Invasive species

Invasive species have proved problematic globally for indigenous reptile and amphibian species, whether they are out-competed for habitat or food resources, predated, or pushed out by habitat modification. In Canada there are several examples.

The American bullfrog (Lithobates catesbeianus) is perennially listed as one of the World’s 100 Worst Alien Invasive Species, escaping cultivation for food to invade places as diverse as Belgium, China, Colombia, Cuba, France, Germany, Haiti, Italy, Jamaica, Japan, Mexico, Netherlands, Puerto Rico, South Korea, Taiwan, Uruguay, Venezuela, and the western United States and British Columbia. From a handful loosed in various B.C. locations decades ago, populations have ballooned in lower Vancouver Island and the Fraser Valley east of Vancouver. As a generalist predator, bullfrogs eat everything from crayfish to salamanders, snails, snakes, hatchling turtles, small mammals, birds—and even other bullfrogs. Its favoured food, however, is other frogs, particularly young ones—a problem for endangered B.C. species like the Red-legged Frog and Oregon Spotted Frog. It is also prolific: a single female can deposit 20,000 eggs annually, a high proportion of which will survive, in part because most animals find bullfrog eggs and tadpoles distasteful. After establishing in a water body, bullfrogs eliminate native frog species through both predation and competition; by the time they begin to feed on their own kind, it’s usually too late for species they’ve displaced to recover, and the opportunistic and fast-dispersing bullfrog will have spread to nearby wetlands, a hop ahead of any would-be control. In collaboration with the University of Waterloo, Environment Canada, private landowners, and various regional non-profit organizations, the B.C. Ministry of the Environment squelched a small invasion near Osoyoos in B.C.’s Okanagan Valley.

In Victoria, B.C., the Italian wall lizard (Podarcis muralis), released from a small zoo, has become an out-of-control invasive inside of a decade, reaching absurd densities in many urban and peri-urban areas. Although these have been studied to some degree, the invasion’s effect on indigenous Northern Alligator Lizard remains unknown.

In Ontario and Quebec, a European genotype of the Common Reed (Phragmites australis australis) has taken over wetlands and shorelines at a record pace in recent years, crowding out native cattail with dense stands that are impossible for small animals like reptiles and amphibians to move through, and converting what previously functioned as aquatic breeding and foraging habitats into semi-terrestrial wastelands. In southern Ontario, where wetland loss has already been extensive, the invasion of Phragmites is threatening the remaining wetlands that provide critical habitat for several species of reptiles and amphibians. For example, Phragmites has caused the loss of almost all breeding habitat for the Fowler’s Toad in the Long Point area, resulting in the precipitous decline of this population — one of only three populations of this species in Canada. The creation of additional breeding habitat for this species has slowed these declines, but the fate of this population remains precarious. Phragmites has also been identified as a serious threat to Spotted Turtles and Blanding’s Turtles, which depend on the shallow wetland habitats that are disappearing rapidly in southern Ontario in the wake of the invasion.

Further Reading

Allan, G. M., C.J. Prelypchan, and P.T. Gregory. 2006. Population profile of an introduced species, the common wall lizard (Podarcis muralis), on Vancouver Island, Canada. Can. J. Zool. 84: 51–57. doi:10.1139/Z05-176

Bolton, R.M and R.J Brooks. 2010. Impact of the seasonal invasion of Phragmites australis (Common Reed) on turtle reproductive success. Chelonian Conservation and Biology 9:238–243.

Greenberg, D.A. and D.M Green. 2013. Effects of an invasive plant on population dynamics in toads. Conservation Biology 27: 1049–1057. doi: 10.1111/cobi.12078.

Murray R.G., V.D. Popescu, W.J. Palen, and P. Govindarajulu. 2015. Relative performance of ecological niche and occupancy models for predicting invasions by patchily-distributed species. Biological Invasions. doi:10.1007/s10530-015-0906-3