Agroecology

I wrote the following articale about a decade ago for an independent study course on sustainable agriculture. The writing style is very academic, but the content is worth the read if you’re interested in agroecology and family farming.


Overview
animal-prodAgroecosystems are agricultural ecosystems; they include plants, animals and humans in a biotic community. A farmer manages an agroecosystem by removing products that have economic value and introducing inputs to maintain acceptable levels of productivity. If you visually compare a farmer’s field with a natural ecosystem in the same geographic area, you will notice the field has fewer plant species, its soil surface has been disturbed, and it has been managed so that nearly all the plants are at the same growth stage. This lesson addresses some of these differences in more depth.

Study Notes
“Any collection of organisms that interact or have the potential to interact form, along with the physical environment in which they live, an ecological system or ecosystem.” (Cox and Atkins 1979, 37) Let’s focus on the word ‘system’ from the definition just cited. A system is composed of individual components; however, studying a system requires attention to more than just the components. It requires study of a new entity that is something more than the sum of the parts. Suppose a simple system is composed of two species and that one species provides some sort of benefit to the other. The resultant organism may be something much different than would have been predicted based on the two component species. As you begin to think about ecosystems, please remember to take a systems approach — that is focus on the complexity of the whole entity.

Definitions
The term agroecosystem was coined to refer specifically to agricultural ecosystems. Susanna Hecht’s article starts from a limited persepective “…at its most narrow, agroecology refers to the study of purely ecological phenomena within the crop field, such as predator/prey relations, or crop/weed competition.” (Hecht 1995, 4) She broadens the scope of her discussion to include the social perspective which we briefly addressed in lesson 1 (definitions of sustainability). Much of the rest of her article concentrates on the approaches agricultural sciences have taken in agroecological studies. Later in the study notes, I will pick up on one of these approaches.

Agroecosystems have been more broadly defined as ecosystems that have been modified or managed by humans. This is reflected in Conway’s definition: “Agroecosystems are ecological systems modified by human beings to produce food, fiber or other agricultural products.” (Conway 1987, 95). The farmer enters the system with the intent of raising the level of productivity so that his/her economic goals can be met: to provide a livelihood for a family. Society supports agriculture because one of society’s goals are achieved: to provide a reasonably priced food supply for non-agricultural workers.

Economically valuable food compounds such as complex carbohydrates, proteins, fats, and oils are produced via biochemical pathways within plants. Humans exists because crops produce organic compounds in excess of their maintenance needs and as a result humans can harvest feed, fuel, and foodstuffs for his or her own needs. The basic process of agriculture is photosynthesis where plants take simple inorganic elements along with energy obtained from sunlight to produce complex organic compounds. A simple equation for photosynthesis is:

6 CO2 + 12 H2O –> C6H12O6 + 6 O2 + 6 H2O
carbon water carbohydrate oxygen water
dioxide

The figure below (copyrighted; unable to display in blog) shows the plant securing energy from the sun. The sunlight striking the leaf activates the process of photosynthesis in organelles with cells of the leaf. Carbon dioxide coming from the atmosphere provides the source of carbon.
agroecosystems
Figure 2.1. Photosynthesis – inputs and compounds produced. (Stoskopf 1981, 5).

Natural Ecosystems vs. Agroecosystems
Some scientists have suggested the viability of an agroecosystem is related to its degree of similarity to an unmanaged, natural ecosystem in the same environment (Hart 1980, 73-74). If we start from this premise, then we need to compare and contrast these ecosystems. Your assigned reading for this lesson lists Odum’s four characteristics that set an agroecosystem apart from other ecosystems: (1) agroecosystems utilize outside sources of energy (other than sunlight) to enhance the productivity, (2) diversity is greatly reduced compared with a natural ecosystem, (3) the dominant plant and animal species are under artificial selection (developed by plant and animal breeders), and (4) the system controls are external (human management) rather than internal via subsystem feedback. Although Susanna Hecht cautions us that this model is based primarily on modern agriculture in the United States, I feel this is an appropriate place for us to start. Let’s look at these characteristics in more detail.

Inputs and Human Management
Natural ecosystems reach a climax community which is characterized by an equilibrium. The equilibrium is nothing more than a balance of inputs and outputs in a fairly closed system. In agroecosystems, humans have altered the physical environment (e.g., soil is cultivated) and harvested a portion of the plant biomass without returning it to the soil to decompose. We are exporting the products of photosynthesis and as a result it is necessary to import inputs to maintain a level of productivity that is acceptable. The viability of our agricultural systems are maintained with inputs in the form of fossil fuels, fertilizers, and pesticides.

fertilizer-applicWe’re going to look at one example in some detail — nitrogen. We will be comparing natural ecosystems and agroecosystems for how nitrogen is used and recycled. But before we can proceed with the comparison, here’s a quick review of plant nutrient requirements with an emphasis on nitrogen.

Plants require fourteen nutrients for development and growth. Of these fourteen, nitrogen (N), phosphorus (P), and potassium (K) are the three elements most commonly applied in inorganic fertilizers. To simply this discussion, we will use nitrogen as an example. Some terms related to nutrient cycling are defined below for those students who have not taken an introductory soils course.

  • Denitrification-Denitrification is the biochemical process that results in the reduction of nitrate (NO3) to gaseous nitrogen (N2). Nitrogen undergoing this process is lost to the air; it is no longer available for plant growth. This most commonly occurs under water logged soil conditions.
  • Immobilization-Immobilization involves the conversion of inorganic forms of nitrogen to organic forms that reside in soil microorganisms. These sources of nitrogen are unavailable for plant growth until the microorganisms die.
  • Leaching-The movement of nitrogen in solution through the soil. For example, rain or irrigation will move nitrate-nitrogen out of the root zone as water infiltrates deeper into the soil.
  • Mineralization-The conversion of nitrogen from an organic form to an inorganic state, thus becoming available for plant growth.
  • N-fixation-The conversion of gaseous nitrogen (N2) to forms that are available for plant growth. The process involves a symbiotic relationship between legumes and soil microorganisms that are capable of fixing nitrogen.
  • Nitrogen fertilizers -Nitrogen applied in fertilizers will be present in one of three forms: 1) ammonia, 2) nitrate, and 3) urea. Most nitrogen used by plants is in the ammonium and nitrate forms.
    • ammonium (NH4+) – Ammonia is readily converted to the ammonium ion (NH4+) and is held in the root zone by the clay-humus fraction which has a negative charge (see organic matter). The ammonium ion can be converted to nitrate through biochemical processes involving soil bacteria.

      nitrate (NO3-) – Some fertilizers contain nitrate sources of nitrogen and the nitrogen in other fertilizers may be converted to the nitrate ion. Notice that nitrate (NO3-) has a negative charge and as a result is not adsorbed by the clay-humus fraction. It is subject to leaching.

      urea CO(NH2)2 – Urea is readily hydrolyzed in the soil and forms ammonium ions.

  • Organic matter (o.m.)-Organic matter that is partially decomposed forms part of the clay-humus fraction of a soil. Clay-humus particles are very small (smaller than sand and silt particles) and have an inherent negative charge associated with them. The clay-humus fraction increases the nutritive holding properties of a soil because cations (ions having a positive charge) such as NH4+ are held within the root zone and therefore are available for crop growth and development. Organic matter also improves the physical structure of the soil allowing greater water retention.
  • Volatilization-The loss of gaseous forms of nitrogen. Denitrification is the most common type of volatilization.

We can assume there is a balance between nutrients taken up by plants and nutrients returned to the soil in an unmanaged, natural ecosystem. Losses of nitrogen due to soil erosion, leaching and volatilization still occur, but at a low rate, and are matched by gains due to precipitation, N-fixation and imports from other ecosystems (see figure 2.2; copyrighted; unable to display in blog)). Nitrogen moves through the ecosystem; this cycling of nitrogen is referred to as the nitrogen cycle.

Figure 2-2. Nitrogen cycling for a natural ecosystem (a), and an agroecosystem (b). O.M. = organic matter. (Tivy 1990, 66).

In an agroecosystem, cultivation disturbs the soil. The organic matter content of soils decreases with time and as a result the nutritive holding capacity (or the affinity for cations such as NH4+) is lowered also. Cultivation not only removes the original plant biomass, but it also causes the remaining organic matter to decompose at a more rapid pace due to increased aeration and higher soil temperatures. The processes of leaching, denitrification, and volatilization may be accelerated. In addition, nitrogen is removed from a field in grain that is harvested, in straw or other crop residue that is removed, and in livestock that feed on forage and fodder crops.

grainThe following table (table 2.1) gives some idea of the relative amounts of nitrogen removed by various commodities. Some of the nitrogen that is removed will be replaced through natural processes such as mineralization. However, over time the fertility of the soil will decline without the input of inorganic nitrogen fertilizers. The timing and quantity of fertilizer applied is only one of many of the management decisions required of farmers of modern agroecosystems. The agricultural producer both determines what inputs are appropriate and produces outputs that have economic and social value to society. The intensity of management varies greatly for different agricultural enterprises.

[The second number in each row is the amount of nitrogen removed per acre.]
Commodity Acre yield Nitrogen (lbs./acre)
Grain bu/acre
corn (includes cob) 150 135
soybean 40 150
wheat 40 50
oats 80 50

Stover/straw tons/acre
corn 4.5 100
soybean 2 90
wheat 1.5 20
oats 2 25

Hay tons/acre
alfalfa 4 180
red clover 2.5 100

Animal manure
(annual production/1000 lbs. of animal weight) Manure yield
(tons) Nitrogen
(lbs.)
cattle 12.6 126
hogs 12 120
Table 2.1. Nitrogen removed from the ecosystem by various crop and livestock components. (adapted from Metcalfe and Elkins 1980, 158-159)

Biodiversity and Natural Selection
Agroecosystems are composed largely of domesticated plants and animals. They can be as simple as a field planted to a single crop (monoculture) or more diverse with several species (polyculture) of plants and animals interacting over time. Natural ecosystems tend to be more complex. A natural ecosystem undergoes many structural and functional changes over time as it progresses from an immature ecosystem to a climax community.

Imagine you are traveling to southwestern Minnesota. What type of ecosystems would you find when you arrive? A farmer planting corn and soybeans would be fairly representative of the agricultural enterprises that characterize this region. The agroecosystem could be described by the production practices and development of the crop during a calendar year. For example, tillage of the soil in the spring is followed by planting corn at the rate of 24,000 plants/acre. Prior to planting, a soil test is performed to determine the need for fertilizer to achieve a specific yield goal. Inorganic fertilizer is applied several times during the growing season. Competition from weeds is reduced through pesticide application and/or cultivation of the inter-row zone. Corn borers find the 100-acre field a good habitat and losses result in a 10 percent reduction in yield. The corn is harvested (120 bu/acre) in the fall after moisture in the grain has dropped sufficiently (low moisture is desired so that grain drying costs are minimized). The soil is tilled yet in the fall leaving limited residue on the soil surface.

Before this land was first tilled (in the mid to late 1800s), wide expanses of native prairie existed. The natural ecosystem of this area consisted of a mix of grass species. Species number was high and diversity within a species existed. The soil was covered year round. Plants matured at different times during the growing season. As plants died, they remained in place and were subject to decomposition. This describes a relatively closed system in which sunlight is the energy source and there are few other inputs other than those resulting from naturally occurring processes such as precipitation. Very little native prairie still exists in the Great Plains of the United States.

I’m sure you’ll agree there are visible differences between native ecosystems and agroecosystems. Hopefully you are also wondering whether or not the greater diversity of the native ecosystem conveys some advantage? The answer to that question is yes. The greater diversity of natural ecosystems provides them with greater stability. A field planted with a single variety of soybeans may be more sensitive to environmental fluctuations than a mature, natural ecosystem. Conversely a natural ecosystem composed of a number of species will display variable responses to environmental stress. For example, plants may differ in their ability to tolerate drought stress. Some species will possess roots that draw water from greater depths than other species, while other species possess physiological processes that reduce cellular damage during periods of water stress. Another mechanism of defense may be simple avoidance; a plant already has produced seed and therefore the impact of the drought is insignificant.

stem-rustPests also are stressors of plants, just as an illness stresses your own body. In the following paragraphs, I use pests (both insects and plant pathogens) as an example from which we can draw further insight into the benefits derived from high levels of diversity. Before we proceed, I again want to define some terms (related terms are boxed together):

  • herbivores-Herbivores are organisms that feed on plant tissue for nourishment. In this lesson, I refer specifically to herbivorous insects.
  • pathogen-A disease-causing organism. Some bacteria, fungi, and viruses are plant pathogens.
  • host-A plant species that provides nourishment for the growth and development of another organism.
  • non-host-A plant species that does not provide nourishment for a specific organism. The non-host is not detrimentally affected by the presence of this specific organism.
  • heterogeneous-A group of individuals that are genetically different. In this lesson, heterogeneity refers to plants of a single species that are genetically different.
  • homogeneous-A group of individuals that are genetically similar. A good example of this is a crop variety — all plants in a variety theoretically are identical.
  • immunity-A non-host displays immunity. The pathogen of concern is unable to produce disease symptoms in a non-host species. (e.g., the organism that causes Dutch elm disease is unable to produce this disease in corn.)
  • resistance-Resistance is displayed by a host plant when it carries genes for resistance and the pathogen has corresponding genes for avirulence. Often no disease symptoms are visible. (e.g., a wheat variety carries the gene Sr6 which conveys resistance to race 15B of the stem rust pathogen.)
  • susceptible-A susceptible host plant will display disease symptoms when infection occurs by a pathogen capable of causing disease. (e.g., cassava plants carrying no genes for resistance to cassava mosaic are infected with the virus causing this disease.)
  • blend-A seed mixture of several varieties of a single crop species.

The presence of non-host plants may act to block dispersal to a host crop for those insect species that are passively dispersed (e.g., wind carried). However, these insects are in the minority. It is more typical for herbivorous insects to be able to move from one field to another and to discriminate between host and non-host plants in a field. Yet, the genetic heterogeneity of a natural ecosystem still conveys an advantage. Simple crop communities (agroecosystems) characteristically have: (1) greater population densities of specialist herbivores, (2) greater colonization rates, (3) greater herbivore reproduction, (4) less disruption of the pest finding the host, and (5) lower mortality rates due to natural enemies than reported for natural ecosystems (Altieri 1993, 258).

Why is an agroecosystem at a disadvantage? There have been several plausible explanations for this including higher incidence of natural predators, reduced concentration of the host, and higher emigration rates by the herbivore. Some studies indeed have shown the resident time, the time the herbivorous insect remains on a host plant, to be shorter when the host exists in a mixture with other plant species. In addition, the distance of emigration of herbivorous insects may be greater after meeting a non-host plant.

Plant disease results from the interaction of a pathogen, host, and suitable environmental conditions. Incidence can range from a single plant to nearly every plant in a field displaying disease symptoms. Some diseases are polycyclic in nature which means that the pathogen produces several generations in a single growing season. The result can be an epidemic.

In a natural ecosystem, responses to a disease-causing organism will include immunity, resistance, and susceptibility because the plant community includes both host and non-host species. Even the reaction of host plants can vary because the host typically is genetically heterogeneous — some host plants will posses genes for resistance while others won’t. Host and pathogen have interacted over long periods of time in natural ecosystems with coevolution occurring in both species. Mutations occur at a low, but steady rates in both the host and pathogen. Occasionally a mutation will provide the host or pathogen with a temporary advantage. Sooner or later a corresponding mutation will occur in the other organism bringing them back into balance. Equilibrium and genetic diversity are the result. Artificial selection of desirable varieties by plant breeders has resulted in high yielding lines, but diversity has been sacrificed for uniformity.

In a farmer’s field, the host species often is a crop variety which is genetically homogeneous. If this variety is popular, this variety can be planted over large acreages in a region. An epidemic of southern corn leaf blight occurred in the United States in 1970. In some areas, corn production was reduced as much as 70 percent. This resulted from the genetic uniformity of the corn crop. Although different hybrid varieties were planted that year, nearly all the corn hybrids contained the same cytoplasm (the cytoplasm carries genetic material in addition to that carried in the nucleus of a plant cell). The cytoplasm conveyed susceptibility to the disease. This incident drew a lot of public attention to the consequences of the lack diversity in American crops.

Conclusion
This package of inputs — improved varieties, fertilizers, pesticides and mechanization — and intensive management has made it possible to farm large acreages of highly productive crops. This has been a success story in terms of producing reasonably priced food for the American public. Modern agriculture also has been characterized by its need for high levels of inputs and the reduced diversity of its agroecosystems. If viability of agriculture depends on emulating natural ecosystems, then significant adjustments will be required of present cropping / animal production systems. People who take this argument believe there are ecological and social costs associated with modern production systems that are not reflected in the price we pay for food.

Required Reading
Susanna B. Hecht, “The Evolution of Agroecological Thought” (in the supplement).

References
Alteri, M.A. 1993. Ethnoscience and biodiversity: key elements in the design of sustainable pest management systems for small farmers in developing countries. Agric. Ecosyts. and Environment 46:257-272.
Conway, G.R. 1987. The properties of agroecosystems. Agric. Syst. 24:95-117.
Cox, G.W., and M.D. Atkins. 1979. Agricultural ecology. W.H. Freeman & Co., San Francisco.
Hart, R.D. 1980. A natural ecosystem analog approach to the design of a successional crop system for tropical forest environments. Biotropica 12(suppl.): 73-82.
Metcalfe, D.S., and D.M. Elkins. 1980. Crop production: Principles and practices. MacMillan Publ. Co., New York.

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