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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population densities <strong>of</strong> existing species. K-strategists have low mobility and are adapted to late successional stages and stable<br />
environments where the carrying capacity is approached and competition is high (Matthews 1976, Rejmánek and Richardson<br />
1996). For perennial plants, the ability to propagate vegetatively has <strong>of</strong>ten been considered important (Newsome and Noble<br />
1986).<br />
Table 1. General characteristics <strong>of</strong> successful vs. unsuccessful invasive organisms. Sources: Newsome and Noble 1986, New<br />
1994, Adair 1995, Rejmánek and Richardson 1996, Williamson and Fitter 1996, Cox 2004, Whitney and Gabler 2008.<br />
Successful invaders<br />
large native range<br />
wide climatic tolerance<br />
abundant in native range<br />
high vagility<br />
high reproductive rate<br />
short generation time<br />
reproduction requiring a single parent<br />
small propagule size<br />
high ecological flexibility<br />
wide physical tolerance<br />
wide genetic variability<br />
larger than related taxa<br />
rapid growth<br />
absence <strong>of</strong> specialised requirements<br />
low susceptibility to attack by other organisms<br />
special competitive mechanisms<br />
r strategists<br />
association with humans (commensal)<br />
Unsuccessful invaders<br />
small native range<br />
narrow climate tolerance<br />
rare in native range<br />
low vagility<br />
low reproductive rate<br />
long generation time<br />
reproduction requiring two parents<br />
large propagule size<br />
low ecological flexibility<br />
narrow physical tolerance<br />
narrow genetic variability<br />
smaller than related taxa<br />
slow growth<br />
specialised requirements<br />
high susceptibility to attack by other organisms<br />
no special competitive mechanisms<br />
K strategists<br />
not associated with humans (not commensal)<br />
Subsets <strong>of</strong> inasive characteristics can be combined to define particular ‘strategies’ possessed by invasive plants. Newsome and<br />
Noble (1986) identified four such strategies, based on the suites <strong>of</strong> ecophysiological characters possessed by different types <strong>of</strong><br />
weeds:<br />
1. Gap-grabbers – early germinators with fast initial growth enabling preoccupation <strong>of</strong> ecological space.<br />
2. Competitors - taller growing (light) or with deeper or more extensive roots (water and nutrients).<br />
3. Survivors – longevity due to resistance to mortality factors or clonal growth.<br />
4. Swampers – mass germinators.<br />
Rejmánek (2000) found that Eurasian and North African <strong>grass</strong> species adventive in eastern and western North America are more<br />
<strong>of</strong>ten those with large rather than small native latitudinal ranges and that the latitudinal size <strong>of</strong> the native range is highly<br />
correlated with the size <strong>of</strong> the introduced range. Species with larger ranges may be more successful because <strong>of</strong> the larger<br />
absolute size <strong>of</strong> the propagule pool and because they are more likely to interact with long distance dispersers (Levin 2006).<br />
Schmidt et al. (2008) highlighted a particular character suite for invasive <strong>grass</strong>es: 1. smaller seed size than the native species; 2.<br />
plastic morphological traits that enable the invader to adjust to water and N deficiencies; 3. faster growth to sexual maturity than<br />
the native species; and 4. ready stem dehiscence at the lower node (i.e. stoloniferous growth form). However Lonsdale (1994)<br />
found no relationship between weediness and height growth, relative growth rate, time to maturity (annual or perennial) or seed<br />
weight <strong>of</strong> exotic <strong>grass</strong>es introduced into northern <strong>Australia</strong>. Instead the successful weeds were most likely to be species judged<br />
to be useful, high performing or persistent in experimental agronomic field trials.<br />
Numerous studies have investigated particular biological traits and their relationships with the success <strong>of</strong> invasive plants.<br />
Hamilton et al. (2005) investigated specific leaf area (the ratio <strong>of</strong> the light-capturing area per unit dry mass), plant height and<br />
seed mass and found significant correlations between invasion success and small seed mass at regional and continental scales,<br />
and between high specific leaf area at the continental scale. Greater environmental heterogeneity at regional levels, with<br />
consequent increased biotic resistance, was invoked as one cause <strong>of</strong> the differences across spatial scales. Rejmánek (1995) found<br />
that invasiveness <strong>of</strong> Pinus species correlated with small seed weight, short juvenile period, short mean intervals between large<br />
seed crops and vertebrate dispersal.<br />
Various attempts have been made to assess the most important invasiveness characters by analysing subsets <strong>of</strong> regional weed<br />
floras. For example, Gassó et al. (2009) assessed invasive success on the basis <strong>of</strong> area occupied in mainland Spain and found that<br />
wind dispersed species were the most invasive, followed by animal dispersed species, and that residence time, when