URBAN SNAKE PROJECT

A gravid female brown snake found underneath a plywood in Perth Amboy, NJ.
A handful of brown snakes I found in an urban green space in Newark, NJ in 10 minutes!

The process of urbanization has largely changed natural landscapes and threatened the survival of wildlife, which draws increasing attention to the conservation of biota in the urban areas. Currently, most work assessing the effect of urbanization on wildlife has been predominately focused on birds and arthropods, with few studies conducted on other taxa, such as the reclusive and elusive reptiles.

DeKay’s brown snake, Storeria dekayi, is a small, semi-fossorial snake species found throughout the Eastern and Central United States. They frequently inhabit urban areas and often form aggregations, which gives us a good opportunity to explore how reptiles respond to urban environments.

Using S. dekayi as a model organism, I aim to:

  • build ecological niche models to predict current and future distribution of S. dekayi, to examine how different environmental variables affect S. dekayi distribution, and to find out if urbanization contributes to the distribution pattern of S. dekayi;

  • compare morphological traits between different S. dekayi populations along the rural-urban gradient to test whether certain traits are selected for urban adaptation;

  • determine if S. dekayi is an urban adaptor with higher population density in urban environments with urban population size estimation;

  • compare community composition of potential prey species between different S. dekayi populations along the rural-urban gradient to test whether urban environment has the potential to change the feeding behavior and dietary structure of S. dekayi;

  • determine preferred microhabitat characteristics by S. dekayi within urban environments via habitat characterization;

  • characterize the genetic structure of S. dekayi populations across New Jersey.

Storeria dekayi Ecological Niche Modeling

I am constructing ecological niche models (ENMs) of Storeria dekayi using georeferenced data points and different environmental and bioclimatic layers. Preliminary results show that the ENMs I constructed with S. dekayi east of the Mississippi River, S. dekayi west of the Mississippi River, and all S. dekayi all differ significantly from random and exhibited high sensitivity and specificity. The bioclimatic variables with the greatest contribution to the models for the east clade, the west clade, and the total data points are the precipitation of the driest month, the mean temperature of the warmest quarter, and the minimal temperature of the coldest month respectively. When the variable of Human Influence Index (HII) is added to the ENM of total data points, it becomes the variable that contributes the most to the model. A jackknife test of variable importance also indicates that HII is a relatively transferable variable, which should be recommended to use to transfer the S. dekayi ENMs (e.g. using together with future climatic variables to predict future S. dekayi distribution).

Figure 1. A graphic representation of the maxent model for S. dekayi throughout its distribution except for Central America. Warmer colors show areas with better predicted conditions.

Morphometric Analyses of Urban Storeria dekayi Populations

I am collecting morphological data of Storeria dekayi from multiple sites with different levels of urbanization across New Jersey, and from the herpetology collection at the American Museum of Natural History (AMNH). Using a linear morphometric approach, preliminary results of my data from the two study sites suggest that S. dekayi display distinct sexual dimorphism. Males have lower ventral scale counts, higher subcaudal scale counts, and longer tails. There is also evidence of sexual dimorphism in terms of head shape, because males and females show differentiated internostril distances and interocular distances. Individuals also display inter-site morphological differentiation in terms of head heights (Figure 2).

Figure 2. Individuals from two sites in central New Jersey show differentiated head height. ANCOVA (SVL × Sex) F = 4.631, P = 0.041.

Using a geometric morphometric approach, I analyzed inter-population variation of four urban and suburban S. dekayi populations (three from my study sites and one from Van Cortlandt Park, New York City preserved in AMNH) with 13 landmarks of the dorsal view of the head (Figure 3). Preliminary results show that it is possible to discriminate the four populations by their head shapes (Figure 4).

Figure 3. 13 landmarks on the dorsal view of the head of S. dekayi used for the geometric morphometrics study.
Figure 4. Canonical variates analysis (CVA) plot using CV1 and CV2. CV1 and CV1 account for 88.5% of the inter-population variation of head shape among four populations. Ellipses are 90% confidence ellipses.

Population Ecology of Urban Storeria dekayi

I am trying to apply the Huggins closed captures form of the robust design model to estimate urban Storeria dekayi population size using capture-mark-recapture data. I identify recaptured individuals based on their unique head markings (Figure 5). Initial results suggest that S. dekayi is an urban adaptor that can sustain exceptionally high population densities in urban environments. For example, an estimate from a green space at my Newark study site gives a rough total population size of 22 ± 8 individuals, and a density of one individual every 17 m2 of space.

Figure 5. A simplified diagram of the urban S. dekayi population estimation process.

Feeding Ecology of Urban Storeria dekayi

Although Storeria dekayi is known to be a diet specialist that primarily feeds on land gastropods and earthworms, feeding ecology of this common snake species have rarely been documented besides several simple observational and stomach content examination attempts. I am applying the quadrat sampling method to survey the snake's potential prey community at my study sites (Figure 6; Figure 7). I also plan to do a series of diet preference tests to investigate if S. dekayi would prefer certain prey species over the others, and if S. dekayi from populations of different urbanization levels display differential prey preference patterns. Preliminary data gathered from quadrat samplings and regurgitated food (consumed preys that are occasionally thrown up by snakes upon capture) from one of my study sites (Perth Amboy) suggest that S. dekayi might display selective predation on different earthworm species.

Figure 6. A PVC quadrat used to sample earthworms.
Figure 7. Some earthworm specimens obtained from one sampling attempt at Highwood Hills Park, Leonia.