Biology Reference
In-Depth Information
web, five ecological guilds previously present are now absent, and other ecological
guilds have become rare (Fritts and Rodda, 1998). Wholesale loss of avian and
mammalian insectivores has apparently resulted in an increase of spiders (Fritts and
Rodda, 1998; Rodda et al., 1999b) and changes in their web-making behaviors
(Kerr, 1993). The extirpation of volant frugivores has been predicted to lead to
losses of pollinator and fruit-dispersal services to native plants, leading to long-
term changes in floral composition (Savidge, 1987b); extirpation of insectivores is
expected to increase damaging insect populations, leading to increased rates of
herbivory on native plants (McCoid, 1991; Fritts and Rodda, 1998). Observations
of slowed or failed regeneration in some plant populations (Perry and Morton,
1999; Ritter and Naugle, 1999) are consistent with these predictions, but other fac-
tors (especially high ungulate densities) are also involved, so conclusive evidence
of those secondary effects is not yet available.
Secondary effects have been demonstrated to attend invasion of cane toads in
northern Australia. Subsequent to arrival of the toads, monitor lizards (
Varanus
panoptes
) suffered dramatic decline, apparently from preying on the toxic new
arrivals (Doody et al., 2006). This removed the most significant source of nest pre-
dation on the river turtle
Carettochelys insculpta
, increasing its nest-success rate by
20%. Doody et al. (2006) hypothesized that similar secondary effects would benefit
sea turtles and other native species subject to heavy predation from
V. panoptes
,
potentially leading to a cascade of trophic effects as yet unstudied.
The success of
Boiga irregularis
on Guam illustrates an additional secondary
ecological effect of considerable importance. Early expectations were that snake
abundance would abate once its food source of native birds declined. However, that
did not happen because the snake population is now maintained by supremely
abundant alien vertebrate species, the most important of which are the lizards
Carlia ailanpalai
,
Hemidactylus frenatus
, and
Anolis carolinensis
(E.W. Campbell,
1996; Fritts and Rodda, 1998; McCoid, 1999; Rodda et al., 1999b, c). In this
instance, the secondary effect is not from the snake itself but from the alien prey
organisms that allow it to maintain high densities and continue cropping native prey
to extinction. This effect from the alien prey base is maintained because the repro-
ductive rates of the alien lizards far exceed those of the snakes (Fritts and Rodda,
1998), making them a reliably available resource.
A similar alien-prey boost to an invasive snake predator has been proposed else-
where: high population densities of the alien frog
Rana perezi
on the Balearic
Islands are thought to maintain high population densities of the alien snake
Natrix
maura
(Moore et al., 2004a). This snake is thought to be the primary threat to the
survival of the endangered endemic frog
Alytes muletensis
(Alcover et al., 1984;
Tonge, 1986), and the latter is largely limited to rugged upland areas in which both
N. maura
and
R. perezi
are scarce (Moore et al., 2004a).
This augmentation of food resources for alien predators by alien reptiles and
amphibians may be of more common occurrence than currently appreciated
because many species of both taxa can attain tremendous population densities and
biomass (Burton and Likens, 1975; Gosz et al., 1978; Rodda et al., 2001; Rodda
and Dean-Bradley, 2002; Gibbons et al., 2006), including in their introduced ranges
