A critical threshold exists when the relationship between the amount of suitable habitat and population density or probability of occurrence exhibits a sudden, disproportionate decline as habitat is lost. Critical thresholds are predicted by a variety of modeling approaches, but empirical support has been limited or lacking. We looked for critical thresholds in two pool‐breeding amphibians that spend most of the year in adjacent upland forest: the spotted salamander (Ambystoma maculatum) and the wood frog (Rana sylvatica). These species were selected because of their reported poor dispersal capacities and their dependency on forest habitat when not breeding. Using piecewise regression and binomial change‐point tests, we looked for a relationship between the probability of occupancy of a site and forest cover at five spatial scales, measuring forest cover in radial distances from the pond edge of suitable breeding ponds: 30 m, 100 m, 300 m, 500 m, and 1000 m. Using piecewise regression, we identified significant thresholds for spotted salamanders at the 100‐m and 300‐m spatial scale, and for wood frogs at the 300‐m scale. However, binomial change‐point tests identified thresholds at all spatial scales for both species, with the location of the threshold (percent habitat cover required) increasing with spatial scale for spotted salamanders and decreasing with spatial scale for wood frogs. Thresholds for spotted salamanders occurred at ∼30% forest cover at spatial scales of 100 m or less, with 41% cover at 500 m, and with 51% habitat cover at 1000 m. Thresholds for wood frogs ranged from 88% habitat cover at 30 m from the pond edge, declining to 44% habitat cover within 1000 m. These patterns might be explained by the different winter dispersions of these species. Knowing whether a species has a critical threshold, and at what level of cover and at what spatial scale it exists, would be essential for conservation of habitat‐sensitive species.

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We conducted a 2-year study of terrestrial habitat use by spotted salamanders (Ambystoma maculatum), bluespotted salamanders (A. laterale), wood frogs (Rana sylvatica), and eastern newts (Notophthalmus viridescens) within blocks of forest enclosed by drift fencing and pitfall traps in the vicinity of a single breeding pond. Adult winter densities within forest habitat <100 m from the breeding pond were low for all species (range 0.1-1.9 individuals / 100 m 2 , n = 6 enclosures). During our 2-year study, at least 40% of wood frogs, 52% of blue-spotted salamanders, and 60% of spotted salamanders wintered >100 m from the breeding pond. Males tended to winter closer to the breeding pond than did females. Adult wood frogs and eastern newts were largely absent from upland forest adjacent to the breeding pond during spring and summer, but they entered these areas in significant numbers to winter. Analyses of net flow of amphibians resulting from fall movement suggest that summer and winter densities of Ambystoma salamanders remained similar, while there were large increases in wood frog and eastern newt densities within terrestrial habitats near the breeding pond during fall. These results support a growing body of evidence that maintenance of suitable terrestrial habitat beyond 100 m from breeding pools is important for maintaining pool-breeding amphibian populations. Narrow protected buffer strips around breeding ponds might be even less effective than previously thought due to the disproportionate representation of males within these areas.

Spotted salamander (Ambystoma maculatum) distributions were studied within eighteen 272-m2 field enclosures in upland deciduous forest adjacent to two breeding ponds. The enclosures were subsequently used in experiments involving (i) removal of small-mammal burrows to determine if burrow abundance influenced salamander density and (ii) manipulation of salamander densities to determine if increased density was associated with decreased growth, fecundity, or probability of re-emerging to breed. Mean density of salamanders wintering in enclosures <65 m from the breeding ponds was 1.0/100 m2 (SD = 1.1/100 m2, n = 17, range = 0–4.0/100 m2). Density declined as distance to the nearest breeding pond increased, and the sex ratio was skewed sharply toward males (8:1) wintering close (<65 m) to the pond but not farther from the pond (1:1). Males residing within the enclosures weighed more than males wintering farther from the breeding ponds. Migrating salamanders were over 3 times more likely to leave burrow-removal plots than control plots prior to the summer inactive period. Salamanders housed at high density (15/enclosure) were less likely to re-emerge to breed than were salamanders housed at low density (5/enclosure), but we observed no effects on growth or fecundity. Results suggest that conspecific distribution and burrow availability might affect the density of spotted salamanders within terrestrial habitats.

We sampled blood from free-living spotted salamanders (Ambystoma maculatum) to test whether differences in the concentrations of a stress hormone (corticosterone) were associated with different qualities of breeding and migration habitat. Spotted salamanders are forest specialists that migrate to vernal pools to breed, and upland habitat degradation may have sub-lethal effects on animals that lead to population declines. An individual's level of physiological stress may function as a biomonitor for sub-lethal effects, and thus as a biomonitor for ecosystem quality. We compared unstressed (baseline) and stress-induced corticosterone concentrations in spotted salamanders: (1) at sites that differed in amount of forest loss; (2) during breeding migration across forest habitat versus pavement; (3) in microhabitats that varied in soil drainage and canopy cover. Removal of large amounts of terrestrial habitat surrounding a breeding pond was correlated with lower baseline (in males) and stress-induced corticosterone concentrations, which may indicate healthy individuals with a reduced ability to respond to additional stress or individuals experiencing chronic stress. Male salamanders migrating across pavement had elevated baseline corticosterone concentrations compared to animals migrating through a forest, consistent with an acute stress response. However, concentrations of corticosterone did not differ between individuals in microhabitats with canopy cover and well-drained soil versus those in microhabitats with no canopy cover and/or swampy soil. This endocrinological technique may be one useful measure of a population's health, helping to identify populations where further ecological study is recommended to evaluate conservation concerns.