The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 15

petiolata populations are less resistant to a specialist herbivore but equally
resistant to generalists when compared to native A. petiolata populations.
Willis et al. [99] found no differences in the sizes of biennial species, Carduus
nutans, Digitalis purpurea, Echium vulgare and Senecio jacobaea , from native
(United Kingdom and continental Europe) and alien (Australia and New
Zealand) habitats. These authors suggested that post invasion evolution of
increased size is not common, and observed patterns may be due to a plastic
response to the new environment.
Evidence both for and against EICA has been shown, and that evidence sup-
porting EICA is contingent upon much of the evidence questioned for the ERH
(above), as the two differ only in the mechanism of increased competitive abil-
ity (ecological versus evolutionary). However, factors other than the absence
of enemies might contribute. Rogers and Siemann [100] investigated the
effects of simulated herbivory and resource availability on a native tree, Celtis
laevigata and an introduced tree, Sapium sebiferum . These authors examined
the effect of different levels of simulated herbivory on the growth of S. seb-
iferum and C. laevigata at different levels of nitrogen and light. It was found
that simulated leaf herbivory negatively affected growth of C. laevigata and
had no effect on growth of S. sebiferum. S. sebiferum was able to compensate
for leaf damage in all resource conditions. They concluded that phenotypic
plasticity in combination with low levels of herbivory in the invaded range
were likely to have contributed to S. sebiferum invasiveness.
Müller-Schärer et al. [101] opined that invaders cannot be completely
released from enemies and proposed to revise the EICA hypothesis to include
generalist herbivore effects. They argued that shifts in the suite of herbivores
to assemblages dominated by generalist enemies should be taken into account.
These authors suggested several antagonistic interactions in the native range.
First, high concentrations of a toxin may repel generalist herbivore and attract
specialist herbivores. Second, plants having lower digestibility-reducing
(quantitative defence) defences are more susceptible to specialist herbivores
and such a defence strategy is likely to have adverse general impacts on
growth. This occurs because, according to Müller-Schärer et al. [101], spe-
cialist herbivores are likely to be absent in the introduced range, and plants are
more likely to be attacked by generalist herbivores, resulting in an increased
toxin concentration. Obviously this logic runs counter to decreases in toxin
concentration in introduced ranges as predicted by EICA hypothesis.
General-purpose genotype (GPG) hypothesis
The term 'general-purpose-genotype' was coined by the late Herbert G. Baker
working at the University of California, Berkeley. The GPG originally referred
to a species possessing characteristics that allowed it to colonize a wide vari-
ety of habitats - characteristics such as a plastic growth response, r-selected
life-history traits, and tolerance for a variety of environmental conditions