Ecological Disturbance


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•  Disturbance influxes the structure and rate of change within ecosystems

Environmental disturbances may cause fluctuations to the structure and rate of change within ecosystems

  • Environmental disturbances may be natural or artificial (human induced) in origin

Natural Disturbance

A natural environmental disturbance may give rise to secondary succession – where one ecosystem is replaced by another

  • Secondary succession occurs when succession starts on existing soil following the upheaval of a pre-existing ecosystem
  • This upheaval results in the removal of existing biota and allows a new ecosystem to develop on the site of the old
  • Because the soil is already developed, dominance is usually achieved by the fastest growing plants

The progression of secondary succession can be summarised as follows:

  • An environmental disturbance, such as a bushfire or earthquake, destroys the pre-existing climax community
  • Grasses and herbaceous plants are the first to grow back as the soil is already present (no pioneer species required)
  • Fast growing trees will develop to their fullest, while shade tolerant trees will develop in the understory
  • Eventually the fast-growing trees may be overtaken by larger, slower-growing trees as the ecosystem reverts to its prior state

Secondary Succession

secondary succession

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•  Consideration of one example how humans interfere with nutrient cycling

Deforestation is the permanent destruction of a forest via the removal or clearance of trees

  • This human-induced degradation of forests is driven primarily by a need for timber and cleared land for agricultural purposes

Deforestation disturbs the normal nutrient cycling within the region in a number of ways:

  • Less trees means there is less evapotranspiration, meaning there is less moisture in the air (less precipitation)
  • Fewer trees means less litter (due to less defoliation), which reduces the production of humus (so less nutrients in soil)
  • There will be a rapid loss of nutrients from leaching, but less chemical weathering of rock (due to less water in soil)
  • The soil will become acidic and release iron and aluminium to form an infertile ferrilitic soil (nutrient poor)
  • The soil layer will become increasingly thin and easily eroded once the trees have been cleared
  • The infertile soil will prevent vegetative growth, reducing biodiversity and further nutrient cycling
  • Logging operations may also alter the distribution of plant species by removing the canopy and increasing light exposure
  • Removal of the canopy also results in an increased loss of nutrients from the soil via runoff

The Effect of Deforestation on Nutrient Cycling


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•  Investigation into the effect of an environmental disturbance on an ecosystem

Environmental disturbances are caused by natural or artificial disruptions to a normal ecosystem, including:

  • Fire breaks in bush lands or regions damaged by bushfires
  • Outer boundaries of population settlements or regions bordering roads
  • Dams and artificial rivers and creeks (e.g. irrigation sites)

The effect of an environmental disturbance on an ecosystem can be measured in a number of ways:

  • Population density (using the Lincoln index via the capture-mark-recapture technique)
  • Species diversity and richness (using the Simpson’s reciprocal index)
  • The presence and distribution of indicator species (to measure levels of pollution)
  • Canopy coverage and relative light intensity (could measure with a lux meter)
  • Biomass (via the average width of tree stems at a specified height)
  • Edaphic factors such as soil erosion (via depth), water retention (via drainage), pH and nutrient content

Measurements taken from a disturbed area need to be compared against measurements taken from an undisturbed control

  • This enables the investigator to statistically calculate both the effect and magnitude of the environmental disturbance