Alien Species.vp - IUCN

Sarah Hayden Reichard and Peter White
one introduction every 50 years. In contrast, approximately 4,988 species (both angiosperm
and gymnosperm) have been introduced to the islands since European colonization, a rate of
about 22 taxa per year (St. John, 1973). At least 869 of the introduced species have established
in the last 200 years (Wagner et al., 1990). At the recent rate of introduction, native
communities are swamped and overwhelmed with new species that are often very aggressive.
Hawaii, because of its isolation and island ecology, may be an extreme case, but the rate of
introductions carried out by humans over the past several hundred years around the world is
clearly far higher than the “natural” rate of dispersal.
The related objection may also be answered with a similar explanation. The rate at which we
are dispersing plants is not natural. For most of human history, humans moved relatively few
plants around by migration by foot, pack animals, or small boats and across relatively small
distances. It did not involve hundreds of jumbo jets and container ships moving people and
cargo around daily. And yes, humans are invasive organisms. But a harmful invasive plant
transported by an invasive animal does not change its effect.
Finally, it should be pointed out that natural migrations usually occur into contiguous areas
and within natural barriers within the “coevolutionary envelope,” in which they are usually
accompanied by the simultaneous movement of their natural enemies, including specialized
pathogens and herbivores that attack and/or feed on one or a few plant species by contrast. Most
harmful invasions are across broad discontinuous landscapes.
Objection 5. Anti-invasive policies are bad for the nursery industry.
This statement is not accurate for a number of reasons. First, consumers have expressed a
preference to be informed of invasive ability so that they do not purchase invaders (Reichard
and White, in press). If the nursery industry wants to continue to be perceived as a “green
industry,” it will need to recognize this. Second, invasive plants are a small part of the sales of
most nurseries, so removing them from sale is unlikely to have a significant effect on their
bottom line. Third, removing invasive plants from sale could actually stimulate sales, if
handled correctly, because replacement plants would need to be promoted and sold. Countless
landscapes use Hedera helix (English ivy), an invasive species along the west coast and parts of
the east coast of North America, as a groundcover. What if consumers are encouraged to
remove that species and plant with another non-invasive species? Finally, advertising that a
nursery is selling only non-invasive species could draw customers in, as the survey discussed
earlier revealed.
Objection 6. I can grow invasive species because I can prevent them from setting seed or
growing vegetatively outside my property.
This is a naïve, although perhaps well-intentioned, thought. It is possible that some smaller
plants may be controlled for a time by removal of seed heads and confinement of rhizomes, but
it is impossible for larger plants and over a long period of time. Larger plants may grow to a size
beyond the reach and capabilities of the grower. And over time, the grower may forget, be on
vacation or ill during fruiting time, or move away and sell the property to a less informed or
vigilant individual. A destructive invasive species should not be grown.
Objection 7. Restrictions against invasive plants may be needed, but they should only be
done on a regional or state level.
The answer to this is yes – and no. It is true that most species do tend to invade only certain areas
of a large country. For existing invaders it may be possible to restrict their sale only in the
regions where they can escape and establish outside of cultivation. Each bioregion could have a
coalition of ecologists and horticulturists to discuss which species could be reasonably
removed and the timeframe in which to do it. However, for new species not yet introduced or
established in the country, but judged likely to escape and become a pest in some region, the
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