Many evolutionary and ecological factors can have strong and significant effects in small isolated populations when compared to larger, more connected populations. For example, isolated populations of endangered species are often at greater risk for extinction because of the potential loss of genetic diversity and the negative effects of inbreeding and inbreeding depression. Reduced levels of gene flow among isolated populations may also lead to differentiation in important ecological factors such as diet. The eastern massasauga (Sistrurus c. catenatus) is an endangered snake that is found in isolated populations throughout eastern North America. In this dissertation, I address both genetic and ecological consequences of these snakes existing in small isolated populations.
In Chapter 2, I used data from 19 microsatellite loci to assess levels of genetic variation and to compare rates of historical and contemporary gene flow among populations of the these snakes. Overall, massasauga populations display high levels of genetic differentiation and differ by an order of magnitude in genetic effective population size. Historical and contemporary migration rates are low and similar in magnitude. There is little evidence that populations have experienced a sharp decline in population size and a comparative modeling approach favors a model of long-term drift-migration equilibrium.
In Chapter 3, levels of inbreeding were accessed in these populations and I tested for the presence of a heterozygosity-fitness correlation between individual multilocus heterozygosity (MLH) and an estimate of fitness, body condition This analysis revealed significant variation in mean MLH and mean body condition among populations but no correlation between them. This result was true even after controlling for non-genetic factors such as sex, season of capture and year of capture. Tests for inbreeding were also negative and suggest snakes are mating randomly within each population.
Finally, in Chapter 4, I used stable isotope analysis of carbon and nitrogen to study the patterns of variation in diet over time and space across the range of this species. I employed a set of novel techniques in isotope biology to assess the relative importance of among-population or –individual variation and then calculated Manly alpha selectivity indices to determine if prey preference exists for mammalian prey species. Stable isotope analysis revealed diet is variable among populations with shrews and voles being among the most important prey items. A preference for shrews was observed in two of three populations and an avoidance of voles was observed in all three populations.
Overall, my results indicate populations have been isolated for thousands of years and that recent habitat fragmentation caused by humans has had little effect on the patterns of genetic variation in this species. Although populations have been isolated over evolutionary timescales they appear to have retained a large enough size to avoid inbreeding and the negative effects of inbreeding depression. The lack of any correlation between genetics and fitness implies genetics may be less important for the overall fitness of these snakes when compared to ecological factors. Stable isotopes appear to be a viable method for studying diet in snakes and this analysis represents an important first step in understanding if variation in body condition is a result of variation in diet.