Impacts of exotic weeds on wildlife

Abstract

In the last two decades, there has been an increasing realisation worldwide that invasions by exotic weeds are second only to landscape destruction through clearing and development as a threat to biodiversity. Recognition of the massive impact of exotic weeds 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 displacement of the native flora. Displacement directly reduces the population of native plant species, which may become endangered or extinct in all or part of their ranges.
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 that make them unsuitable for generalist herbivores such as wallabies and possums. The populations of these animals therefore fall in proportion to the replacement of the native flora by unpalatable exotic weeds. The arthropod fauna (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 arthropod fauna, both in diversity and actual numbers, and unrecorded local and widespread extinctions are undoubtedly happening as a result.
Other flow-on effects on wildlife result from the absence in weeds of particular features of native plants, such as nectaries. For example, nectar-feeding birds, bats, and gliders will be 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 fruit-eating bird populations, with flow-on effects to their predators and competitors.
The reduction in arthropod numbers has further flow-on effects on parasitoid and predator species, including spiders, wasps, lizards, birds and insectivorous mammals. These feed on a variety of prey, so are not critically affected by the loss of any single species, but are affected by the loss of groups of species and the overall fall in abundance of invertebrates. Populations of species in higher trophic levels can therefore be expected to fall in proportion to the replacement of native flora by exotic weeds.
In summary, invasion of an ecosystem by exotic weeds has enormous flow-on effects on the wildlife, and these effects need to be more widely appreciated and understood.