The biological bulldozer

Phytophthora dieback, colloquially known as the ‘biological bulldozer’ because of its clearing of areas of susceptible native vegetation, is now widespread in native forests, woodlands and heathlands of south-western and south-eastern Australia, including Tasmania. It is caused by the oomycete Phytophthora cinnamomi, which was introduced in soil attached to trees imported for horticulture following European settlement. The oomycete infects plants through the roots and kills by blocking the uptake and circulation of water and nutrients. The disease spreads naturally through ground water, but much more rapidly by human transfer of infected plants and soil. Deaths of infected plants can reduce vegetation cover by up to 50% and alter species composition, facilitating invasion by introduced weeds. These in turn can increase fuel loads and change the frequency of fire in infected areas. Such radical changes in plant communities reduce the range of food and shelter available for local animals, so animal communities are affected too.

In the botanically rich areas of south-western Australia, eucalypts in the subgenus monocalyptus (especially Eucalyptus marginata or jarrah in Western Australia) and at least 40% of the understorey plants are susceptible. Large areas of the jarrah forest are known as ‘black gravel’ or ‘graveyard’ sites as they have been so severely impacted by P. cinnamomi over the last 70–100 years: they are essentially denuded of many of the plants and animals they supported (Plate 10.1).