The cane toad (Rhinella marina) is a large amphibian species native to South and Central America. It was intentionally introduced to various regions outside its natural range, primarily to control agricultural pests. However, instead of effectively managing pests, the cane toad became a significant environmental problem in these new habitats.
Invasive Characteristics
Several biological traits contribute to the cane toad’s success as an invasive species. They exhibit a rapid reproductive rate, with females laying large clutches of eggs, ranging from 8,000 to 35,000 per spawn. These toads can also undergo multiple breeding cycles annually, particularly in favorable conditions, contributing to quick population growth.
Cane toads are highly adaptable, thriving in diverse habitats, including grasslands, forests, and urban areas. Their diet is broad, as they are opportunistic feeders consuming almost any terrestrial animal they can swallow. This includes invertebrates like insects, small vertebrates, carrion, and even pet food. The absence of natural predators in many introduced environments allows their populations to expand unchecked.
Direct Impacts on Native Species
Cane toads inflict direct harm on native wildlife through their potent toxins. They secrete bufotoxins from parotoid glands behind their eyes, which are highly poisonous to most animals that attempt to consume them. These toxins can cause rapid death in predators that bite or ingest the toads.
Numerous native predators have experienced severe population declines following the arrival of cane toads. For instance, northern quolls have suffered significant losses, with declines exceeding 90% in some affected areas. Other species vulnerable to bufotoxin poisoning include goannas, snakes, and freshwater crocodiles.
Beyond toxicity, cane toads compete with native amphibians and other ground-dwelling animals for essential resources. They vie for food sources such as insects and small vertebrates, and also compete for shelter and breeding sites. This competition can lead to reduced food availability and suitable habitats for native species, impacting their populations. Adult cane toads also act as opportunistic predators, consuming native invertebrates and small vertebrates like lizards and frogs.
Ecosystem-Wide Disruptions
The presence of cane toads triggers broader, cascading effects that disrupt entire ecosystems. The decline or loss of keystone predators, particularly those susceptible to bufotoxin poisoning, can lead to significant alterations in food webs. For example, the reduction in populations of large reptilian predators, such as goannas and certain snakes, can result in an increase in their previous prey species, leading to a trophic cascade.
This imbalance extends beyond direct predator-prey relationships, impacting overall biodiversity within affected areas. The shift in predator assemblages can lead to increased predation pressure on smaller mammals and other prey groups by opportunistic predators unaffected by toad toxins. Additionally, the sheer biomass of cane toads can alter nutrient cycling by consuming vast numbers of invertebrates, acting as a nutrient sink. This pervasive presence and resulting systemic changes contribute to long-term ecological imbalance.