15 - Plant invasion and soil seed banks: control by water and nutrients

Summary

It appears that few, if any, introduced species became established in undisturbed native plant communities prior to European settlement of Australia (Specht, 1981a). The invasion of introduced vascular plants into the disturbed mediterranean-type ecosystems of southern Australia has been almost continuous since European occupation of the continent. Some five to six introduced plant species have become naturalised every year since 1880 (Ewart, 1930; Specht, 1981a), a rate of invasion which can be projected back almost to the beginning of the nineteenth century (Kloot, 1985, and this volume).

Invasive plants flourished in the savanna communities (grasslands, savanna woodlands and savanna open-forests) on the more fertile soils, often replacing the indigenous ground stratum. In contrast, fewer invasions occurred in the sclerophyll communities (heathlands, mallee scrubs, woodlands and openforests) that occur extensively in south-western Australia and occupy almost half of the mediterranean-type landscapes of south-eastern Australia. The soils associated with the sclerophyll communities are very low in plant nutrients.

Over the last 200 years, and especially over the last 50 years, urban and agricultural developments have expanded rapidly in the mediterranean-climate region of Australia. Over the latter 50-year period the discovery and amelioration of nutrient deficiencies of major (phosphorus, nitrogen, potassium and sulphur) and minor (copper, zinc, manganese and molybdenum) elements in these sclerophyll ecosystems has led to rapid agricultural development on these soils, with concomitant invasion of introduced plants.