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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As yet there appears to be no evidence that N. neesiana possesses allelopathic properties. However N. neesiana does have a<br />
unique phytolith pr<strong>of</strong>ile (see below) that may confer more robust resistance to herbivory.<br />
Rapid evolution<br />
Evolutionary aspects <strong>of</strong> invasive species have been poorly explored and are little understood (Lee 2002, Callaway and Maron<br />
2006, Whitney and Gabler 2008). Removed from the environment in which they evolved, new immigrant plant populations are<br />
subject to new selection regimes, founder effects, genetic drift and new hybridisation possibilities, so should be “prime<br />
candidates for ... evolutionary changes”, and there is now substantial evidence <strong>of</strong> rapid evolution in populations <strong>of</strong> a diverse<br />
array <strong>of</strong> invasive plants (Whitney and Gabler 2008 p. 570). Many <strong>grass</strong>es set seed in their first year, and most are capable <strong>of</strong><br />
reproducing when two years old, so invasive <strong>grass</strong>es are capable <strong>of</strong> more rapid evolution than many other plants.<br />
Strong new selection effects have demonstrably led to rapid evolution <strong>of</strong> weeds (Callaway and Maron 2006). Herbicide<br />
resistance is one <strong>of</strong> the most troublesome recent manifestations, and has developed in a large number <strong>of</strong> <strong>grass</strong> species (Preston<br />
2000). Similarly genetically based “adaptive breakthroughs” (Cox 2004 p. 61) have sometimes been responsible for relatively<br />
inocuous exotic species becoming serious inavaders. Some species have become invasive after rapid evolution <strong>of</strong> new genotypes<br />
with altered seed dormancy or earlier reproduction (Cox 2004), by hybridisation and other mechanisms. Apart from alterations<br />
due to deliberately imposed anthropic selection (weed management, including herbicides and biological control), evolved trait<br />
changes within 20 or fewer years are documented for several species (Whitney and Gabler 2008).<br />
Plant invasions typically result from one or few founder events, so the genetic variance <strong>of</strong> the invasive populations is usually<br />
surmised to be much reduced compared to populations in the native range. If the population remains small over several<br />
generations, genetic drift can result in loss <strong>of</strong> further variation (allelic diversity and heterozygosity), and the population is said to<br />
pass through a genetic bottleneck. Inbreeding species are likely to lose more genetic variation during a population bottleneck<br />
than outbreeding species. Inbreeding depression is uncommon in Poacaeae (Groves and Whalley 2002) so genetic bottlenecking<br />
may rarely have any impact on their invasions. But genetic bottlenecks also may reduce the potential benefits <strong>of</strong> genetic<br />
outcrossing and favour self-fertilisation (Cox 2004). Combined with strong selection, the surviving population may become<br />
highly adapted to the new environment and can strongly differ, genetically and phenotypically, from the conspecific native<br />
population (Callaway and Maron 2006). Superior competitive abilities, such as larger stature or greater vigour, may be acquired<br />
via evolutionary ‘trade-<strong>of</strong>fs’ involving the loss <strong>of</strong> traits, such as predator defences, that no longer provide an advantage<br />
(Callaway and Maron 2006).<br />
Characters that increase dispersal ability are likely to be favoured at the expanding periphery <strong>of</strong> the invaded range (Cox 2004).<br />
Thus the evolution <strong>of</strong> higher levels <strong>of</strong> self-fertilisation, apomixis and vegetative reproduction are frequent in invasive species,<br />
partly because their small initial populations restrict opportunity for out-breeding (Cox 2004). Watsonia meriana (L.) Mill., var.<br />
bulbillifera (J.W. Matthews and L. Bolus) D.A. Cooke is rare in its native South Africa, but is by far the dominant form <strong>of</strong> the<br />
plant in its introduced range in <strong>Australia</strong> (Cooke 1998), where the main means <strong>of</strong> dispersal appears to be movement <strong>of</strong> the<br />
vegetative cormils, produced in clusters on culm nodes, along roadsides by graders and other machinery (Parsons and<br />
Cuthbertson 1992). In this case, a rare native form with high vegetative dispersal ability has become the dominant form in the<br />
invaded range. In contrast, if the invaded system is severely geographically constrained, an invasive species may evolve reduced<br />
dispersal ability that minimises the loss <strong>of</strong> propagules in sink areas (Whitney and Gabler 2008).<br />
Increased levels <strong>of</strong> out-breeding are also recorded in invasive species, possibly because selection regimes in the new<br />
environment favour some <strong>of</strong> the recombinant genotypes (Cox 2004). Outbreeding species have high levels <strong>of</strong> cross fertilisation<br />
between individuals, so have a more diverse array <strong>of</strong> phenotypes, theoretically capable <strong>of</strong> occupying a wider range <strong>of</strong> habitats.<br />
Invasion by obligatory outbreeder is constrained, however, because a breeding population requires multiple individuals.<br />
It is widely recognised that invasion success may depend on the genetic substrate <strong>of</strong> the source populations. Single source<br />
introductions <strong>of</strong> a limited number <strong>of</strong> individuals are <strong>of</strong>ten assumed, so founder populations are thought to have passed through<br />
genetic bottlenecks, but multiple introductions from multiple source populations may be more usual (Petit 2004). For example<br />
Echium plantagineum L. populations in <strong>Australia</strong> seem to be the result <strong>of</strong> the mixing <strong>of</strong> genetic material from different European<br />
sources (Petit 2004). Genetic drift acting alone on the founder population can result in successful invasion, but this is probably<br />
exceptional.<br />
Broad environmental tolerance and genetic plasticity in the founder populations are <strong>of</strong>ten invoked as the mechanisms behind<br />
successful invasions, but do not stand up to examination, although possession <strong>of</strong> high levels <strong>of</strong> additive genetic variance (i.e.<br />
variance related to phenotype) for invasive traits in source populations has been demonstrated in a number <strong>of</strong> studies (Lee 2002).<br />
Simple directional selection on such traits is likely to explain many successful invasions, with genotype x environment<br />
interactions in the invaded resulting in diversification <strong>of</strong> the available phenotypes (Lee 2002). Lag phases, which precede the<br />
expansion phase <strong>of</strong> a new invasive organism (Shigesada and Kawasaki 1997), are associated with so-called ‘sleeper weeds’ in<br />
<strong>Australia</strong> (Groves 1999, Grice and Ainsworth 2003) and may be attributable to the slow accumulation <strong>of</strong> such variance (new<br />
mutations, etc.). Epistatic genetic variance (involving interaction between genes) could also generate new phenotypes on which<br />
selection could act (Lee 2002).<br />
There is strong evidence that polyploidy increases the colonising ability <strong>of</strong> plants (De Wet 1986). Many invasive plants are<br />
allopolyploid (hybrids retaining chromosomes <strong>of</strong> both parent species), and hybridisation (inter- or intra-specific) can generate<br />
more highly invasive genotypes (Lee 2002). Grasses in particular display high levels <strong>of</strong> hybrid and polyploid speciation. The<br />
invasive <strong>grass</strong>es Sorghum halepense (L.) and Bromus hordeaceus L. are both the result <strong>of</strong> hybridisation, the latter with later<br />
chromosome doubling (Cox 2004). Spartina anglica C.E. Hubbard, a sterile amphidiploid (tetraploid) evolved by chromosome<br />
doubling from S. X townsendii H. and J. Groves (Cox 2004), and is a more vigourous species that apparently displaces it in<br />
Victoria (Walsh 1994), as well as native Spartina spp. in many areas <strong>of</strong> the Northern Hemisphere (Cox 2004, Petit 2004). S.<br />
anglica is extremely geneticallydepauperate, unlike most allopolyploids which have large diversity because <strong>of</strong> the multiple<br />
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