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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origins <strong>of</strong> their parents (Petit 2004). The normally strongly outcrossing S. alterniflora Loisel., introduced from the east coast <strong>of</strong><br />
North America to San Francisco, rapidly evolved high rates <strong>of</strong> self-fertilisation in its new evironment (Cox 2004). Stebbins<br />
(1972) recorded the invasive nature <strong>of</strong> an artificial Ehrharta erecta Lam. autopolyploid he created by colchicine treatment and<br />
released on the Berkeley campus <strong>of</strong> the University<strong>of</strong> Califorina. Stipeae as a whole apparently consists largely <strong>of</strong> species<br />
resulting from frequent and widespread hybridisation <strong>of</strong> divergent elements (Johnson 1972, Tsvelev 1977).<br />
Single genes or a few genes might effect invasiveness or weediness traits. Sorghum halepense, one <strong>of</strong> the world’s worst weeds<br />
(Parsons and Cuthbertson 1992), has few genes affecting weediness that distinguish it from non-invasive grain Sorghum spp.<br />
(Lee 2002). When S. halepense (tetraploid) pollinates the diploid Grain Sorghum, S. bicolor (L.) Moench, sterile triploids are<br />
produced that are weedy in successive crops (Parsons and Cuthbertson 1992).<br />
Selection pressure after naturalisation can produce phenotypes with altered morphology, physiology and phenology, or with<br />
greater plasticity in response to environmental variables (Lee 2002) and these changes can occur within periods as short as a few<br />
plant generations (Cox 2004). For example invasive populations <strong>of</strong> Echinochloa crus-galli (L.) P. Beauv. in Canada have<br />
evolved greater catalytic efficiency <strong>of</strong> some enzymes, which compensates for the poor adaptation <strong>of</strong> their C 4 photosynthetic<br />
system to the cold climate (Lee 2002, see her references). Selection can occur in response to environmental gradients, the<br />
resident biota and control activities (Lee 2002). Weed mimicry <strong>of</strong> crop species e.g. by E. crus-galli (Lee 2002), is another<br />
example <strong>of</strong> relatively rapid evolutionary adaptation, well known in <strong>grass</strong>es (Barrett 1983).<br />
Release from predation and competition in the invaded environment removes some selection pressures on the invading plant and<br />
may release characters associated with defense mechanisms from evolutionary canalisation (phenotype limitation due to the<br />
constraints imposed by developmental pathways) and result in rapid evolution (Lee 2002). This has occurred with Silene latifolia<br />
Poir. in North America which has apparently allocated resources, previously used in defence, to enhanced reproduction (Whitney<br />
and Gabler 2008). Rapid evolution can also occur if plant predators and specialist herbivores expand or shift their host<br />
preferences to consume invasive plants (Cox 2004).<br />
Complex patterns <strong>of</strong> evolutionary change should be expected in each particular invasion, with particular traits favoured at<br />
different stages <strong>of</strong> the invasion, and possible reversals <strong>of</strong> trait changes (Whitney and Gabler 2008).<br />
Complementary evolutionary changes may also be expected in the invaded community – the more serious the invader, the<br />
greater the selective pressure it imposes – and those that have been investigated can also occur rapidly, on timescales <strong>of</strong>