Every clearing operation, dam, pipeline, irrigation project or use of pesticides and fertiliser causes changes to the the "balance of nature".  Ecosystems are complex but this complexity is what keeps them stable.  Through their actions humans frequently simplify the environment, with the numbers of plants and animals no longer kept in check through natural interactions.  The question of how much simplification is possible before land degradation results is currently under investigation. 

All these simplifications are cumulative and eventually threaten the viability of the ecosystem.  The problem for managers is that the cumulative degradation which occurs can't be measured, and therefore the impact can't be managed.

Sooner or later the ecosystem can no longer cope, and it declines rapidly.  This threshold is the point where cumulative impacts cannot be assimilated without the ecosystem showing symptoms of degradation.  

Desertification

One type of degradation is desertification.  This is defined as a drop in land productivity of 10% or more.  The conversion of rangelands (uncultivated land used for animal grazing), cropland, or irrigated cropland to desert-like land can occur quite rapidly.  Moderate desertification causes a drop in productivity of between 10% -25 %, severe being 25%-50% and very severe desertification being over 50%.

In the west of NSW saltbush shrubland was consistently overgrazed during the end of the 19th century.  The result was a widespread replacement of