On the Spot

How do you stop a bullfrog invasion? Part 1: Early Detection

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Figure 2. American bullfrog (Rana catesbeiana)

The American bullfrog (Rana catesbeianabullfrog), a southeastern US native, is a large, robust frog that was initially introduced for the purpose of farming meaty frog legs. The North American market disagreed with the idea and the bullfrogs were released and expected to perish in the wild. Instead, bullfrogs began to proliferate and truly establish themselves as an invasive species and serious ecological threat. Bullfrogs prefer deeper, heavily vegetated ponds where they prey on smaller organisms including insects, spiders, fish, birds, and other frogs in the water column. Unfortunately, some habitat they use overlaps with the preferred habitat of our precious and endangered Oregon Spotted Frog (Rana pretiosaOSF). The result of this shared habitat use is a need to understand how bullfrogs are impacting OSF populations. The OSF Recovery Team would like to know if bullfrogs are threatening to invade or are currently present at high value OSF locations. If not yet present, we want to determine where the bullfrogs are and then predict where and when they may arrive at key OSF habitats. If they are present, we want to understand how bullfrogs are impacting OSF fitness, whether by competition, disease, predation, or otherwise. If they are posing a significant threat, we want to determine the best method of control. As funding for the maintenance of long-term restoration projects is limited, determining the most cost-effective bullfrog control method is a priority. 

 

When an invasive species establishes a population and reaches carrying capacity, the economic costs of control reach a maximum. The optimal time to control an invasive species is during the “lag phase” before the population explodes. The best way to do this is by “Early Detection and Rapid Response” (Figure 1). In the case of bullfrogs, early detection means call surveys. Call surveys involve listening to and recording all frog calls heard within five minutes at specific survey locations along a route. In 2018, we had call survey routes in Abbotsford, Chilliwack, and Agassiz. Call surveys are carried out several times at each location during the bullfrog breeding season. Call surveys are only conducted on days with suitable weather (sunny, 25 degrees Celsius or warmer, <5km/hr winds). Comparing the call survey results from year to year provides a model of the bullfrog invasion allowing the team to predict which OSF locations may be threatened. 

Figure 1. Invasive Species Establishment and Management Curve (https://weedwise.conservationdistrict.org/resources/report-weeds)