Ambystoma gracile (Baird, 1859)
H. Bradley Shaffer
1. Historical versus Current Distribution. Northwestern salamanders (Ambystoma gracile) are found from northern California (extreme northern Sonoma County) north to southeastern Alaska. Two subspecies are often recognized (Petranka, 1998), although recent allozyme data suggest that this may not be warranted (Titus, 1990). Within this range, the species is limited to mesic habitats of the Coast Range and Cascade Mountains (Nussbaum et al., 1983; Stebbins, 1951). Habitats range in altitude from sea level to 3,110 m (Nussbaum et al., 1983). The current distribution presumably resembles the historical distribution, but no serious analysis of the species has been undertaken. Almost certainly, populations have been lost as the Pacific Northwest has been developed for urban and agricultural uses; however, ponds constructed for stock and other uses have probably improved habitat in some instances.
2. Historical versus Current Abundance. Few data exist and those that do are contradictory. Aubry and Hall (1991) found northwestern salamander numbers to be substantially lower in regrowing forest, while Corn and Bury (1991) found little correlation between salamander abundance and stand age once clearcuts began to grow over (see also Petranka, 1998).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. The breeding season for northwestern salamanders varies based on latitude and altitude. In low elevation Pacific Northwest populations, egg deposition occurs from January–April (Henry and Twitty, 1940; Watney, 1941; Snyder, 1956; Licht, 1969c, 1975b; H.A. Brown, 1976b; Eagleson, 1976; see also Petranka, 1998), with peak activity in late February in the Seattle area (Snyder, 1956). Breeding occurs from June to late August in higher elevation (1,300–1,676 m) lakes in Oregon (Snyder, 1956) and British Columbia (Eagleson, 1976). Breeding migrations can occur when ice remains on ponds (Snyder, 1956; Eagleson, 1976; Nussbaum et al., 1983). Breeding lasts 1–7 wk; at any given site the length of the breeding season is variable and apparently linked to the rate at which water temperature increases in the pond or lake (Snyder, 1956; Eagleson, 1976).
ii. Breeding habitat. Adults breed in semipermanent and permanent wetlands, lakes, and slow-flowing streams and rivers.
i. Egg deposition sites. Vary depending on whether adults have metamorphosed or are neotenic. Metamorphosed females attach egg masses to submerged objects such as tree limbs or branches and cattails, from 0.5–1 m below the water surface. Over time, symbiotic algae invade egg masses, providing eggs with oxygen (Patch, 1922; Carl and Cowan, 1945; see also Petranka, 1998). Neotenic adults tend to lay eggs in smaller, looser masses (Henry and Twitty, 1940; Snyder, 1956; but see Knudsen, 1960) directly on the wetland bottom. Depending on water temperature, hatching occurs 2–9 wk after laying (Slater, 1936; Watney, 1941; Licht, 1975b; H.A. Brown, 1976b). Egg masses are firm gelatinous structures 80–150 mm in diameter, and individual ova are 1.5–2.5 mm in diameter (Nussbaum et al., 1983).
ii. Clutch size. Egg masses are large (5–15 cm long, 5–8 cm wide), contain anywhere from 30–270 eggs (Slater, 1936), and generally have a firm jelly layer. There is some discrepancy concerning differences in egg masses produced by paedomorphic versus transformed individuals (Petranka, 1998).
i. Length of larval stage. Hatchlings average about 8 mm (Licht, 1975b), and larvae 1–7 d old were 15–20 mm (Bishop, 1943). Most larvae overwinter and metamorphose at 12–14 mo, but some overwinter a second year; montane populations may overwinter a third year (Watney, 1941; Snyder, 1956; Licht, 1975b; Eagleson, 1976; see also Petranka, 1998). Some animals are neotenic and apparently never metamorphose (see "Neoteny" below). Under laboratory conditions, high food levels increase larval growth rates and decrease time to metamorphosis but do not affect size at metamorphosis or the tendency to be neotenic (Licht, 1992). Larvae of both sexes have identical growth rates.
ii. Larval requirements.
a. Food. Hatchlings initially feed on cladocerans, copepods, and ostracods, gradually adding prey items to their diet as they can swallow them. Larger prey include amphipods, insect larvae, annelids, mollusks, amphipods, anostracans, and snails (Efford and Tsumura, 1973; Licht, 1975b). Northwestern salamander larvae will eat anuran tadpoles, but apparently avoid toxic bufonid larvae (Peterson and Blaustein, 1991).
b. Cover. Larvae seek cover in terrestrial vegetation or in shallows in the presence of predatory fishes (Neish, 1971). Sexually mature paedomorphic animals tend to burrow in the substrate and flee into deeper water in fishless lakes, but flee into shallow water and burrow less often in fish-containing lakes (Taylor, 1983b).
iii. Larval polymorphisms. Unknown and unlikely given the attention dedicated to northwestern salamanders.
iv. Features of metamorphosis. Larvae metamorphose at sizes ranging from 47–74 mm SVL under laboratory conditions (Eagleson, 1976), and size frequency analysis from field populations suggest that this accurately reflects sizes in the field.
v. Post-metamorphic migrations. For terrestrial adults, migration takes place from their breeding ponds to upland sites. Adults and terrestrial juveniles are assumed to spend most of the year in rodent burrows (Nussbaum et al., 1983).
vi. Neoteny. Nearly all populations contain neotenic (paedomorphic) individuals (Licht, 1992). The percentage of neotenic animals has been suggested to be linked to altitude, with higher elevation populations metamorphosing less frequently (Snyder, 1956; Sprules, 1974a,b; Eagleson, 1976; see also Petranka, 1998); recent experimental data has brought this correlation into question (Licht, 1992). There are no anatomical differences in cloacal anatomy between transforming and nontransforming adults, suggesting that the two forms should be capable of interbreeding (Licht and Sever, 1991).
D. Juvenile Habitat. Juvenile northwestern salamanders are thought to become sexually mature 1–2 yr after metamorphosis (Efford and Mathias, 1969), although this has not been validated with accurate field studies. Terrestrial juveniles likely have habitat characteristics similar to terrestrial adults.
E. Adult Habitat. Varies depending on whether adults are terrestrial or paedomorphic. Little is known about the habits of terrestrial adults. As is typical of many ambystomatids, males migrate to breeding sites before females (Nussbaum et al., 1983). The habitat characteristics for neotenic adults are similar to those for larvae, but the larger neotenic adults can presumably take a greater range of prey.
F. Home Range Size. Unknown.
G. Territories. Unknown.
H. Aestivation/Avoiding Dessication. Unknown; may not be necessary in the moist coastal climate of the Pacific Northwest.
I. Seasonal Migrations. Terrestrial adults migrate when snow and ice are still present at high elevation sites (Nussbaum et al., 1983). The terrestrial ecology of northwestern salamanders has not been well studied.
J. Torpor (Hibernation). Important for terrestrial individuals but apparently unstudied.
K. Interspecific Associations/Exclusions. Unlike many ambystomatid salamanders, northwestern salamanders can coexist with predatory fishes; they apparently monitor fishes and alter their behavior to avoid predation (Efford and Mathias, 1969; Taylor, 1983a,b). Salamanders in fishless habitats are diurnal and exhibit weaker escape responses (Taylor, 1983b). In the presence of fish, larvae concentrate in shallow water where they seek refuge (Taylor, 1983a). Larger larvae and neotenic adults escape into shallow areas when fish are encountered. Both larvae and eggs are eaten by rainbow trout (Efford and Mathias, 1969). Rough-skinned newts (Taricha granulosa) co-occur in ponds with northwestern salamanders in some sites (Efford and Mathias, 1969) where they have similar diets and densities. In mountain ponds and lakes in Mount Ranier National Park, northwestern salamander larvae tend to occur in sites that are larger, deeper, lower in elevation, higher in organic content, with more flocculent sediment and coarse woody debris, when compared with long-toed salamander (Ambystoma macrodactylum) larvae.
L. Age/Size at Reproductive Maturity. Variable depending on whether animals are at low or high latitudes and altitudes and whether adults are terrestrial or neotenic. The youngest age at which animals could be reproductively mature appears to be 2 yr (after 1 yr as a larva and 1 yr as a juvenile); some animals take several years. Animals become sexually mature at about 70–75 mm SVL (Petranka, 1998). If the lifespan of 5 yr inferred by Efford and Mathias (1969) is accurate, it implies that most animals breed once or at most a few times.
M. Longevity. Unknown, for terrestrial animals. Neotenic animals live about 5 yr (Efford and Mathias, 1969).
N. Feeding Behavior. Neotenic adults feed on a wide range of aquatic invertebrates and tadpoles (Efford and Tsumura, 1973; Licht, 1975b; Peterson and Blaustein, 1991). Feeding behavior for terrestrial adults is apparently unstudied but probably includes insects and other invertebrates that inhabit grasslands and forest floors.
O. Predators. Predatory fishes (Efford and Mathias, 1969; Sprules, 1974a; Taylor, 1983b). Rough-skinned newts (Taricha granulosa) have been observed attempting to eat northwestern salamander eggs (Pearl, 2003).
P. Anti-Predator Mechanisms. Aquatic animals will seek shelter in vegetation and littoral zones (Neish, 1971; Taylor, 1983a,b; see also Petranka, 1998). Metamorphosed individuals have conspicuous parotid glands and poison glands along the tail. They assume a characteristic "head-down" defensive posture when challenged by predators (Brodie and Gibson, 1969).
Q. Diseases. Unknown. In experimental containers, embryos are affected by ambient UV-B radiation (Blaustein et al., 1995).
R. Parasites. Larvae, including paedomorphic adults carry heavy burdens of the nematode Chabaudgolvania waldeni (Quimperiidae; Seuratoidea), with 40–100% infection rates in four British Columbia lake populations (Adamson and Richardson, 1991).
4. Conservation. Northwestern salamanders remain common over many parts of their range. Their current distribution appears to resemble their historical distribution, but undoubtedly populations have been lost as the Pacific Northwest has been developed. Ponds constructed for livestock and other uses have probably improved habitat in some instances. While logging may reduce populations (Aubry and Hall, 1991), once clearcuts began to grow over, there is little correlation between salamander abundance and stand age (Corn and Bury, 1991).
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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