Agro-ecosystem: characteristics and models-abiotic and abiotic factors in rainfed and irrigated  ecosystem

Characteristics of agro ecosystem are given below.

·            The energy input to agroecosystems includes not only natural energy (sunlight) but also processed energy (fossil fuels) as well as human and animal labor

·            Biodiversity in agroecosystems is generally reduced by human management in order to channel as much energy and nutrient flow as possible into a few domesticated species.

·            Evolution is largely, but not entirely, through artificial selection where commercially desirable phenotypic traits are increased through breeding programs and genetic engineering.

·            Widespread use of synthetic chemical pesticides has boosted farm production worldwide, primarily by reducing or eliminating herbivorous insect pests. Use of organochlorine broad-spectrum pesticides such as DDT, BHC have made biomagnifications in successive level of trophic level in food chain. Secondary pest outbreaks associated with the use of many traditional pesticides are common due to the elimination of natural enemies or resistance of pests to chemical control.

Figure 1. Functional component of an agroecosystem

Abiotic factors in rainfed and irrigated  ecosystem

Abiotic components of agroecosystems include

1. Temperature

2. Soil

3. Water

4. Relative humidity

5. Light

6. Wind

7. Composition of the atmosphere

The variations in the abiotic components of an environment can act as stress factors on plants. In a genetically diverse population, some individuals will be better adapted to these stress factors and thrive, while others may not survive. In this way, environmental influences exert selective pressures on crop populations.

Habitats located at higher altitudes are commonly associated with particular abiotic factors, including low carbon dioxide availability and high variation in precipitation, light, soils and temperature. Likewise, other ecogeographic niches are likely to contain ‘portfolios’ of abiotic factors. For instance, semi-desert regions are associated with shallow sandy soils, low rainfall and temperature extremes. Just as these abiotic factors can be clustered in various ecogeographic regions, so corresponding adaptations may appear in portfolios of genetic diversity

Nutrient deficiencies or toxicity may be particularly important in determining the survival and productivity of crop varieties in the agroecosystem.  Soils may be deficient in N, P or K, as well as secondary and micronutrients such as magnesium, sulphur, zinc and boron.  In contrast, iron, manganese and aluminium may occur in such high quantities as to cause toxicity. Nutrient availability may be related to soil pH and precipitation regimes.  Information on soils can help identify soil-related constraints and explain current management practices

Biotic factors in rainfed and irrigated  ecosystem

Biotic factors include

1.   Parasitic and herbivorous pests

2.   Competition from other plants

3.   Favourable (symbiotic) relationships with other organisms

4.   The farmers who manage these factors

1.       Parasitic and herbivorous pests

Herbivorous animals, including mammals, birds and arthropods, may act as predators on crop plants, while viral, bacterial and fungal diseases harm crops through parasitic relationships.  Crop genetic diversity is an important means of minimizing the threat of these pests in an agroecosystem.  Crop plants’ vulnerability to particular pests may vary with agromorphological characteristics like plant height, pubescence or time to maturity, in addition to the variability in specific genetic traits for pest resistance.  Crop genetic variation, and hence phenotypic variation, may also attract a diversity of other organisms into the agroecosystem, including the natural enemies (predators or parasites) of pests.

Crop plants and their pests have adapted to each other over time in a process called co-evolution. One of the most important aspects of co-evolution for on-farm conservation is crop plants’ resistance to pests (conversely, the ability of pests to overcome host resistance), which depends upon the development of new genetic diversity.  The genetic diversity evolved by crops and pests through co-evolution is particularly complex because both are genetically variable over time and space. Indeed, the diversity of pest-induced stresses on a particular crop is often closely correlated with diversity in the crop’s resistance.

The complexity of crop-pest interactions in agroecosystems is increased by their seasonal or annual variability.  Pest populations fluctuate with changing climatic conditions, farmer inputs and host resistance.  In addition, pests can be highly mobile, especially with assistance from humans. This ease of mobility, coupled with favorable conditions, may engender widespread epidemics, with severe effects on host populations.

2.     Competition from other plants

Competition with other organisms may also foster crop genetic diversity.  Weeds are the primary competitors of crop plants of concern to farmers.  Weeds can reduce or inhibit growth.  Crops and weeds within the same agroecosystem can have similar requirements in terms of water, light and nutrients – the essential resources plants need to survive.

3.       Favourable (symbiotic) relationships with other organisms

Organism interactions within an agroecosystem are not always competitive and may be neutral, commensal or mutualistic.  Crops cultivated together in an intercropping system may have faced selection pressures to develop complementary needs, using different resources or using them at different times.  Crops have also adapted to take advantage of symbiotic relationships with non-plant organisms, such as insect pollinators and, in the case of leguminous plants, nitrogen-fixing Rhizobium bacteria.

4.       The farmers

The farmers who manage these factors in terms of irrigation, nutrient input, pest control, land preparation, mixed/relay cropping and other practices are also a biotic component of agroecosystems.  These factors vary over time, with seasonal, annual and stochastic changes, and in space, from the micro-environmental to the ecoregional scale. As a result, local landraces adapt to the particular conditions of their immediate ecogeographic setting.  These adaptations to local environmental stresses are likely to be reflected in the genetic composition of landraces over time. For instance, in dry areas, irrigated crops face far less natural selection for drought tolerance than those relying solely on rainfall. 

          Farmers have developed ways of manipulating the environment to respond to the abiotic and biotic stresses their crops face. The threats can be associated with local climates, seasonal changes, or the effects of pathogens; the responses may be simple or complex, temporary or permanent, traditional or modern.

Environmental stresses and possible responses by farmer

Environmental factor	Possible farmer response to alter environment
Extreme cold	Crop sheltering, frost coverage
Extreme heat	Crop shading
High clay content/poor drainage	Removal of hardpans, addition of drainage lines
High sand content/rapid drainage	Addition of water retention lines
High gravel/rock content	Removal or rock material
High or low pH	Fertilizers, soil additives
Low nutrient content	Fertilizers, soil additives, intercropping, crop rotation with legumes
High aluminium or salt content	Fertilizers, soil additives
High precipitation/     Waterlogged soils	Addition of drainage lines
Low annual precipitation	Irrigation systems/ water harvesting
Low seasonal precipitation	Temporary/seasonal irrigation systems
Desertification	Sand barriers
High erosion potential	Flattening field slopes, developing terraces
Low light intensity	Thinning possible shade
Long/short photoperiod	Agroforestry, crop rotation
Strong local winds	Plant/build windbreaks, agroforestry
Pests	Pesticides, physical barriers, intercropping, crop rotation
Diseases	Avoidance of conditions favourable to disease, fungicides, crop rotation
Plant competition	Weeding, reduced plant spacing, herbicides

One important agroecosystem management strategy is the use of inter- and intraspecific crop diversity to mediate potential environmental stresses.  If a crop population has a diverse genetic make-up, the risk of its being entirely lost to any particular stress, such as temperature extremes, droughts, floods, pests and other environmental variables, is reduced.  Different crops and varieties may differ in their vulnerability to specific threats (e.g. traits for resistance to a specific disease).  In addition, vulnerability to stresses may vary with the crop’s level of maturity, from the planting to post-harvest stages, particularly in the case of pests, to which even post-harvest yields may still be at risk.  Crops with different planting times and times to maturity give the farmer the option to plant and harvest crops at multiple points in the season to guard against total crop loss to environmental threats.