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Northern Leopard Frog

Limiting Factors

Northern Leopard FrogOver the last 30 to 50 years, amphibian populations have declined dramatically around the globe, a relatively large number of species are in jeopardy. This is true of many species found in Canada, including the Northern Leopard Frog. In this, and other, species, declines can often be explained in relation to natural population fluctuations, extremes in climatic conditions or anthropogenic factors such as habitat loss, acidification, contaminant releases and the introduction of non-native species. However, in many other cases the declines cannot be explained and have occurred in relatively pristine environments or in areas where there has been no recent obvious disturbance. Such declines are often characterized by abrupt, massive and widespread population declines of one or more species. At the same time, populations of other amphibian species occupying the same area and similar habitats show no signs of decline.

In the past few years, comprehensive research and monitoring programs have been initiated world-wide to determine the extent and causes of amphibian declines. Among the factors being examined are disease, the reduction in the ozone layer and accompanying increase in ultraviolet radiation, changing weather patterns and climatic extremes, acid rain and chemical contaminants, the introduction of exotic species, and habitat loss or fragmentation. The following sections outline factors, which may be applicable to declines in Northern Leopard Frog populations in Alberta including Climate, Disease, Habitat Loss and Fragmentation, Livestock Activity, Road Kills, Water Management, Harvesting, Introduction of Game Fish and Exotic Species, as well as Contaminants and Wetland Acidification.

Climate:  Climatic conditions have a major influence on Northern Leopard Frog populations. In general, during wet years there will be increase in the amount of breeding and overwintering habitat allowing populations to expand and providing opportunities for dispersal and colonization of unoccupied areas of suitable habitat. In drier years, available habitat will diminish causing a reduction in population levels. Some local populations will also become extinct because of a lack of habitat. For example, drought in the 1930s caused a general decline in amphibian populations in southern Alberta. Drought has been suggested as a possible reason for the decline and disappearance of Northern Leopard Frog populations in Washington, Oregon, Idaho, Montana and Colorado.

During the late 1970s and 1980s, drought conditions prevailed over much of southern Alberta. The accompanying loss of wetland habitat probably caused general population declines and the extinction of some local populations in that area during this period. Drought, however, has been dismissed as the cause of the abrupt and widespread population declines observed in southern Alberta since 1979.

Disease:  Mass mortality of amphibians from disease is not uncommon and may be a natural feature of the biology of a species, or it may be induced by environmental stressors. Amphibians are known to be susceptible to a variety of diseases including many diseases of fish. Mortality in Northern Leopard Frogs has often been associated with the condition of "red leg", which is not a disease itself but rather a condition of kidney failure. It is often associated with infection by Aeromona hydrofila, a naturally-occurring and widespread bacterium. Ordinarily, this pathogen only affects individuals whose immune systems have been weakened by stress, and not entire populations. 

However, there is little conclusive evidence that disease was responsible for the abrupt and dramatic population declines observed in Alberta and elsewhere. The lack of calling males in the Red Deer area during the spring of 1979 suggests that mortality occurred in the fall or winter. This is similar to the pattern of mortality observed in Wisconsin. Incidences of overwintering mortality were noted in the Clearwater River in 1976. Similar to other locations, mortality in the Clearwater River was associated with the condition of "red leg". In this case, however, only individuals were affected and healthy populations persisted in the area for the next two years.

PondHabitat Loss and Fragmentation:  Habitat loss and fragmentation can impact Leopard Frog populations at two levels.  Locally, populations are dependent on a variety of habitat types to meet the annual requirements of various life history stages. The loss of any one of these habitats or the impairment of movement between habitat types could result in the demise of the entire population. Relative to many other amphibians, this requirement for a seasonal mosaic of habitat types makes Northern Leopard Frogs particularly vulnerable to habitat loss or alteration. On a regional basis, many amphibian populations exist as metapopulations, represented by a set of linked but geographically discrete local populations occupying suitable habitats. Local populations will fluctuate because of environmental factors and natural stochastic mechanisms and local extinctions may occur. But regionally, populations will be maintained through dispersal of individuals between populations and recolonization of vacant habitat. However, habitat loss can result in populations becoming isolated or separated by greater distances.  This can limit immigration from neighbouring populations, which can lead to a decrease in the fitness of individuals in the isolated population because of reduced gene flow and increase the likelihood that the isolated population will become extinct because of random population fluctuations.

Over the last half of this century, wetland drainage has been extensive in southern Alberta.  For example, it is estimated that 60% of the wetlands have been lost in the aspen parkland, and over 90% of wetland margins in the prairie and parkland have been impacted by agricultural activities. The extent to which wetland loss and alteration have impacted Northern Leopard Frog population in the province is unknown, but in some areas population declines have no doubt been substantial.

Livestock Activity:  Livestock have been present at a number of sites in southern Alberta where Leopard Frogs have been found, and livestock disturbance is considered a threat to frog populations at Prince's Spring, Empress Creek and Sexton Creek. Livestock activity has resulted, or could result, in the following disturbances: 1) grazing and trampling have reduced vegetation cover and created drier soil conditions in riparian areas, reducing the quality of summer habitats; 2) cattle trampling along the shores and in the water have caused an increase in water turbidity, which could have negative effects on tadpole and egg development; and 3) cattle movements along the shoreline and wetlands could potentially dislodge or trample egg masses. Livestock defecation in or around overwintering ponds could result in increased nutrient loading in the pond and increase the likelihood of winterkill.

Road Kills:  In Minnesota and Ontario, road kills account for the death of a large number of Northern Leopard Frogs, particularly in the spring and fall when frogs are moving between overwintering and breeding sites. Until recently, road kills were not thought to be a major concern in Alberta.

Water Management:  The raising and stabilization of water levels to enhance recreational activities or to improve habitat for other species, such as fish or waterfowl, can lower the habitat potential for frogs by reducing the amount of wetland vegetation. Drawdowns of wetlands for vegetation management can impact tadpoles if the drawdown occurs before transformation. Similarly, flooding of artificial wetlands in the spring could dislodge eggs. Dams can also reduce winter stream flow levels, which could increase overwintering mortality.

There are concerns that the destruction of beaver dams could reduce available Leopard Frog habitat in the Cypress Hills.

Harvest:  Throughout its range, the Northern Leopard Frog has been collected for teaching and research needs, bait and even food. In Alberta, commercial harvest has long been illegal, so the extent of collecting has probably been limited to the capture of frogs for fish bait and to the collecting and raising of tadpoles and frogs by children. In the past, these were probably relatively benign activities, but with the recent population declines, any loss must be considered unacceptable. The collecting of frogs by children has been noted recently in Medicine Hat and at Writing-on-Stone Provincial Park.

With the recent designation of the Northern Leopard Frog as an endangered species it is now illegal to collect frogs for any purposes. With the recent designation, ongoing efforts will have to be made to inform the public, particularly children, that collecting could be detrimental to frog populations.

Introduction of Game Fish and Exotic Species:  In many parts of western North America, declines of amphibian populations have been linked to the introduction of game and other species of fish. Fish introductions are also thought to pose a threat to Northern Leopard Frogs. The species is believed to select against fish-bearing water bodies for spawning as many fish species are known to prey on tadpoles. Game fish are also known to prey on overwintering frogs. 

The extent to which sport fish introductions have impacted Leopard Frog populations in the province is unknown, but the matter is a cause for concern. This could potentially have had dire consequences for the important Leopard Frog population occupying the site.

Contaminants and Wetland Acidification:  Because of their use of subcutaneous respiration, use of water for egg laying, reliance on both terrestrial and aquatic environments, and the different positions of larvae and adults in the food chain, amphibians are particularly vulnerable to a variety of contaminants, including pesticides. There is, however, no evidence linking Northern Leopard Frog declines in Alberta to use of pesticides or other toxic compounds. Research in Wisconsin also failed to establish a link between pesticide use and population declines, although research has pointed out the need for further study into the possible role of herbicides in the decline.

The distribution of many amphibian species is related to the acidity of wetland habitats, and concerns have been raised that acid rain could lower the pH of wetlands causing population declines in many species. Laboratory studies have shown that wetland acidification can lead to increased mortality in embryos and disrupt sodium and chloride balances and increase the rate of heavy metal uptake in larvae. Decreased pH levels can also have indirect effects by altering food chain dynamics and habitat characteristics in wetlands.

Reprinted from Alberta Wildlife Status Report No. 9 (1997), with permission from Alberta Sustainable Resource Development.

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