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Genetically Modified Organisms as Invasive Species? 305
of others (Kennedy and Smith 1995). Thus, if impact is measured in terms of
a change in ecosystem function, then no significant impacts have been
detected and, indeed, seem unlikely (Timms-Wilson et al. 2002; Griffiths et al.
2003).
17.5.2 Potential Impacts in Plant Populations
Transgenes which would alter the invasiveness of plants, ultimately causing
the extinction of other species, would be unacceptable. Fitness and invasiveness
are sometimes used interchangeably in the literature, yet this is not
always appropriate. In fact, only when a species is invading a habitat for the
first time is it appropriate to equate fitness and invasiveness – under those circumstances,
population growth rate (fitness) can be used as a measure of how
invasive that genotype is. When considering the invasion of transgenes into
wild populations, enhanced fitness may result in a change in gene frequencies
but not necessarily a change in invasiveness. This latter attribute depends
upon the factors responsible for limiting and regulating the population. For
example, the extent to which the enhanced fecundity of Bt wild sunflowers
(Snow et al. 2003) will enhance invasiveness depends upon the extent to which
wild sunflowers are seed-limited and the herbivores remain susceptible to the
transgene products. Rates of co-evolution may be quite rapid in response to Bt
genes (Shelton et al. 1993; Ferré and van Rie 2002), in which case any increases
in fitness may be transient. If wild sunflower populations are seed-limited and
herbivore populations remain susceptible, then enhanced fitness may translate
into increased abundance. A review of seed sowing experiments aimed at
unravelling the extent to which natural populations are seed-limited (Turnbull
et al. 2000) demonstrated that approximately 50 % of seed augmentation
experiments showed some evidence of seed limitation; this tends to occur in
early successional habitats and with early successional species. In other
words, if succession proceeds, any changes in abundance resulting from
enhanced fecundity may also be transient. The most likely problems to occur
are in disturbed (agricultural) habitats.
17.5.3 Potential Impacts in Animal Populations
Theoretical scenarios have been raised in which the release of transgenic fish
could have very detrimental impacts on wild populations, and one of these is
the Trojan gene hypothesis. This hypothesis illustrates how data on relative
fitness for part of the life cycle could mislead as to the potential long-term
impact of released transgenic salmon. If transgenic escapees have a mating
advantage but viability of offspring is reduced, then models predict that the
transgene will spread through the wild population but that reduced viability