Invasive toads shift predator–prey densities in animal communities by removing top predators

Abstract

Although invasive species can have substantial impacts on animal communities, cases of invasive species facilitating native species by removing their predators have rarely been demonstrated across vertebrate trophic linkages. The predictable spread of the invasive cane toad (Rhinella marina), however, offered a unique opportunity to quantify cascading effects. In northern Australia, three species of predatory monitor lizards suffered severe population declines due to toad-induced lethal toxic ingestion (yellow-spotted monitor (Varanus panoptes), Mertens' water monitor (V. mertensi), Mitchell's water monitor (V. mitchelli). We, thus, predicted subsequent increases in the abundance and recruitment of prey species due to the reduction of those predators. Toad-induced population-level declines in the water monitor species approached 50% over a five-year period spanning the toad invasion, apparently causing fledging success of the Crimson Finch (Neochmia.phaeton) to increase from 55% to 81%. The consensus of our original and published long-term data is that invasive cane toads are causing predators to lose a foothold on top-down regulation of their prey, triggering shifts in the relative densities of predator and prey in the Australian tropical savannah ecosystem.

Invasive cane toads might initiate cascades of direct and indirect effects in a terrestrial ecosystem

“…Direct population level impacts of cane toads have not been restricted to a single goanna species but a multitude of native predators in terrestrial and aquatic ecosystems including other monitor lizards, crocodiles, snakes and quolls (Shine 2010;. Doody et al (2006Doody et al ( , 2009Doody et al ( , 2013Doody et al ( , 2015 demonstrated that cane toad-induced decline of goannas can have cascading effects on species not predicted to be directly affected by cane toads such as small lizards, tree snakes, freshwater turtles and grain-eating birds in riparian systems in northern Australia. Cane toads have also been reported to have direct suppressive effects on their invertebrate prey, and to compete with nesting birds for burrows (Boland 2004;Greenlees et al 2007;.…”

“…Given the extent of their reported direct impacts, we contend that it is likely that cane toads have also had indirect impacts on species which have strong interactions with species that have declined following the toad invasion. Indeed, given the multitude of possible indirect interaction pathways they could potentially disrupt (Doody et al 2009(Doody et al , 2013(Doody et al , 2015, we suspect that the invasion of cane toads has affected a much greater range of taxa than has thus far been reported.…”

“…Due to poisoning of individuals, populations of marsupial quolls, monitor lizards (i.e., goannas), freshwater crocodiles and some snake species have undergone marked declines following the arrival of toads (Shine 2010;. Because cane toads have driven declines in predator populations, it follows that diminished levels of predation could lead to increases in abundance or survivorship of prey species in areas where cane toads have suppressed the abundances of predators (Doody et al 2013(Doody et al , 2015.…”

“…Based on prior knowledge of cane toad biology, cane toads' suppressive effects on varanid lizard populations (Doody et al 2009(Doody et al , 2014(Doody et al , 2015 and varanid lizards' predatory effects on populations of smaller lizard species (Olsson et al 2005;Doody et al 2012Doody et al , 2015, we tested the following predictions: (1) toad activity should decrease with distance from AWP and be greater in the vicinity of dams where toads are abundant than tanks where toads are rare; (2) because encounter rates between toads and varanid lizards should decrease with distance from AWP, varanid lizard foraging activity should increase with distance from AWP and should be greater in the vicinity of tanks where toads are rare than dams where toads are abundant when distance from water is set to the mean distance; and (3) because of reduced predation by varanid lizards, the abundance of small lizards (Scincidae and Agamidae) should be greater in the vicinity of dams where toads are abundant than tanks where toads are rare.…”

Claw morphometrics in monitor lizards: Variable substrate and habitat use correlate to shape diversity within a predator guild

“…Guild members are characterized as generalist, opportunistic predators possessing a degree of dietary overlap (Losos & Greene, 1988; Shine, 1986). The spread of the invasive cane toad ( Rhinella marinas ) has decreased populations of many varanid species across northern Australia (Doody et al., 2009; Doody, James, et al., 2014; Doody, Mayes, et al., 2014; Doody et al., 2017; Doody, Soanes, et al., 2015; Shine, 2010), potentially changing the nature of these guilds.…”