Population‐level declines in Australian predators caused by an invasive species

Abstract

The cane toad Bufo marinus has been migrating westward across northern Australia since its introduction as a biological control agent in 1935. It has been implicated in the widespread decline of many native frog-eating predators. To investigate the impacts of this invasive species on native predatory reptiles, annual surveys were conducted from 2001 to 2007 to document variation in the relative abundances of three varanid lizards (Varanus mertensi, Varanus mitchelli and Varanus panoptes) and one crocodile Crocodylus johnstoni species known to consume toads. In addition, the indirect effects of this variation on one agamid lizard Amphibolurus gilberti, a known prey item of V. panoptes, were also examined. Surveys were performed at two sites in northern Australia before and after the arrival of B. marinus. Significant declines in the relative abundances of all three species of varanid lizard were observed following toad arrival. Declines in the abundance of V. panoptes, V. mitchelli and V. mertensi at the two sites ranged 83-96, 71-97 and 87-93%, respectively. In contrast, A. gilberti increased by 23-26%; whereas there were no significant population-level declines in C. johnstoni despite observations of individual effects (i.e. several dead crocodiles with B. marinus in their stomachs). These findings suggest population-level changes in Australian lizards caused by an invasive species.

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

“…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.…”

“…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;.…”

“…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.…”

The utility of repeated presence data as a surrogate for counts: a case study using butterflies

“…Often, though, abundance measurements are desired to monitor the longer-term dynamics of a population or focal suite of species. In particular, relative abundance data are important in identifying species in decline (Browne and Hecnar 2007;Doody et al 2009), the rise of invasive species (Engeman and Whisson 2006;Harrington et al 2008), the success of management strategies (Coelho and Manfrino 2007;Homyack and Haas 2009), as well as geographical differences in population dynamics (Okuda et al 2009). Presence data are already used to predict abundance in gridded plot surveys (Conlisk et al 2009).…”