What do I mean by agriculture and agricultural lands? In this book, I will take the word agriculture to mean the landuse that is based on the clearing of landcover and the cultivation of the soil at least once. Agricultural landuse is a significant disturbance to natural ecosystems. It differs from pastoralism and some aspects of forestry that only harvest the natural ecosystems.

In Australia, all agricultural landuse is heavily dependent on fossil fuels and is part of a market-driven, export-oriented industry. There is no subsistence agriculture in Australia. As a landuse, agriculture always results in a very distinctive landcover type. Whatever landcover was there before is converted or transformed into either crops or pastures. This landcover conversion is usually irreversible.

A useful way to think about the change that results from agricultural landuse is to regard the landcover conversion as a replacement process. The original landcover is replaced by a different landcover. The original biological abundance and diversity are removed or destroyed, usually by clearing and burning, and a biologically much-simplified community substituted. The many original plant species are replaced by far fewer domesticated species, the crops and pastures. The many original animal species are eliminated and replaced by domestic herbivores, the cattle and sheep. In ecological terms, the objective of agriculture is to focus the entire productivity of the landscape into only the preferred species; wheat, sheep and so forth. The numbers of remnant or invading species, be they weeds or pests, are controlled by cultivation or chemicals.

By far the largest proportion of the agricultural lands in Australia have had the original landcover replaced with crops and pastures. There is a much smaller proportion that has undergone an additional landcover conversion. If the desire is to increase the total production from agriculture, two options present themselves. The first is to increase the area under cultivation, to extensify agriculture. The alternative is to intensify agriculture by artificially supplying the land with water (irrigation) or with nutrients (fertiliser), or both. In this way, the productivity per unit area of land is substantially increased so the overall production is also increased. Thus, the areas of agricultural lands that comprise improved or fertilised pastures and the areas of irrigation represent a landcover that has not only been replaced but which also requires maintenance and continued expenditure of water nutrients and fossil fuels.