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Red-legged Frog (Rana Aurora)

Limiting Factors and Threats

In British Columbia, the distribution of the Red-legged Frog is restricted to relatively low elevations. Most records are from elevations below 500 m, although the species has been reported from localities up to 1040 m in elevation. The species reaches the northern limits of its natural distribution in southern British Columbia. Its present northern distributional limits are probably a result of the glacial history of the area and not a reflection of its physiological or ecological tolerance limits, as attested by two isolated northern populations, in Haida Gwaii and southeast Alaska – the Alaska population and possibly also the Haida Gwaii population are the result of recent introductions. The species may still be expanding its range northwards along the Pacific coast, but altitudinal barriers (and possibly interactions with the Columbia Spotted Frog or other species) pose limits to its range expansion inland.

Throughout the species’ Canadian range, populations are threatened by human activities and land-use practices. Anthropogenic threats include habitat fragmentation, draining of wetlands, loss and modification of forest habitats, removal of riparian vegetation, pollution of breeding habitats, and the introduction of exotic and other non-native organisms to aquatic habitats. Epidemic disease is a concern for anuran populations in general, and synergistic interactions among various factors, human-induced and natural, are probably common and can affect amphibians in unpredictable ways. Global climate change can exacerbate all these effects.

 

Habitat Loss and Fragmentation

The distribution of the Red-legged Frog in Canada overlaps the most populated parts of the province in the Lower Fraser Valley and on the southern and eastern Vancouver Island (Figure 3). Over the past century, habitats in these areas have been lost due to draining of wetlands and the conversion of forests to agricultural lands and urban developments. Most of the development has been in lower elevation areas extensively used by this species. Urban development, in particular, continues to expand at a rapid rate in the southwestern part of the province, including parts of Vancouver Island, Lower Fraser Valley, and the Gulf Islands (see Section on “Habitat Trends”). Within less populated parts of the species’ range (on the northern and western Vancouver Island, Sunshine Coast, and areas northward along the coast), forestry activities are extensive and continue to modify habitats.

In addition to habitat loss and alteration, human activities contribute to increased need for roads and to fragmentation of habitats. Habitat fragmentation is of particular concern for these frogs that undertake seasonal migrations and require forested areas for foraging, in addition to wetland breeding sites. Many amphibian populations are organized spatially as some form of metapopulations, and dispersal is essential for maintaining the viability of such populations (Marsh and Trenham 2001 and references therein). In other areas, habitat fragmentation has been shown to contribute to local declines and disappearances of forest-dwelling, pond-breeding amphibians that rely on dispersal among habitats and subpopulations (R. sylvatica, Ambystoma maculatum; Gibbs 1998). Green (2003) compared population trend data and demographic parameters of a large number of amphibian species and populations and concluded that “curtailment of recolonizations in an obligately dispersing species with highly fluctuating populations and high frequencies of local extinctions, such as pond-breeding amphibians, is likely to be affected rapidly and catastrophically by habitat fragmentation“ (p. 341). These considerations apply to the Red-legged Frog in general, although details of its population fluctuations and dynamics in space and time are unknown.

The loss of habitats to agriculture and urbanization are more or less permanent, whereas the effects of forestry are more temporary, provided that the forest cover is allowed to regenerate and wetlands are not degraded. However, vast tracts of land over much of the species’ Canadian range are affected by forestry. Therefore, the responses of this species to forestry activities and its ability to co-exist on forestry lands are of utmost importance in assessing threats to populations and their vulnerability within the species’ Canadian range.

 

Effects of Forestry Practices

Logging results in generally drier conditions and alters the microclimate and structure of the forest floor, and hydrology of wetlands. The concern is that these changes create unsuitable conditions for frogs, resulting in reduced opportunities for foraging and hibernation, in barriers to dispersal and migration movements, and in fragmentation of habitats. If wetlands are altered or drainage patterns changed, the reproductive success could be affected.  The Red-legged Frog is associated with moist forest conditions and is frequently found in older forests; however, it is not restricted to older forest age-classes (reviewed in Blaustein et al. 1995). In Oregon, Cole et al. (1997) detected no effects attributable to logging, burning, herbicide application on the capture success of the Red-legged Frog; however, capture rates varied greatly both among years and sites, making comparisons problematic. In southern Washington, this species was more abundant in mature than in young forest stands (Aubry and Hall 1991). In another study in Washington, in managed, second-growth Douglas Fir (Pseudotsuga menziesii) forest, this species was more abundant in rotation age stands (50 – 70 years) than in all younger stands, where only a few captures took place (Aubry 2000). These stands also had the highest species richness of amphibians. Aubry (2000) concluded that maximizing the percentage of rotation age forest in managed stands is beneficial for this and other species of amphibians.

In the Clayoquot Sound area on the west coast of Vancouver Island, Beasley et al. (2000) found that the percentages of occupancy of wetlands by aquatic-breeding amphibians, including the Red-legged Frog, was similar in areas disturbed by past logging and in undisturbed areas. However, wetlands surrounded by clearcuts were more likely to dry up in the summer than were those in more undisturbed areas. The relative abundance of amphibians or their survivorship characteristics in the two types of habitats were not investigated.

On northern Vancouver Island, Chan-McLeod (2003b) investigated the effects of clearcut logging on movements of the Red-legged Frog. Radio-tagged frogs were introduced to experimental plots at the clearcut – old growth forest interface, and their movements were monitored for a period of several weeks outside the breeding season in the summer and autumn. New clearcuts (less than 12 years old) posed barriers to movements of the frogs, which moved mostly towards and within the forest. The use of clearcuts for movements varied depending on weather, and permeability of clearcuts was greatest during wet, cool conditions and least during dry, hot conditions. Most movements into the clearcut occurred during rainy days. The frogs failed to use unbuffered (without a fringe of trees) creeks as movement avenues within clearcuts. In another experiment on northern Vancouver Island, Chan-McLeod and Moy (in review) investigated the use of residual trees and patches of trees (ranging in size from 0.07 ha to 2.7 ha) within a logged matrix by this species; the logging patterns were a result of standard variable-retention, operational procedures. Over a 3-day period of the trials, individual frogs that were experimentally released at the bases of residual, single trees or in small tree patches were more likely to leave than were individuals released in larger tree patches. In addition to large patch size, the presence of a stream within the patch was associated with an increased residency time by frogs.  While moving through the logged matrix, individual frogs did not use the residual tree patches as stepping stones, but rather entered the patches more or less at random; directional movements towards a patch occurred only from short (< 20 m) distances away. The authors suggested that residual trees grouped together in patches of 0.8 ha to 1.5 ha, especially when located at sites with streams, facilitate movements of these frogs.

The above studies indicate that logging, whether clearcut or variable-retention, alters movement patterns and poses barriers of varying permeability to movements of the Red-legged Frog. These effects can be mitigated to some degree by adjusting the spatial configuration of cut areas and the size and location of residual tree patches. However, the retention of larger areas of older forest is also desirable and may be essential for the long-term persistence of populations. The effects of logging on foraging success and survivorship of this species have not been studied.

 

Pollutants

Pools, ponds, and other wetland habitats act as sinks for various pollutants, resulting in the exposure of aquatic-breeding amphibians to contaminants during critical periods in the early development (Vitt et al. 1990). Amphibians from heavily cultivated areas show mutagenic effects and developmental abnormalities in eastern Canada (Bonin et al. 1997), and wind-borne agricultural pesticides have been implicated in the population declines of the California Red-legged Frog and several other aquatic-breeding amphibians in California (Davidson et al. 2002). Contamination of aquatic breeding sites with nitrate and nitrite, resulting from run-off of fertilizers and sewage, has recently been recognized as a problem for amphibians (reviewed in Rouse et al. 1999, Halliday 2000). The exposure of early developmental stages to even low concentrations, considered safe for humans, can result in behavioural changes, developmental abnormalities, or mortality (Marco and Blaustein 1999, Marco et al. 1999). Tadpoles of the Red-legged Frog suffered acute effects when raised in water with nitrite (Marco et al. 1999).

In British Columbia, the range of the Red-legged Frog overlaps agricultural and farm lands in the Lower Fraser Valley, and poor water quality appears to affect hatching success (De Solla et al. 2002a). De Solla et al. (2002a) examined survivorship and development of two species, the Red-legged Frog and the Northwestern Salamander, in the intensively farmed Sumas Prairie area where these amphibians occupy drainage canals and other aquatic habitats that are exposed to agricultural run-off. Hatching success of both species was reduced in enclosures within agricultural sites (up to 9% and 34% in two years, respectively) when compared to that within reference sites at the periphery of the agricultural area (85% or higher). However, when reared in the laboratory in water from these sites, the hatching success was not significantly different between the two types of sites. The authors suggested that laboratory conditions failed to adequately mirror conditions in the field, where water quality could be poorer due to constant influx of contaminants, lack of artificial aeration, and variable temperature regimes. They concluded that high ammonia concentrations and low concentrations of dissolved oxygen were likely factors for the observed differences in hatching success under field conditions. In contrast, residues of organochlorine pesticides, which were widely used in the area in the 1970s, and polychlorinated biphenyls (PCBs) appear not to be a problem for this species (De Solla et al. 2002b). Concentrations of these compounds in the eggs of the Red-legged Frog from the Sumas Prairie area were relatively low and similar among agricultural and reference sites.

A wide range of chemical substances present in the environment potentially interferes with hormone signals during sensitive developmental periods of amphibians (Crump 2001). Exposure to sex steroids or their mimics can alter the operational sex ratio and reproductive characteristics, whereas exposure to thyroid hormones or their mimics can alter developmental timing and metamorphosis. Where it occurs in the vicinity of populated areas and farmlands, the Red-legged Frog is potentially exposed to endocrine-disrupting compounds, but whether these substances are a problem for this species has not been studied.

 

Introduced Species

The introduction and spread of non-native species, particularly sport fish and the Bullfrog (Rana catesbeiana), are of concern for the persistence of native amphibian populations throughout western North America and are thought to be a contributing factor to declines in some areas (Hayes and Jennings 1986, Stebbins and Cohen 1995). The modification of habitats by human activities, including altered hydrological and temperature regimes and forest clearing, creates conditions suitable for the establishment and spread of the Bullfrog and other exotic species.  The occurrence of the Red-legged Frog shows a negative correlation with introduced fish in the Puget Lowlands of Washington State, and in enclosure experiments tadpoles failed to survive in the presence of predatory fish (Adams 1999, 2000). Predatory sport fish have been introduced to permanent water bodies throughout the range of the Red-legged Frog in British Columbia, and restocking of lakes remains common. Many of the stocked lakes originally lacked fish; as a result native amphibians using these lakes are poorly adapted to survival in the presence of predatory fish (Wind, manuscript in review).

Both adults and tadpoles of the Bullfrog have been shown to prey on native anurans and their life stages, but the adverse effects of the Bullfrog on the Red-legged Frog are probably largely indirect and involve complex interactions. Using experimental enclosures, Kiesecker and Blaustein (1997, 1998) demonstrated a shift in microhabitat use and activity by Red-legged Frog tadpoles in the presence of Bullfrog larvae or adults: Red-legged Frog tadpoles reduced their activity and increased the time spent in shelters when exposed to Bullfrogs or their chemical cues. Developmental timing and body size at metamorphosis were also altered in the presence of Bullfrogs, but survivorship to metamorphosis was reduced only if the tadpoles were exposed to a combination of factors (either Bullfrog larvae and adults, or bullfrogs and fish, Smallmouth Bass, Micropterus dolomieui; Kiesecker and Blaustein 1998). When exposed to chemical cues from Bullfrogs, Red-legged Frog tadpoles from areas of syntopy (where they had co-occurred with the Bullfrog since its introduction about 60 years previously) showed behavioural differences when compared to tadpoles from allopatric populations; these differences included reduced activity and increased use of shelters. The authors inferred that tadpoles from allopatric populations failed to recognize the Bullfrog as a threat and thus behaved inappropriately in their presence, so increasing their risk of mortality from predation. Kiesecker et al. (2001) showed that the dispersion of food affects interactions of tadpoles of the Red-legged Frog with the Bullfrog: adverse effects on growth and survivorship of the Red-legged Frog in the presence of Bullfrogs occurred only when food was distributed in a clumped pattern but not when it was widely scattered. The above studies emphasize the complexity of the interactions of native anurans with Bullfrogs.

Negative effects of the Bullfrog on native frogs have been inferred from correlations of Bullfrog population increases with population declines of native species (Hayes and Jennings 1986). However, in wetlands in the Puget Lowlands, Washington State, Adams (1999) found that the presence of the Red-legged Frog was more closely correlated with habitat structure, including pond permanence, and the presence of introduced fish than with the presence of the Bullfrog. In enclosure experiments, the survival to metamorphosis of both the Red-legged Frog and the Pacific Treefrog (Pseudacris regilla) in permanent pools tended to be low when compared to temporary pools, regardless of the presence or absence of Bullfrog tadpoles (Adams 2000). Adams (2000) suggested that the effects of the Bullfrog on the Red-legged Frog were largely indirect and might augment other factors, such as changes in hydrology and the presence of both native and introduced fish predators in breeding habitats.

In British Columbia, the Bullfrog is presently found in the Lower Fraser Valley, where they were first recorded in the 1940s, on southern and southeastern Vancouver Island from Victoria north to Parskville, and in the southern Okanagan Valley (Govindarajulu 2003). On the Saanich Peninsula on southern Vancouver Island, a dramatic range expansion of the Bullfrog has occurred within the past six years since monitoring began in 1997. Control efforts, consisting of the removal of adults in an attempt to reduce the breeding population, are in progress along the periphery of the distribution and are hoped to limit further spread of the species. In addition to disturbed areas, Bullfrogs occupy relatively undisturbed water bodies, such as small, wooded lakes in the vicinity of Victoria. A study is in progress to investigate the effects of Bullfrogs on native amphibians, including the Red-legged Frog, through field and enclosure experiments, but complete results were unavailable at the writing of this report (P. Govindarajulu, pers. comm.). Preliminary results show that on the Saanich Peninsula the Red-legged Frog is mainly found in lakes and ponds that lack Bullfrogs, possibly indicating past displacement; the two species presently co-occur in a small number of water bodies in this area.

 

Epidemic Disease

Outbreaks of epidemic disease, including new, emergent diseases caused by chytrid fungi and iridoviruses, are an important, widespread factor threatening amphibian populations. Skin disease caused by a chytrid fungus has been implicated in amphibian declines worldwide and can have devastating effects on populations over a large area over a relatively short period (Daszak et al. 1999). This disease appears to be caused by a single species of the fungus (Batrachochytrium dendrobatidis), which is capable of infecting a wide range of amphibian species (Nichols 2003). A number of species of Rana are known hosts to the fungus, but there are no reports of infections for the Red-legged Frog (Speare and Berger 2002). In British Columbia, this fungus has been isolated from both the Northern Leopard Frog and Oregon Spotted Frog (L. Friis, pers. comm.), but to date no outbreaks of chytridiomycosis has been reported from the province. Other pathogenic microorganisms infecting amphibians include Aeromonas bacteria, which cause red-leg disease in stressed animals, and various pathogenic iridoviruses.

 

Atmospheric Changes

Some amphibians have been shown to be sensitive to exposure to ambient ultraviolet-B radiation (UV-B: 280 – 320 nm in wavelength) during early development, and elevated UV-B levels, resulting from stratospheric ozone depletion and habitat modifications, have been suggested as a contributing factor to amphibian population declines (Blaustein et al.1994a). Hatching success of the Red-legged Frog was unaffected by ambient UV-B levels in studies conducted in Oregon (Blaustein et al. 1996) and British Columbia (Ovaska et al. 1997). However, in the same study, hatching success of this species was reduced under slightly elevated UV-B regimes, while the sympatric Pacific Treefrog (Pseudacris regilla) showed no similar reduction in survival and appeared to be more tolerant. Belden and Blaustein (2002) found that the exposure of embryos of the Red-legged Frog to ambient UV-B levels affected subsequent larval growth and development, although direct mortality did not occur. These effects included smaller body size of exposed tadpoles one month after hatching and retarded early growth rate. The authors concluded that such sublethal effects may already occur in nature under the present, ambient UV-B levels.

Global climate change is predicted to be associated with drier summers, increased incidence of droughts, and alterations in hydrological conditions (Gates 1993). All these factors are expected to stress amphibian populations, influencing their movement and activity patterns, and resulting in loss and deterioration of breeding habitats. Drying of breeding wetlands and increased barriers to movements are likely to be most important effects of global climate change to populations in British Columbia. Water temperatures would have to increase considerably to directly affect embryonic survival, even of cold-adapted species such as the Red-legged Frog, in British Columbia (see section on Physiology). However, these northern populations will become increasingly important as reservoirs of genetic variation, if declines occur farther south within the species’ range.

 

Interactions and Synergistic Effects

Under natural conditions, individual stressors rarely act alone but interact with other stressors and background conditions, which modify and sometimes enhance their effects. For example, Kiesecker and Blaustein (1998) experimentally demonstrated synergistic interactions between two invasive organisms (the Bullfrog and fish) and between different life-stages of the Bullfrog (adults and larvae) on survivorship of the Red-legged Frog. Synergistic effects have been shown to occur for various anurans between pathogenic fungi and UV-B radiation (Kiesecker and Blaustein 1995) and among various pollutants, the competition intensity, the predator environment, and the hydroperiod of the breeding sites (Boone and Semlitch 2001, 2002). Global climate change is likely to accentuate habitat fragmentation and interact with other threat factors.

Other amphibians that are sympatric with the Red-legged frog have exhibited declines. The Western Toad, Bufo boreas, has disappeared from several localities on Vancouver Island for reasons that are largely unknown (Davis and Gregory 2003), however there is no evidence of similar declines of the Red-legged frog from these localities. The Oregon Spotted Frog, Rana pretiosa, has declined throughout its range in western North America, including British Columbia. It is presently known from only a few localities in the Lower Fraser Valley. Loss and degradation of breeding habitats due to agriculture, urban developments, and other human activities and invasion of aquatic habitats by introduced, invasive plants and animals are thought to be largely responsible for the declines of this species in British Columbia (Haycock 2000).