The spatial ecology of cane toads (Bufo marinus) in tropical Australia: Why do metamorph toads stay near the water?

Abstract

Cane toads (Bufo marinus) have invaded large areas of Australia, killing many native predators as they have done so. The metamorph stage of the life cycle -the first terrestrial phase, immediately after transformation from the tadpole -is critical for ecological impact (because these animals are small enough to be prey for many native predators) and for potential control of toad populations (because small body size renders metamorphs vulnerable to desiccation).To quantify the spatial and temporal distribution of metamorph toads, and the biotic and abiotic factors that might affect their distributions, we surveyed toad breeding sites in Australia's wet-dry tropics (Adelaide River floodplain, NT) in both the wet season and the dry season. Metamorphs were concentrated close to the water's edge during the dry season, especially at midday when desiccation rates were highest. During the wet season, metamorphs were widely dispersed through the landscape. Our surveys indicate that abiotic factors (risk of desiccation) are most favourable for metamorph toads close to the pond edge, but biotic factors (food supply, and risk of competition and cannibalism) are most favourable away from the water. Operative temperatures were spatially homogeneous and sublethal, and so are unlikely to influence metamorph distribution. Desiccation risk fluctuated on a diel cycle as well as seasonally. We predict that metamorph toads benefit from dispersing as soon as desiccation risk allows them to do so, and hence the distribution of metamorph toads will shift dynamically in response to weather-mediated changes in rates of evaporative water loss.

Abiotic and biotic influences on the dispersal behavior of metamorph cane toads (Bufo marinus) in tropical Australia

“…Access to prey did not modify toad distributions or activity levels, though factors such as a toad's body size and the presence of conspecifics affected metamorph dehydration rates and (perhaps for this reason) their movement patterns in response to the experimentally imposed treatments. Although we were unable to investigate the influence of high temperatures (owing to a logistical inability to impose temperatures above 321C), the thermal independence of dehydration rates over the range we measured suggests that moisture rather than temperature may be the key factor limiting metamorph dispersal, and thus in causing pondside aggregations in metamorph toads (Lillywhite and Wassersug, '74;Heinen, '93;Child et al, 2008).…”

“…In fitness terms, one of the advantages accrued by dispersing away from the pond may be the better feeding opportunities (reflecting two effects: higher prey resources and lessened competition; Child et al, 2008). In proximate terms, however, the young toads' dispersal is driven at least partly by another disadvantage of high conspecific density: the threat of predation by larger conspecifics.…”

“…We calculated total distance travelled (the sum of all excursions) and average distance per excursion as well as total time, and mean time per visit, spent in the pond margin area. We placed thermochron-implanted agar models (for details, see Child, 2007;Child et al, 2008) in three randomly chosen arenas within each treatment for each trial session to measure rates of evaporative water loss (EWL), and to check that those rates differed among treatments in the ways we intended.…”

Larval alarm pheromones as a potential control for invasive cane toads (Bufo marinus) in tropical Australia

“…Controls (bore water) were run at the same time, in adjacent ponds containing tadpoles from the same clutch as those in the treatment pond. Close to metamorphosis, tadpoles tend to stay in the shallows; and metamorphs remain near the water's edge for a few days post-emergence (Child et al 2008b). We inspected the shorelines twice daily (in the morning and in the afternoon) to collect metamorphs within a few hours of emergence.…”

Impact of the invasive cane toad (Bufo marinus) on an Australian frog (Opisthodon ornatus) depends on minor variation in reproductive timing

“…For example, smaller metamorphs are more vulnerable to mortality sources such as desiccation, predation and parasite attack (Cohen and Alford 1993;Kelehear 2007;Child et al 2008). Thus, manipulating native frog densities might oVer novel approaches to reduce the numbers of toad metamorphs surviving to enter the terrestrial landscape.…”