Life Histories of Gopherus polyphemus: Quantifying Fecundity and Survivorships of Populations in Resource-Poor and Resource-Rich Environments

It is a well known fact that the life history of an organism is influenced by many sources, including resource availability, geographical location, disease, and predation rates (Hathaway 2012). Such factors can and do influence individual growth rates and population growth rates by affecting survival and reproductive rates within the population. In order to better understand how life histories are affected by resource availability, the fecundity and survivorship of Gopher tortoises, Gopherus polyphemus, were examined by comparing the two life history traits in two different Gopher tortoise populations, one in a resource rich environment, and the other in a resource poor environment. It was hypothesized that resource availability would have an overall positive correlation to survivorship, a positive correlation to clutch sizes of females of the same size, and a negative correlation on egg diameter. 
Gopher tortoises, regardless of resource environment, are subject to a Type III survivorship curve, with juveniles experiencing high levels of mortality. However, Gopher tortoises in a resource rich environment experience higher survival rates than those in resource poor environments, as demonstrated in Figure 1. This increased survival can drastically effect overall population growth, as higher survival means that more individuals of each age class are present and able to reproduce, contributing more and more to population growth (Table 1). While sexual maturity is not directly linked to age in gopher tortoises, those surviving longer are more likely to reach the required size (carapace length of 200-240 mm) in order to reproduce (Hathaway 2012). Thus, increased survival rates can positively affect reproductive rates and overall population growth.

Figure 1. Survivorship curves of Gopherus polyphemus originating from resource rich environments and resource poor environments.

Table 1. Life history table displaying the fecundity and survivorship probabilities for Gopherus polyphemus originating from resource rich and resource poor environments

Radiographs of female Gopher Tortoises from the two different populations were examined. The radiographs allowed the size of each female (as measured by carapace width) and the clutch size of each female to be determined. As predicted, for females of comparable sizes, those in resource rich environments produced larger clutch sizes, as seen in Figure 2. T-test values calculated in Excel determined that the mean clutch size differences between the two populations were significantly different, possessing a p-value of 0.0000850. These results demonstrated the positive correlation between resource availability and the reproductive output of individual females. Figure 2. Mean Carapace Width of Female Gopher Tortoises vs. Mean Clutch Size for individuals originating from resource rich environments and resource poor environments.

The radiographs collected were also examined to determine the mean egg diameters of the clutches for the females originating in the resource poor and resource rich environment. Due to environmental tradeoffs, it would be easily assumed that egg diameter would decrease with increasing clutch sizes, due to limitations in resources. However, the results, as seen in Figure 3, demonstrated that no significant difference between mean egg diameters existed between the two populations. The calculated t-test value further supported this