Northern Leopard Frog
Over the last 35 years, Northern Leopard Frog
(Rana piplens) populations have declined dramatically over much of the
species' range in North America. Abrupt population declines were first noted in Alberta in 1979. Since then, populations appear to have been
extirpated over much of central Alberta and are absent or greatly reduced in southern Alberta. Only a handful of viable breeding populations currently remain in southeastern Alberta. Because of its virtual disappearance from the province, the Northern Leopard Frog has been designated as an
endangered species under the Alberta Wildlife Act.
Since 1990, considerable effort has been expended trying to locate Northern Leopard Frogs in Alberta. Known breeding populations have also been routinely monitored and studies have been carried out on two of the breeding populations to provide more information on dispersal and the general
ecology of the species in the province.
The Northern Leopard Frog requires a mosaic of habitat types to meet the annual requirements of all life history stages. Generally, separate sites are used for breeding and overwintering. However, in some cases breeding and overwintering may occur in the same pond. This is particularly true in
spring fed wetlands. In Alberta, Northern Leopard Frogs are typically associated with clear water that is relatively fresh to moderately saline. Other parameters of water quality, however, vary widely between sites.
Breeding occurs in shallow and warm standing water associated with permanent and
semi-permanent wetlands, springs, dugouts, borrow pits, lakes, beaver ponds, and the backwaters and oxbows of rivers. Temporary ponds and shallow lakes that are unsuitable for fish, and that contain water until late July or August are considered to be the most favourable spawning sites for Northern Leopard Frogs. Most breeding ponds contain a mixture of open water and emergent vegetation. Although at some sites in Alberta, breeding ponds are totally encompassed by emergent vegetation. Leopard Frog tadpoles are generally poorly adapted to cope with currents and thus can develop successfully only in slow reaches of streams or backwaters.
Following breeding and transformation, adult, juvenile and young-of-the-year frogs move to summer feeding areas. Most often, these areas are located along the margins of
water bodies. Smaller, immature frogs appear to be more closely tied to water; adults will occasionally venture some distance from water in the summer. This difference may be related to the greater surface-area-to-volume ratio of smaller frogs, which makes them more prone to desiccation.
Preferred summer feeding habitat generally consists of open and semi-open areas with shorter vegetation. Areas lacking vegetation or with closely-clipped vegetation, such as found on heavily-grazed pasture, are avoided by frogs. Areas with tall, dense marsh vegetation, grasses or extensive shrub cover are also avoided, particularly by smaller frogs.
Unlike most of Alberta's other species of amphibians, Northern Leopard Frogs overwinter in water. Overwintering frogs require well-oxygenated water that does not freeze to the bottom during the winter.
Hibernacula are most often located in springs, streams, spillways below dams, or in deeper lakes and ponds. Frogs have been found hibernating under rocks, logs, leaf litter or vegetation, or in depressions in sand or mud. Shallow breeding ponds and lakes are unsuitable for overwintering because of the depletion of oxygen in the water column, which can lead to winterkill. In southern Alberta, springs appear to provide critical overwintering habitat during periods of drought when deeper permanent water sources become limited.
Northern Leopard Frogs emerge from overwintering sites shortly after the ice begins to melt and narrow strips of open, warmer water form along the shoreline. Increasing temperature probably acts as a stimulus, causing hibernating frogs to become more active and to move from deeper overwintering areas to shallow water areas along the shore. It has been noted that migration from overwintering sites begins when water temperatures reach 9 to
12 degrees Celsius. Short overland migrations may be made between overwintering sites and breeding ponds. In some instances, breeding may also occur in overwintering habitats.
The timing and duration of breeding also appears to be temperature dependent. In Alberta, breeding occurs from late April to late June and breeding behaviour may occur while ice is still on
ponds. Cooler air temperatures can suppress breeding activity, resulting in two or more mating periods and several size classes of larvae.
Sexually-mature males arrive at the breeding pond first and call while floating on the surface of the water. Males typically congregate in small groups in the warmest parts of the breeding ponds. Sexually-mature females are attracted to the calling of the males and arrive within a few days to a few weeks after the males. Egg laying occurs shortly thereafter. Egg masses are typically laid in shallow water on vegetation, branches, and, rarely, on the pond bottom.
Females are believed to produce only one clutch per year and it is thought that all eggs are laid in one mass. Egg masses may contain between 600 and 7,000 eggs.
The time of hatching is variable and is also dependent on water temperature. Cool weather can extend hatching to 14 days or more after egg laying. Factors such as flooding,
droughts or periods of cold temperatures can produce high mortality of eggs, including failure of entire clutches.
Tadpoles remain close to egg masses after hatching but disperse after a few days. Time to metamorphosis is temperature, and, perhaps, density dependent, ranging from 60 to 90 days and may occur over several weeks at a single site. Mortality, however, can be complete if breeding ponds dry up before tadpoles become fully transformed. In Alberta,
transformation generally occurs in July and early August.
Annual adult mortality has been estimated to be approximately
Following breeding, adults may remain in the vicinity of breeding ponds or disperse to summer feeding ranges. Usually these feeding ranges are located near water. Adults appear to establish home ranges in the summer. In late summer or early fall, adults begin to migrate from summer feeding areas to overwintering sites. Northern Leopard Frogs are active in Alberta until October, and exceptionally into November. Little is known about the overwintering ecology of Northern Leopard Frogs.
Following transformation, young-of-the-year frogs disperse away from natal ponds.
Reprinted from Alberta Wildlife Status Report No. 9
with permission from Alberta
Sustainable Resource Development.