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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Disturbance, defined as any process that creates open ground, changed habitat or altered resource availability (Hobbs 1989 1991,<br />
Mack and D’Antonio 1998, Lockwood et al. 2007) is universal at a range <strong>of</strong> spatial and temporal scales, so gaps in vegetation<br />
and fluctuating resource pools are always available. Under this definition, disturbance includes reduction or lack <strong>of</strong> normal<br />
disturbance to which the community is adapted, e.g. reduced fire frequency, removal <strong>of</strong> grazing, or loss <strong>of</strong> burrowing mammals,<br />
can provide fluctuating resources, as native plants senesce or seed banks decay without replacement. Elimination <strong>of</strong> perturbation<br />
in disturbance-dependent systems is one <strong>of</strong> the most serious ‘disturbances’ they can suffer (MacDougall and Turkington 2007).<br />
Many plant communities and species require disturbance, particularly for regeneration (Hobbs and Huenneke 1992). Whether or<br />
not the gaps or unused resources created by disturbance can be taken up by an exotic species therefore depends on their size,<br />
their spatial and temporal availability, the pool <strong>of</strong> available species and the requirements <strong>of</strong> the particular species (Prieur-Richard<br />
and Lavorel 2000). Whether or not a particular species invades is dependent on the disturbance characteristics: its mgnitude and<br />
severity,duration, predictability,distribution in space and time and synergistic effects on other disturbances (Lockwood et al.<br />
2007). When the parameters <strong>of</strong> the disturbance are suitable, exotic species that possess resource utilisation traits not present in<br />
the native flora <strong>of</strong>ten seize the advantage (McIntyre et al. 1995). Invasions <strong>of</strong> exotic plants can be greatly increased by<br />
combinations <strong>of</strong> disturbance such as nutrient addition and soil disturbance (Cale and Hobbs 1991).<br />
Early successional stages have greater pools <strong>of</strong> unused resources and less competition, so are more susceptible to invasion (Davis<br />
et al. 2000). Resource poor environments are not invaded by many exotic species (Cox 2004). Greater species diversity generally<br />
corresponds with more complete resource usage, so diversity theoretically confers invasion resistance by limiting resource<br />
fluctuation.<br />
Weed invasions are commonly associated with extreme resource fluctuations. One example is the invasion <strong>of</strong> Buffel Grass<br />
Cenchrus ciliaris L. in arid <strong>Australia</strong>. Prior to 1974 it was apparently naturalised in a few small areas in the semi-arid inland, but<br />
in the unusually wet season <strong>of</strong> 1974-5 it spread along flood plains and “run-on areas” across large areas <strong>of</strong> the arid zone (Moore<br />
1993 p. 316).<br />
Nutrient enrichment is <strong>of</strong>ten a significant contributory cause <strong>of</strong> invasions (Milton 2004). For example, King and Buckney (2002)<br />
compared total N and P, and concentration <strong>of</strong> Na, K, Ca and Mg cations in soils <strong>of</strong> 16 urban bushlands and 8 national parks in<br />
Sydney and the proportion <strong>of</strong> exotic plant species present in the vegetation. All soil nutrients were significantly higher in urban<br />
areas, and gradients from low to high invasion, and low to high concentrations, were correlated. The correlations were best<br />
explained by a combination <strong>of</strong> nutrients rather than any single nutrient. Direct manipulative studies have demonstrated that<br />
nutrient addition without other disturbance can cause weed invasion.<br />
Other studies show that both nutrient enrichment and other disturbance are required to alter community composition. Hobbs<br />
(1989) experimented in a range <strong>of</strong> heathland, shrublands and woodlands by digging the soil, adding fertiliser, both digging and<br />
fertilising, and adding seeds <strong>of</strong> Avena fatua L. and Ursinia anthemoides (L.) Poir. Very similar results were obtained in all<br />
communities. Avena responded to digging with little fertiliser effect. Ursinia showed little response to any treatment. Digging or<br />
fertiliser alone had little effect on resultant weed biomass, but a strong effect together. These results were attributed to better<br />
seed survival and more safe germination sites in the dug areas, a probable small increase in nutrient availability in dug treatments<br />
and extreme nutrient limitation for the weeds. Hobbs (1989) therefore argued that certain types <strong>of</strong> disturbance do not<br />
significantly increase resource availability and do not make a community more invasible. In a long-running experiment Davis et<br />
al. (2000) demonstrated a strong relationship between the levels <strong>of</strong> disturbance and nutrient enrichment and the mean aggregate<br />
cover <strong>of</strong> 54 plant species deliberately sown into a Derbyshire, UK, <strong>grass</strong>land.<br />
Fluctuating resources theory posits that resource removal or impoverishment will not cause an invasion. This was tested by<br />
Kreyling et al. (2008) who examined the effects <strong>of</strong> drought and heavy rainfall on simple experimental plant communities by<br />
counting individual plants that invaded from the matrix vegetation. Heavy rainfall increased invasibility while drought decreased<br />
invasibility. Higher diversity in the experimental plots (4 spp. vs. 2 spp.) decreased invasibility, and the two effects acted<br />
independently and were additive. Furthermore, several species that invaded were dependent on the functional groups present in<br />
the artificial communities or the nature <strong>of</strong> the particular weather event. Thus the predictions <strong>of</strong> both niche theory and fluctuating<br />
resources theory were supported.<br />
Enrichment or limitation <strong>of</strong> any resource including available space, soil nutrients, water and light may facilitate invasion.<br />
However the basic ecological processes that enable invasion are no different to those that enable native plants to regenerate or<br />
occupy new areas (Davis et al. 2000). In ecological time, an organism that is unable to change its distribution in response to<br />
environmental change must either evolve or become extinct.<br />
The theory <strong>of</strong> fluctuating resources predits that greater susceptibility to invasion occurs: 1. immediately after resource<br />
enrichment or following a decline in rate <strong>of</strong> resoure usage; 2. when a disturbance increases resource supply or reduces resident<br />
vegetation sequestration <strong>of</strong> resources; 3. when the interval between resource enrichment and resident vegetation sequestration is<br />
long; 4. when grazing is introduced, paticularly in high-nutrient areas; 5. that there is no necessary relationship between<br />
community plant diversity and invasion resistance and 6. that there is no general relationship between average community<br />
productivity and invasion resistance (Davis et al. 2000).<br />
In temperate <strong>Australia</strong>n native <strong>grass</strong>lands the theory therefore predicts that: 1. areas subject to more intense or frequent resource<br />
enrichment are more prone to invasion (e.g. areas with soil disturbance, areas where the existing vegetation has died or been<br />
killed, floodplain areas, areas subject to fertiliser drift, areas more subject to anthropogenic N enrichment such as roadsides,<br />
urban areas, etc.); 2. fires in autumn create greater susceptibility to invasion than fires in spring, since autumn fires mean a<br />
longer interval between the growth period <strong>of</strong> the dominant <strong>grass</strong> Themeda triandra and the resource enrichment, providing the<br />
opportunity for winter growing N. neesiana to sequester more nutrients, light and space; 3. more intense fires intensify invasion<br />
because they create a greater increase in nutrient supply and a longer period <strong>of</strong> reduced resource capture by native plants; 4.<br />
drought will facilitate invasion, especially if it breaks in the period preceding the main N. neesiana seedling establishment and<br />
growth periods; 5. grazing <strong>of</strong> long-ungrazed areas will facilitate invasion since it makes available nutrients that were previously<br />
locked up by the native plants, and 6. <strong>grass</strong>lands with greater native species richness will be more resistant to invasion only if<br />
they are characterised by more complete and total resource utilisation.<br />
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