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Concept of agroecosystem- based on ecological principles- based on our understanding of natural ecosystems

Structure of natural ecosystems- Ecosystem defined as a functional system of complementary

relations between living organisms and their environment,delimited by arbitrarily chosen boundaries, which in spaceand time appear to maintain a steady yet dynamicequilibrium

- A structural components of ecosystems are biotic factors,living organisms that interact in the environment, andabiotic factors, non-living physical and chemicalcomponents such as soil, light, moisture and temperature

Structure of natural ecosystems:

Levels of organization� - Ecosystems can be examined in terms of a hierarchy of

organization of their component part (e.g. human body)

� 1. Individual organism� also known as autecology or physiological ecology

� it is concerned with how single individual of a speciesresponse to the factors of the environment and how theorganism’ particular degree of tolerance to stresses in theenvironment determine where it will live (e.g. banana -restrict to tropical humid environments with a particularset of conditions, whereas strawbery plant is adapted to amuch more temperate environment)

� 2. Population

� groups of individuals of the same species

� understanding of population ecology is important indetermining the factors that control population sizeand growth, which is related to the capacity of theenvironment to support a particular population overtime

� 3. Community

� - populations of different species always occur together in mixtures

� - an assemblage of various species living together in a particular place and interacting with each other

� - an important aspect of this level is how the interactions of organisms affect distribution and abundance of the different species that make up a particular community (e.g. competition between plants in a cropping system or the predation of aphids by lady beetles are examples of interaction at this level in an agroecosystem)

� 4. Ecosystem

� - include abiotic factors of the environment inaddition to the communities of organisms in a specificarea

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Levels of ecosystem organization applied to an agroecosystem

The farm in the context of itswater shed

Ecosystem

Community

Population

Organism

Individual crop plant

Monoculture of the cropplant

Polyculture of intercroppedplants, along with otherorganisms

Structural properties of

communities� 1. Species diversity� - Number of species that occur in a community

� 2. Dominance and relative abundance� - In any community, some species may be relatively abundance

and others less abundant� - The species with greatest impact on both biotic and the biotic

components of the community is referred to as the dominant species

� 3. Vegetative structure� - Terrestrial communities are characterized by the structure of

their vegetation, which determined mostly by the form of the dominant plant species and

� 4. Trophic structure

� - Every species in a community has nutritive needs

� - How these needs are met in relation to other species determines a structure of feeding relationships or ‘trophic structure’

5. Stability� - Over time, the species diversity, dominance structure,

vegetative structure, and trophic structure of a communityusually remain fairly stable, eventhough individual organismsdie and leave the area, and the relative sizes of populations shift

� - Unchanged even if some kind of ‘disturbance’ such as fire,flooding killed many members of a species in the community,the community will recover and return to something close to theoriginal condition and species composition

� - Because of this ability of communities to resist change and tobe resilient in response to disturbance, communities (and theecosystems of which they are part) -are said to possess theproperty of stability

FUNCTIONING OF NATURAL

ECOSYSTEMS� It is important to understand these processes in order to address

the concepts ecosystem dynamics, efficiency, productivity anddevelopment, especially in agroecosystems where function candetermine the difference between the success and failure of aparticular crop or management practice

� Two most fundamental processes in any ecosystem are the flowof energy among its parts and the cycling of nutrients

� 1. Energy flow

� Each ind organism in an ecosystem is constantly using energy tocarry out its physiological processes, and its sources of energymust be regularly replenished

� Ecosystem energy flow. The size of each box represents the relativeamount of energy flowing through that trophic level. In theaverage, only about 10% of the energy in a trophic level istransferred to the next trophic level. Nearly all of the energy thatenters an ecosystem is eventually dissipated as heat.

Producers‘Plant’-energy isstored intochem ica l

bonds of thebiomass

Net primaryproductivity-energy plant use to main tainthem selves-expressed inte rm ofkilocalories persquare meter peryear

Herbivores-primary consumer

Carnivores Top carnivores

HeatHeat

Heat

Decomposerbiomass andheat

Decomposerbiomass andheat

Decomposerbiomass andheat

SUN

RESPIRATION

DECOMPOSITIONAND WASTEDFOOD

Return toso il

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� 2. Nutrient cycling� - In addition to energy, organisms require inputs of matter to maintain

their life functions

� - In a form of nutrients containing a variety of crucial elements and compounds (is used to build cells and tissues and the complex organic molecules required for cell and body functioning)

� - Linked to the flow of energy

� - The biomass transferred between trophic levels contains both energy in chemical bonds and matter serving as nutrients

� - Many nutrients are cycled through ecosystems

� - The most important are water and macronutrients: Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P), Sulfur (S)

� - Some nutrients exist in a form of readily available

Green plants

Atmospheric carbon (CO2)

Herbivores& predators

Soil carbon(organic matter)

Deadandnonlivingorganicmatter

Fossilfuels

Combustion

Cabonification

Respiration RespirationPhotosynthesis

Fire

Decay

DeathDeathandwaster

The carbon cycle

Ecosystem change

� - constant state of dynamic change� - if the disturbance is so intense and the ecosystem are not able to

resist change the ecosystem may lose their stability� - Eg. Clear cut of forest into a monoculture crop

AGROECOSYSTEM� - human manipulation and alteration of ecosystems for the

purpose of establishing agricultural production makesagroecosystem very different from natural ecosystems

� - however, the processes, structures and characteristics ofnatural ecosystems can be observed in agroecosystems

� - agroecosystems differ from natural ecosystems in severalaspects:

� 1. Energy flow� - altered greatly by human interference� - inputs are derived from human source and often not

sustaining

� 2. Nutrient cycling

� - minimal in agroecosystem

� - considerable quantities are lost from the system with the harvest or as result of leaching or erosion due to great reduction in permanent biomass levels held within the system

� - farmers rely much on petroleum based nutrient input to replace these losses

� 3. Population regulating mechanisms

� - due to the simplification of the environment and areduction in trophic interactions, populations of cropplants or animals in agroecosystems are rarely self-reproducing

� - human inputs (e.g. seed or control agents), oftendependent on large energy subsidies, determine populationsizes

� - biological diversity is reduced, trophic structures tend tobecome simplified and many niches are left unoccupied

� - the danger of catastrophic pest or disease outbreak is highdespite the intensive human interference

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� Outbreak of leaf eating caterpillar - oil palmplantation

� 4. Stability

� - due to their reduced structural and functionaldiversity in relation to natural ecosystems,agroecosystem have much less resilience than naturalecosystems

� - a focus on harvest outputs upsets any equilibriumthat is established and the system can only besustained if outside interference (in a form of labourand external human inputs is maintained)

Important structural and functional differences between natural ecosystems and agroecosystems

Natural ecosystem Agroecosystems

Net Productivity Medium HighTrophic interactions Complex Simple, LinearSpecies diversity High LowGenetic diversity High LowNutrient cycles Closed OpenStability (resilience) High LowHuman Control Independent DependentTemporal permanence Long ShortHabitat heterogeneity Complex Simple