ECOSYSTEMS
Agriculture Systems
O Andrén, TSBF-CIAT, Nairobi, Kenya
T Kätterer, Department of Soil Sciences, Uppsala, Sweden
r 2008 Elsevier B.V. All rights reserved.
Introduction
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 (soilless)
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.
The Agroecosystem
Fig. 1 is an attempt to summarize the characteristics of an agroecosystem as compared to most natural ecosystems. Note that this
comparison is between a typical natural ecosystem and a typical, high-production agroecosystem.
Encyclopedia of Ecology, 2nd edition, Volume 2 doi:10.1016/B978-0-444-63768-0.00313-9 401
Abiotic Constraints
Just like natural ecosystems, agroecosystems are constrained by climate and soil properties – maize does not grow in Northern
Sweden. However, climate can be modified, that is, in dry climates one can irrigate (with surface- or groundwater), and soil
properties can be modified through, for example, liming, organic matter amendments, and fertilization. Too high water tables can
be lowered through ditching or tile draining.
Nutrients
Highly productive agroecosystems need high inputs of plant nutrients (nitrogen, phosphorus, potassium, and other elements) to
replenish the nutrients removed with the exported products. These inputs can be delivered either as commercial fertilizer,
recirculated sewage sludge and ash from garbage burning, or manure from cattle, pigs, poultry, etc. All sources have their
advantages and disadvantages. Commercial fertilizers are well defined, low in pollutants such as heavy metals (although exceptions
exist), hygienically safe, and are concentrated, easy to transport, and rapidly available to the plant when applied in the field.
However, production and long-range transport of fertilizers is energy consuming, and the concentrated product increases the risk
for too high doses, leading to environmental pollution. An even greater problem is that a large part of the farmers of the world
cannot afford to buy enough fertilizer to maintain soil fertility and obtain good yields. In all, world N fertilizer production in 2001
was slightly less than 90 Mt, very unevenly distributed. In sub-Saharan Africa, only 1.1 kg fertilizer nitrogen is used per person and
year, whereas in China the corresponding value is 22 kg.
In theory, recirculation of nutrients from waste of the exported products seems to be ecologically sound. In practice, there are a
number of problems. First, sewage sludge mainly consists of water, which either must be removed (requires energy) or transported,
which is expensive and impractical. Second, sewage sludge contains harmful bacteria, human parasites, etc. and has to undergo
hygienic treatment. Third, and most severe, is the problem with contaminants, such as heavy metals and organic toxins. Therefore
recirculation of sewage sludge and garbage incineration ash is strictly regulated in most countries. In this perspective, replacement
of nutrients using newly produced fertilizer can be a better solution from an environmental viewpoint.
Naturally, animal manure produced on the farm should be and is recycled to soil as much as possible. Compared with
fertilizers, manure has the advantage of containing organic matter, which improves soil structure. On the other hand, manure
contains mostly water (expensive storage and transportation, heavy machinery needed for spreading), and it will lose nitrogen
through ammonia emission, both at storage and spreading.
Crops, Varieties, and Cropping Systems
The vegetation found in an agroecosystem is usually divided into crop and weeds, where weeds are unwanted trespassers, which
traditionally have been regarded only as negatives. More recently, this view has been modified, and weeds, particularly weedy
border zones can be accepted to some extent, as biodiversity enhancers and refuges, for example, for beetles.
The crops used today are products of many years (in some cases millennia) of plant breeding, and properties selected for are
usually productivity, product quality, pest resistance, etc. This directed selection, in recent years augmented by direct manipulation
of DNA, is one of the main differences between agro- and natural ecosystems. Crop species and varieties are being redistributed all
over the world; maize, a staple food in Africa, comes from Central America, common West European and North American cereals
such as wheat come from the Middle East, etc. This breeding and distribution of improved crops, together with improved
Natural
Plant biodiversity Soil biodiversity
Plant production
Soil cultivation Decomposition rates
Primary consumers
Potential for nutrient loss
Agricultural
Plant biodiversity Soil biodiversity
Plant production Soil nutrient status
Soil cultivation Decomposition rates
Primary consumers Plant disease
Potential for nutrient loss
Solar energy
Rainfall
Nutrient input
Seed input
Pesticide input
Migration
Human control
Solar energy
Rainfall
Nutrient input
Seed input
Pesticide input
Migration
Human control
Carbohydrates
proteins
Other ecosystem
services
Carbohydrates
proteins
Other ecosystem
services
Soil nutrient status
Plant disease
Fig. 1 Similarities and differences between typical natural and high-production agricultural ecosystems. Inputs of energy, mass, and control (left),
comparison of selected ecosystem properties (center), outputs (right). Note that cattle, etc. are not included as primary consumers here.
(Bold ¼ markedly higher value than in the other ecosystem type. Italic ¼ very low.)
402 Ecosystems: Agriculture Systems
cultivation/fertilization techniques probably is the main reason for the global success of the human species (three billion in 1960,
probably nine billion in 2050). For example, world grain production was 631 Mt in 1950, and in 2000 it had increased to
1840 Mt.
Herbicides, Pesticides, and Fungicides
To reach the goal of high production of crops of good quality, weeds (unwanted plants), pests (unwanted animals), as well as
fungal, bacterial, and viral diseases must be kept in check. A monoculture crop is vulnerable to attacks, since one (or a pair) of the
pests that enter a field will have a high concentration of food with no transport stretches in between. Potential predators may be
absent, since they may need a litter layer on the ground for reproduction, which does not exist in the field, etc. Repeated
monocultures may build up specialized pests, such as plant parasitic nematodes. Crop rotations (switching crops from year to year
according to a predetermined pattern) can successfully deal with many pests and diseases, and careful soil cultivation can reduce
weed problems. Intercropping (growing two or more crops together, such as barley/clover) may also help.
However, most fields will benefit from occasional chemical (or biological) pesticide/herbicide treatment. These types of
agrochemicals have a somewhat dubious reputation among laymen and perhaps also ecologists (DDT, Agent Orange, mercury,
etc.). Three things should be kept in mind, though. First, the substances and formulations used today are thoroughly tested before
approval, and their side effects and the fate of their decomposition products are well known. Second, chemical warfare is common
in natural systems – all successful plant species present today have at least some chemical defense against microorganisms and
pests. Third, which alternatives do we have? A failed crop in a well-fertilized field will lead to high risks for nutrient losses to the
environment. A failed crop in poorer conditions may lead to starvation for the farmer and her family.
Alternative methods, such as increased cultivation, hand weeding, or biological pest reduction by introduction of predators all
have their advantages and disadvantages, but there is no `silver bullet' available. In summary, an integrated approach with a
combination of methods is the solution, and modern agriculture has moved and is moving in this direction. Of course, for
commercial reasons it can be profitable to cultivate, for example, `organic' crops (without fertilizer or pesticides) to obtain a higher
price, but from an ecological or environmental viewpoint this approach is not necessarily better.
Agriculture can thus be classified according to the use of agrochemicals, for example, biodynamic, organic, integrated, and
industrialized farming. Biodynamic farming forbids the use of conventional agrochemicals and replaces them with exotic
homemade concoctions, and organic farming a priori forbids conventional agrochemicals. None of these farming systems is firmly
based on scientific evidence; instead they are based on a green view of nature that leads to the banning of certain chemicals.
Integrated and industrial farming can also be called `conventional', where economic, legal, and environmental constraints limit
the end goal, maximum productivity, and profitability. The main difference between the latter two is that integrated is more
environmentally concerned (reduced pesticide use, use of biological pest reduction methods, etc.), and industrialized is more
leaning to maximum production with whatever means available, with a minimum of environmental concerns. It should be noted
that `conventional' and particularly `industrialized' are somewhat derogatory terms, mainly used by those negative to these
approaches.
Migration
Natural ecosystems, for example, East African savannas, can be subjected to major migrations of large herbivores that annually
move long distances, following the seasonal changes in rainfall and consequential grass growth. Most natural ecosystems are less
subjected to migrations, but, for example, in Northern forests at least migratory birds occur seasonally.
In agroecosystems, migration is usually kept to a minimum. Measures are taken to keep large or small grazers out from the
cropped field. In some regions, wild grazers are exterminated (or close to extinction – Western European agricultural regions) and
in other regions crop fields are guarded or fenced. However, migration is a component in animal husbandry; cattle is often shifted
between pastures, which are given time to recover. Nomadic herding of cattle (Sami people, Masai) is similar to the savanna
migrations mentioned above; the cattle and herdsmen follow the annual cycles in grazing opportunities.
Biodiversity
In a cereal monoculture, plant biodiversity is extremely low – if weed control is successful there may be only one species present, a
highly specialized and genetically homogeneous wheat variety. This is not common in natural ecosystems, although it can occur in
extreme environments. As mentioned above, this means that a pest can have a field day if it can reproduce in the field (or migrate
into the field at a large scale).
However, agricultural monocultures still are common and continue to produce good yields. There are several reasons for this.
First, there is no simple relation between biodiversity, productivity, or ecosystem stability. A plant monoculture that is well
adapted, grows under good conditions, and has a reasonable resistance to pests and diseases can survive and produce well. This is
exactly what a highly productive agricultural field is – a well-adapted monoculture. The crop variety has been selected for high
production under a number of years with different weather (and on different soils) in a region. A variety that would demand
intensive treatment with herbicides, pesticides, and fungicides will not be economical and will be rejected.
Ecosystems: Agriculture Systems 403
Second, the low plant diversity reduces animal diversity in the stand, but perhaps less than one would expect. In a cereal
monoculture stand, there can be hundreds of species of insects, mites, springtails, snails, slugs, etc. In the soil under a monoculture
the biodiversity is almost always extremely high, though usually lower than in natural systems. Thousands, perhaps millions of
bacterial species, tens to hundreds of species of earthworms, enchytraeids, soil insects, springtails, mites, spiders, millipedes,
flagellates, amoebae, blue-green algae, etc. can be found. There are no consistent indications that soil functions such as organic
matter decomposition is hampered by a low biodiversity under monocultures – a given plant residue will decompose at the same
rate under a monoculture as under mixed plants, if soil temperature and moisture are the same.
Third, the last line of defense is the crop protection measures that the farmer takes. For example, in several countries there is a
sophisticated monitoring and prediction system for aphid outbreaks. Aphids suck the sap from the crop leaves, but they are also
vectors for crop diseases. Therefore their hibernating stages are enumerated, weather is monitored, and if the conditions are `right'
the farmers are recommended to spray the fields with an insecticide (or a more specific aphicide) with dose x at date y. In less
technically developed regions, experience and skill is a substitute for the model projections, but the principles are the same. It
should also be mentioned that in spite of these defenses, pest insects, pathogens, and weeds still reduce worldwide crop yields
considerably, and there is a great potential for improvements.
Other Ecosystem Services
The main ecosystem service from agricultural systems is simply to `feed the world'. This simple fact is easily forgotten in the richer
parts of the world. However, even in Europe, which for centuries has been thoroughly under agriculture, there are other ecosystem
services that are appreciated. In the forest-dominated northern Europe, agriculture actually contributes to biodiversity and
landscape diversity. Without agriculture, the forest would cover all land area – the only open areas at lower altitudes would be the
lakes and rivers (and the newly clear-cut forest areas, rapidly covered by shrubs). The European rural landscape in general, that is
so refreshing for the city-dweller, is an agricultural product.
Fig. 2 Example of farmer's decisions regarding N management for a maize crop in sub-Saharan Africa, using a decision support system for
organic N management depending on resource quality, expressed as N, lignin, and soluble polyphenol content. General decision matrix (top), more
detailed for N economy in a maize cropping system (bottom). Modified from Vanlauwe B, Sanginga N, Giller K, and Merckx R (2004) Management
of nitrogen fertilizer in maize-based systems in subhumid areas of sub-Saharan Africa. In: Mosier AR, Syers JK, and Freney JR (eds.) Agriculture
and the Nitrogen Cycle. 124p. SCOPE 65. Washington Island Press.
404 Ecosystems: Agriculture Systems
In other areas of the world, where the agricultural land is not sufficient to properly feed the population, other ecosystem
services become relatively less important. However, if agricultural productivity can be increased, some agricultural land can be
returned to savanna, forest, or other natural or seminatural states – which would be another type of service from the
agroecosystem.
Since the agroecosystem is managed, and more or less sophisticated machinery and management skills are in place, it can easily
be converted according to new demands from the society. If the quality requirements are met, agricultural fields can be used for
recycling organic waste and ashes, and even for drawing nutrients out of sewage water. Conversion to energy crops is not too
difficult (grasses, sugar beet, willow, sugarcane, etc.). Another demand from society, to sequester carbon in the soil to reduce CO2
in the atmosphere, has recently received much attention. Increasing soil carbon content usually has beneficial effects for soil
structure, water-holding capacity and general fertility, and C sequestration, perhaps even with direct payments per ton C
sequestered to the farmer, is a new potential service.
The Intelligent Choices
As mentioned in the introduction, an agroecosystem has a purpose. It is designed to obtain certain goals, and the state of the
system at any given point in time is a consequence of an array of intelligent choices by the farmer, complementing the border
conditions set up by weather and soils, etc. The following decision matrix (Fig. 2) illustrates how decisions made by a maize
farmer in sub-Saharan Africa can be supported by basic science knowledge. Note that the chemical analyses are not necessary for
every farmer and decision. Instead, typical values for the different organic resources are estimated, and the individual farmer uses
the rule of the thumb based on these estimates.
In the upper part of the Fig. 2, the general decision matrix is shown. Let us assume that we have leaves from a tree, which we
know have a low N content and less than 15% of lignin. Then we should mix the leaves with fertilizer or add to compost. Now, in
the lower part of Fig. 2 we can see that if we look in more detail at the N economy of a maize system, we have other options –
maybe add the low N material to the cattle corral (kraal/boma) to trap urine N or feed to livestock to produce higher quality
organic inputs. Organic resources belonging to the third column from the left could be fed to livestock and the manure thus
produced could belong to the first or the second organic resource class, depending on the management of that manure.
All over the world, farmers make these kinds of choices, based not only on biophysical knowledge and constraints, but also on
economic and sociopolitical opportunities and constraints. An agroecosystem is not only controlled by farmers, but also by the
society the farmer operates in. Subsidies can make growing products that have no market an intelligent choice for the farmer; lack
of money can make fertilization impossible, even if it would be profitable in the long run, or real or imaginary environmental
concerns from the society can force a farmer to, for example, abandon fertilizer use, cereal cropping, or pig farming.
Summing up, the agroecosystem, although limited by climatic constraints, is a product of decisions made by generations of
farmers, supported by advice from agronomists and extension workers – all within a societal context of values, traditions, and
legislation. In fact, the present and future agroecosystems are at least equally dependent on the societal context as on the climate
and soil. However, the organisms involved are, as in any ecosystem, products of millions of years of evolution, and crop and
animal breeding has only contributed with small, although important changes to the germplasm.
See also: Aquatic Ecology: Abundance Biomass Comparison Method; Eutrophication. Behavioral Ecology: Herbivore-Predator Cycles; The
Marginal Value Theorem in a Nutshell; Thermoregulation in Animals: Some Fundamentals of Thermal Biology. Ecosystems: Coral Reefs.
Evolutionary Ecology: Metagenomics; Natural Selection; Phylogenomics and Phylogenetics. General Ecology: Plant Ecology; Allopatry.
Terrestrial and Landscape Ecology: Thermodynamic Properties of Landscape Cover
Further Reading
AndrÅn, O., Lindberg, T., Paustian, K., Rosswall, T. (Eds.), 1990. Ecological Bulletins 40: Ecology of Arable Land - Organisms, Carbon and Nitrogen Cycling. Copenhagen:
Ecological Bulletins.
Brussaard, L., 1994. An appraisal of the Dutch program on soil ecology of arable farming systems (1985–1992). Agriculture, Ecosystems and Environment 51 (1–2), 1–6. and
following papers.
Clements, D., Shrestha, A. (Eds.), 2004. New Dimensions in Agroecology. Binghamton: The Hawort Press, Inc, p. 553.
Eijsackers, H., Quispel, A. (Eds.), 1988. Ecological Bulletins 39: Ecological Implications of Contemporary Agriculture. Copenhagen: Ecological Bulletins.
Kirchmann, H., 1994. Biological dynamic farming – an occult form of alternative agriculture? Journal of Agricultural and Environmental Ethics 7, 173–187.
Mosier, A.R., Syers, J.K., Freney, J.R. (Eds.), 2004. Agriculture and the Nitrogen Cycle. SCOPE 65 Washington: Island Press, p. 124.
New, T.R., 2005. Invertebrate Conservation and Agricultural Ecosystems. Cambridge: Cambridge University Press, 354pp.
Newman, E.I., 2000. Applied Ecology and Environmental Management, 2nd edn. Blackwell Science.
Vanlauwe, B., Sanginga, N., Giller, K., Merckx, R., 2004. Management of nitrogen fertilizer in maize-based systems in subhumid areas of sub-Saharan Africa. In: Mosier, A.R.,
Syers, J.K., Freney, J.R. (Eds.), Agriculture and the Nitrogen Cycle. Washington Island Press, p. 124. SCOPE 65.
Woomer, P.L., Swift, M.J., 1994. The Biological Management of Tropical Soil Fertility. Chichester: Wiley.
Ecosystems: Agriculture Systems 405
Relevant Websites
http://www.cgiar.org— Consultancy Group on International Agricultural Research.
http://www.fao.org— Food and Agriculture Organization of the United Nations.
406 Ecosystems: Agriculture Systems