MAKE LIFE INTERESTING
by Whit Gibbons
September 15, 2003
past summer I saw newspapers with pictures of a man holding an ugly-faced
piranha in Mississippi, a woman who found a tropical caiman in South
Carolina, and college students standing in a patch of water hyacinth
in Florida. The carnivorous fish, the crocodilian, and the aquatic plant
are natives of tropical America, not the southeastern United States.
In short, they are introduced species. Most environmentalists rate habitat
destruction as the single greatest cause of biodiversity loss on every
continent. Introduced, invasive species run a close second and are fast
becoming a modern scourge.
An invasive species is one brought to a region, usually from another
continent, that successfully establishes itself. The most nefarious
invasive species are those we are unable to control while they proceed
to eliminate native species, destroy habitats, or just become a nuisance.
Pleas are made daily to federal and state governments to set controls
on one invasive threat or another.
One fact about invasive species is that few generalizations can be made
about what will determine success or failure. The findings can be contradictory,
as in a review of three studies of invasive plants that identified certain
seed characteristics common to successful invaders in Great Britain.
In the three studies, one revealed that having large seeds made a plant
more likely to be successful. Another study found that small seeds were
the key, and the third said seed size did not matter. So much for figuring
out which exotic plants will be most likely to populate England.
Another example is the Brazilian pepper tree, a major pest in southern
Florida due to its supplanting many native varieties. According to one
authority, Floridians had kept Brazilian pepper trees as ornamental
plants for decades with no problems, until they suddenly began to grow
wild and create environmental havoc. No one had predicted they might
become a problem.
In another ecological paradox, one group of ecologists noted that introduced
species did not fare well when biodiversity of native species was high.
In contrast, another team provided evidence that the number of invading
exotics increased in proportion to increasing numbers of native plant
species. In other words, we have no idea whether any guiding ecological
principles can be applied to predicting whether an introduced species
will become a dominant part of the landscape or simply disappear.
Major laws and regulations are being proposed about how we should deal
with the present-day pervasiveness of introduced species. The solutions
are not going to be easy ones, biologically or politically, although
if we do not do something, many environments will be changed in ways
that are unquestionably negative from most perspectives.
A variety of issues confound the situation for anyone who cares to start
philosophizing about introduced species. First, not all introduced species
have bad reputations. A prime example is the honeybee, introduced from
Europe. Second, most major environmental impacts on natural habitats
of exotic species are a result of domesticated animals that we brought
here on purpose. Feral burros destroy parts of the Mojave Desert; cattle
overgraze native vegetation over millions of acres; and feral swine
move like baby bulldozers through the vegetation of southern swamplands.
All these qualify as introduced species that are invasive. Should we
take care of these identifiable and controllable species before we worry
about the new ones coming in?
Another viewpoint involves our perspective of when an introduced species
constitutes a problem. Do we simply have a shortsighted view? Over evolutionary
time, wouldn't all invading species eventually be incorporated into
the habitats they have invaded? Won't all of them eventually be controlled
naturally, without our intervention? Are we being too impatient in indicting
some species simply because they have modified our view of the way we
think the world should be right now?
A final point to consider is that most of us who are complaining about
introduced, invasive species are really part of the problem. After all,
our ancestors invaded the North American continent at least within the
past millennium or so, which is not a long time on the evolutionary
scale. Maybe then was the time to consider the exotic, invasive species
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