Distribution of Redback Salamanders in a Fragmented Landscape
Personnel: Amy Sheldon, David Capen
Fragmentation of once continuous natural landscapes by human alteration
creates isolated "islands" of suitable habitat for plants and animals. Loss
of habitat continuity may lead to isolation of populations, genetic drift,
and local extinctions. I studied the distribution of redback salamanders
(Plethodon cinereus) in Addison County, Vermont, a region once entirely
forested, but now primarily agricultural, with remnant forest patches
throughout the landscape. My objective was to test the hypothesis that the
distribution of redback salamanders was related to size and isolation of
I created a GIS coverage of forest patches from recent aerial photographs and calculated nine variables that described the size, shape, and isolation of each patch. More than 1,000 forest patches comprised 26% of my 27,900-ha study area. Average patch size was 7.1 ha, and the mean distance between forested patches was 113 m. I searched for salamanders in 51 forested patches selected to represent a range of patch sizes and suitable habitat conditions for redback salamanders. Redback salamanders were found in 61% of these patches, and eight of nine patch variables were significantly different for these patches verses patches where salamanders were not detected (Table 1).
I used logistic regression to develop statistical models that discriminated between patches where redback salamanders were present and those where redback salamanders were not present. Two models correctly classified salamander presence or absence in a patch 80% and 76% of the time. Patch area, distance to the next nearest forest patch, and presence of a core area (defined as area 100 m from the forest edge) were the best predictors of salamander presence in a forest patch. Large patches closer to another patch with a core area present were most likely to have salamanders present.
Table 1. Measures of forest patches where redback salamanders were present or absent in Addison, Bridport and Shoreham, Vermont, 1995.
PRESENT ABSENT Patch Variable Mean SD Mean SD P * AREA (ha) 74.8 257.4 4.8 5.3 0.0001 PERIM (m) 6149.2 10076.3 1150.5 819.8 0.0001 SHAPEI 2.5 1.5 1.6 .4 0.0005 COREA 40.6 206.5 0.3 1.1 0.0008 COREAI 9.4 17.1 1.5 5.4 0.0011 NUMCORE 1.3 1.5 0.2 0.4 0.0003 FRACTAL 1.3 0.05 1.3 0.04 0.6159 PROX 622.2 1684.5 30.8 83.4 0.0026 NEAR 85.7 109.2 229.5 182.7 0.0008 * Wilcoxon Rank Sum Test.
|Updated: 28 June 2000|