THE POPULATION STATUS AND HABITAT SELECTION OF THE

TIMBER RATTLESNAKE (Crotalus horridus) IN EAST-CENTRAL ILLINOIS

C. Drew Foster

Department of Biological Sciences, Eastern Illinois University, Charleston, IL

Abstract

The timber rattlesnake, Crotalus horridus, is a top-level predator inhabiting the forests of eastern North America. Communal denning and anthropogenic alteration of suitable land surrounding den sites make C. horridus a vulnerable species to extinction or extirpation. Habitat alteration and fragmentation are believed responsible for population declines observed in 20 states, including Illinois where it is protected as a threatened species. Presently in Illinois, C. horridus is found primarily in association with the majority of the remaining forestland in the extreme southern portion of the state and along the Mississippi River bluffs. In this study, I determine the size of a relict population of C. horridus in Clark County, Illinois, and habitat selection by individuals in this forested island surrounded by row crop agriculture. Collected individuals are marked and implanted with transmitters that allow relocation and temperature monitoring throughout the activity season. I will describe their habitat and activity range (ha) in comparison to the various habitat types available. Through a better understanding of this species’ activity and habitat selection patterns, I will be able to recommend an efficient management strategy for the conservation of this species in Illinois.

Introduction

Over 90% of historical habitat types in Illinois have been altered, exacting a negative effect on the top predators in this state [6,7]. In Illinois, C. horridus (Figure 1) is found in forestland of the extreme southern portion of the state and along the Mississippi River border counties [9]. Populations of C. horridus have vanished from much of Illinois as land has been converted from forest to farmland (Figure 2).  A landowner survey, commissioned by the Illinois Department of Natural Resources (IDNR) in 2000-2001, compiled credible C. horridus sightings centering around an area in Clark County locally referred to as “Rocky Hollow” (Figure 3).  Habitat use in C. horridus has been described as dense deciduous forest with a great amount of leaf litter, relatively few fallen logs, and a relatively open canopy [2,3,4,12].

	Figure 1.  Timber rattlesnake (Crotalus horridus)
	Figure 2. Agricultural field fragmenting and surrounding the forest habitat.
	Figure 3.  A typical sandstone outcropping at the “Rocky Hollow” site.

Purpose of research

To locate and quantify a relict population of C. horridus in Clark County, Illinois.

To determine the sex ratio and population sustainability (e.g., > 8 mature females; [3]).

To determine the location/structure of the associated communal hibernacula.

To determine the home range of individuals.

To determine habitat selection of individuals during the activity season.

Methods

Collected individuals are marked:

1. By clipping a unique combination of ventral scales [1].

2. With a passive integrative transponder (PIT tag; [8]).

3. With a radio-transmitter, surgically implanted into adult snakes [5,10] which will allow for relocations through the activity season which include thermistors that provide snake core body temperature (± 0.5 0C). 

Their mass (± 0.1 g), snout-vent length (SVL; ± 0.5 cm), and tail length (TL, ± 0.5 cm) is recorded. 
Gender of each individual is determined using a cloacal probe. At initial capture and relocations the coordinates of the snake’s position is recorded using a global positioning system and general habitat features and microhabitat (1 m² plot centered on the snake) are assessed and compared to available habitat types and sizes (ha).  Home range for each snake is calculated using the minimum convex polygon method [11], based on relocation data. Appropriate statistical analyses are applied to all data. 

Results

The IDNR landowner’s survey documented rattlesnake sightings in the "Rocky Hollow" area as recent as 2001.  Despite not finding C. horridus during the first season of searching, I found four new county records and fourteen post-1980 records [9].

Literature Cited

[1]   Blanchard & Finster. 1933. Ecology 14:334-347.

[2]   Brown. 1992. Pp. 251-258 in Biology of the Pitvipers. Selva, Tyler, TX.

[3]   Brown. 1993. Herpetological Circular No. 22, SSAR. Lawrence, KS. 78 pp.

[4]   Fogell. 2000. Univ. of Nebraska Master’s Thesis. Lincoln, NE. 91 pp.

[5]   Hardy & Greene. 2000. Sonoran Herpetol. 13:110-114.

[6]   IDENR. 1994. IL Dept. Energy & Nat. Resources. Springfield, IL.

[7]   Iverson et al. 1991. IL Council Forestry Develop. & INHS. Springfield, IL. 24 pp.

[8]   MacGregor & Reinert. 2001. Herpetol. Rev. 32:170-172.

[9]   Phillips et al. 1999. INHS. Champaign, IL. 282 pp.

[10] Reinert. 1992. Pp. 185-197 in Biology of the Pitvipers. Selva, Tyler, TX.

[11] Rose. 1982. J. Herpetol. 16:253-269.

[12] Walker. 2000. Purdue University Master’s Thesis. West Lafayette, IN. 45 pp.

Acknowledgments

I would like to thank Dr. Stephen J. Mullin for all of his assistance, Robert Szafoni for initiating this project, IDNR for financial support, all of the landowners for allowing their land to be searched, especially one Harold East (R.I.P.) for having invited me into his home for conversation on more than one occasion, J. Brian Towey, Erin Barding, William James Jordan IV, Paul Hampton, Pat Enstrom, Erin Casey, Jennifer Fish and Jacob Hesterman, all for trekking across my topographically diverse site in search of this elusive species, and my fiancé Jennifer Chasteen for putting up with my antics.