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