Is a species (possibly) exotic only if introduced by humans? Certainly
invasive species can come by natural means -- presumably when the land
bridge between Siberia and N. America emerged from the sea there was an
invasion of new species (including humans). Less dramatic natural events can
bring in new species, or they may arrive because they evolve longer flight
ranges or greater temperature tolerances.
I think that we are getting too wrapped up with irrelevant rigor in this
discussion. A species is exotic if it is outside its normal range, and it is
invasive if its local population is growing. Those of you familiar with my
work on fuzzy logic may detect that this is a fuzzy definition -- we cannot
draw a sharp distinction between native and exotic, some species are more
exotic than others, and there are degrees of invasiveness.
Is the distinction ecologically meaningless? Not if it has value in
understanding an ecosystem. For example, sometimes tropical species show up
off the Atlantic coast of Canada due to entrainment in warm-core rings in
the Gulf Stream. They are exotics, rarely found, but they can have an impact
on the ecosystem.
Bill Silvert
----- Original Message -----
From: "James Crants" <jcra...@gmail.com>
To: <ECOLOG-L@LISTSERV.UMD.EDU>
Sent: segunda-feira, 10 de Maio de 2010 16:51
Subject: Re: [ECOLOG-L] Ecology Terminology and associated phenomena
Colonizing species etc
In our conversation, Matthew Chew argued that the distinction between native
and exotic species is ecologically meaningless. A species does not have
higher fitness because it is dispersed by humans instead of other agents.
Most species dispersed by humans fail utterly in the new environment to
which they were dispersed. Very few species are evolutionarily specialized
for human-mediated dispersal ...
