Critical Thresholds Associated With Habitat Loss for Two Vernal Pool‐breeding Amphibians

Abstract

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.

Quantifying the Road-Effect Zone: Threshold Effects of a Motorway on Anuran Populations in Ontario, Canada

“…Although the total amount of forest within 500 and 1000 m of the edge of the survey ponds was uncorrelated with distance to Highway 401, there was a high correlation between the amount of forest within 500 and 1000 m of Highway 401 that could be reached without crossing Highway 401 (accessible forest) (Eigenbrod et al 2008b) and distance to the highway (r = 0.84; P <0.0001 and r = 0.73; P <0.0001, respectively for the two distances). The amount of forest in the landscape is known to be the best single predictor of both spring peeper and wood frog abundance (e.g., Homan et al 2004, Porej et al 2004, Gagné and Fahrig 2007, with both species dependent on forests for foraging and breeding habitat. Because the near-continuous flow of traffic on Highway 401 means that it is likely a near-complete barrier to the movement of anurans, the reduced forest cover available within 500-1000 m of ponds near the highway without crossing the highway likely has led to diminished populations of forest obligate species at these ponds.…”

Frog Virus 3-Like Infections in Aquatic Amphibian Communities

“…As the dispersal ability of adult wood frogs is poor (Homan et al, 2004), it is unlikely that the geographic movement of the virus occurs through movement of wood frogs. An alternative explanation, hypothesized by Harp and Petranka (2006), is that virus spread between ponds could occur through the movement of virus-contaminated sediment by humans or animals.…”

Conserving biodiversity in managed forest landscapes: The use of critical thresholds for habitat

“…Thus, species that are more sensitive to anthropogenic changes should have a higher critical habitat threshold (around 40%) than more generalist species (around 10%) (Betts et al 2007, Betts andVillard 2009). It appears, however, that these models, established at the landscape scale, are more suitable for species with relatively large home ranges (in the order of several hectares), while those with smaller home ranges might obtain more complex responses (Homan et al 2004).…”