Biology Reference
In-Depth Information
Invasive species are usually thought to comprise a relatively modest subset of all
alien species (Williamson and Fitter, 1996), but this conclusion bears two important
caveats. First, this view may partly reflect our limited anthropocentric perspective,
and it is certainly a function of the degree to which we have attempted to identify
invasives. When investigated from the standpoint of impacts on other species, such
as native insects, it may turn out that far more alien species have negative ecological
effects than we currently appreciate and should be viewed as invasive pests. Hence,
our impression of the percentage of alien species formed by invasive pests may rise
with passing time and increased research effort, as suggested by recent findings
indicating higher rates of establishment (Kraus, 2003c) and spread (Jeschke and
Strayer, 2005) among animals than earlier predicted (Williamson and Fitter, 1996).
Second, although most pests prove invasive in many or most areas where intro-
duced, some species prove invasive or pestiferous in only one a few localities but
appear harmless in most areas where introduced. There are a number of examples
of this phenomenon, such as the traveller's palm,
Ravenala madagascariensis
, that
is widely and benignly planted throughout the tropics but has become an invasive
pest in the Mascarene Islands (Cronk and Fuller, 1995). Consequently, one must be
careful in extrapolating from an observation of non-invasiveness in one locality to
infer safety in other areas. Because of our imperfect knowledge of the ecological
consequences of mixing biotas, caution is required in asserting that any alien spe-
cies poses no hazard. Prudence and expanding scientific understanding both dictate
that the burden of proof lies on those who would argue than an introduction is
harmless. This has practical consequences for designing effective management
responses for invasive species, a point that will be discussed at greater length in the
final chapter.
Two Misconceptions
One sometimes hears claims that the introduction of alien species is a normal, if
not always positive, phenomenon that does not merit concern. One such argument
is that introducing alien species serves to increase biological diversity (or “biodi-
versity”) within a region. Because establishment of an alien species increases the
total number of species - naively thought to equate to biodiversity - alien species
are good, the argument goes. This argument is fallacious for two reasons. First,
biodiversity is not measured as just the summary number of species in an area but
also includes some measure of the relative abundances of the assembled species.
Diversity is not enhanced when one species dominates over everyone else. If many
(native) species are present but rare and one (invasive) is supremely common,
biodiversity is relatively low, even if the number of species is one greater than it
was prior to the invasion. This is exactly how invasive species tend to behave, so
they frequently decrease biodiversity. Secondly, the scale at which biodiversity is
measured is crucial. In particular, one must carefully distinguish among diversity
measures at different geographical scales. Obviously, increasingly larger regions
