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

correlated with significant reduction in native plants, but high native cover had no such relationship (Morgan 1998d). However
native plant species richness per se does not appear to inhibit exotic plant invasion in these grasslands (Morgan 1998d). Damage
to the cryptogam crust appears to be one factor that facilitates invasion (Scarlett 1994), presumably because it enables more rapid
seed burial and therefore decreases exposure of exotic seeds to fire and predators (Morgan 1998d), but possibly also because of
the nutrient fluxes that result.
At the landscape scale, high levels of fragmentation by roads and proximity to agricultural land and urban areas makes native
grassland more invasible (Williams 2007).These factors are important drivers of propagule pressure, that increase movement of
seeds from established weed infestations into the surrounding landscape (Melbourne et al. 2007). Similarly, the factors with the
most influence on the extinction of native plants from grassland remnants in the Victorian basalt plains have been found to be the
road density of the surrounding landscape, and long intervals between fires, which are general indicators of habitat degradation
(Williams et al. 2006), and probably represent good indicator measures of the invasibility for such grasslands.
Apart from factors that affect colonisation pressure, the extent and nature of resource fluctuation is probably the main
determinant of what species invade (Morgan 1998d). In effect, exogenous disturbance is “a special case” of environmental
heterogenity (Melbourne et al. 2007 p. 78) – the greater the severity and range of such disturbances, the greater the invasibility
of the area.
Impact
Impact has been defined as the effect that an invader has on the invaded system once established (Melbourne et al. 2007),
however this approach ignores effects associated with the establishment phase, and longer term effects post-establishment.
Immediate impacts during the establishment phase might be substantial (e.g. lethal toxicity to animals) and long-term effects
might include changes to the abiotic environment (e.g. changes to soil pH). The approach also distances any analysis from
whatever management is targetted at the invader, and ignores effects that persist after its eradication or removal (Mgobozi et al.
2008). Furthermore such an approach glosses over the complexities of evolutionary change that are likely to occur in the invader
and the invaded system as they interact (Whitney and Gabler 2008). Since very few plant invaders are ever eradicated, ultimately
both the invader and the invaded system adapt to accomodate each other, and since there is more adaptive potential in the
community that the single invader, the impact will eventually decay, or, if these adaptive changes are themselves considered to
be impacts, may continue to slowly increase (Fig. 1).
Impact
Time
Figure 1. Hypothetical impact of an invasive species on the invaded community over time. Overall impact rises at some rate
during the establishment and spread phases, then the rate of increase of impact declines as the invader occupies all suitable
habitat and areas. Impact reaches a plateau after which it either slowly declines, as previously hidden effects of adaptation and
evolutionary compensation in the invaded community gradually reduce the effects, or continues to slowly increase if these
compensatory changes are themselves considered to be components of impact.
Impacts of invasive plants can be negative, positive or neutral, and in a particular invasion are typically composed of a mixture
of such effects on different system components; so perceived impact on biodiversity is highly dependent on the measures of
biodiversity that are assessed (Groves 2002). Effects can be identified at hierarchical levels from genetic, through individual to
community and ecosystem, and can include extinction of other species, reduced abundance of native species and facilitation of
other species. At one extreme, mere presence of an alien plant in a natural ecosystem may not be tolerated by humans (Groves
2002), even if the plant is accomodated in the system without other apparent negative effects.
Transformer species
Plants that cause major and permanent (or difficult to reverse) changes in invaded communities have been called “transformers”
(Henderson 2001) or “ecosystem engineers” (Byers et al. 2002), although the latter term is usually applied to animals (Cox
2004). Transformer species often have a habit, life form or phenology not present in the invaded community (Woods 1997).
Henderson (2001 p. 253) defined a transformer as a plant which can “as monospecies dominate or replace any canopy or
subcanopy layer of a natural or semi-natural ecosystem, thereby altering its structure, integrity and functioning”. The definition
is that of Swarbrick (1991), but not the terminology.
Henderson (2001) contrasted ‘ruderal’ and ‘agrestal’ weeds, which “invade mainly sites of severe human disturbance”, with
other invasive plant species – the implication being that invasion by transformers does not require such disturbance. Rather, such
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