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Contents
1.  What you can do
2.  Water
3.  Ecology
4.  Amphibians
5.  Environmental Issues
6.  Keystone species
7.  Get Wet!-
     Field Study Ideas

8.  The Zoo Experience
9.  Frogs & Friends
10. Case Studies
11. Resources
12. Glossary

Case Studies

1. Global Amphibian Decline
2: The Decline of the Leopard Frog
3: The Introduction of the Cane Toad to Australia
4: The Puerto Rican Crested Toad - A Species Survival Plan
5: The Bullfrog - A Species in Decline
6: Blanchard's Cricket Frog

CASE STUDY 5: THE BULLFROG - A SPECIES IN DECLINE
(Level: 7 : 10 acad : 10 appl : 11 acad : 11 appl : 12 acad )

The bullfrog (Rana catesbeiana) is the largest frog in North America, capable of reaching sizes of 90-200 mm (snout to vent) at maturity. The bullfrog is a voracious eater, consuming anything smaller than itself. They will eat mice, birds, other frogs and are well documented for cannibalism on juvenile frogs of their own species.

Eggs are laid in permanent water, as a large surface film of 6,000-20,000 eggs. Many of these may die before hatching into tadpoles, with predation and UV radiation the primary cause of death. Tadpoles require 2-3 years to metamorphose into a fully formed bullfrog. During this period they are preyed on by snakes, invertebrates, fish, turtles, birds and small mammals. Transformation from the tadpole stage occurs in July/August, and takes an additional 1-3 years to produce a mature breeding bullfrog.

Bullfrog populations face many challenges to survival. Bullfrogs hibernate in water. Under the ice, oxygen is severely depleted; a major cause of death for tadpoles and adults alike. Because they are not freeze tolerant, rivers and lakes that are drained or drawn down can ice to the bottom, resulting in frogs freezing. In addition, adult bullfrogs are preyed upon by many predators, such as snapping turtles and herons, that appear to be attracted to the commotion that the male bullfrogs make when calling, defending territories and mating. This results in males being selectively removed from the population.

Researchers are investigating the widespread decline of this large frog throughout Ontario, but the causes are not fully understood. When ready to breed, bullfrogs are at their prime size for the frog leg industry. Harvesters tend to catch breeding (chorusing) males since they are the largest and easiest to locate. This results in a faster, more devastating population decline. The commercial harvest of bullfrogs for their meaty legs was prohibited in 1995 when research indicated not only a severe decline in bullfrog numbers from accessible areas, but a decrease in the size of bullfrogs found. Additional stresses result from development for cottages which deplete the quality and quantity of the bullfrog's prime shoreline habitat.

Nurseries and water garden companies often import frogs for release into native ponds. Bullfrogs and tadpoles from Florida and Quebec are brought to Ontario, along with bacterial, viral and genetic concerns if these animals are released. By comparison, bullfrogs in Ontario grow slower than those in the U.S. It is not known whether this is a result of genetic differences or adaptations to local environments, with a cooler climate and a shorter growing season.

Frog legs - A Case for Decline

Despite the large and abundant populations of frogs that existed in many countries around the world, some species are experiencing declines. Unlike the Puerto Rican crested toad, the reason for the loss of frogs from most areas is clear… predation. Not predation from an introduced species, but predation from humans. Frogs are harvested for their long, meaty legs and are sold to the restaurant trade as a delicacy. France alone, imports 400 million frogs from Indonesia; a country with no controls on its annual harvests. Until recently, India and Bangladesh were suppliers, contributing 20 million frogs to the frog leg market annually. Their populations have now crashed. The frog populations of Mexico, without restrictions to harvest, have also disappeared. These countries are not alone. The bullfrog capital of the world, Rayne, Louisiana was once famous for producing the largest bullfrogs in the world. This town’s love for bullfrogs and the export of its bullfrogs to North American markets has been responsible for local population declines and collapse of the industry. When populations decline in one region, harvesters shift to more pristine ecosystems to meet consumer needs. At current levels, the harvesting of frogs is not sustainable and will continue to deplete populations on a worldwide scale. The Ontario government has closed the commercial bullfrog harvest and it is hoped that this protection will allow Ontario's bullfrog populations to recover.

Note: Adopt-A-Pond has a conservation poster available which celebrates the bullfrog as a flagship species.

Link: Bullfrog predation has been implicated as a contributing factor to the decline of cricket frogs on Pelee Island (see case study 6).

Link: Bullfrogs have been introduced to several regions as released pets or as a commercially farmed food source. These introduced frogs are predators on native species (see case study 3 demonstrating the impact of introduced species).

Questions:

  1. How should the threats to the bullfrog populations be addressed, particularly concerning the harvesting of adults?
  2. What are the reasons for the northern limit of the bullfrog?
  3. Bullfrog populations are prominent throughout the world. Discuss why the introduction of non-native bullfrogs into Ontario may be harmful to native populations?
  4. A member of the World Bank has approached your organization to review a proposal to "farm" North American bullfrogs in Brazil. Discuss the benefits and dangers of this project.
  5. Possible benefits and dangers:

    Benefits

    • reduce impact on native frogs
    • protein source in protein-poor areas
    • employment

    Dangers

    • Escapes into the wild
    • most operators go bankrupt because they cannot compete efficiently (7 years to market from tadpole to frog).
    • bankrupt businesses release frogs which devastate local fauna
    • disease potential for native frogs
    • conversion of local balanced wetlands to polluted rearing ponds

     

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