Cold Blooded Invasives
The impacts of invasive species in general has been documented since the late 1800’s, however, it wasn’t until nearly 100 years later that the effects of introduced herpetofauna (reptiles and amphibians) were seriously discussed. Reptiles and amphibians have been translocated across the globe as part of the pet and food trades, for use as biological controls, and accidentally as stowaways in cargo transports. These introductions have had major ecological impacts and in some cases, such as the Florida Everglades, the problem is possibly getting worse.
Two incidents that led to major ecological consequences would bring the importance of invasive herpetofauna to the forefront: the introduction of cane toads (Rhinella marina) to Australia in 1935, and the introduction of the brown tree snake (Boiga irregularis) to Guam. These two species have become such nuisances in their respective introduced areas, that they have been considered the most problematic and reviled invasive species in the world.
Toad’s Wild Ride
In the 1930’s, Australian sugar cane growers were in desperate need of some means of controlling beetles responsible for damaging their crops. Farmers saw the option of importing cane toad as preferable to having to spray their crops with expensive and dangerous pesticides. The cane toad originates from Central and South America and had previously been exported to Puerto Rico and subsequently Hawaii as part of biological pest control programs. In 1935, 102 toads were exported from Hawaii to Australia. Although they did not produce the effects sugar cane growers were hoping for, the toads were very successful at adapting to their new Australian habitat.
Most amphibians are gape limited predators, meaning they will eat whatever fits in their mouth, and cane toads grow to very large sizes allowing them to exploit a wide variety of Australia’s native wildlife. Cane toads, like many other amphibians are explosive breeders and a single female can lay approximately 30 000 eggs multiple times a year. The large glands behind their head secrete large amounts of the poisonous bufotoxin, making the toads inedible and poisonous to many predators. Cane toads have caused a significant decline in the populations of many native species, especially predatory species who are killed as a result of trying to eat the toads. The toads are still expanding their range into novel sections of Australia. (Check out Team Bufo’s website for more info)
A thought experiment posed by Ross A. Alford showed that cane toads are so prolific at reproducing that if only 1% of all eggs survived to reproduce, then by 1986 there would have been 10^80 toads in Australia. That would be as many toads as there are atoms in the universe, and the resulting ball of toads would have a diameter 24000 times larger than that of the Milky Way Galaxy.
In the late 1940’s the brown tree snake was accidentally introduced to the island of Guam. It is believed that the snakes were introduced by the US military during WWII when snakes stowed away in transport planes or cargo traveling from Papua New Guinea, Australia, or the Solomon Islands to American military bases in Guam.
When the brown tree snake arrived in Guam it became a veritable snake in paradise. Guam had an abundance of bird, mammal, and herpetofaunal species due in part to its lack of arboreal predatory species. In other words species that could fly or climb trees could live in the trees with no risk of them or their offspring being eaten. That all changed once the brown tree snake arrived.
Over the first 35 years of their existence in Guam, the population of tree snakes is estimated to have increased by 1 million percent. At their peak density in the mid-1980’s there were an estimated 2 million snakes on the tiny island and there were approximately 100 snakes/Ha. By comparison most large snake species have natural densities of 1-10/Ha. Additionally, during this peak density period the common prey of the brown tree snakes had a combined density of 26/Ha, meaning there were four snakes for every individual prey animal. Rapid extinctions and extirpations of 9 out of 12 bird species, 2 of 3 bat species, and 6 of 12 lizard species native to Guam were concurrent with this period of peak density. Not surprisingly, these extinction/extirpation events are believed to be the result of the introduction of the brown tree snake. Snake populations have since declined, however, survivors have adapted to their loss of preferred prey species by turning to unusual food sources including things like dog food, rotting carcasses, etc.
The brown tree snake has been implicated in disturbance of reproduction and seed dispersal of endemic plant species. Prior to the predatory impacts of the brown tree snake, birds were the major pollinators and means of seed dispersal on Guam. The introduction of brown tree snakes has, therefore, indirectly jeopardized the plant biodiversity of the island.
The brown tree snake has also had considerable economic impacts to Guam. Being tree snakes these animals commonly climb onto power lines causing an estimated 86 power outages per year. This translates into a loss of approximately $6 million per year to Guam’s power utility and $1 million per year to commercial customers. Additionally, these snakes have cost Guam approximately $25 000 per year in health care costs from snake bites (an estimated 26 per year), and $4.6 million per year in conservation and control measures.
Meanwhile in Canada….
Although the harsh winters typically protects Canada from the invasion of exotic herpetofauna, it is not immune to invasions from within. The North American bullfrog (Lithobates catesbeianus) is native to much of eastern Canada and the USA; however, it has been introduced to British Columbia, as well as Mexico, Cuba, Jamaica, and parts of South America, Europe, and Asia. Bullfrogs were initially brought to BC in the 1930’s to be farmed for food. Frogs then either escaped these farms and/or were released into the wild. Populations of bullfrogs are now established in the Lower Mainland, and the southern Vancouver Island, bullfrogs are also found in the South Okanagan.
Bullfrogs are a large aggressive species capable of eating a wide range of prey. As with cane toads, bullfrogs are explosive breeders and will eat any prey they can fit into their mouth including other amphibians, snakes and lizards, and small mammals and birds. In BC bullfrogs represent a threat to native amphibian species, either through predation and/or competition. Additionally, bullfrog tadpoles produce chemicals in their skin that make them unpalatable to predatory fish species that would normally eat tadpoles.
Bullfrogs (Lithobates catebeianus) have also aided in the dispersal of infectious amphibian pathogens contributing to current global amphibian population declines. Bullfrogs are among a number of amphibian species that exhibit resistance to the virulent chytrid fungus (Batrachochytrium dendrobatidis aka Bd) and may act as carriers of disease, transmitting these disease to species in their invaded ranges. Outbreaks of chytrid in North America, South America, and Europe now threaten numerous amphibian species and are suspected to have occurred as a result of transmission from bullfrogs or other carrier species (i.e. African clawed frogs or cane toads). Similarly, fears exist that salamanders imported through the pet trade could result in the spread of the salamander chytrid disease (Batrachochytrium salamandrivorans aka BSal) in North America, where it could decimate salamander populations.
A Costly Introduction
Herpetofauna are particularly difficult to get rid of because of the risk that many removal methods pose to cause negative impacts to native reptile and/or amphibian species. Although removal of isolated populations is possible, it is often very costly. For example, in England the eradication of an isolated bullfrog population from a single 80m x 20 m pond cost $72000.
Furthermore, problems caused by invasive species can promote apathy or even hatred of herpetofauna in general. Public culls (e.g. “toad-busting”, etc) are often not effective methods of control and there is often a general inability of the public to differentiate native from invasive species. This can result in negative impacts to already imperiled native species and undermine conservation efforts of species in need of protection.
It is important to remember that populations of reptiles and amphibians are currently experiencing precipitous global declines and that while a few species may be problematic in their introduced ranges they are not representative of herpetofauna on the whole. Additionally, these animals have made their way to these novel habitats through human activities and they themselves are not at fault for the destructive actions they have caused.
Contributed by Dr. Dale Jefferson, Data and Communications Officer CWHC National Office.