Ecology Workshop

HUMAN IMPACTS

The human population seems to resemble the pattern of logistic growth. Humans are associated with the following effects (among others) on the biosphere;

  • loss of rainforests
  • loss of biodiversity
  • insecticides appearing at the Poles
  • global warming
  • ozone hole
  • overfishing
  • extinctions
  • land degradation

Since Europeans arrived, Australia has lost roughly 10 % of native mammals, 75 species of feral vertebrates have been introduced, and half the forest have been cleared. Some of these changes have been intentional and others not.

An example of an unintended change to the Australian is salination. Salination is a process of soil degradation in which the dissolved salt content near the soil surface increases to concentrations which cannot be tolerated by plants, causing them to die.

Salination can be a natural process but it can also be accelerated or caused by human activity. Human-mediated salination affects 20,000 km2 of land - and this figure in on the rise. Many of the affected regions are areas in which tree cover has been removed to facilitate extensive agricultural use.

Salts are a natural component of soils - they come from the sea, they are leached from rocks, but they are usually adsorbed onto soil particles and do not move around.

Within most soils there is a water table - a layer of water overlaying impervious bed rock. The level of this varies from soil to soil.

The level of this water table is determined by the combined influence of rain, the uptake of moisture by plants and the evaporative loss of water from the soils. The water table may rise when there is an increase in rain.

Salts dissolve in the water. As the table rises it becomes more concentrated as salts adsorbed onto soil particles are dissolved. When the water level gets to within a couple of metres of the soil surface, capillary action draws the water up to the surface. In low parts of the land, the water poisons the plant-life. When nothing can grow, the soil becomes bare - this is called a salt scald.

Natural salt discharge can create zones of high salinity - indeed of saturated salt solutions. Areas, for example in and around Lake Eyre, are natural, salt-rich inland seas.

In affected farmlands, the farm dams can become salinated.

This process has been exacerbated because when vegetation is cleared to allow the land to be used for agricultural purposes, there is a reduction in the uptake of precipitating rain by plants. The roots of crop plants typically penetrate less deeply into the soil and so are less likely to prevent the rising water table from getting into the top couple of metres.

The result is that more water accumulates within the soil and the water table rises killing plants.

(Refer Fig 45.13 pg 1011 Knox et al)