Biology Reference
In-Depth Information
stand within a few feet of a large python in Everglades National Park in Florida and
not be able to see it. In contradistinction, this problem does not apply to many of
the mammals and plants that have been the targets of successful control operations.
It does apply to other invasive mammals, such as rats, that have frequently been
successfully eradicated, but success in those instances has come only after many
years devoted to development of reliable baiting, trapping, and poisoning methods.
Even eradication of the larger mammals, such as goats and pigs, has required devel-
opment or refinement of novel control methodologies over the past few decades.
Development of effective control methods may be possible for many alien reptiles
and amphibians as well, but those research efforts have barely begun. Methods
devised for brown treesnakes, for example, have taken almost two decades of effort
and are not yet perfected.
High reproductive rates characterize many herpetological species, including
several of the most notorious invasives. Amphibians can often have clutches of
hundreds or thousands of eggs, making their intrinsic growth capabilities obvious,
but even species with rather small clutch sizes can expand their numbers rapidly.
For example, the brown treesnake has a modest clutch size of only 3-12 eggs/brood
and 1-2 broods/year (Rodda et al., 1999b; Rodda and Savidge, 2007). Nonetheless,
in a favorable environment it was able to march across Guam and largely extinguish
its native bird community in slightly less than 40 years. Similarly, the direct-
developing coqui can produce perhaps 100-120 eggs/year. By frog standards this
is rather modest. Nonetheless, it was able to explode across the landscape of Hawaii
Island in less than five years, sometimes increasing from a few calling males to
large populations in only six months (Kraus and Campbell, 2002). Many invasive
reptiles and amphibians have reproductive capabilities greater than these two noto-
rious invaders, and it seems that intrinsic ability to rapidly establish and expand
populations is likely to be a severe constraint to controlling many herpetological
invasions.
These high reproductive capacities, coupled with the relatively small size of most
herpetofauna, frequently result in populations that occur at high densities. This is
especially true for alien populations, which typically escape many of the biotic
factors - parasites, predators, and competitors - that may constrain their numbers in
their native ranges. To return again to the examples just cited, brown treesnakes have
been found to occur at densities on Guam that are the highest known for any terres-
trial snake species (Rodda et al., 1999c), attaining 50-100 snakes/ha. Similarly,
coqui on Hawaii Island occur at approximately three times the maximum densities
attained in their native Puerto Rico, reaching 28,000-89,000 frogs/ha (Woolbright
et al., 2006). Such numbers will be daunting for any control operation but are likely
to be common for many herpetological invasions. All three of these biological obsta-
cles make it difficult to put all individuals in a targeted population at risk, a precondi-
tion for successful eradication operations (Bomford and O'Brien, 1995).
Social obstacles operating among the general public, government officials, and
scientists may be no less important in constraining successful control operations but
are not often discussed. One or more of four such obstacles are likely to apply to
many herpetological invasions: disbelief that either a problem or a solution exists,
