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 forest patches.

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