The introduction of the cane toad (Rhinella marina) to Australia represents a textbook case of a failed biological control attempt. In 1935, a small group of cane toads was brought from Hawaii and released in Queensland to protect sugar cane crops from destructive insect pests. The specific targets were the larvae, or “grubs,” of the Greyback Cane Beetle (Dermolepida albohirtum) and French’s Cane Beetle (Lepidiota frenchi), which eat the roots of the cane plants. This plan, intended to solve an agricultural problem, instead created an immense ecological disaster.
Failure to Control the Target Pests
The project failed due to a biological mismatch between the toad and its intended prey. The cane toad is a ground-level, nocturnal forager, hunting primarily on the soil surface after dark. This behavior conflicted with the ecology of the cane beetles, which spend their adult lives in locations inaccessible to the toads.
Adult Greyback Cane Beetles are found high up in the cane stalks and canopy, out of the toad’s reach. Furthermore, the most damaging life stage—the grub that eats the cane roots—remains underground, completely inaccessible to the surface-dwelling toads. The beetles are often active at different times or in different locations than the toads prefer, ensuring minimal overlap between the two species. The toads did not effectively control the pest population, meaning the initial agricultural problem persisted while a new ecological problem was created.
Unchecked Population Growth and Dispersal
The cane toad’s rapid population growth exacerbated the failure of the control program. Females are highly fecund, capable of laying between 8,000 and 35,000 eggs in a single clutch. These reproductive events can occur multiple times per year, giving the toad a numerical advantage over native Australian amphibians.
Northern Australia lacked the natural checks that regulate toad numbers in their native South American range. There were no co-evolved predators or diseases to control the population, allowing the species to multiply unchecked. This reproductive success fueled a rapid geographical expansion, with toads at the invasion front evolving traits like longer legs for faster movement. The warm, wet climate proved highly suitable, facilitating rapid colonization into the Northern Territory and beyond.
Toxicity and the Impact on Native Predators
The most damaging consequence of the cane toad’s introduction is its toxicity, which has caused declines in native predator populations. The toad possesses large parotoid glands behind its eyes that secrete a complex venom known as bufotoxin. This toxin contains compounds, including bufadienolides, which are lethal when ingested.
Native Australian predators, such as monitor lizards (goannas), Northern quolls, and certain large snakes, had no evolutionary history with this defense and are susceptible to the poison. These predators often attempt to consume the toads whole or bite them, resulting in a fatal dose of bufotoxin. The mortality of these predators has created an ecological cascade, disrupting food webs across the continent.
Ecological Disruption Through Competition and Predation
Beyond toxicity, the sheer biomass and generalist habits of the cane toad cause ecological disruption through competition and opportunistic predation. Cane toads are adaptable, generalist feeders that consume almost anything, including insects, small vertebrates, and carrion. This broad diet puts them in direct competition with native fauna, such as small frogs, lizards, and ground-dwelling birds, for food resources.
The toads’ overwhelming numbers allow them to outcompete native species for shelter and breeding sites, stressing local populations. They also act as predators, consuming native invertebrates and small vertebrates, thereby altering the composition of local food webs. The failure of the cane toad was not a single failure, but a multi-faceted environmental catastrophe driven by reproductive success, toxicity, and ecological dominance.