Serious threats to the terrestrial habitat of red-legged frogs include:
- Reduction of forested habitat; narrow buffers around riparian or wetland areas are insufficient.
- Replacing natural vegetation with turf grass.
- Introduction of urban or suburban developments. These are particularly detrimental per unit area, likely because they bring associated hazards, including lower water quality.
Wetland habitat: is important for a relatively short portion of the frog’s life, but it is critically important then. Eggs take about 5 weeks to hatch and the hatchlings need another 3 to 4 months to metamorphose into frogs (Brown 1975). Several features of wetland habitat had negative effects on egg masses.
Egg masses declined significantly with various forms of water pollution indexed by increasing pH, decreasing dissolved oxygen and greater conductivity. Egg mass associated with extremes of these measures were consistently the lowest observed, averaging less than 1 egg mass per site.
Mean egg mass also declined significantly with increasing surface water temperature. Northern red-legged frog eggs are more susceptible to overheating than most other frog eggs so require relatively stable water temperatures (e.g., Frogwatch). That likely is one reason why wetlands bordered by forest and shade are more productive.
Northern red-legged frogs attach their egg masses to stalks of emergent vegetation (e.g., rushes and sedges) in quiet water of little or no flow (Storm 1960; Briggs 1987; Richter and Azous 1995). Some sites in the Little Campbell watershed host too little emergent vegetative to be productive. Average emergent class for sites with mean egg mass of <3 averaged 1.15, but were 2.63 for sites averaging more than 3 egg masses. Emergent class 1 is 0 to 10%; emergent class 3 is 25 to 50%.
The tall stature and robust leaves of non-native, invasive, reed canary grass (Phalaris arundinaceae) create a dense vegetative mat that suppresses other plant species and can be a barrier to small amphibians, like northern red-legged frogs. American bullfrogs are sufficiently robust to be unimpeded. Abundant Phalaris apparently constrains success of small frogs beyond limiting movement (Hayes 1994; Platt 2014). Rittenhouse (2011) found that the pulse of organic matter created by large stands can create anoxia. Bullfrogs cope better at lower levels of oxygen than do red-legged frogs. Currently, there is little evidence that Phalaris is depressing red-legged frog abundance in the watershed. Left unchecked, however, expanding Phalaris will encourage a decline in red-legged frogs.
Climate change and development will gradually reduce the water available to wetlands. Detrimental impacts on frog reproduction were not apparent in the 6 years of data reported here, but negative trends are well established. During summer the Little Campbell River already flows both ways, to and away from the ocean, as it dries out part way along. Unlike rivers that supply Vancouver’s drinking water, the Little Campbell is not replenished by deep snowfalls each year. Aquifers play a much larger role in the water flow of the Little Campbell. All water is connected. Currently, the sources of the Little Campbell are being depleted. Observation well #7 in Langley shows an average decline in aquifer water level of 0.225 m per year (see http://www.env.gov.bc.ca/soe/indicators/water/groundwater-levels.html), but the rate has been increasing since 1983. Actions to slow the decline are likely as drinking water declines.
Other observation wells reflecting water levels in the Little Campbell River show similar patterns. Analysis of data indicates dropping water levels are not due to changes in precipitation but are the result of overuse of groundwater. A Rocha intends to monitor wetland depths more frequently in the continuing studies.
Serious threats to the wetland habitat of northern red-legged frogs include:
- Water pollution. Some wetlands already are seriously polluted.
- Warming water levels. This trend will worsen as the pattern in Figure 3 continues.
- Climate change will augment the trends of less water and warmer water.
Currently there is no established trend of reduction in emergent plants. Reed canary grass, however, spreads aggressively. Patient restoration can change that in limited, key areas. Experience at A Rocha Canada’s Brooksdale Environmental Centre indicates that cutting down Phalaris and planting red alder will eventually reduce Phalaris.
Introduced species present in the watershed that feed on northern red-legged frogs include pumpkinseed sunfish and bullfrogs. No efforts have been made to survey the pumpkinseed sunfish. They are a much smaller threat than the bullfrogs. Bullfrogs are an unwise and largely unintended introduction to the watershed. They are the largest frog in Canada and eat any animal that fits into their mouths. Red-legged frogs fit. Bullfrogs have completely or almost completely eliminated red-legged frogs from Stanley Park, areas of Delta and southern Vancouver Island.
Bullfrog in Campbell Valley Park – Photo by Anthea Farr
Bullfrogs were seen on 95.6% of the 90 visits to sites having 3 to 20 as peak egg mass values. Bullfrogs were seen much less often at sites having >20 as peak egg mass values (22.4% of 67 visits). Sites showing good egg production were less often exposed to bullfrogs. Sites having peak egg mass values of <3 had bullfrogs present on 32% of 93 visits, suggesting that these sites were favourable to neither species.
Compared to red-legged frogs, bullfrogs do better in warmer temperatures, lower levels of oxygen, higher levels of Phalaris and moderate levels of pollution. In short, current trends all favour bullfrogs over red-legged frogs. Without some changes in these trends red-legged frogs in the Little Campbell watershed appear destined to follow the demise of populations documented in Stanley Park, Delta and southern Vancouver Island.
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Brown, H.A. 1975. Reproduction and development of the Red-legged frog, Rana aurora, in Northwestern
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Bunnell, C., F. Bunnell, A. Baylis, and C. Juteau. 2016. Long-term monitoring of northern red-legged frog (Rana aurora aurora) in the Little Campbell River – 2009-2014.
COSEWIC 2004. COSEWIC assessment and update status report on the Red-legged Frog Rana aurora in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 46 pp. (www.sararegistry.gc.ca/status/status_e.cfm
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Township of Langley. 2008. Township of Langley Water Management Plan. http://www.waterbucket.ca/okw/sites/wbcokw/documents/media/86.pdf