Alien Species.vp - IUCN

Vivian Parker
is evident that priorities for control measures must be based on a sound understanding of the
true ecological impacts to biodiversity from invading species, and not simply be based on
economics or expediency.
Plant invasions and global change
The issue of IAS is as complex as any other in biology today, and is worthy of our greatest
efforts to understand, in order to respond in ways that will not result in further harm to the
environment. The effects of industrialization of the planet are beginning to be felt in synergistic
ways, most notably global climate change. Many scientists now believe the Earth’s biota are
headed for a massive extinction event (e.g., Ehrlich and Wilson, 1991). Evidence that animals
are moving away from the equator in response to rising temperatures is increasing daily. Plant
populations, however, historically have been found to experience tremendous time lags in their
responses to climate changes, with some forest communities lagging by 1000 years or more
(Davis, 1987; Webb, 1987). Palynological (fossil pollen) evidence suggests that for short-lived
herbaceous plants, local populations which cannot evolve physiological adaptations quickly
enough to adapt to climate fluctuations tend to die out, while other populations build up in new
localities where conditions are closer to the optimum for the species. The ability to disperse into
new habitats is the most critical factor for plant survival in response to global changes.
In this regard, the global movement of seeds and propagules by humans may actually
enhance opportunities for the future development of plant communities. This is an area that is
ripe for research. Whittaker (1977) determined that “Diversity evolves as a self-augmenting
phenomenon: evolution of species diversity provides resources (and controls) that make
possible additions of further species to the community.” The individualistic nature of species
distribution has been well described by Whittaker (1967) and confirmed by the palynological
studies of Davis (1987). Communities are shifting assemblages rather than tightly determined
units, responding to many variables in the environment including stochastic events. Davis
(1987) suggests that “alternative communities” have arisen many times during the interglacial
times of the Quaternary period, which occurred perhaps 18 to 20 times, or roughly once every
100,000 years and lasting only 10,000 to 20,000 years.
To what extent does the present mixing up of the planet’s biota present an opportunity for
heightened diversity after the dust settles on the present extinction spasm? To my knowledge,
this possibility has not been addressed anywhere in the current literature concerning plant
invasions.
Further underscoring this potential is the fact that the primary mode by which continents and
islands have become colonized and populated with unique and rich biological assemblages is
through migration and subsequent speciation (Futuyma, 1986). This mode is so fundamental to
evolution that attempts to predict, quantify, and evaluate the effects of biological invasions
have confounded ecologists; after all, the distinction between native and alien species is
arbitrarily defined by temporal and spatial scales of human derivation.
Impacts from invasive species also need to be analyzed in terms of their global distribution.
Many plants that are invasive in one country are actually rare or threatened with extinction in
their country of origin (e.g., Carpobrotus edulis, Carduus nutans, Lepidium latifolium). If
optimum conditions are found in a new locale and a previously threatened species is found to
thrive there, it does not make sense to attempt to extirpate it – particularly when conclusive
evidence for strong or long-term effects is lacking, as is usually the case with plant invaders.
Some scientists have distanced themselves from consideration of these issues, arguing: “If
we admit that this is a natural evolutionary process, it excuses us from the incentive to
implement conservation measures”. On the contrary, I am not suggesting that we suspend
conservation efforts at all. We should do everything we can to protect and conserve rare species
and biodiversity, and to prevent further introductions of invasive species. However, if we do
not halt the present trajectory of habitat destruction across the planet, we may wind up with
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