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

experimental manipulation of species diversity, or by assembling simple experimental communities, have given conflicting
results (Prieur-Richard and Lavorel 2000). Dukes (2002) assembled combinations of native and naturalised species from four
functional groups (annual grasses, perennial grasses, early season forbs, late season forbs) in grassland microcosms and seeded
them with the invasive Centaurea solstitialis. He tested single species, 2 combinations of 4 spp., and one combination each of 8
and 16 spp., with all combinations containing an equal number of species from each functional group. Above-ground biomass of
C. solstitialis decreased rapidly with increasing diversity but reached an asymptote of c. 100 g m -2 with only c. 4 species. C.
solstitialis allocated significantly more of its biomass to reproduction in 1 year-old communities than in newly established
communities. The resident species produced less biomass in the 1 year old compared to the newly established microcosms,
corresponding with increased dominance of C. solstitialis. This was contrary to the expectation that the increased resource
availability due to disturbance in the newly established microcom would favour the weed. The species most effective in
suppressing C. solstitialis growth was, like the weed, an annual late-season forb. These findings suggest that at the community
scale, diversity reduces invasibility by increasing competition for limited resources, either because of the presence of individual
competitive species or as a collective response of the resident species. Dukes calculated “impactibility”, as the percentage
change in biomass of resident species divided by invader biomass and found that as species richness increased communities
became more impactible but less invasible.
More recent investigations of invasibility have usually explicity incorporated extrinsic properties that relate to human
environmental impact. Gassó et al. (2009) found that invasive plant richness in mainland Spain was significantly positively
correlated with the proportion of built-up land and the length of roads and railways in an area, and negatively correlated with
distance from the coast, altitude and annual rainfall. The factors negatively correlated with invasibility reflect in part the level of
anthropogenic disturbance. Thus land that has been impacted by human activities is generally more highly invaded and more
invasible than semi-natural or natural areas.
Melbourne et al. (2007) proposed that invasibility of a community is dependent on its temporal, spatial and invader-driven
environmental heterogeneity, ‘invader-driven’ heterogeneity encompassing the effects of the invader itself on the environment.
Thus small, relatively homogeneous areas are more ‘species saturated’ and have lower invasibility than large, more
heterogeneous areas, which are able to accomodate more invasive species without losses of natives; and a successful invader can
modify the invaded environment, altering its invasibility by other species. Environmental heterogeneity theory can be viewed as
a broadening of the theory of fluctuating resources (Kreyling et al. 2008).
Dunstan and Johnson (2006) argued convincingly that the spatial scale of a community is a critical variable in determining its
invasion resistance. Where a community occupies a small area, the variability within the area decreases with increasing species
richness, but when the area of a community exceeds a critical size, increasing richness increases variability. This pattern results
from the well-known vulnerability of small populations to extinction through stochastic factors. The invasibility of the system is
strongly dependent on its variability – less variable communities being more invasion resistant. Their approach partially
reconciles a number of competing theories but also presages “a much larger continuum of possible relationships between
richness, stability (both persistence and resilience), invasion resistance, species invasion/extinction, and area than have
previously been explored” (op. cit. p. 2849)
Nevertheless the invasibility of an area in relation to a particular invasive plant depends on proximity to invasion sources, the
availability of dispersal mechanisms or vectors that can deliver propagules into the community, and the existence of suitablyresourced
patches or openings in which the organism can establish, survive and reproduce (Hobbs 1989). Thus invasibility has
been demonstrated to be influenced by factors such as landscape situation, edge effects and the size and type of community
(Morgan 1998d), and is increased by disturbance and high resource availability (Hobbs and Heunneke 1992, Levine et al. 2003).
For example, in studies of urban open forest and woodland in Sydney, King and Buckney (2001) found the highest number of
exotic species in the soil seed bank and the above-ground vegetation was at the edges, that the vegetation was a very poor
indicator of seed bank contents, 84% of the exotic species not being present in it, and that lack of suitable conditions (nutrient
enrichment or other disturbance), rather than lack of propagules, was probably restricting the establishment of the exotics in
areas away from edges. Structure and density of vegetation may restrict propagule entry, and integrity of the soil crust may
restrict invasion, despite nutrient addition (Hobbs 1989). Predators, pathogens and competitors in the community may confer
invasion-resistance, rather than plant species or the vegetation community, while symbionts and mutualists may act as invasion
faciltators (Davis et al. 2000).
Increased susceptibility to invasion in general has been found in areas with strong, temporally-varying change that creates
abundant under-utilised resources, or to which such resources are anthropogenically supplied in short-term fluxes (Rejmánek
1989, Davis et al. 2000, Cox 2004). Areas without such fluxes appear to be generally less invasible the greater their plant species
or functional group richness at small spatial scales (Cox 2004, Melbourne et al. 2007). The environments with greatest plant
diversity are generally the most nutrient impoverished, and may therefore be the most susceptible to anthropogenic nutrient
enrichment and consequent increase in invasibility: this may in part explain Lonsdale’s (1999) findings that high plant diversity
is associated with greater invasibility (Davis et al. 2000). Increased plant diversity may confer invasion resistance only in highly
stable environments subject to very limited disturbance.
In Australia as elsewhere in the world, temperate native grasslands communities that lacked co-evolved large, herding ungulate
graziers have proved to be highly invasible by exotic plants when subjected to continuous grazing by introduced livestock
(Crosby 1986, Mack 1989). The effects of livestock movement and on nitrogen cycling are important factors (Milton 2004). In
the temperate grasslands of south-eastern Australia, both physical and chemical soil disturbance, and particularly nutrient
enrichment, can increase invasibility at patch, community and landscape scales (Morgan 1998d). When not overgrazed, speciesrich,
high quality grassland is in general less weed-invasible (Beames et al.(2005 citing Hector et al. 2001). Appropriately
managed (frequently burnt or conservation grazed) Themeda triandra grasslands are more resistant to N. neesiana invasion than
those that are poorly managed or where the dominant Themeda triandra is allowed senesce (Hocking 1998). When T. triandra
dies, “areas of dead grass are quickly occupied by exotic species, against which a healthy T. triandra sward provides
considerable defence” (Lunt and Morgan 2002 p. 183). At the patch scale, resistant grasslands tend to have high cover of healthy
T. triandra. In a T. triandra grassland at Evans St., Sunbury, burnt 9 months before surveying, a high cover of weeds (i.e. >40%)
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