc.

Ecological research performed in agricultural systems has many advantages compared with research in most natural ecosystems. For example, there are a number of long-term field experiments running, although originally designed for, for example, crop production response to fertilizer dose, that can give us a 30-year integration of what has happened, for example, to organisms in the soil under different conditions. Further, agricultural fields are 'homogenized', that is, trees, larger stones, etc. are removed and regular soil cultivation evens out differences in topsoil properties over time. However, even after many years of cultivation, a fairly high variability in soil properties remain, which is the incentive for 'precision farming', where soil and crop properties are measured at high resolution (m2), and management is based on these measurements. For ecological research, this is an opportunity, since any given hectare will yield numerous observation points, each helping us to answer questions such as: Why does this particular location yield more wheat, or why is more water present at that location?

Another advantage is that agricultural crops often have a short lifespan and a small size, compared with, for example, forest trees. Often, an experiment can be started when the soil is bare, and a single crop can be followed from sowing, through harvest, and finally when the stubble is plowed down at the end of the growing season. This life cycle can take a century for a tree in a northern forest, which, to add insult to injury, also may contain several other plant species. Therefore it is not surprising that a considerable part of modern ecological theory (predator-prey interactions, general soil ecology, above- and belowground plant growth dynamics, organic matter decomposition, nutrient mineralization, etc.) is based on work performed in agricultural land, and that the reluctance of ecologists to work in agricultural systems that was obvious 30 years ago seems to have vanished.

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