Amphibians (class Amphibia)
The members of this group are mostly characterized by their aquatic lifestyles in freshwater systems and only a few are terrestrial. Almost all are predatory on invertebrates, other Amphibians and other small animals. They are usually introduced as biological controls but failure in the form of population explosion of the exotic predators ensues in most cases; many are also transported out of their native ranges as pets. Amphibians are torpid under low temperatures and require plenty of water in their habitats. In the United States for example, attempts in Southern States are usually quite successful due to favourable habitats and natural conditions, such as high temperature and humidity all year-round.
Example:
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Name: Cane Toad
Order: Anura (Frogs)
Family: Bufonidae (Toads)
Scientific name: Bufo marinus |
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The largest toad in the world, Cane Toads are heavily built, with dry warty skin and poisonous large swelling parotoid glands on the shoulder. They move in short hopping gaits, but are fast colonizers --- the initial introduction in 1935 proved a fiasco as Cane Toads spread from the Northern Territory to New South Wales, devastating local fauna in their diverse habitats --- they are equally at home in coastal heathlands as rainforest edges, marshes and urban gardens. They are also established in Florida, the islands of Guam, Samoa, Hawaii, Puerto Rico and the Northern Mariana Islands, where it is regarded as a nuisance species. Now the Cane Toad has attained a circumtropical distribution. The initial aim of introduction, which is for the Toad to act as a biological control of two pests of the sugar cane crop, Grey-backed Cane Beetle and Frenchie Beetle, was never fulfilled; the pest thrived along with the Cane Toad.
These Giant Toads come in different colours, from grey to yellow to rufous, but all are large, strong, thick-skinned and predatory, measuring 15cm on average, with the record size being 24cm long and weighing 1.3kg. In contrast to most Australian native Frogs they sit upright and have become quite diurnal in Australia. Cane Toads eat almost anything at hand and in their stomachs have been found carrion, all sorts of insects including crickets, beetles, honey bees, termites and other amphibians and small snakes, even smaller mammals. Their large appetite and generalist feeding habits make them an adaptable species, and they are killing off native amphibians and insects in an unstoppable rampage.
Cane Toads, in addition, can breed almost anywhere with water. Their environmental tolerance is remarkable. They can put up with a salinity of 15% in the water and can live within the range of temperatures from 5 to 40ºC. They can also survive with 50% loss of water, a state of dehydration that can be fatal for many animals. Their parotoid glands secrete highly toxic chemicals and this poison has been found to vanquish many of the continent's top predators like the Freshwater Crocodile, Goanna (a large species of monitor lizard native to Australia), poisonous snakes of the Cobra family such as the Tiger Snake, Red-bellied Black Snake and Death Adder, even large mammals like Dingo, Quolls (marsupial cats) and human. On Guam, pet dogs and cats are commonly poisoned from biting these Toads, and a large varanid monitor lizard is suffering a decline from attempted predation on the Toad.
Prolific breeders, Cane Toads lay up to 30000 eggs per year in stark contrast to most Australian native Frogs, which lay only about 1000 annually. Their reproductive keenness enable them to colonize Australia so quickly it was hardly believable, and they now occupy a range of more than 500000 square kilometres there. Currently no control method has been found to be specific to the Giant Toad, but scientists are endeavouring in coming up with diseases that may hold the Toad population in check.
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Name: African Clawed Frog
Order: Anura (Frogs)
Family: Pipidae
Scientific name: Xenopus laevis |
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Used panglobally as a laboratory amphibian, the African Clawed Frog is large, reaching 12cm in length, and are generally selected because of their hardiness. They can tolerate high salinity up to 40% seawater, are generally resistant to many diseases, and can survive in a large range of pH values and temperatures. Aestivating during dry periods to avoid death from dehydration during such periods, they are principally aquatic otherwise, and have a long lifespan of up to 20 years. Tadpoles become sexually mature within a year of birth, and adults are capable of migration overland, as a result of which they can spread widely during floods that dislodge individuals from the ponds and rivers they were living in.
Native to the Cape region of South Africa, Clawed Frogs have been introduced extensively into USA and settled in California by the 1960s, where it has become quite problematic, competing and preying on local species as well as being toxic to some predators. Despite being aquatic they take terrestrial prey as well --- the scope of impact is yet to be perfectly assessed but this noxious species is not likely to be eradicated before long, as biological controls are not known and the species is quite intractable to start with.
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Name: North American Bullfrog
Order: Anura (Frogs)
Family: Ranidae (True Frogs)
Scientific name: Rana catesbeiana |
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This is the largest true frog in North America, and measure up to 20 cm in length. Generally these frogs are greenish with blotches on the back, and have characteristic external tympanums as large circular patches behind the eyes.
A native of eastern states, the Bullfrog has been introduced into California and Mexico in the 1900' s and since then has been responsible for driving some of the West Coast amphibians and Reptiles to the brink of extinction, as they are highly efficient predators on other frog species and their tadpoles. Bullfrogs are cannibals and feast on a wide range of food items including snakes, insects and other frogs, particularly those of their own kind --- there are even reports of them eating bats. Their cannibalism and gluttonous universality in food choices sustain high populations during shortages of many of its prey species, as adults turn to young Bullfrogs and tadpoles for food. Native species are thus increasingly likely to be extirpated by this exotic predator in the West coast of America and Southwestern France, into which they have only been recently introduced. In the absence of natural predators of the river systems in eastern North America with which the Bullfrogs have coevolved, like Snapper Turtles, Garter Snakes and Pikes, their populations can soar unchecked in their acquired ranges. In addition, the Bullfrog also produces tarichatoxin in their skin, which is one similar to that of Puffer Fish, rendering them inedible to many predators.
The decimating effect is particularly severe in native Leopard frog species such as the endemic R. chiricahuensis and R. yavapaiensis, both of which have suffered drastic population losses from direct predation by the Bullfrogs. The highly aquatic Mexican Garter Snake (Thamnolis eques), whose habitat overlaps with the Bullfrogs by a large degree, are highly susceptible to Bullfrog predation when young, and adults have been reported with severe inflammation of the tail from Bullfrog bites. The Bullfrog has been shown also to limit the ranges of other Ranids (True Frogs) through vying for food sources.
