Australian ecosystems are generally all underpinned by soils with quite low nutrient levels. This is a consequence of the Australian continent's great geological age, on average, and overall lack of volcanic activity that renews the rocks and therefore the soils that derive from the rocks. Combined with this our dry climate and bushfire prone vegetation where nutrients are volatised by bushfires and lost rather than being returned to the soil.
Australia's ecosystems are therefore quite different to those of Europe. European ecosystems are characterised by relatively low biodiversity with a small numbers of aggressive plant species dominating many landscapes. In contrast, Australian ecosystems are characterised by high biodiversity and where no single species dominates. Compare the pine forests of Europe where, Pinus radiata dominates a region that encircles the Earth, to the Australian Eucalytps and Wattle which are extraordinarily diverse groups of plants.
Indigenous plants are an integral part of the local ecosystem and there is a complex web of interactions between them, other indigenous plants, disease micro-organisms, mammals, birds and insects. As a result of the combined effects of these checks and balances no one species can mount a prolonged and sustained 'take over bid' of an ecosystem. The high biodiversity is therefore sustained.
When you plant an exotic plant, or sometimes even an Australian native plant of distant origin, in that same ecosystem equivalent checks and balances on the species are very unlikely to be present. As a result it is possible for that species to mount a successful and prolonged 'take over bid' of the ecosystem. The new species displaces the indigenous species and reduces biodiversity.
Environmental weeds behave in a very similar manor to human cancer cells so it is also appropriate to describe them as environmental cancer.
As the environmental weed infiltrates the surrounding indigenous vegetation and spreads throughout it indigenous plant species are progressively lost until little else remains but the weed. Indigenous mammals, birds and insects soon become locally extinct as their food sources and preferred nesting sites etc. disappear. They are often replaced by feral animals such as rabbits, foxes and rats.
It is important to note here that the process of invasion by environmental weeds is aided by:
- Land clearing
- Soil disturbance, e.g. excavations
- Dispersal of propagates (seeds, bulbs, rhizomes, stems, leaves) through:
- Mowing and slashing
- Earth moving
- Livestock & pets
- Wild animals
- People - shoes &clothing
This huge loss of biodiversity results in a greatly 'downsized' and simplified ecosystem that does not provide any where near the same level of 'ecosystem services'. Such an ecosystem has no where near the resilience of the original one. It is much more susceptible to continued degradation such as further weed invasion, salinity, erosion and loss of productivity. Once an ecosystem is degraded to this extent it requires huge amounts of effort and expense to halt the decline. It is rarely possible to restore the ecosystem to the same level of biodiversity, resilience and productivity.
Prickly pear (Opuntia) is in our history books as one of the most invasive weeds ever imported into Australia. It had a devastating impact on life in rural eastern Australia during the early part of the 20th century. Special acts of Parliament were passed to enforce control measures in an attempt to halt its spread through Queensland and New South Wales. The story started over two hundred years ago.
Click here to read the full story on the North West Weeds website.
A subset of environmental weeds pose a major threat to agriculture, as well as the environment and biodiversity. Consequently the CaLP Act declares these species as 'noxious weeds'. Landowners, whether private or public, have a statutory duty to control or eradicate noxious weeds from their property and/or road sides adjoining their properties.
For the current lists of declared noxious weeds in each state and territory click the links below:
Weeds Australia has an excellent website where you can find detailed information about a huge number of weeds.