Biology Reference
In-Depth Information
4.
Number
. The minimum number of independent introductions of a species to a
jurisdiction is given, when known or able to be estimated from the literature. By
“independent” I mean separate colonizations from outside the locality in ques-
tion, including division of a single lot of animals for release into separate areas
at the same time. This is meant to exclude the phenomenon of spread of the alien
species once successfully established in the new region (whether done by “natu-
ral” dispersal of the species or by further saltational movement of animals by
humans), although it is possible that I have inadvertently counted as independent
introductions a few instances of what are, in reality, translocations of existing
stock. It is sometimes difficult to distinguish between the two from the literature
available.
Instead of explicitly providing numbers of separate introductions, authors fre-
quently summarize observations of alien herpetofauna verbally as “commonly”,
“frequently”, or “occasionally” seen, for example. I have operationally rendered
“some”, “occasional”, and “a few” as a minimum of “2”; “several” as “3”, and
“many”, “numerous”, “frequent”, or “common” as “4” in the following table. The
value in this column frequently defaults to “1” because most authors don't specify,
even in this approximating verbal fashion, the number of introductions they are
discussing, especially when considering incidences of escaped or released pets.
Sometimes I have been able to estimate a minimum number of introductions by
counting numbers of independent introduction locations on maps provided in origi-
nal articles. In such instances, each location counted as only a single introduction,
even though many more may have occurred at some localities. This option could be
used, for example, in obtaining numbers of
Trachemys scripta
introductions to
some European countries because of active herpetological atlas projects in that
region. Again, note that I have adopted a conservative stance in estimating numbers
of introductions, so the number given in this column is a bare minimum estimate of
introduction intensity based on literature information. It is not intended to be, nor
can it be interpreted as, a precise measure of number of introductions because those
are impossible to know in most instances. For example, it is impossible to know the
real number of times a species such as
Trachemys scripta
has been released (no
doubt in the tens or hundreds of thousands) because not all releases are known or
reported in the literature. Nonetheless, including a minimum estimate of number of
independent introductions does provide some measure of relative propagule pres-
sure across taxa and pathways.
5.
Pathway
. The reason for introduction is included in the database only if
explicitly mentioned in one or more of the supporting literature citations or if it
could be reasonably inferred from data presented or something said in those
citations. For all other instances, even if the general pathway of introduction for
a species is well-known, that information cell was left blank. For example,
Ramphotyphlops braminus
has been introduced around the world in nursery
materials, and many authors have pointed to this introduction pathway - and to
no other - in explaining its recent arrival to their particular geographic area.
