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

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