Biology Reference
In-Depth Information
wildlife refuge in southeastern Arizona met with marginal success (Rosen and
Schwalbe, 1996b). Adults and juveniles were targeted by hand-capture, spearing,
and use of aquatic traps for several short-duration intervals during the summer
months. Although large numbers of frogs were removed each year and mean mass
of adults declined through time, frog numbers consistently rebounded between
years, suggesting that effort and technology were insufficient to impact the popula-
tion (Rosen and Schwalbe, 1996b). An attempt to eradicate bullfrogs from nearby
Sycamore Canyon, Arizona, is ongoing and, again, involves targeting of adults and
tadpoles (Kahrs, 2006). Another effort at bullfrog removal from a site in Washington
State resulted in numerous dead bullfrogs but no summary of what effect this effort
had on the population (Hays et al., 1999).
In Germany, bullfrogs have inhabited relatively few areas and have been suc-
cessfully eradicated in at least three locations, as mentioned above. An eradication
program involving pond drainage, electrofishing, and netting of adults and tadpoles
has also been attempted for the largest series of German populations near Karlsruhe
(Veenvliet and Veenvliet, 2002; Reinhardt et al., 2003), but the population persists
(Ficetola et al., 2007b). It remains to be seen whether bullfrogs can be eradicated
from Germany as a whole or whether long-term control will be required at the
larger populations. The potential for bullfrog control or eradication in southwestern
France is being assessed through a four-year research and public-education pro-
gram that began in 2003 (Detaint and Coïc, 2006). In this effort, cover traps and
catfish traps have been used to collect adults and tadpoles effectively, but it remains
to be seen whether these can be deployed broadly enough to effect eradication of
metapopulations, which have spread extensively throughout the region (Ficetola et
al., 2007b). Failure to do so will result in constant immigration from nearby popula-
tions and re-establishment in treated areas. In Venezuela, local control of bullfrogs
has involved shooting adults, spreading lime in water bodies to kill tadpoles, and
clearing vegetation (Díaz de Pascual and Chacón Ortiz, 2002). Interest in bullfrog
control seems to be growing in a wide variety of locations, judging by the number
of websites mentioning this goal. Although many of these initiatives refer to their
proposed or ongoing activities as “eradication” operations, they are in fact local
control operations that may, if competently executed, result in very localized
removal of populations. Complete eradication from jurisdictions may be unlikely
except in the few instances, like Britain, that have readily defensible borders, few
and small populations, and well-executed control operations. Otherwise, this frog's
reproductive capabilities and the threat of repeated introduction or recolonization
from adjacent untreated areas are likely to thwart most “eradication” efforts.
Some of the poor success to date in controlling bullfrogs may be explainable
because recent work has shown that population growth rate in bullfrogs is most
influenced by tadpole development rate and by early postmetamorphic survival rate
(Govindarajulu et al., 2005). Consequently, removal of adults - as most control
operations have so far done - may only increase survivorship rates among recent
metamorphs by reducing the level of cannibalism they suffer. Modeling suggests
that control efforts would better be directed toward removal of recent metamorphs
in the autumn months (Govindarajulu et al., 2005), and coupling that effort with
