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Invasion by exotic species

Natural communities may be invaded by species that do not occur naturally in the area, but have been introduced either deliberately or accidentally by humans. Such species may be plants or animals and are referred to as exotic. While some introduced species may remain in low numbers on the margins of the natural community, others may establish vigorously, changing local conditions and outcompeting native species.

While our Australian Botanic Garden woodland contains a high proportion of exotic plant species, many of these are agricultural weeds that were introduced during the 19th and 20th centuries. In the absence of agricultural practices involving ongoing large scale soil disturbance, and intensive stock grazing, most of these species have become less abundant over the last 20 years, or remain restricted to disturbed sites such as roadside margins or periodically mown areas e.g. *Lolium perenne.

The conservation-management conditions of the last two decades have benefited a few species however, and these pose the main weed threats to the long-term survival of the woodland. The main threat is the African Olive *Olea europea subsp. cuspidata. Other threats are posed by Chilean Needle Grass *Nassella neesiana, which was only noted in the woodland in 1991,  St Johns Wort, *Hypericum perforatum and Heliotrope, *Heliotropium amplexicaule. Unlike the agricultural exotics, these species have all invaded the woodland and have spread in the last 20 years; active management is needed to control them. Rhodes Grass *Chloris gayana is persistent and may be difficult to eradicate in grassland areas, but is not generally invasive.

Interestingly there are some genera that have both weed and native species in the woodland e.g. *Sida rhombifolia and Sida corrugata, *Solanum nigrum and Solanum cinereum, *Chloris gayana and Chloris ventricosa, *Sporobolus africanus and Sporobolus creber.

Introduced animals include rabbits, hares and garden snails (which may impact on native species through predation - grazing and herbivory), foxes (through weed seed dispersal), and Honey bees (possible pollination impacts).

Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.