Brent Cornell

    
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

    
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

    
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