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

For invasion to occur there must be not only fluctuating resources or resource enrichment, but propagule pressure (Davis et al.
2000). Disturbance (including the absence of particular disturbances), resource enrichment and propagule dispersal events are
often correlated and can occur as part of the same event (e.g. intensive grazing by a flock of sheep contaminated with N.
neesiana seed). Determining the proximate cause of invasions is thus complicated, but it is necessary to investigate the
contribution of each of these factors to succesful invasions in order to devise optimal management practices.
Propagule pressure
Propagule pressure is the number of propagules dispersed into a given area and may be more important than any other factor in
determining the success of a potential invader (Williamson and Fitter 1996, Lonsdale 1999, Levin 2006). Propagule pressure is
equivalent to the factorial combination of the number of introduction events and the number of individuals per event (Lockwood
et al. 2009). Where the availability of propagules is low, recruitment is always limited (Levin 2006) and when populations are
small, there is a reduced likelihood that they will survive (Lockwood et al. 2009). Rejmánek (2000 p. 498) examined various
proxy measures for propagule pressure and concluded that, as a general rule, initial population size and the number of
introduction attempts determined the success of an invasion: the “most robust but ... trivial, generalization in invasion ecology”.
However a recent metaanalysis (Hayes and Barry 2008) found a significant association between the number of released or
arriving individuals or the number of release/arrival attempts and establishment success only for animals, and considered the
proposition to be ‘untested’ for plants.
Propagule pressure is a complex function, based on fecudity, dependent on propagule dispersal mechanisms and the availability
and incidence of dispersal agents, and ultimately determined by the ability of the propagules to find suitable habitat and establish
new populations (Williamson and Fitter 1996). In situations where an invasive species is already present and reproducing, use of
the term “propagule rain” may be preferable (Lockwood et al. 2009). Where more than one potential invader is being considered,
e.g. with community-level processes, the combination of propagule pressures is better termed ‘colonisation pressure’ (Lockwood
et al. 2009).
Plant taxa that achieved successful ancient long distance (transcontinental) dispersal have similar characteristics to modern
invasive plants: high propagule dispersability, large propagule production by multiple, large populations, wide geographical
range and major presence in their communities (Levin 2006). Species with large native ranges tend to be more abundant and
produce more propagules per unit area so have a greater chance of becoming invasive elsewhere, purely based on the propagule
pressure they exert (Levin 2006). Substantial propagule pressure is required to overcome the genetic and demographic liabilities
of small populations (Levin 2006).
Vacant niches and competitive exclusion
Under the theory of competitive exclusion and niche displacement, a more competitive invader can occupy the niche previously
occupied by a native species, and an empty niche is open to invasion. The theoretical underpinnings of this approach are derived
from community assembly theory, based on island biogeography (Woods 1997, Seabloom et al. 2003, Cox 2004). Shea and
Chesson (2002) reframed the theory in the context of community ecology. Niche theory and dispersal assembly theories may be
contrasted with ‘neutral community assembly’, which predicts that the characteristics of a potential entrant to a community have
a neutral effect on the possibility of it becoming a part of that community, in particular, each species is equally likely to
reproduce (Leigh 2007).
The ‘storage effect’ is a related concept that incorporates the temporal and spatial variation of niche elements. “The invader must
be able to take advantage of times or locations in the landscape where the environment favours its population growth over that of
the resident species, and store those gains in time or space in such a way that they are not eroded too much in unfavourable times
or locations” (Melbourne et al. 2007 p. 84). ‘Storage’ can consist of a seed bank, a population of adult plants or a dormant tuber.
Richer communities supposedly have more niches, both filled and vacant (Prieur-Richard and Lavorel 2000). Resources that are
unsequestered by existing native species represent elements that could contribute to an ‘empty niche’. An invader might use
resources in a different way to the native species or at different times, without interfering with other species, and so could
theoretically occupy a previously empty niche. Or it might, through competitve processes, sequester resources that would
otherwise by used by the resident species.
Species that are ecologically similar to an invader are often lacking in successfully invaded communities (Mooney and Drake
1989, Systad 2000), suggesting that vacant niches are often present and that competitive exclusion often repels invasions.
However various authors have argued that there is little evidence for the existence of vacant niches (Newsome and Noble 1986,
Prieur-Richard and Lavorel 2000) and that the competitive superiority of plant invaders to native species has “rarely been tested
experimentally” (Seabloom et al. 2003 p. 13384).
Invasive plants that establish in dense populations must displace other plants and alter community composition, unless they
occupy an unfilled niche, but if the niche was unfilled there should be no displacement, the invader is not a problem, and it
successfully integrates into the existing community. The invasive species may displace another plant with a similar niche, or may
be widely competitive and have the potential to completely restructure or replace a community, alter its successional dynamics
or directly interact with disturbance regimes (Woods 1997, Mack and D’Antonio 1998), and thus alter the niche space of many
community components. A community or one of its members may repel even a superior invasive competitor “because of the
priority effect that established residents have over invaders” (Systad 2000 citing Case 1990 1991). Alternatively a native species
may persist on sites where they have unusual competitive advantages, e.g. native grasses persist on sites with serpentine soils,
but have been largely replaced by exotic grasses in California (Melbourne et al. 2007 citing Harrison 1999).
Other author have argued that the niche concept itself is “a circular argument empty of mechanism and process” (Wedin 1999 p.
193). Niche theory is largely contradictory, since niches are multidimensional hyperspaces defined by their occupation by a
species population in relationship with all the other organisms in the community (Herbold and Moyle 1986, my emphasis). A
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