Bullfrogs- coming to a pond near you…..
An invasive threat to local pond ecology that comes hopping on hind legs.
Recognized in the top 100 most invasive species, the American Bullfrog, Lithobates catesbeianus, is now reaching the Campbell River area. I recently spent an afternoon in a large wetland in the Merville area and was shocked at the amount of large and voracious Bullfrogs I saw. This species is an invasive threat that has been wreaking havoc on Vancouver Island for some time now. Initially brought to lower Vancouver Island in the mid 1930’s in a failed attempt to farm them for the Frog Leg market, they were subsequently released and began to colonize. At first contained to lower Vancouver Island, they have made their way North steadily. According to UVic graduate student Purnima Govindarajulu, their range on the Island is expanding by about five kilometres a year.
I was surprised to hear a while ago that they were in this area, and most people I talk to don’t realize it. I do a lot of diving and photography in swamps so I decided I would go and look for some first hand. In the first swamp I went in I found no sign of them. But in a much larger system a few kilometers away, which is a fantastic wildlife area of many acres in size, I found plenty. In just a 100 meter stretch of shoreline I counted hundreds. I didn’t see any truly giant ones (I could hear them tho), but plenty of small and medium ones.
A frog like no other.
Being almost fully aquatic frogs, the adults seldom leave the water, Most frogs and toads in BC forage on land. Bullfrogs prefer shallow, warm ponds and lakes with lots of vegetation. Due to the 1-3 year tadpole stage the breeding ponds must be permanent. Bullfrogs may travel short distances over land, establishing themselves in new ponds. This leads to the slow and continuous expansion of their range.
Eating their way to the top.
Large frogs, the size of small puppies are a particular threat to all native ecology in the pond. Besides the typical frog food of insects and other small invertebrates, they also eat birds, small mammals, snakes, and other native frogs. Basically anything that can fit in their wide mouths. It would be horrific to see a frog eating a cute little duckling, but the main is the fact that they eat other frogs. Native frogs are little more than a bite-sized snack for Bullfrogs, and the evidence seems to be pointing to declines in the native Red-legged Frog and Pacific Chorus Frog populations. Western Toad populations have declined in some places, particularly in the Western United States, but this is mostly due to habitat loss. The effects on toad populations by invasive bullfrogs is not determined. Because they have very few natural predators here they are quickly becoming established, and the ripple effect is still to be determined.
More than just an eating machine.
Another potential threat comes in the form of Batrachochytrium dendrobatidis, the highly contagious fungus responsible for a worldwide decline of amphibians. This has been responsible for over a 100 species to disappear forever and many more pushed to the edge of extinction. Testing in British Columbia has revealed that the fungus is always present in bullfrog infested ponds, while in ponds with no bullfrogs, the fungus may or may not be present.
What can be done.
There are regional efforts in place to control or eradicate the frogs, but with their voracity and appetite it is a hard thing to do and even low numbers of frogs can cause big problems. Wholesale poisoning of ponds is not practical without causing the loss of other species. Selective destruction of bullfrogs is time consuming and the costs would be in the millions of dollars, without any guarantee of success. Habitat alteration and prevention of spread are two ways that will probably have the greatest results, especially in areas that have little to no current populations.
If you see Bullfrogs in waterways around Campbell River and especially North of here you can report them to the BC Frog watch group. http://www.env.gov.bc.ca/wld/frogwatch/