Impacts of exotic weeds on wildlife
In the last two decades, there has been an increasing realization worldwide that invasions by exotic weeds are second only to landscape destruction (through clearing and development) as a threat to biodiversity. This recognition is only slowly filtering down to on-ground land managers and the general public, where there is still a view that all plants are more or less equally desirable. The major impact of exotic weeds is through displacement of the native flora, directly reducing native plant populations to the point where some species become endangered or extinct.
There are also very serious flow-on effects to the wildlife. Every native plant has a large associated fauna feeding on it, but most weeds contain bitter-tasting and/or poisonous chemicals which make them unsuitable for generalist herbivores such as wallabies and possums. These animals therefore disappear as the native flora is replaced by unpalatable exotic weeds. Arthropods (mites and insects) are often highly species-specific, and also cannot transfer to the invading weed. Replacement of native vegetation by exotics thus results in an enormous reduction in the insect life, both in diversity and actual numbers, and unrecorded local and widespread extinctions are undoubtedly happening as a result.
The reduced insect numbers has further flow-on effects on parasitoid and predator species, including spiders, wasps, lizards, birds and insectivorous mammals. These feed on a range of prey, so are not critically affected by the loss of any single species but are affected by the overall drop in insect numbers. Their numbers will therefore also fall in proportion to the replacement of native flora by exotic weeds.
There are other flow-on effects on wildlife. For example, nectar-feeding birds, bats, and gliders are seriously affected by the replacement of eucalypts, grevilleas and bottlebrushes by exotic trees or shrubs. Conversely, the exotic trees camphor laurel and Brazilian pepper produce abundant small fruit, resulting in increases in some fruit-eating bird populations, with flow-on effects to their predators and competitors.
In summary, invasion of an ecosystem by exotic weeds has enormous flow-on effects on the wildlife, and this needs to be more widely appreciated and understood