Basic ideas - Succession, agriculture and biodiversity

Key Words:
Agriculture - food crops - new habitats - heathland - grassland - coppicing - biodiversity - monoculture - stability - regeneration - pollution

 

Main points

  • Agriculture has major impacts on plant succession because the whole purpose of agriculture is to deflect plant productivity away from non-food crops to food crops - either human food or animal food.
  • Humans have deliberately managed ecosystems and in the process created entirely new habitats such as heathland, downland, fens, meadows etc. Several examples are shown below:
  • Heathland - acid sandy soils often have heather as a seral stage, replaced by gorse, then birch and pine.
    • Common management is to burn the heather as soon as it gets too mature and woody. This prevents succession.
    • The heather has underground roots which, provided the fire is not too hot, will regenerate to give fresh young shoots for grouse and permit grass to seed in to provide grazing for sheep.
  • Grassland - a lot of agricultural land is grassland. Permanent grazing prevents woody plants from regenerating.
    • Some plants are not eaten, such as thistles or ragwort. These have to be removed by hand.
    • Traditional grazing involves uneven distribution of animal urine and faeces. This provides a variety of different soil environments and gives rise to a diversity of plant types.
    • More modern grassland may be sprayed with fertiliser or seeded with just one grass type.
  • Coppicing - sustainable temperate woodland management involves coppicing of chestnut and hazel, ie chopping down young growth over a regular cycle.
    • The trees are cut down in rotation of about 7 years. This provides timber in thicknesses useful for furniture making, fencing, etc.
    • Mature oak standards are left to shelter the coppice and provide larger timber, originally for house and shipbuilding.
    • Areas of cleared and regenerating woodland combine with mature coppice to give considerable niche diversity.
  • Agriculture reduces biodiversity. Agricultural communities (eg potato fields) are simple. This means a pest species has a large amount of food available with few predators. If it is an introduced pest, it may have no predators. This makes a farmers field a very unstable ecological community.
  • In addition, monocultures (areas of only one species) are very prone to disease spread, due to the high density of the plants.
  • The natural biodiversity which has built up over millenia helps to ensure stability. Humans upset this balance through
    • clearance of diverse communities and
    • the introduction of exotic species.
  • The ability of a community to recover from total destruction is linked to the diversity of the community, the stability of the soil and the alteration to the environment that occurs during clearance.
  • Rainforest cleared in small areas and then left can regenerate. Upland tropical areas cleared, farmed and then abandoned cannot.
  • In any well adapted climax community, pollution tends to reduce biodiversity eg freshwater streams.

 

Imagery


New Forest heath - a deflected climax
Even "natural" areas like the New Forest in Hampshire (above) have a strong human impact. Grazing and regular heather burning have created a subclimax of heath and common with a scattering of trees. If this area was abandoned, the forest climax vegetation would rapidly re-establish.
Grazing land - a subclimax

Agricultural landscapes are characterised by their ecological simplicity. The many layered complex ecosystems of the climax forest have been replaced - usually by a single layer of monoculture crop.

Traditional farming methods relying on mixed farming, regular crop rotation and low intensity production encourage a greater biodiversity than modern intensive methods.

 

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