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

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