Northern Leopard Frog
|Taxonomic Notes: This species was placed in the genus Lithobates by Frost et al. (2006). However, Yuan et al. (2016, Systematic Biology, doi: 10.1093/sysbio/syw055) showed that this action created problems of paraphyly in other genera. Yuan et al. (2016) recognized subgenera within Rana for the major traditional species groups, with Lithobates used as the subgenus for the Rana palmipes group. AmphibiaWeb recommends the optional use of these subgenera to refer to these major species groups, with names written as Rana (Aquarana) catesbeiana, for example.|
H. Vannoy Davis
© 2005 California Academy of Sciences (1 of 40)
Rana pipiens Schreber, 1782
James C. Rorabaugh1
1. Historical versus Current Distribution. Northern leopard frogs (Rana pipiens) occurred historically from Newfoundland and southern Québec, south through New England to West Virginia, and west across the Canadian provinces and northern and central portions of the United States to British Columbia, Oregon, Washington, and northern California (Stebbins, 1985; Conant and Collins, 1998). The species has been introduced at Lake Tahoe, California (Jennings and Hayes, 1994b), western Newfoundland (Maunder, 1997; Conant and Collins, 1998), Vancouver Island (Alberta Fish and Wildlife Division, 1991), and elsewhere. In the southwestern states, the species generally occurs at higher elevations, such as the mountains of northern and central Arizona and New Mexico. Geographic variation is discussed by Moore (1944, 1949a,b) and Pace (1974), however some morphs described by these authors have since been erected as separate species. Both burnsi (lacking dorsal spots) and kandiyohi (specked appearance) color morphs are known from areas centered in western Minnesota (Merrell, 1965; see also Dapkus, 1976; McKinnell et al., 2002).
Although northern leopard frogs presently occur throughout most of their historical range, population declines and loss since the 1960s (Gibbs et al., 1971) or earlier have resulted in extirpation from some areas, particularly in the western 2/3 of the species’ range. In Canada, populations of northern leopard frogs have declined dramatically in British Columbia (Orchard, 1992), and as of 2002 they were only known to be extant at a single site (Creston Valley; British Columbia Frogwatch Program, 2002). The species has disappeared from central Alberta and is greatly reduced in southern Alberta (Roberts, 1992; Seburn, 1992; Wagner, 1997; Takats and Willis, 2000), where declines were first noted in 1979 (Roberts, 1981). In Saskatchewan, populations reached a low in the early to mid 1970s (Seburn, 1992; Didiuk, 1997). The range of the species has contracted in Manitoba (Vial and Saylor, 1993), where a die-off in 1975–‘76 resulted in “heaps” of dead and dying frogs up to 1 m high on the shores of frog ponds (Koonz, 1992). Declines have occurred in northern Ontario (Oldham and Weller, 1992) and the Richelieu River Valley of Québec (Gilbert et al., 1994). However, populations in New Brunswick, Nova Scotia, Prince Edward Island, and Northwest Territories show no evidence of decline (Green, 1997c; McAlpine, 1997a). Introduced populations in western Newfoundland apparently have disappeared (Maunder, 1997a). Reports of extirpations and range contractions are common in the western United States, where the species has disappeared from about 95% of their historical range in California (M.R. Jennings, 1995) and most historical localities in Washington, where they are probably now limited to two sites in the Crab Creek drainage of Grant County (Leonard et al., 1999; McAllister et al., 1999). In Arizona, northern leopard frogs are absent from most historical localities (Clarkson and Rorabaugh, 1989; Sredl et al., 1997; Meyer and Mikesic, 1998). Northern leopard frogs may be extirpated from historical localities in Oregon (St. John, 1985; Storm, 1986; Leonard et al., 1993), apparently are absent over much of western Montana (Reichel and Flath, 1995; Werner and Plummer, 1998), and may be extirpated from the middle Rio Grande Valley of New Mexico and Texas (Jackson, 1992; Degenhardt et al., 1996) and portions of eastern and north‑central Colorado, including Rocky Mountain National Park (Corn et al., 1989, 1997; Stebbins and Cohen, 1995). In the western states, declines or extirpations have also been noted in Nevada (Panik and Barrett, 1994; R. Jennings, personal communication), southeastern Wyoming (Corn et al., 1989; Stebbins and Cohen, 1995), Grand Teton National Park (Koch and Peterson, 1995), areas of Utah outside Cache and Kane counties (L. Colburn, personal communication), Targhee and Caribou National Forests, Idaho (Clark et al., 1993; Burton, 2001), and southwest of Twin Falls, Idaho (McDonald and Marsh, 1995). Northern leopard frogs are relatively rare and appear to have declined in the Greater Yellowstone Ecosystem (Patla et al., 2000). Call count survey results suggest that either the range of northern leopard frogs has contracted in North Dakota or current sampling techniques are inadequate to detect frogs (Bowers et al., 1997). No reports of declines or extirpations are known from South Dakota or Nebraska (Vial and Saylor, 1993; but see McLeod, 2002). Declines and extirpations were first observed in portions of the Midwest (Wisconsin, Michigan, and northeastern Illinois) in the late 1960s or early 1970s (Rittschof, 1975; Hine et al., 1981; Mierzwa, 1998a), at the same time declines were noted in Manitoba and Saskatchewan. Northern leopard frogs apparently declined in Indiana from 1948–‘93 (Minton, 1998) and in northwestern Indiana, possibly due in part to habitat degradation (Brodman, 2002). The species has declined in Ohio in Hamilton County (Davis et al., 1998) and east of Cleveland away from the lake plain (Matson, 1999). In the northeastern states, northern leopard frogs are still common, but have been extirpated in developed areas such as New Haven, Connecticut; Cuttyhunk Island, Massachusetts; and Providence, Rhode Island (Klemens, 1993). Populations may be recovering in some areas (Seburn, 1992; Stebbins and Cohen, 1995; Didiuk, 1997; Casper, 1998). Northern leopard frog populations and habitats are dynamic (Lannoo, 1998a; Skelly et al., 1999), and local extirpation or apparent declines may not be permanent. Thus, the relationship of reported declines or local extirpation to the long‑term viability of the species on a regional or rangewide basis is often unclear (Corn, 1994a; Green, 1997c). However, consistent, widespread reports of population loss are alarming, particularly in the western but also the central portions of the species’ range.
2. Historical versus Current Abundance. Much information exists concerning historical versus current distribution, however, relatively few data are available to compare historical versus current abundance at extant localities. At the two localities at which northern leopard frogs have been seen in recent years in California, only one individual was observed at one site and 8–10 at the other during 1990–‘94 (Jennings and Hayes, 1994b). In Arizona, northern leopard frogs are relatively common in livestock tanks at and near Stoneman Lake, Coconino County, but are scarce elsewhere (M. Sredl, personal communication). At Rocky Flats Environmental Technology Site, Colorado, northern leopard frogs were only recorded at one site, where they exhibited a relatively low vocalization index (Nelson, 1999). Lannoo et al. (1994) report that since 1900, northern leopard frogs have declined by 2–3 orders of magnitude in Dickinson County, Iowa. However, in northeastern Illinois, the species was considered "exceedingly abundant” (Kennicott, 1855), is still common today, and numbers of northern leopard frogs at Glacial Park, Illinois, appear to be on the increase (Mierzwa, 1998a). The species is rare in Missouri (Vial and Saylor, 1993). Leopard frog harvests in Minnesota have declined from 100,000 lbs/yr in the early 1970s to approximately 1,000–2,000 lbs/yr in recent years (Moriarty, 1998), suggesting population declines. Breckenridge (1944) noted northern leopard frogs were scarce in the prairies of western Minnesota during a drought in 1931–‘34, but populations rebounded rapidly with the return of rain. At the Edwin S. George Reserve in Michigan, northern leopard frogs were abundant and green frogs (Rana clamitans) were rare in the 1950s; from the 1970s through 1992, the reverse was true (Collins and Wilbur, 1979; Skelly et al., 1999). Northern leopard frogs are one of the least abundant amphibians in Jasper County, Indiana (Brodman and Kilmurry, 1998). Northern leopard frogs are common, but declining in Wisconsin (Mossman et al., 1998). In the Great Lakes region, northern leopard frogs currently are uncommon or rare in many places, but can be locally abundant (Harding, 1997). In New England, northern leopard frogs may be the predominant species in a narrow habitat strip, but are rare regionally (Klemens, 1993). Hinshaw (1992) notes that in Maine the species is typically less abundant than pickerel frogs (Rana palustris). Northern leopard frogs were common in southwestern Ontario, but declined from 1992–‘93 (Hecnar, 1997). They are considered uncommon and of restricted distribution in the Northwest Territories (Fournier, 1997); and in Québec, northern leopard frogs are widespread but not abundant (Lepage et al., 1997). Predicting local abundance and distribution requires knowledge of the landscape in terms of all the habitat needs of northern leopard frogs, which include overwintering, breeding, and upland post-breeding habitats, as well as corridors among them (Pope et al., 2000).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
&nb sp; i. Breeding migrations. In New England and Michigan, northern leopard frogs occupy grassy areas or damp wooded areas well away from water in summer, but breed and hibernate in aquatic sites (Wright, 1914; Noble and Aronson, 1942; Dole, 1965a; Merrell, 1970; Hinshaw, 1992; Klemens, 1993). Frogs migrate to breeding areas following emergence from hibernation (Dole, 1968). In Minnesota, sexually mature frogs move up to 1.6 km or more from hibernacula to breeding sites. Migration occurs at night or during the day when air temperatures approach 10 ˚C. Juvenile frogs tend to remain at hibernacula, which are relatively large, deep bodies of water (Merrell, 1970). Breeding and overwintering may occur in the same pond (Wagner, 1997). There is little evidence for breeding migrations in western North America (but see Simmons, 2000).
&nb sp; ii. Breeding habitat. Northern leopard frogs breed in a variety of aquatic habitats, such as quiet or slow‑moving water along streams and rivers, wetlands associated with lakes or tidal areas, permanent or temporary pools, beaver ponds, and human‑constructed habitats such as borrow pits, agriculture, and cattle ponds (Zenisek, 1963; Roberts, 1981; Klemens, 1993; Degenhardt et al., 1996; Wagner, 1997; Orr et al., 1998; Werner and Glennemeier, 1999). Permanent water is not necessary for breeding (Collins and Wilbur, 1979; Bonin et al., 1997b), and agricultural ditches and flooded fields may serve as breeding sites in the Midwest (Walker, 1946; Smith, 1961). However, at the Edwin S. George Reserve, Michigan, northern leopard frogs were never found to breed in temporary ponds and were almost never found at ponds in closed-canopy forests. In that area, location and number of breeding populations and habitats were dynamic. However, ponds occupied during 1967–’74 and 1988–’92 tended to be closer to potential source populations than ponds that were never occupied (Skelley et al., 1999). Near Ottawa, Ontario, mean pH, the amount of spawning habitat, amount of post-breeding summer habitat within 1 km, and the number of sites with leopard frogs calling within 1.5 km of a core breeding pond all contributed substantially to explaining observed variability in relative frog abundance at core ponds. Relative abundance was higher in core ponds with lower mean pH (Pope et al., 2000).
Northern leopard frogs may be absent or rare where large populations of bullfrogs (Rana catesbeiana) occur (Hammerson, 1982b; Jennings and Hayes, 1994b; Lannoo et al., 1994; Davis et al., 1998). Emergent or submergent vegetation may enhance habitat for oviposition and cover during the breeding season (Jennings and Hayes, 1994b). Waters no more acidic than pH 6.0 are optimal for fertilization and early development (Schlichter, 1981). In the Pacific Northwest, northern leopard frogs do not breed in bodies of water devoid of vegetation (Nussbaum et al., 1983). In Arizona, these frogs breed most often in constructed earthen cattle tanks (Sredl and Saylor, 1997). In New Mexico, northern leopard frogs breed in irrigation ditches in addition to natural aquatic sites (Degenhardt et al., 1996). In the Rocky Mountains of Colorado, breeding occurs in natural or mamade lakes and ponds (Corn and Fogelman, 1984). In Wisconsin, northern leopard frogs breed in many habitats, but are most common in open country (Mossman et al., 1998). Orr et al. (1998) found breeding northern leopard frogs in northeastern Ohio often associated with pools or ponds in early stages of succession and typically where fish were absent. Increasing beaver populations and the ponds they create in New Brunswick have benefited northern leopard frog populations (McAlpine, 1997a). In southwestern Ontario, absence of fish was associated with presence of northern leopard frogs (Hecnar, 1997). In Québec, habitats with the greatest numbers of egg masses, in decreasing order of importance, were a wet meadow, a shallow marsh, and an abandoned (unplowed) meadow (Gilbert et al., 1994). In agricultural areas of southern Québec, northern leopard frogs were closely associated with wooded habitat strips (Maisonneuve and Rioux, 2001). At Lac Saint Pierre, Québec, high densities of frogs were found in habitats that were close to the marsh line, had a tall herbaceous stratum and high richness, and had low moss cover (Beauregard and Leclair, 1988). Calling northern leopard frogs are associated with both permanent and intermittent water bodies in Québec (Bonin et al., 1997b).
Breeding is generally restricted to a relatively short period in the spring, but Scott and Jennings (1985) reported eggs and small tadpoles during April–July and September–October in New Mexico. Breeding occurs later at higher elevations (Corn and Livo, 1989) and in northern areas as well (Breckenridge, 1944). In Colorado, initial breeding activities seemed to be related more to temperature than to precipitation. Oviposition followed the onset of male chorusing by 2–3 d and corresponded to periods of warm weather (Corn and Livo, 1989). In Wisconsin, eggs are found from early April to May (Watermolen, 1995). Breeding in the northeastern states occurs in March–May (Klemens, 1993). In Québec, northern leopard frogs were found calling from 2 May–5 July, with peak calling from 2–23 May (Lepage et al., 1997). In Glen Canyon, Arizona, northern leopard frogs probably deposit egg masses during late April to early May (Drost and Sogge, 1993).
&nb sp; i. Egg deposition sites. Egg masses are deposited in a tight, oval mass, and several egg masses may be communally attached to emergent vegetation or, less commonly, may lie on the pond bottom in shallow, slow‑moving, or still water (Wright, 1914; Breckenridge, 1944; Wright and Wright, 1949; Corn and Livo, 1989; Gilbert et al., 1994; Degenhardt et al., 1996). Based on a review of pertinent literature, Pope et al. (2000) characterized northern leopard frog preferred spawning habitat as sites with non-acidic water, 10–65 cm water depth in full sun, usually on the north side of the pond, and with emergent, non- broad-leaved vegetation for attachment of egg masses.
&nb sp; ii. Clutch size. Egg masses are reported to contain between 645–7,648 eggs (Livezey and Wright, 1947; Hupf, 1977; Corn and Livo, 1989; Gilbert et al., 1994; Watermolen, 1995). Post‑spawning ovarian quiescence lasts 1–2 mo in the northeastern states. In Wyoming and Colorado, females probably do not produce multiple clutches (Corn and Livo, 1989). Time to hatching is correlated with temperature and ranges from 2 d at 27 ˚C to 17 d at 11.4 ˚C (Nussbaum et al., 1983; see also Volpe, 1957b).
&nb sp; i. Length of larval stage. Variable depending on latitude and weather, with metamorphosis typically occurring approximately 3–6 mo following egg deposition (Merrell, 1977; Hinshaw, 1992). Time from hatching to metamorphosis is about 50 d (7 wk) in northwestern Iowa (Lannoo, 1996). Crowding and interaction may reduce growth rates of tadpoles where they are especially dense, in hatchling aggregations, and in pools in the final stages of drying (Richards, 1958; West, 1960; Gromko et al., 1973; John and Fenster, 1975; Alford, 1999). In the laboratory, high tadpole density or limited availability of food slowed both growth and developmental rates. Blocking corticoid synthesis reversed growth suppression caused by high density, but did not alter effects of density on developmental rates (Glennemeier and Denver, 2002). Overwintering of tadpoles has been documented in Arizona (Collins and Lewis, 1979) and Nova Scotia (DeGraaf and Rudis, 1983).
&nb sp; ii. Larval requirements.
&nb sp; &nb sp; a. Food. Tadpoles feed on attached and suspended algae, other attached plant and animal material, and occasionally on dead animals (e.g., Hendricks, 1973; Seale, 1982a).
&nb sp; &nb sp; b. Cover. In Michigan, northern leopard frog tadpoles survive and grow much better in open-canopy ponds as opposed to closed-canopy ponds. Addition of food to closed-canopy ponds dramatically increases tadpole survival and growth. Closed-canopy ponds tended to be slightly cooler, and pH and dissolved oxygen are lower (Werner and Glennemeier, 1999). Northern leopard frog tadpoles are poorly adapted for life in fast‑moving water, and thus are most often found in quiet backwaters and pools (Roberts, 1981). Gulping of air at the pond surface begins to increase when dissolved oxygen drops below about 4 ppm (Wassersug and Seibert, 1975). Aerial respiration is more energetically expensive than aquatic respiration and can slow growth and development if food is not abundant (Feder and Moran, 1975). Tadpoles are active during the day and exhibit behavioral thermoregulation (Casterlin and Reynolds, 1978; see also Noland and Ultsch, 1981). Tadpoles do not school per se, and laboratory observations using spatial affinity as a recognition assay could not demonstrate sibling recognition (Fishwald et al., 1990). Determinants of larval behavior include responses to both intrinsic (body size, age) and extrinsic (group size) cues. Tadpoles in pairs or groups generally are more active and occur more often in the open water column than lone tadpoles. Younger tadpoles tend to be less active and use the open water column less than older tadpoles (Golden et al., 2001).
&nb sp; iii. Larval polymorphisms. None described.
&nb sp; iv. Features of metamorphosis. Metamorphosed frogs are 18–50 mm SUL (Merrell, 1977; Nussbaum et al., 1983; Leclair and Castanet, 1987; Hinshaw, 1992; Degenhardt et al., 1996).
&nb sp; v. Post-metamorphic migrations. Newly metamorphosing northern leopard frogs disperse away from their breeding wetlands. Frogs can move up to 800 m in 2–3 d (Dole, 1971) and have a tendency to move to the edges of permanent bodies of water (Merrill, 1977; Cochran, 1982). During the metamorphic period, at least a few frogs will leave wetlands every night; mass emigrations can occur following heavy rains (Bovbjerg, 1965; Dole, 1971). Dole (1972a) cites evidence for celestial orientation in newly metamorphosed northern leopard frogs.
D. Juvenile Habitat. During post‑metamorphic dispersal in summer, juvenile frogs can be found in upland forests and meadows, whereas adult frogs remain closer to water (Dole, 1971; see also Dole, 1972a,b). Juvenile frogs are also sometimes encountered at temporary bodies of water (Smith, 1961).
E. Adult Habitat. Adults require suitable habitats for breeding, upland foraging areas after leaving the breeding ponds as well as sites for overwintering (see “Breeding habitat,” and "Seasonal migrations" above, “Torpor [Hibernation] " below), and, where these habitats are different, require habitat corridors connecting them. Upland areas used as post-breeding habitat in summer are typically grassy areas, meadows, or fields but can include other sites such as peat bogs and perennial forage crops (e.g., grass, alfalfa, and clover). Post-breeding summer habitats usually do not include barren ground, open sandy areas, heavily wooded areas, cultivated fields, especially those that have been cut recently, heavily grazed pastures, or closely mowed lawns (Wright and Wright, 1949; Werner and Glennemeier, 1999; Pope et al., 2000; Mazzerole, 2001). Northern leopard frogs in the relatively arid landscapes of southwestern Alberta may be limited by lack of suitable upland post- breeding habitats and lack of dispersal opportunities (Roberts, 1992). Near Chicago, despite presence of abundant suitable breeding ponds, northern leopard frog abundance was low. However, after grasslands were restored around ponds, frog populations increased dramatically (K.S. Mierzwa, personal communication, in Pope et al., 2000). Orr et al. (1998) suggest that in northeastern Ohio, the northern leopard frog is a colonizing species that takes advantage of habitats in early stages of succession but does poorly in later successional stages where predation and competition are more intense. Allozyme frequency and rapid amplified polymorphic DNA analysis of northern leopard frog populations in Utah and Arizona showed relatively large differences among populations, suggesting microgeographic differentiation was occurring and that each population may be locally adapted (Kimberling et al., 1996a,b). In a rural landscape of agriculture and fragmented wetlands and forests in Indiana, northern leopard frogs were likely to occur in more isolated forest patches that were in close proximity to wetlands. A sensitivity to isolation from wetlands suggested an important role of recolonization in the distribution of the species (Kolozsvary and Swihart, 1999). Larger leopard frogs may be less susceptible to desiccation and more capable of using arid environments such as mined peat bogs (Mazerolle, 2001). In agricultural and urbanized regions of southwestern Minnesota, reduced landscape connectivity caused by habitat fragmentation and loss negatively affected amphibian assemblages that include northern leopard frogs (Lehtinen et al., 1999).
F. Home Range Size. In Michigan, Dole (1965b) found that daily movements of adults are usually < 5–10 m in wet pastures and marsh, but home range was not calculated. Both adults and juveniles wander widely during wet weather (Dole, 1971). Home ranges may include breeding sites, hibernacula, and upland foraging and dispersal areas (Merrell, 1970). Displaced northern leopard frogs will home and apparently use olfactory and auditory cues, and possibly celestial orientation, as guides (Dole, 1968, 1972a).
G. Territories. Because egg masses are typically confined to an area much smaller than that occupied by calling males, Pace (1974) suggested that relatively few males are involved in fertilizing egg masses. Short, terminal sounds in the call sequence have an aggressive or spacing function among males (Pace, 1974). These observations suggest territoriality among calling males.
H. Aestivation/Avoiding Dessication. Northern leopard frogs apparently are active throughout warm periods and seasons. No periods of aestivation have been described. Dole (1967) described the role of dew and substrate moisture in the water balance of northern leopard frogs. Maximum thermal tolerances were addressed by Hutchison and Ferrance (1970).
I. Seasonal Migrations. Four types of seasonal migrations have been described, as follows: (1) spring migration of adult frogs from hibernacula to breeding sites; (2) movements of newly metamorphic animals from breeding sites to new areas; (3) movements from summer locales to hibernacula; and (4) passive migration of tadpoles in waterways following heavy rains that carry them to new areas (Merrell, 1970). Both adult and juvenile frogs may also move in response to warm rains. Pope et al. (2000) characterize northern leopard frogs as a species dependent upon landscape complementation, i.e., linking together different landscape elements through movement to complete their life cycles. This is well documented in eastern populations, however, descriptions of landscape complementation in western North America are lacking. Simmons (2000) notes that in Washington, northern leopard frogs are believed to move among overwintering, spring breeding, and summer feeding sites, but she finds little supporting documentation. In the Great Lakes region, adult frogs hibernate in deep water, which protects them from freezing, but move to shallow water for breeding (Harding, 1997). In Michigan, newly metamorphosed animals disperse overland and nocturnally to new locales, particularly during warm, wet weather. Young leopard frogs commonly move ≤ 800 m from their place of metamorphosis; three young males established residency up to 5.2 km from their place of metamorphosis (Dole, 1971). In the Cypress Hills, southern Alberta, young-of-the-year northern leopard frogs successfully dispersed to downstream ponds 2.1 km from the source pond, upstream 1 km, and overland 0.4 km. At Cypress Hills, a young-of-the-year northern leopard frog moved 8 km in 1 yr (Seburn et al., 1997). Streams are important dispersal corridors for young frogs (Seburn et al., 1997), and vegetated drainage ditches can enhance connectivity among seasonal habitats (Pope et al., 2000). Rainfall or humidity may be an important factor in dispersal, because odors carry well in moist air, making it easier for frogs to find other wetland sites (U. Sinsch, 1991). In Iowa, post‑metamorphic frogs emigrated from breeding ponds in July, possibly in response to internal cues (Bovbjerg, 1965). During heavy, prolonged rains in Michigan, northern leopard frogs moved in a direct line until daybreak, in contrast to the short, winding movements at other times. Following such excursions, frogs usually returned to their typical areas of use (Dole, 1965b). In Minnesota, post‑breeding frogs disperse to a variety of wetland types. In the fall, frogs move once again back to hibernacula (Breckenridge, 1944). In New Brunswick, adult and juvenile northern leopard frogs' use of peat bogs peaked in August, corresponding to juvenile dispersal from breeding ponds (Mazerolle, 2001). During migrations in Minnesota, large numbers of frogs are crushed by automobiles on roadways (Breckenridge, 1944; Merrell, 1970). Northern leopard frog populations within 1.5 km of roads are negatively affected by road mortality (Carr and Fahrig, 2001). At Ithaca, New York, of 44 recaptured northern leopard frogs, the maximum recorded distance from the first place of capture was 137 m (Ryan, 1953).
J. Torpor (Hibernation). Northern leopard frogs typically hibernate in ponds and lakes (Nussbaum et al., 1983), where they may sit on the bottom under rocks or logs or in shallow pits in silt substrates. Overwintering frogs may bury themselves in the mud (DeGraaf and Rudis, 1983; Cunjak, 1986; Harding, 1997) or they may aggregate into mounds over underwater spring heads (Lannoo, 1996). Northern leopard frogs overwintering in caves in Indiana remained active (Rand, 1950). In New England, northern leopard frogs hibernate from October or November to February or March, but may emerge on warm days in winter (DeGraaf and Rudis, 1983). In Minnesota, northern leopard frogs were found hibernating in January in a shallow (46–61 cm deep) outflow from a dam (Breckenridge, 1944); however, northern leopard frogs typically hibernate in deep, well-oxygenated water that does not freeze solid (Cory, 1952; Wagner, 1997). In the Lamoille River, Vermont, northern leopard frogs were found in well- oxygenated water in a common map turtle (Graptemys geographica) hibernaculum (Ultsch et al., 2000). In a laboratory setting, northern leopard frogs held in water at 1.5 ˚C remained submerged and motionless with limbs held loosely from the body. Frogs began to move and resurfaced at water temperatures of 7–10 ˚C (Licht, 1991). Northern leopard frogs are freeze intolerant. Frogs frozen at -2 ˚C survived no longer than 8 hr (Layne, 1992). Overwintering northern leopard frogs are intolerant of severely hypoxic or anoxic waters (Pinder, 1985) and will winterkill (Manion and Cory, 1952). Burnsi morphs appear to be more tolerant of temperature extremes than normal color patterned animals (Merrell and Rodell, 1968; Dapkus, 1976).
K. Interspecific Associations/Exclusions. In a competitive exclusion experiment, northern leopard frog tadpoles exhibited higher survival rates than spotted frogs (Rana pretiosa), suggesting that the former might be capable of displacing the latter (Dumas, 1964). In the Pacific Northwest, spotted frogs historically co-occurred with northern leopard frogs at many localities, but the former has since disappeared from most sites (Nussbaum et al., 1983). Water conditioned by other taxa did not inhibit the growth of northern leopard frog tadpoles (Akin, 1966). Northern leopard frog populations may be eliminated by predators such as American bullfrogs (Lannoo et al., 1994) and fishes (Bovbjerg, 1965; Lannoo, 1996). However, at Point Pelee National Park, Ontario, after extinction of American bullfrogs, observations of northern leopard frogs declined, while those of green frogs increased (Hecnar and M’Closkey, 1997). When European carp (Cyprinus carpio) invaded a pond in Washington, they ate all aquatic vegetation and left the site denuded, and northern leopard frogs subsequently disappeared (Corkran and Thomas, 1996). In the Caribou National Forest, southeastern Idaho, northern leopard frogs were not observed to commonly breed with tiger salamanders (Ambystoma tigrinum) or boreal chorus frogs (Pseudacris maculata). In comparison to these species, northern leopard frogs were more often associated with ponds that were less isolated and more likely to be seasonal or permanent (Burton, 2001). In Arizona, northern leopard frogs occur with Woodhouse's toads (Bufo woodhousii), red-spotted toads (B. punctatus), Mexican spadefoot toads (Spea multiplicata), Chiricahua leopard frogs (R. chiricahuensis), tiger salamanders, canyon treefrogs (Hyla arenicolor), and striped chorus frogs (Pseudacris triseriata; Clarkson and Rorabaugh, 1989; Drost and Soggee, 1993), and, until recently, occurred with American bullfrogs at one site (personal observations). In areas of sympatry between northern leopard frogs and plains leopard frogs (R. blairi) in Nebraska, the former is found in clear, sandy bottom streams, the latter in turbid, silty streams (Lynch, 1978; see also Cousineau and Rogers, 1991; see also Hardy and Gillespie, 1976). Hybridization (usually < 5%) occurs between northern leopard frogs and plains leopard frogs in areas of sympatry in Colorado, Nebraska, and South Dakota (Lynch, 1978). At Pawnee, Weld County, Colorado, northern leopard frogs occur with chorus frogs (Pseudacris sp.) and plains spadefoot toads (Spea bombifrons; Corn et al., 2000). In Cuyahoga Valley National Recreation Area, Ohio, relatively large breeding choruses of northern leopard frogs occurred in areas with large choruses of American toads (B. americanus), spring peepers (P. crucifer), striped chorus frogs, green frogs, and wood frogs (R. sylvatica; Varhegyi et al., 1998). In the 1940s–'50s at Bacon Swamp in Indiana, northern leopard frogs were common, as were eastern gray treefrogs (H. versicolor) and spring peepers (Minton, 1998). At the Edwin S. George Reserve, Michigan, northern leopard frogs co-occurred in wetlands with American toads, American bullfrogs, green frogs, wood frogs, spring peepers, eastern gray treefrogs, four species of mole salamanders (Ambystoma), and eastern newts (Notophthalmus viridescens), but bred after salamanders in sympatric habitats (Collins and Wilbur, 1979). Near Ithaca, New York, northern leopard frogs occurred with green frogs and bullfrogs (Ryan, 1953). Wright (1914) found northern leopard frogs in amplexus with pickerel frogs and American toads near Ithaca. Pace (1974) found leopard frogs often calling in association with chorus frogs and spring peepers. At Lac Saint Pierre, Québec, northern leopard frogs occurred with wood frogs, American toads, and American bullfrogs (Beauregard and Leclair, 1988). Wood frogs typically breed in woodland ponds and northern leopard frogs breed in more open areas; however, within agricultural areas of southern Québec, wood frogs typically occur in shrubby strips, whereas northern leopard frogs are more closely associated with wooded strips of habitat (Maisonneuve and Rioux, 2001). Where the two occur in sympatry, competition may occur (Smith‑Gill and Gill, 1978; Werner, 1992), but the importance of competition varies among years (DeBenedictis, 1974). In the laboratory, when northern leopard frog and wood frog tadpoles were reared separately in the absence of predators, the former grew faster. However, when reared together, wood frog tadpoles grew faster. Addition of caged predators (larval dragonflies or mudminnows) to pens containing both species reversed competitive interactions—the leopard frog larvae grew faster. Competition also resulted in increased mouth width in northern leopard frog tadpoles, although this effect was eliminated when predators were introduced (Relyea, 2000). In open-canopy ponds in Michigan, growth rates of leopard frog tadpoles in the presence of wood frog tadpoles was only 60–63% of that in ponds without wood frogs. There was no indication that northern leopard frogs influenced growth rates of wood frog tadpoles. Wood frogs bred earlier, so their tadpoles were larger, and leopard frog tadpoles are probably less active than wood frog tadpoles. Werner and Glennemeier (1999) suggest these factors may explain the competitive dominance of the wood frog in their experiments. Overlap in prey species length suggests a potential for competition between northern leopard frogs and green frogs in New Brunswick. In that study, northern leopard frogs were more terrestrial and occupied denser vegetation than green frogs (McAlpine and Dilworth, 1989).
L. Age/Size at Reproductive Maturity. In southeastern Québec, female northern leopard frogs reached sexual maturity at 2 yr and ≥ 60 mm SUL; however, 55% of males were mature at age 1 (Gilbert et al., 1994). In Wisconsin, males may mature within 1 yr of metamorphosis, but probably more commonly after 2 yr (Hine et al., 1981). Females probably do not mature until 2–3 yr post‑metamorphosis (Rittschof, 1975; Merrell, 1977; Hine et al., 1981; Hinshaw, 1992). Force (1933) suggested northern leopard frogs reach sexual maturity at age 3 in northern Michigan. In Wyoming, females matured at age 3 in high elevation populations and at age 2 in lower elevation sites (Baxter, 1952). In Minnesota, frogs matured more rapidly in populations of relatively low density (Merrell, 1977). Ryan (1953) suggested that near Ithaca, New York, a few frogs matured in the same year in which they metamorphosed.
M. Longevity. In southwestern Québec, 51 of 53 frogs collected were 2 yr old or less. The two other frogs were 3 and 4 yr old (Leclair and Castanet, 1987). Dole (1971) reported a 5-yr-old individual from Michigan. Captives have lived as long as 9 yr (Russell and Bauer, 1993).
N. Feeding Behavior. Prey of metamorphosed frogs include a variety of terrestrial invertebrates such as insect adults and larvae, spiders, slugs, snails, leeches, sowbugs, and earthworms (e.g., Knowlton, 1944; Linzey, 1967; Hedeen, 1970). As well, larger frogs take vertebrates such as spring peepers and striped chorus frogs (Harding, 1997), small northern leopard frogs (Russell and Bauer, 1993), small birds and snakes (Breckenridge, 1944; DeGraaf and Rudis, 1983), fish (Leonard et al., 1993), and bats (Creel, 1963). Prey items correlate with peak abundances in insect species (Linzey, 1967). Drake (1914) reported that almost 90% of stomach contents of Ohio specimens consisted of insects and spiders. Foraging success of northern leopard frogs in Itasca State Park, Minnesota, was correlated with both the total distance traveled during the foraging bout and the distance to the nearest conspecific neighbor (Wiggins, 1992). Northern leopard frogs, as with most frog species, typically use visual cues to detect and locate prey, but Shinn and Dole (1978) provide evidence that olfactory cues are also used. Gut passage times vary from 12–24 h in active adults, 48–96 h in overwintering animals (Gossling et al., 1980).
O. Predators. In the Pacific Northwest and Québec, northern leopard frog tadpoles are preyed upon heavily by garter snakes (Thamnophis elegans and T. sirtalis), while adults are preyed upon by snakes, birds, and other small carnivores (Nussbaum et al., 1983; Russell and Bauer, 1993). Cannibalism has been documented (Borland and Rugh, 1943; Nussbaum et al., 1983). Predators in the Great Lakes region include green frogs, American bullfrogs, various species of snakes, hawks, waterfowl, herons, raccoons, foxes, mink, otters, and humans (Breckenridge, 1944; Harding, 1997). American bullfrogs prey on northern leopard frogs in New Brunswick (McAlpine and Dilworth, 1989). Invasion of American bullfrogs in Iowa is correlated with decline of northern leopard frogs (Lannoo et al., 1994; Lannoo, 1996). Northern leopard frogs apparently have disappeared from the Columbia National Wildlife Refuge, Washington, possibly as a result of American bullfrog predation (Leonard et al., 1993). In mesic regions, northern leopard frogs use a variety of seasonal habitats, including upland sites, and may wander far from water after breeding; American bullfrogs are more likely to remain close to the water’s edge (Hecnar and M’Closkey, 1997). In arid areas, northern leopard frogs are more likely to stay close to water and therefore are probably more susceptible to predation by American bullfrogs. A large population of northern leopard frogs has been severely reduced by predation at Garlock Slough, Iowa, after a variety of fish species was stocked there by state fisheries biologists (Bovbjerg, 1965). Largemouth bass (Micropterus salmoides) feed on young-of- the-year northern leopard frogs inhabiting the littoral regions of permanent wetlands and lakes (Cochran, 1982; see also Cochran, 1983). Crayfish may feed on northern leopard frogs and may have adversely affected populations in California (Jennings and Hayes, 1994b) and other western states. Under laboratory conditions, northern leopard frog eggs and tadpoles were readily eaten by marbled salamanders (Ambystoma opacum) and eastern newts (Walters, 1975).
P. Anti‑Predator Mechanisms. Northern leopard frogs seek shelter in water when threatened (Russell and Bauer, 1993); if in upland areas, they hop in an erratic pattern and then conceal themselves in vegetation (Harding, 1997). The resemblance of the species to pickerel frogs, which have skin secretions that repel predators, may deter some predators from pursuing northern leopard frogs (Harding, 1997). During summer in Minnesota, frogs that are disturbed are less likely to jump into water and remain submerged (Merrell, 1970). Recently metamorphosed northern leopard frogs crouch or cease to move in the presence of active eastern garter snakes (Heinen and Hammond, 1997). In the laboratory, as predator density increased, the proportion of time tadpoles are active and swimming speed declined, presumably as a means to avoid predation. Larger tadpoles, which are less vulnerable to predation, were more active in the presence of predators than small tadpoles (Anholt et al., 2000). In the presence of caged predatory dragonfly (Anax sp.) larvae, northern leopard frog tadpoles developed a deeper tail fin, deeper tail musculature, and shorter body in more natural pool and tank experiments, but not in a laboratory setting. Deeper tail fins are correlated with greater swimming speeds and improved ability to escape predators (Relyea and Werner, 2000; Van Buskirk, 2001).
Q. Disease. Massive die‑offs attributable to disease and winter freezing and associated oxygen deprivation have been recorded (Cory, 1952; Koonz, 1992; Harding, 1997). Dead and moribund frogs are often symptomatic for “red-leg,” a bacterial infection (Aeromonas hydrophila), and other gram‑negative and occasionally gram‑positive bacteria (Glorioso et al., 1973; Koonz, 1992; Harding, 1997; Faeh et al., 1998); however, the infectious agents are normal inhabitants of frog environments, and frogs may only become symptomatic when immune competence is compromised (Crawshaw, 1992). Intestines of mostly healthy northern leopard frogs and tadpoles from North Dakota and Minnesota contained Aeromonas hydrophila and 29 species of Enterobacteriaceae (Hird et al., 1983). Faeh et al. (1998) report isolation of polyhedral cytoplasmic amphibian viruses (in the iridovirus family) from northern leopard frogs, but they were not identified as a cause of lesions or illness. Renal carcinomas caused by a herpes virus are most evident during the winter and spring, and transmission is thought to occur via frog urine when adults congregate at breeding ponds (Hunter et al., 1989; Faeh et al., 1998). Hunter et al. (1989) believed the prevalence of tumors was increasing in the 1980s in Minnesota. Gibbs et al. (1966) reported intestinal tract cancer in northern leopard frogs. During a period from hibernation emergence until immune function is restored, frogs are particularly susceptible to diseases (Maniero and Carey, 1997).
Low pH affects fertilization and development of northern leopard frog eggs. Adult northern leopard frogs exhibited high mortality when maintained for 10 d in water of pH 5.5. Immune function declines during hibernation (Cooper et al., 1992), leaving northern leopard frogs more susceptible to low pH (Vatnick et al., 1999). Northern leopard frog tadpoles exposed to pH < 4 died, and those exposed to pH < 5.6 for > 24 hr experienced high mortality. Prior to death, tadpoles exposed to low pH exhibited caudal curling, thoracic swelling, and failure to retract the yolk plug (Watkins-Colwell and Watkins-Colwell, 1998). Leopard frog abundance may be low in areas where waters are either acidic or basic (Pope et al., 2000). Endogenous gut bacteria persist through hibernation and may be a source of systematic infection when frogs emerge in the spring in an acidic environment (Leonard et al., 1999).
Large numbers of malformed northern leopard frogs (mostly limb abnormalities) and some other malformed anurans recently have been observed in Minnesota (Helgen et al., 1998; Souder, 2000; Lannoo et al., in press). Malformed northern leopard frogs have also been reported from Arizona, Québec, Wisconsin, Ohio, South Dakota, Vermont, Maine, and other states. Meteyer et al. (2000) report that 86% of recently metamorphosed northern leopard frogs from Minnesota, Vermont, and Maine exhibited hindlimb deformities. They describe these malformations in detail and suggest that “developmental events may produce a variety of phenotypes depending on the timing, sequence and severity of the environmental insult.” In five Vermont counties, 7.5% of recently metamorphosed northern leopard frogs exhibited external malformations; observed malformation rates varied seasonally and annually (Levey, 2000). Some malformed frogs in Le Sueur County, Minnesota, had apparent parasitic cysts in the thigh muscles (Helgen et al., 1988). Infections of a parasitic trematode (Ribeiroia sp.) have been implicated in limb malformations of Pacific treefrogs (Pseudacris regilla) in California and may be a contributing factor in malformations observed in other amphibians (Johnson et al., 1999), including northern leopard frogs in Arizona (Sessions et al., 1999). However, Gilliland and Muzzall (1999, 2002) concluded that trematodes were not the cause of deformities in southern Michigan. Visiting the "hottest of the Minnesota malformed frog hotspots," and control sites, Lannoo et al. (in press) concluded that where Ribeiroia metacercariae were found, they likely cause malformations, but there were two important disconnects: some "control" sites contained Ribeiroia, but malformations were not present in high numbers, and at some "hotspots," malformations were present in the absence of Ribeiroia metacercariae.
Possible causes of malformations include pesticides, retinoids, heavy metals, increased ultraviolet light, mechanical perturbation including parasitic infestations and predation attempts, and estrogen mimics (Sessions and Ruth, 1990; Kao and Danilchik, 1991; Ankley, 1997; Ouellet et al., 1997b; Helgen et al., 1998; Johnson et al., 1999; Sessions et al., 1999). Agricultural contaminants are the suspected cause of deformities observed on the St. Lawrence River Valley, Québec (see discussion below), although variation in the proportion of deformities among sites was too large to conclude that there was a difference between control and pesticide-exposed habitats. Conspicuous deformities interfered with swimming and hopping and likely constituted a survival handicap (Ouellet et al., 1997). Deformed frogs from Minnesota exhibited normal chromosome number and morphology, and selectively stained metaphase plates had the normal number of nucleolar organizer regions, providing no evidence that affected frogs had damaged genetic material (Reister et al., 1998; Horner et al., 2000). Deformed frogs in Canada weighed less than normal frogs, while both deformed and normal frogs from sites of high deformity rates exhibited shorter body length, head width, and femur and forelimb length than frogs from sites with low incidence of deformities (Gallant and Teather, 2001).
A growing body of evidence suggests northern leopard frogs can be adversely affected both acutely and via sublethal symptoms by pesticides and other chemicals. However, with a few notable exceptions, as yet there is little evidence that concentrations of chemicals present in the environment are contributing to population decline or increased levels of malformations. Northern leopard frog embryos exposed to low levels of three insecticides and six herbicides commonly used in Canadian forests and croplands hatched at the same time and with the same hatching success as control animals. Experimental embryos did not exhibit higher levels of tadpole deformities than controls. Experimental tadpoles were paralyzed, but gradually recovered after exposure was terminated (Berril et al., 1997). Northern leopard frogs may be vulnerable to agricultural chemicals during spring runoff due to their habitat of overwintering in permanent water bodies in close contact with bottom sediments (Kaplan and Overpeck, 1964; Didiuk, 1997). During metamorphosis, resorption of the tail may cause mobilization of bioaccumulated pesticides and subsequent toxicity (Cooke, 1970). Streams, rivers, and reservoirs in mid-western corn-growing regions may be exposed to high levels of the herbicide atrazine. In the laboratory, high doses of atrazine caused deformities in northern leopard frog larvae and respiratory distress and cessation of feeding in adult frogs, but these doses were considerably higher than concentrations found in North American surface waters. Thus, direct toxicity of atrazine may not be a significant factor in recent declines of northern leopard frogs (Allran and Karasov, 2001). Furthermore, atrazine and nitrate fertilizers at concentrations found in the environment, and interactions between the two, do not appear to pose a substantial direct toxicity threat to northern leopard frog tadpoles (Allran and Karasov, 2000). Atrazine may not be present in the environment at levels that result in acute toxicity, but there is new evidence that sublethal effects to frogs are occurring. Atrazine disrupts endocrine function and, even at very low concentrations of 0.1 ppb, cause retarded gonadal development, hermaphroditism, and oocyte growth in male northern leopard frogs. Atrazine contamination is widespread in the United States and can be present in excess of 1 ppb in precipitation and even in areas where it is not used (Hayes et al., 2002b,c). Glennemeier (2001) and Glennemeier and Denver (2001) examined the effects of a polychlorinated biphenyl (PCB) congener, 77-TCB, on northern leopard frog tadpoles. Effects included decreased feeding rates, reduced whole-body corticosterone content, and altered competitive interactions with wood frog tadpoles. Corticosterone is important in mediating the negative growth response of northern leopard frog tadpoles to increasing larval densities, and also affects development, morphology, and response to adrenocorticotropic hormone. The results suggest negative population-level consequences from sublethal effects in PCB or other pollutant-contaminated environments. However, no significant correlation between frog densities and severity of PCB contamination was found in contaminated wetlands. Tissue concentrations of PCBs in the field were much lower than in sediments (Glennemeier and Begnoche, 2002). Northern leopard frog tadpoles exposed to high levels of PCB 126 exhibited elevated incidence of edema, reduced swimming speed and growth, and 100% mortality before metamorphosis. Few deformities were observed. However, sublethal effects were not apparent at PCB concentrations that occur in the Green Bay ecosystem, Wisconsin (Rosenshield et al., 1999). The pre-emergent herbicide acetochlor interacts with thyroid hormone in northern leopard frog tadpoles, accelerating thyroid hormone induced metamorphosis and countering the effects of corticosterone (Cheek et al., 1999). Survival declined, the prevalence of deformities increased, and growth and development slowed in northern leopard frog tadpoles exposed to un-ionized ammonia concentrations in excess of 1.5 mg/L. This level is higher than that measured in waters of the Fox-River-Green Bay ecosystem, but lower than for pore sediment water. Leopard frogs may be exposed to hazardous levels of un-ionized ammonia when they hibernate on the bottom or while buried in sediments (Jofre and Karasov, 1999). Among northern leopard frog tadpoles, growth rates slow and mortality rates increase when exposed to copper sulfate, used to control nuisance algal blooms (Lande and Guttman, 1973).
Intensive ultraviolet (UV) exposure in the laboratory can result in embryonic mortality and abnormal development (Higgins and Sheard, 1926). However, larvae are more sensitive than embryonic stages; ambient UV-B levels were found to be lethal to northern leopard frog tadpoles in the absence of shade or refuge (Ankley et al., 2000; Tietge et al., 2001). At 50–60% ambient sunlight and in laboratory exposure to UV radiation, incidence of hindlimb malformations increased. However, due to uncertainties in dose extrapolation, the significance of the results in explaining malformations observed in the field is unclear (Ankley et al., 2000). There is growing evidence that the deleterious effects of UV radiation and chemicals may interact or be additive. In the laboratory, northern leopard frog tadpoles exposed to the pesticide s-methoprene exhibited a deformity rate of 2.1%, whereas those exposed to both UV and s-methoprene had a deformity rate of 8.7%. No deformities were observed in the control group (Akins and Wofford, 1999). Exposure of northern leopard frog tadpoles to UV-A, simulating a fraction of summertime, midday sunlight in the northern latitudes, significantly increased the toxicity of fluoranthene (Monson et al., 1999).
A fungal disease, chytridiomycosis, implicated in declines of anurans in Australia and Central America (Berger et al., 1998) and elsewhere, has recently been documented in the United States (Milius, 1998) and as a contributing factor in mass mortality of northern leopard frogs in the Colorado Rockies in the 1970s (Carey et al., 1999). Additional work is needed to clarify the role of chytridomycosis in this and possibly other observed declines of northern leopard frogs.
Diana and Beasley (1998) discuss the toxicology of northern leopard frogs and other amphibians. Underhill (1966) noted an incidence of spontaneous caudal scoliosis in northern leopard frog tadpoles. One of us (M.J.L.) knows of a spring-fed wetland in northwestern Iowa where scoliotic animals are found regularly. The physiology and developmental biology of these animals have been studied extensively (Gibbs et al., 1971; Feder and Burggren, 1992); northern leopard frogs are commonly used as a model organism in laboratory studies.
R. Parasites. Parasitic cysts have been noted in the thigh muscles of northern leopard frogs in Minnesota (Helgen et al., 1998), and trematodes may be contributing to observed limb malformations in some regions (Sessions et al., 1999; see "Disease" above). Brooks (1976b) examined helminth faunas in northern leopard frogs (see also Pollack, 1971). Gilliland and Muzzall (1999) found 12 species of helminths in 43 northern leopard frogs from southern Michigan. Helminth infestations have also been investigated in North and South Dakota (Goldberg et al., 2001) and New Brunswick (McAlpine, 1997c; McAlpine and Burt, 1998). In New Brunswick, helminth species richness was greatest in adults. Helminths that infect the host via skin penetration are most abundant in larger frogs with greater epidermal area (McAlpine, 1997c). A tetrathyridia (Mesocestoides sp.) was found in a single specimen of northern leopard frog from Jefferson County, New York (McAllister and Conn, 1990). The respiratory tracts of ranid frogs are susceptible to infection by Rhabdias spp., a group of lung worms (Baker, 1978a). Brooks (1976b) examined helminth faunas in northern leopard frogs (see also Pollack, 1971). Gibbs et al. (1971) suggested that northern leopard frogs remain healthy despite serious parasite loads.
4. Conservation. In Canada, the Southern Mountain population of northern leopard frogs (British Columbia) is listed as Endangered and the prairie population is designated a Species of Special Concern, but the eastern population is not considered to be at risk (Committee on the Status of Endangered Wildlife in Canada, 2002). Northern leopard frogs have no status under the U.S. Endangered Species Act. States and provinces often have species designations that include northern leopard frogs (e.g. the species is Threatened in Alberta and red-listed in British Columbia) or protect the species from hunting.
Causes of decline and extirpation are many, and several probably interact to exacerbate adverse effects. In the western states and provinces, where declines have been most evident, introduction and spread of non-native predators has played an important role. But frogs are often absent from seemingly pristine habitats in the west, particularly at high elevation, suggesting chytridiomycosis, UV radiation, or other less obvious causes. Relationships among stressors are complex; susceptibility to and virulence of diseases can be affected by many factors. Stressors such as contaminants, acidic rainfall, changes in climate or microclimate, or increased UV radiation can cause immunosuppression (Carey et al., 1999, 2001; Lips, 1999). Also, seemingly pristine forests of the western states are often quite altered from predevelopment conditions (Dahms and Geils, 1997) due to a long history of logging, fire suppression, and grazing. The intensity of wildfire in the forests of the western states in recent years highlights just how altered western forest ecosystems have become. How these changes have affected northern leopard frogs is unclear.
In Saskatchewan, Manitoba, Ontario, Québec, and the Midwestern states, widespread declines began in the 1960s or ‘70s, and observations of large die-offs during that time suggest disease or chemical insults as the proximate cause. However, in these areas, leopard frogs clearly depend on a varied landscape that includes overwintering aquatic habitats, breeding ponds, upland foraging areas, and corridors to move among these habitats. This dependence, combined with likely metapopulation structure, makes this species especially susceptible to fragmentation and loss or alteration of habitats on a landscape level. In the Midwest, massive land-clearing and draining of wetlands for cultivation has occurred over the last century and a half. For example, Indiana’s forests and wetlands have been reduced by about 78% and 86%, respectively (Miller, 1993; Hartman, 1994; Kolozsvary and Swihart, 1999). On the other hand, closed-canopy forest has increased significantly at the Edwin S. George Reserve in Michigan since 1937, and similar ecological succession is occurring over other portions of eastern North America due to abandonment of agricultural lands and possibly other causes (Skelley et al., 1999; Werner and Glennemeier, 1999). Northern leopard frogs do poorly in closed-canopy forest (Werner and Glennemeier, 1999). Alteration of habitats has, and no doubt will, continue to contribute to changes in northern leopard frog distribution and abundance. Recent widespread observations of malformed frogs in this region suggests frogs are also under assault from chemical or other stressors. The species is faring better in the eastern states and provinces, but habitat loss, degradation, and fragmentation have caused localized declines and extirpations in that region, and malformed frogs suggest additional stressors are present as well.Crafting a comprehensive rangewide conservation plan for northern leopard frogs would be a difficult task due to incomplete knowledge of why the species is declining, regional differences in biology and threats, and the daunting challenge of initiating and coordinating conservation across many jurisdictions and the large range of the species. As a result, conservation would probably best be addressed at a regional or population level with coordination and communication among regions.
1James C. Rorabaugh
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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