Alien Species.vp - IUCN

Vivian Parker
natural areas as if they are agricultural systems may result in similar outcomes, resulting in
simplification of natural areas (Auld and Tisdale, 1986; Groves, 1989; Hobbs and Humphries,
1995).
Large numbers of reproducing individuals should greatly improve the probability of random
mutations occurring that might result in superior adaptations conferring greater evolutionary
fitness. As a species we have only been observing nature using the tools of modern science for a
very short period of time; the opportunities to observe evolution in action may be found at the
boundary between populations increasing exponentially and those on the very edge of
extinction. For example, the molecular mechanism responsible for morphological mutants in
the invasive plant yellow toadflax (Linaria vulgaris) has recently been elucidated (Cubas et al.,
1999). The additive effects of global warming and ozone depletion provide an additional
incentive to search for evolutionary effects. In fact, genomic instability in plants resulting from
elevated ultraviolet radiation has recently been demonstrated (Ries et al., 2000). Rather than
placing roadblocks to what may be mechanisms for self-repair in damaged ecosystems,
ecologists should be working to maintain stability in ecosystems that are at risk.
While examining the contradictory evidence for the “slow competitive elimination” models
of communities, Chesson and Case (1986) noted “they are theories of conservation of the
existing species pool and do not address the question of how that pool comes about in the first
place”. Perhaps it is time for ecologists and evolutionary biologists to begin to address this
question in the context of invasive species.
Competition and plant invasions
Studies by paleoecologists of plant community development over geologic timeframes have
shown that, for flowering plants, “communities can accumulate higher and higher numbers of
rarer and rarer species, with no obvious ceiling on species richness” (Knoll, 1986). Empirical
evidence (Lonsdale, 1999; Williamson, 1999; and Stohlgren et al., 1999), suggests that
continental habitats can indeed absorb additional plant species without reducing native species
diversity (richness and evenness). This is presumably due to high resource availability and a
wide variety of other gradients.
Supporting experimental evidence for this has also been demonstrated. A four-year study in
Minnesota by David Tilman (1997) found that, after adding up to 54 species of seeds to several
experimental plots, the plots seeded with the most species were 83% higher in total species
richness than the control plots, while total cover of the pre-existing species remained stable.
This suggests that the new species were simply filling in unoccupied sites. While not
discounting the role of interactions between species, Tilman concluded that plant species
composition, abundance, and diversity are strongly limited by the ability of plants to disperse
seeds. Thus, anything that interferes with this process is likely to result in declining plant
populations.
Although invasive plant populations can displace numbers of native species at the stand or
alpha diversity level, evidence has shown that at the landscape scale (beta and gamma
diversity), with increased area, native diversity increases in both richness and equitability
(Forcella and Harvey, 1983; Crawley 1987). As Parker et al. (1999) point out, “the fact that one
can measure a large response to an invader in a few small quadrats constituting a fraction of a
species’ range may have little to do with a true population impact.” Indeed, areas rich with
native plant diversity are often rich with alien plant diversity as well (Stohlgren et al., 1999;
Lonsdale, 1999). And, no extinctions have ever been documented to plants as a result of
competitive displacement from another, introduced plant species (Harper, 1965; Crawley,
1987). These are significant and important distinctions to be raised.
Nevertheless, protection of habitat is the greatest insurance that viable native plant
populations will remain to provide the necessary seed sources for recruitment and dispersal,
regardless of the source or type of global changes that occur.
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