Literature review: Impact of Chilean needle grass ... - Weeds Australia

negative depending on the particular animal group or species (Grice 2006). Invasive plants may be utilised by animals in the
same wide range of ways as native plants and can degrade or enhance the habitats of animals (Low 2002, Vidler 2004). Weeds
provide food and shelter for vertebrates (Low 2002) including vermin such as rabbits and foxes (Parsons and Cuthbertson 1992)
and native and introduced birds (Loyn and French 1991). Peter (2000) for example provided details of shelter and nest site
provision by Lycium ferocissimum Miers, and utilisation of its fruit by native and exotic birds. Fruit and seeds are widely eaten,
along with foliage, while nectar, pollen, roots and other plant parts may be exploited. Invasive plants may be used when nothing
else is available. Loyn and French (1991 p. 138) noted that weeds used as food by birds “may be better than nothing, but not as
good as the native plants they displace”. Grice (2004a) noted that existing studies provide little indication of the importance of
the plant for the diet of the animal utilising it, nor the impact of the feeding on the plant.
Invaded habits may be less suitable for animals in many ways. Habitat degradation in invaded areas may mean loss of food or
shelter or alteration to physical conditions that make the site unsuitable for habitation. Much of the literature has focused on how
the behaviour and ecology of individual native animals is altered by different characteristics of the invasive and native plants
(Levine et al. 2003), Valentine (2006) found that areas invaded by Cryptostegia grandiflora Roxb. ex R.Br. (Asclepiadaceae)
were less suitable for lizards. Invasive plants may simply out-compete food plants of endangered animal species (Vidler 2004).
In general they can be expected to support a different suite of primary consumers to the plants they displace (Grice 2004a).
Weeds may be toxic to animal species. Groves (2002) discussed impacts on animal health, but mentioned only livestock related
cases. Little appears to be known about poisoning of native animals by invasive plants in Australia but some examples are on
record. The decline of the Richmond Birdwing, Ornithoptera richmondia (Gray) (Lepidoptera: Papilionidae) in Australia is
partly due to the introduced Dutchman’s Pipe vine Aristolochia elegans Mast. (Aristolochiaceae), a close relative of the native
food plants. A. elegans is highly attractive to ovipositing female butterflies but the young larvae are poisoned and do not survive
(Sands and Scott 1996, Braby 2000). Another vine, Cryptostegia grandiflora is possibly toxic to reptiles (Valentine 2006).
Environmental weeds can also reduce access to breeding, nesting or feeding sites (Vidler 2004). Vertebrates that consume the
fruit and seeds of exotic plants can in turn can become important dispersers of propagules. Dispersal of fleshy-fruited weeds by
birds is particularly important in this respect (Loyn and French 1991). Carr (1993) provided a list for Victoria of some
naturalised plants and the indigenous and exotic animals which disperse their seed following ingestion of fruit or seed. The
complexities of impact may take many years to run their course. Synergistic effects, e.g. when a bird dispersed weed facilitates
invasion by other bird dispersed weeds (Grice 2004a) may continue almost indefinitely in environments where new adventives
are always appearing.
Impacts on detritivores and decomposers have been much less investigated (Levine et al. 2003).
Phenology of native species
Phenology is the study of the relationship between climate and the temporal variation of the lifecycle of an organsim. Any
modification of microclimate caused by an invasive plant may affect a range of other species. Increased shade due to riparian
invasions of the exotic Siam weed Chromolaena odorata (L.) R.M. King and H. Rob. (Asteraceae) in South Africa have reduced
soil temperatures and altered the sex ratio of locally breeding Nile crocodiles (Leslie and Spotila 2001). Any plant examples?
Facilitating other invasions
Invasive plants can facilitate the invasion of other plants. Fixation of N by legume weeds can profoundly alter soil conditions to
the detriment of native species and potential benefit of new invaders (Levine et al. 2003). Australian Acacia species invasive in
South African fynbos make the environment far less suitable for fynbos plants (Versfeld and van Wilgen 1986) and thus more
suitable for other species. As previously mentioned the attraction of furit eating animals to a weed food source may facilitate
consumption of the fruit of other weed species and dispersal of their seed.
Genetic changes
Loss of genetic diversity in particular plant species or populations resulting from weed invasion are probably widespread, but
have rarely been investigated, in part due to the lack of or difficulty of appropriate techniques (Adair and Groves 1998).
Probably the most common impacts occur where weed invasion destroys small, isolated or remnant populations that often
possess more extreme geneotypes than core populations. Invasive perennial grasses have played a role in reducing the genetic
variance of Rutidosis leptorhynchoides in temperate Australian grasslands by contributing to the extinction of local populations
(Groves 2004).
On a world basis genetic diversity information for grasslands is entirely inadequate and the genetic composition of only a very
small proporrtion of species has been investigated. Aspects of genetic diversity requiring investigation include its spatial
distribution, structural and functional attributes and processes under various disturbance regimes (Aguiar 2005). Inadequate
baseline data makes assessment of genetic biodiversity impacts very difficult.
Hybridisation is another threat. Hybridisation of indigenous species with exotic garden escapes, accidentally introduced exotics
and other indigenous species established outside their native range have all been reported in Australia, along with exotic-exotic
hybrids (Carr 1993). For example hybrids or possible hybrids between the Argentinian Nicotiana glauca Graham (Solanaceae)
and the natives H. suaveolens Lehm. and N. velutina H.-M. Wheeler have been found in Victoria (Carr 1993, Jeanes 1999a). The
likelihood of hybrid vigour and the possibility of hybridogenic speciation are particular concerns (Carr 1993). The presence in
Australia of a set of Nassella species from different areas of the Americas and their introduction into an environment inhabited
by a large set of native stipoids provides unique circumstances that may allow novel gene flows, with unpredictable effects.
Disturbance regimes and successional pathways
Invasive plants may act directly as disturbance agents but they can also modify the response of the community to disturbance.
The literature survey of Mack and D’Antonio (1998) found numerous studies in which plant invasions led to subsequent
alteration of disturbance regimes. Such changes included alteration of physical or biological attributes of the disturbance (e.g.
enhancement or suppression of fire and erosion) and changes in the responses of other plants. Invasion by species that interact
strongly on disturbance regimes can often produce “discrete state changes in ecosystem structure and function” (Mack and
87