Biology Reference
In-Depth Information
species of amphibians worldwide (Berger et al., 1998; Daszak et al., 1999, 2003;
Speare and Berger, 2000; Garner, 2005; Lips et al., 2006; Skerratt et al., 2007).
Earliest known presence of this fungal infection is from the frog
Xenopus laevis
in
Africa, and this suggests that the fungus may have begun its global spread with the
widespread export (resulting in frequent release) of
X. laevis
for laboratory and
pregnancy testing in the 1930s (Weldon et al., 2004). Infection in
X. laevis
is
typically asymptomatic (Weldon, 2004), as it is in the American bullfrog,
Rana
catesbeiana
(Mazzoni et al., 2003; Daszak et al., 2004). This latter frog has been
widely exported, farmed for food, and escaped or released into the wild in a large
number of countries (Bury and Whelan, 1984); and
Batrachochytrium
has been
documented in feral bullfrog populations in many parts of its introduced range
(Hanselmann et al., 2004; Garner et al., 2006). Both alien frogs are, hence, efficient
potential vectors of the fungus to naive, native frog faunas, and current evidence
suggests their widespread transportation and release may be a contributing source
to the global explosion of the disease in the past two decades. Consistent with this
hypothesis is that the first documented occurrence of
Batrachochytrium
in Great
Britain is at a site in Kent having the only breeding population of
R. catesbeiana
in
the country, as well as a feral population of
X. laevis
(Cunningham et al., 2005;
Fisher and Garner, 2007). Although movement of these two species may have been
responsible for starting and abetting this amphibian pandemic, it is clear that a large
number of widely traded amphibians can serve as vectors for
Batrachochytrium
and
that the amphibian trade generally, whether leading to feral introductions or not,
must be viewed as highly inimical to the continued persistence of uninfected
amphibian faunas (Fisher and Garner, 2007).
Daszak et al. (1999) pointed out the likelihood that other amphibian disease
organisms besides
Batrachochytrium
have been transported with the wide-
spread introduction of alien bullfrogs and cane toads, but this reasonable sup-
position remains uninvestigated. However, iridoviruses of the genus
Ranavirus
have been implicated in numerous amphibian mortality events across North
America in the past decade (Green et al., 2002; Jancovich et al., 2005), and
genetic evidence suggests these viruses to have been derived from widely intro-
duced sport fish, with subsequent spread across western North America due to
the common use (and escape or release) as fish bait of alien larval
Ambystoma
tigrinum
(Jancovich et al., 2004). Outbreaks of disease caused by
Ranavirus
affect a diversity of frog and salamander species, including some endangered
forms (Jancovich et al., 1997).
At least one protozoan has been vectored to Australian frogs by introduction of
Bufo marinus
, and it has been able to expand to areas beyond the invasion front of
the toad (Delvinquier, 1986; Delvinquier and Freeland, 1988a). Effects on native
anurans are unknown. A variety of other protozoan parasites has arrived with
B. marinus
from its native range but are not yet known to infect native amphibians
(Delvinquier and Freeland, 1988). Vectoring of alien helminths to new hosts via
introduced lizards has been documented in Hawaii (Goldberg and Bursey, 2000a;
Goldberg et al., 2004c), but effects on native taxa are non-existent because Hawaii
lacks native lizards. These examples demonstrate the potential for introduced
