Describe the differences between natural ecosystems and agro-ecosystems

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14/10/05                                                                                                                                 Henry Munby

Describe the differences between natural ecosystems and                  agro-ecosystems

An ecosystem is by definition the processes and interactions of the biotic community (living organisms) and inorganic component (physical and chemical features) of a particular environment. In a natural setting a stable, or climax ecosystem represents a state of natural equilibrium, whereby all occupant species compete for resources, and energy and nutrient cycles are balanced. Human farmers effectively out-compete most natural species for resources, and through select harvested species export energy and nutrients for consumption elsewhere; this is an agro-ecosystem or plagioclimax.

        An ecosystem unaffected by man has a structure characteristic of its climatic region, for example deciduous oak woodland in the U.K has a typical biomass of 30Kgm-2, and an average productivity of 1.2Kgm-2 per year. This reflects the maximum mass of flora and fauna that can be supported by the climate and soil. However, when such woodland is cleared and tilled by man new characteristics appear. Arable crops have a far lower average biomass, wheat is typically 1.6Kgm-2, with productivity falling to 0.6Kgm-2 per year. This is caused by the loss of the multi-storey vegetation that photosynthesises at maximum scale and optimum rate, and in arable fields there is not the habitat to support faunal species such as woodpeckers and other woodland birds, resulting in lost animal mass. Livestock farmland is characterised by similar primary productivity as grassland, but again this is far below the potential for the temperate climate and fertile soils. In addition, livestock farming results in a far greater proportion of the biomass being animal matter (large ruminants especially), unlike natural ecosystems where plant species can grow to a higher density and to greater mass when there is no human intervention to grow only grass as feedstock. The latter characteristic is less noticeable in countries with high population densities where a higher energy yield from arable crops is required, for example the necessity for widespread rice growing in Bangladesh.  

        Diversity and richness of plant species is with few exceptions lower in agro-ecosystems than in natural ecosystems. By the nature of farming it is far more efficient in terms of harvesting time and bulk sale to grow monocultures of a single plant species and to remove the diversity of weeds and pests that threaten crop yield. Furthermore, the initial clearing of land for farms removes a large majority of varied native species. Hence, in agro-ecosystems the remaining soil fauna, microorganisms, and any surviving weeds provide the only species diversity. For example in the U.K a wheat crop will only support the insects or animals (i.e. field mouse) that feed on wheat, and any species that can then feed on these primary consumers. Some non-native crops such as rape have very few consumers, and so support an even lower diversity of fauna. Also, the initial loss of the oak tree canopy and rich vegetation has removed the habitat for most bird species, like the jackdaw and tawny owl, and for small mammals such as the dormouse and hedgehog. The result is an agro-ecosystem far more sterile than natural ecosystems, lacking in plant and animal communities and crude numbers of organism species.

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        All ecosystems, natural and agricultural, have two major processes: a flow (of energy) and a cycle (of nutrients). Both processes allow the linking of ecosystem components and are driven by the Sun’s energy. Ecosystems in natural equilibrium have open energy flow, with energy being exchanged across the boundaries of most systems in the form of solar energy and heat. In natural ecosystems the only source of energy is the direct insolation, and the energy is fixed by photosynthesising producers in the form of plant glucose. Conversely, agro-ecosystems have numerous indirect inputs of energy, and farmers import energy in order to ...

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