An Analysis of Preferred Habitat for the Common Garter Snake

By Ian Kinloch MacGregor and David Edward HellerMay 25, 2005

Introduction

• The common garter snake (T. sirtalis), is found in the area around Groton.

• The Groton area has many habitats

– Forest

– Vernal Pools

– Field

– Residential Area

Introduction Cont.

• What is the preferred habitat of the common garter snake?

– They are known to find shelter under cover.

• Logs

• Trees

• Leaf litter

• Shrubs

Introduction Cont.

• Snakes bask in the field and in sunny spots in the forest (Kaplan 2002)– The field habitat is a common place to find garter

snakes• This is prime basking area.• Garter snakes prey upon many different species

that live in the field.– We have found snakes in both the forest and the

marsh as well• The garter snake prefers moisture or a nearby source of

water (Conant 1998).

Introduction Cont.

• We used wood cover boards as traps.– The cover board should provide a good

environment for the garter snake.– We planned to check them to determine

where their preferred habitat is.

• Although the traps are affective,– Snakes have been seen in the fields and

forest • Possibly basking or hunting.

Introduction Cont.

• The variables that we are measuring:– Abundance of common garter snake– Distribution of common garter snake– Vegetation of potential garter snake habitats

• Measured subjectively

– Distance from water to potential habitat– Ground Cover

• Measured subjectively

– Other points of interests• Canopy cover, presence/absence of prey items, ect.• Measured subjectively

Natural History of the Common Garter Snake (T. sirtalis)

• Habitat: Common garter snakes are very widespread, highly adaptable and can survive extreme environmental conditions. Common garter snakes are found in a wide variety of habitats, including meadows, marshes, woodlands, and hillsides. They tend to prefer moist, grassy environments. They are often found near water, such as near the edges of ponds, lakes, ditches, and streams, and are common in suburban and urban areas with plenty of cover (debris, boards, vegetation, logs, or rocks). (http://www.enature.com/fieldguide/showSpeciesRECNUM.asp?recNum=AR0121)

Natural History Cont.

• Diet: Garter snakes typically eat earthworms, amphibians, leeches, slugs, snails, insects, crayfish, small fish and other snakes. They seem immune to the toxic skin secretions of toads and can eat them without harm. Occasionally small mammals, lizards, or baby birds are eaten as well. Common garter snakes find their prey using their excellent sense of smell and their vision. They use several different hunting methods, such as peering, craning, and ambushing to capture their prey. The different techniques describe the way the snakes move while they hunt. They immobilize their prey using their sharp teeth and quick reflexes. The saliva of common garter snakes may be slightly toxic to some of their small prey, making it easier to handle them while they are being eaten. Like other snakes, they swallow their food whole. (Conant 1998)

Natural History Cont.

• Reproduction: Live-bearing. Mates mostly late March to early May, occasionally in fall. 7-85 young born late June to August, earlier in Florida, to early October in the North. Young are 5-9" (13-23 cm) long; mature in 2 years. (http://www.enature.com/fieldguide/showSpeciesRECNUM.asp?recNum=AR0121)

• Status in Massachusetts: Common garter snakes are some of the most common and abundant snakes throughout the eastern United States, at least partly because they do well in urban and suburban areas. Despite the fact that they are harmless snakes, they are often persecuted by humans. Pesticide use in some areas has significantly reduced common garter snake populations. Habitat destruction and over-collection for the commercial pet trade have also led to a decline in the number of garter snakes in the wild. Water pollution is a problem for this species, because so much of its food is aquatic. Northern populations are more vulnerable than southern ones, because they hibernate in larger groups (which are easily harvested) and produce smaller numbers of young each year. It is important to continue to monitor populations of 'common' species as declines in their populations can tell us a great deal about environmental health. (http://animaldiversity.ummz.umich.edu/site/accounts/information/Thamnophis_sirtalis.html)

Introduction Cont.

• Hypothesis:– We expect to find that there is a greater

abundance of Garter Snakes in the field than in the forest or marsh.

• We expect that each cover board will attract a similar amount of snakes

– The snakes will have no preference between cover boards, regardless of distance from forest.

» We expect the distance from forest to be negligible since our cover boards are not more than ten meters apart.

Methods• Cover Board Placement: We placed 4 large cover boards, 2 medium

cover boards, and 1 small cover board. All the boards were made of plywood. The small board and 1 medium sized board were placed on the east side of the path, while the other boards were placed on the west side of the path.

• Vegetation Survey: We recorded the types of vegetative structures in the area around the 3 cover board locations. We recorded the types of trees found that were greater than 10 inches DBH. We also noted any special features (leaf litter, ground debris, sunlight levels, ect).

• We also recorded the distance from each study area to the nearest major source of water. We defined a “major” source of water as either a vernal pool, a pond, a river or stream, Lake Romeyn, ect.

Aerial Photograph of Study Area

Digital Photograph of Study Area

Digital Photograph of Cover Boards

Data

Date Location Species Found

22-Apr Cover Board 1 Garter Snake

26-Apr Cover Board 1 Garter Snake

12-May Cover Board 2 Garter Snake

19-May Cover Board 5 Garter Snake

Study Area: Field

Vegetation: Long grasses and shrubs were present, but no trees over 10 DBH were within 10 meters from the plot.

Distance from water: The closest vernal pool was the back of the landfill pool which was 64 yards away.

Ground Cover: Their was plenty of ground cover (grass) around the cover boards.

Note: The boards may not have been out their long enough to attract the snakes. Also, the natural elevation of the field study area has made it so that there is a permanently moist area to the west of the cover boards (this could be due to either trapped moisture or leakage from pipes). We also found lots of slugs, beetles, and other insects under the boards. There is evidence of annual disturbance in the area (mowing).

Study Area: ForestVegetation: There was an abundance of maple, oak and pine trees that

were over 10 inches DBH. The pine trees were in a stand on the east side of the path across from the boards. Maple and oak trees were scattered throughout the area, although the area was dominated by maple trees.

Distance from Water: The shore of Lake Romeyn was 4 yards away.Ground Cover: The was significant ground cover in the form of dead

pine needles, twigs, and leaves. Their were a few logs scattered around the study area. Shrubs and small bushes made up a large percentage of the ground cover in the study area.

Note: We found toads, and a few salamanders in the study area as well as worms, and other insects. There was no evidence of disturbance. The soil was also very moist in this area. Also, due to the trees, there was a limited amount of sunlight that could penetrate the canopy.

Study Area: Marsh

• Vegetation: There were 4 maple trees and 2 pine trees that were over 10 inches DBH within 10 meters of the study area.

• Distance from Water: The shore of Lake Romeyn was 12 yards away. Also, the extended landfill pool was 17 yards away

• Ground Cover: There was extensive leaf litter (maple and oak leaves mostly) with pine needles as well. There were a few shrubs in the area, but there was not any logs or other debris.

• Note: slugs were found in this study area, as well as red backed salamanders. There was no evidence of disturbance in the area. Also, the ground was very moist and there was very little canopy cover.

Conclusions

• Problems: Although our initial experimental design called for 4 different cover board locations, the corrugated iron or tin cover boards were never made available for our study. Although we did manage to place boards in the field, forest, and wetland habitat, we did not have enough boards in the forest and wetland locations to make a significant analysis. Also, rain and cold temperatures during the month of May inhibited snake movement and hunting during the night. Without warm temperatures to goad the snakes into moving, the cover boards yielded almost no data. Also, it should be noted that capturing snakes using cover boards is very difficult. Although it is the best way to take population surveys of snakes in the field, it helps if the boards have been established for a long time in an area that is well known to have snakes. There is no evidence that suggests that cover boards in an area attract snakes, but there is no evidence to dispute the notion either.

Possible Outcomes

• Garter Snakes are found to be more abundant in the field.– The field is an important habitat for the snakes to

bask or hunt.– There is not enough sunlight in the forest.– Snakes spend most of their time hunting in the field or

along the edge• Garter Snakes are found to have an equal

distribution.– The snakes are generalists and have no particularly

preferred habitat. Traps could be set anywhere to catch this species.

Possible Outcomes cont.

• No snakes were found.– Cover boards are not useful traps. – There are possible confounding variables that we have not

measured. – There was no optimal weather. – Snake population is declining.

• There was a higher abundance of Garter Snakes in the forest.– The forest is an important habitat and the preferred habitat of the

Garter Snake. – The field acts as a garter snake barrier.– There is more cover in the forest which attracts more garter

snakes

Possible Outcomes cont.

• There was a higher abundance of Garter Snakes in the marsh.– The marsh is the preferred habitat– The snakes need moisture at all times.– The snakes have a higher success rate when

hunting in the marsh.

Improvements

Increase the length of the study to ensure better weather. More snakes may be caught if the study is done from late March to early May, which is when the snakes breed. Well established cover boards may also yield better results.

Literature Cited• Conant, R., J. Collins. 1998. Peterson Field Guide to Reptiles and

Amphibians of Eastern and Central North America. Houghton Mifflin. Boston.

• Kaplan, M. 2002. Herp Care Collection: Not-So-Common Garters. Updated Agust 4, 2004. Accessed May 20, 2005. http://www.anapsid.org/garters.html

• National Wildlife Federation. Reptiles and Amphibians: Common Garter Snake. 2003. Accessed May 21, 2005. http://www.enature.com/fieldguide/showSpeciesRECNUM.asp?recNum=AR0121)

• University of Michigan Museum of Zoology. Animal Diversity Web. 1995. Last updated 2005. Accessed May 26, 2005. http://animaldiversity.ummz.umich.edu/site/accounts/information/Thamnophis_sirtalis.html