Pheromones can cull an invasive amphibian without releasing survivors from intraspecific competition

Attempts to cull an invasive species often create a paradoxical situation, whereby the consequent reduction in invader densities frees the survivors from intraspecific competition—and hence, increases the viability of those survivors. Our laboratory experiments with invasive cane toads (Rhinella marina) show that this backfire effect can occur with pheromone-baited trapping. Eliminating most of the tadpoles from a tank accelerates metamorphosis of the survivors and increases the size (and thus quality) of those metamorphs. Thus, trapping is likely to reduce recruitment only if the program catches all, or almost all, of the tadpoles in a waterbody. In contrast, toad control using the suppression pheromone, either alone or alongside trapping, causes similar rates of mortality as via trapping alone, but the survivors exhibit smaller mass at metamorphosis and a longer, not shorter, larval period. Thus, the combination of pheromone-based suppression and trapping can reduce effective recruitment of cane toads more successfully than can either method when used alone.