Biology Reference
In-Depth Information
I explain this impact in some detail because even in those cases in which an
established alien population does not cause economic or ecological damage, it will
always incur an aesthetic cost. So far as I know, no consideration of aesthetic dam-
age from alien introductions appears in the invasive-species literature, whether for
reptiles, amphibians, or any other taxon. This probably reflects the discomfort that
many biologists would have in discussing such an unquantifiable concept, as well
as the fact that social scientists have barely become involved in research on alien
species. Nonetheless, I suggest that this is a topic deserving of consideration and
future research.
Two remaining points about alien invasions deserve emphasis. First, the effects
of invasions are frequently impossible to predict, although ecological mechanisms
of impact can often be identified and explained retrospectively. This situation may
well remain unchanged: prediction difficulty is a direct result of the inherent com-
plexity of ecosystems formed of the myriad interactions of hundreds or thousands
of species with each other and with their changing physical environments. Our
knowledge of more than a handful of these interactions in any particular ecosystem
is usually rudimentary or lacking entirely, and the large number of possible rela-
tionships involved means that an inordinately large number of direct and indirect
effects may attend the insertion of any particular novel species into such a system.
This complexity has led to invasive-species biology often being a very reactive
science - a post-mortem detailing idiosyncratic consequences of invasions that
were not or could not have been foreseen. These unpredictable consequences make
biotic invasions particularly fascinating and challenging from a scientific perspective,
while simultaneously being disconcerting and difficult to address from a manage-
ment perspective.
A second generality of extremely practical importance is that alien-species
naturalizations are usually irreversible. In most instances, once introductions
have been allowed to establish, no amount of money or effort can change the
situation - much as is widely recognized for other lamentable and irreversible
developments such as death, amputation, or the invention of disco music. This
irreversibility stems from a variety of biological and social reasons whose appli-
cability to reptile and amphibian invasions will be examined in Chapter 4, but
largely reflects the fact that biological entities are self-motivated and not readily
susceptible to control. Irreversibility of invasions imposes tremendous economic
costs in terms of perpetual damage, control, and foregone economic opportunities
(Perrings et al., 2005), a fact not yet widely appreciated by the general public or
its political representatives. In those relatively rare instances when it is feasible
to reduce or remove damaging alien species, doing so typically involves a rapid
response to a new incursion and enormous expenditures of time and money
(examples provided in Mack, 2000; Wittenberg and Cock, 2005). High expense
is incurred because invasive species will frequently occur in high numbers, be
difficult to locate, or both.
Already, thousands of damaging alien species have been introduced worldwide.
The number of recognized plant pests alone exceeds 22,000, of which at least
2,000 are environmental pests (Randall, 2002; updated to >28,000 species at
