Bullfrogs and other Super Species – Will They Soon Dominate Our Planet?

Super species are the phenomenally successful invasive creatures—animals, plants, and microbes—that are dominating ecosystems around the globe. Feral pigs are relentlessly trampling across Europe, North America, and Australia. Jellyfish are dominating the world’s oceans, clogging fishing nets. Not to mention the invasive species that are in our own backyards:  house sparrows and eastern gray squirrels.

In Super Species: The Creatures That Will Dominate the Planet (Firefly Books, published October 2010) Garry Hamilton details the fascinating stories of the species that seem to have won the natural selection sweepstakes. Some of these super species include the European green crab, the giant African land snail, the Argentine ant, nutria, zebra mussels, the chytrid fungus, and killer algae.

One of the species he describes, the invasive American bullfrog, especially concerns us at Frogs Are Green. Hamilton contends that bullfrogs are more invasive than Australia’s notorious cane toads. The reasons are many—bullfrogs were shipped around the world for use as biological control agents, as pets, or for sport.

Frog farms have also led to their introduction to nonnative areas. In the late 1800s after gold miners out West ate their way through native frogs, entrepreneurs imported American bullfrogs from back East to satisfy the increasing demand. Eventually farming bullfrogs spread to other parts of the world as well.

As Hamilton describes it, bullfrog farming isn’t easy and many of the frogs in these “farms” were let loose. Most frogs wouldn’t have survived. But like many invasive species, bullfrogs are highly adaptable. Bullfrogs like deep, stable, non-moving aquatic habitats. This describes many human-modified environments: reservoirs, farm ponds, irrigation channels, and even garden water features.

Bullfrogs can survive through cold Ontario winters and the extreme heat of Southern U.S. summers. Female bullfrogs can produce between 6000 and 7000 eggs and as they mature, up to 25,000 eggs per clutch. As carnivorious amphibians, they prey on fish, water beetles, snails, turtles, bats, voles, ducklings, snakes, lizards, and salamanders.

Bullfrogs compete with and prey on native frogs. This is one of the contributing factors to the worldwide decline of amphibians. Bullfrogs may also be helping to spread the deadly chytrid fungus, which is devastating frog populations around the world.

Attempts to deal with invasive bullfrogs have been challenging. But scientists have found that their numbers are fewer in waterways that haven’t been altered by people. As Hamilton writes, “By changing the physical parameters of a freshwater wetland, humans also change the playing field for all life-forms in the ecosystem, and this results in a cascade of ecological readjustments.”

The answer, Hamilton contends, is not to use the old methods such as killing the frogs and draining ponds. Rather, he says that in order to save native frogs, we need to save their habitats. Altering habitats is conducive to an invasion of bullfrogs.

I had mixed feelings reading about invasive bullfrogs. I like coming across them in ponds in the woods of upstate New York (one of their original habitats), their eyes peering just above the water, as they croak a bass jug-a-rum sound. One of my favorite frog books, The Frog Book, written in 1906 by naturalist Mary Dickerson, describes the bullfrog:

If we go rowing on river, lake, pond or park lagoon,  some moonlit night late in late June, we are certain to hear the deep-toned call of the Bullfrog many times. Coming as it does at unexpected intervals  and from unexpected directions, it seems startlingly weird in the quiet of the night. For June nights are quiet. The insect orchestras are not in full swing and the frog choruses have disbanded.

During Dickerson’s time, bullfrogs were less common than other frogs. At that time, they had many natural predators to keep them in control: snakes, otters, hawks, owls, herons, and turtles, and frog farming was mainly in the future. But human intervention tipped the balance.

In Super Species, Hamilton documents the story of many species like bullfrogs in which human intervention and alterations of habitats led to an imbalance. But his tone isn’t hysterical. Some invasive species like the plant kudzu, he contends, don’t actually have much of an impact on local biodiversity despite alarmist news stories. These species may actually be creating a new biosphere from the “rubble of our own destruction.”

This review is part of Ecolibris’s  Green Books Campaign.  Today at 1:00 p.m. ET, 200 bloggers will take a stand to support books printed on recycled or FSC-certified paper by simultaneously publishing reviews of 200 such books. By turning a spotlight on books printed using environmentally-friendly paper, Ecolibris wants to raise the awareness of book buyers to this issue and to encourage them to take it into consideration when purchasing books.