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

Theoretical framework
“Apparently the spheres of competitiveness under which the native vegetation had evolved were irreversibly destroyed by alien
introduction, or at least by the conditions conducive to the alien introduction.”
Raymond A. Evans and James A. Young (1972), on the success of invasive grasses in the inter-mountain areas of the western
USA, in ‘Competition within the Grass Community’, In V.B. Youngner and C.M. McKell (Eds.), The Biology and Utilization of
Grasses. Academic Press, New York.
A number of competing hypotheses and theories seek to explain exotic plant invasions and their impacts on biodiversity. These
concentrate on four elements of the systems: 1. the properties of the invasive plant (‘invasion potential’ or invasiveness) (e.g.
Rejmánek and Richardson 1996, Williamson and Fitter 1996), 2. the properties of the system at risk from invasion (its
‘invasibility”) (e.g. Londsdale 1999), 3. the role of disturbance (Hobbs 1991, Hobbs and Heunneke 1992, D’Anotonio et al.
1999), and 4. dispersal mechanisms and factors (‘propagule pressure’) (Williamson and Fitter 1996, Levin 2006). Adequate
explanation is beset with the same problems faced by ecologists seeking to understand the factors that determine the species
composition of any space, in particular the difficulties of distinguishing between causal processes, the environmental conditions
that modulate them, and the patterns that result (Leigh 2007).
Residence time, the amount of time that an exotic species has spent in its introduced range, is obviously a major determinant of
the extent and impact of an invasion. The longer the residence time, the higher the likelihood that the invader will become
widespread (Hamilton et al. 2005). For example, the minimum residence time of 116 exotic grasses in Venezuela (time since the
first national record of a species) is significantly correlated with the total number of known localities in which each species
occurs (Rejmánek 2000). Longer residence time at the patch scale may also be expected to increase impact, due to alterations in
the density and age structure of the invader population in the patch, and the accumulation of feedback and indirect effects in the
invaded environment.
The ecological mechanisms that enable environmental weed invasions are in general complex and poorly understood (Prieur-
Richard and Lavorel 2000, Levine et al. 2003, Hayes and Barry 2008). Less than 5% of studies on invasive plant impacts
examined by Levine et al. (2003) attempted to determine the processes causing the invasion. Disturbance has “unanimously been
shown to favour plants invasions” (Prieur-Richard and Lavorel 2000 p. 3) but many species appear to be invasive in the absence
of significant anthropogenic disturbance, their success being attributed inter alia to inherently faster growth rates, superior
competitive abilities related to form, phenology, resource exploitation, etc., and the occupation of unfilled structural niches (Carr
et al. 1986, Carr 1993).
However, since each succesful invader and invaded system have distinctive characteristics, unique interactions of multiple
factors are most likely responsible in each case, and single factor explanations are poorly informative (Callaway and Maron
2006).
Some of these hypotheses and theories are explored in more detail in following sections, commencing with invasiveness, then the
enemy release hypothesis and the concept of biotic resistance, the theories of resource enrichment and fluctuating resources with
their emphases on the importance of disturbance as a precursor to invasion, theories related to rules of community assembly
including the ‘empty niche’ concept and competitive exclusion, an examination of the possibilities that rapid evolution of the
invader is a significant contribution to its success, and a discussion of the concept of invasibility of communities and
geographical areas. Finally the parameters involved in determination of the impact of an invasive plant are briefly discussed.
Invasive potential of a species
Many attempts have been made to identify characteristics of ‘weediness’ or what makes some plants more invasive than others
(Rejmánek 1995, Rejmánek and Richardson 1996, Hayes and Barry 2008) and various suites of charactersitics possessed by
successful invasive species have been identified (Table 1).
Each successful invasive species generally possesses a unique subset of these characteristics (Williamson and Fitter 1996).
According to LeJeune and Seastedt (2001 p. 1572) a particular species is invasive when it “encounters habitats in which its
particular suite of traits confers competitive advantage over the native dominants”. Thus the two best predictors of invasion
success are a climate/habitat match of native and exotic range and a history of invasive success elsewhere (Hayes and Barry
2008). Other characteristics significantly associated with invasion success of plants are the date of introduction (residence time),
biogeographic origin, brief juvenile period, growth form, asexual or vegetative reproduction, and flowering period and season
(Hayes and Barry 2008). Environmental weeds (exotic plants invasive in natural ecosystems) tend to possess similar
characteristics, a subset of the following: high input of viable propagules to the envionment, development time 5 year dormancy, high biomass production, dense canopy, efficient long distance (>1 km) dispersal,
allelopathic properties, coloniser of disturbed ground, adapted to fire, broad climatic tolerance and resistance to predation
(Williams and West 2000, after Adair 1995). A similar set of criteria was used to undertake a rigourous weed risk assessment on
N. neesiana as part of the Victorian ‘pest plant prioritization process’ (Morfe et al. 2003) and N. neesiana was classed as highly
invasive.
The invasive potential of a species is often compromised by specialised requirements. These may include the need for particular
symbionts, pollinaters or scarce, rare habitat features. Invasiveness may also be increased by the possession of special
competitive mechanisms, which can include allelopathy or a novel growth form (Newsome and Noble 1986).
R-strategists have a rapid rate of population increase and high mobility, are adapted to unstable environments and early
successional stages, and are able to quickly build up their numbers in areas with high levels of unused resources and low
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