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Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Organismic and Evolutionary Biology
Year Degree Awarded
Month Degree Awarded
Timber Rattlesnake suffered significant range reduction in the past few centuries. Here, I studied the demographics, movement patterns and habitat use of a metapopulation in Berkshire County, Massachusetts and population genetics of northeastern populations. The metapopulation was split into four subpopulations, based on geographic distance and genetic distance. Differences in gender and color morph (yellow and black) ratios were analyzed by subpopulation. Population size estimates were done for each subpopulation and for the metapopulation. Body condition index (BCI) was compared between individuals exhibiting signs of snake fungal disease (SFD) and with those not exhibiting symptoms. A total of 185 individuals was marked, with 32 recaptures, and a 65:35 (male:female) sex ratio. There was no difference in sex ratio by subpopulation (P = 0.23). Color morph did vary significantly among subpopulations (P < 0.0001) with yellow being the dominant color in three subpopulations. SFD was observed in 10.3% of individuals, all males. Three of the infected males were radio-tracked and exhibited healing of lesions with each shed. There was no difference in BCI of individuals due to lesion presence. Six cases of mortality were observed, (three had radiotransmitters) with one predation, one human kill, and four of unknown causes.
Movement patterns can be influenced by many factors, e.g. resource needs that change throughout the year, reproductive condition, and disease. Using radiotelemetry I investigated variation in home range size, 95% kernel density estimates, and maximum distance from a source den. Gravid females moved significantly less often, and used significantly smaller ranges than males and non-gravid females. Individuals used smaller ranges and moved less often during the shedding season than during the active season, supporting a hypothesis that individuals moved farther and more frequently while foraging and mate searching. SFD presence did not affect any movement parameters. Home range size did not vary annually; however, individuals tracked for 4-5 years appeared to use different foraging areas each year, often returning to previously used areas in following years. The results presented here identify key spatial areas, such as basking and foraging areas, for this metapopulation. Habitat selections provide a basis if future management strategies (e.g. headstarting neonates and translocation) are implemented using individuals from this region which should use the same or similar areas for management plans.
Habitat needs often depend on behavior (e.g. foraging, mate searching, gestating), and can vary seasonally and with health condition. I investigated intraspecific variation with regard to health status and sex (male and non-gravid female) using classification tree (CART) analysis, as well as yearly and seasonal variation compared to random available habitat measures using paired logistic regression. Snake fungal disease (SFD) presence and sex were not correlated with habitat selection. Overall, individuals preferred areas of increased rock cover, decreased canopy cover, lower slope, and increased vegetative cover compared to available random sites. Individuals preferred rock outcrops under open canopies during the shedding season, and used open forested areas with high vegetation cover and tree density during the active season. This population is located in one of the largest intact areas of old growth forest in New England, whereas populations in the region inhabiting other areas where the habitat has been severely altered by humans offer difficult management options.
Understanding how genetic variation is distributed within and among populations of a species produces a basis to make conservation management recommendations. Peripheral populations often have lower genetic diversity than core populations and may need artificial gene flow for future population persistence. I quantified the genetic diversity in 16 peripheral Timber Rattlesnake populations in the northeast using 13 microsatellite loci. These populations were all within the peripheral extent of the species’ northeastern range, with several located in the core area of the range in eastern New York and the Appalachian Mountains. Populations were highly differentiated from each other (mean FST= 0.175). There was no correlation between genetic distance and geographic distance (R = -0.0878, P = 0.67). Seven population level clusters were detected (K = 7), all of which corresponded to single peripheral populations, and suggesting that genetic drift has led to population differentiation. Elevated influence of drift is likely the result of regional loss of over 50% of the rattlesnake population in the past few centuries. Within the largest New England area of occurrence, there appears to be a metapopulation structure, with gene flow among nearby den regions. For future population persistence, assisted gene flow or ‘genetic rescue’ might provide a viable management action for the most-at-risk populations. If assisted gene flow is implemented, results presented here should serve as a guide for determining which populations are genetically diverse enough to serve as the best donor populations for imperiled populations.
Stengle, Anne, "HABITAT SELECTION, CONNECTIVITY, AND POPULATION GENETICS OF A TIMBER RATTLESNAKE (CROTALUS HORRIDUS) METAPOPULATION IN SOUTHWESTERN MASSACHUSETTS AND NEW ENGLAND" (2018). Doctoral Dissertations. 1208.