Contrasting Patterns of Inbreeding and Inbreeding Depression in Co-Occurring Spotted Turtle and Eastern Box Turtle Populations

Abstract

Maintenance of genetic diversity is critical for preserving a population's ability to adapt to changing selection pressures. As populations become smaller and more isolated, they are more likely to become inbred, which can lead to inbreeding depression and further population decline. Eastern box turtles and spotted turtles co-occur in remnant oak savanna landscapes in Ohio and Michigan, and while both species have experienced population decline due to habitat loss, remaining spotted turtle populations are smaller and more isolated than box turtle populations; thus, spotted turtles may be more vulnerable to inbreeding depression than eastern box turtles. To test this prediction, we estimated population size, measured the total area occupied, home range size, and baseline physiological stress levels in three populations of both eastern box turtles and spotted turtles. We then compared the inbreeding coefficient and two metrics of reproductive success (egg-hatching success and hatchling survival to overwintering) as potential evidence of inbreeding depression between the two species. We found that while spotted turtles had smaller population sizes, occupied smaller habitat patches, had smaller home ranges, and had higher baseline physiological stress levels than box turtles, the box turtle populations had higher inbreeding coefficients. Moreover, lower rates of egg-hatching success and hatchling survival in the box turtle compared to the spotted turtle populations suggest that the box turtle populations may be experiencing inbreeding depression, whereas inbreeding depression was not evident in the spotted turtle populations. Habitat loss and fragmentation may have contributed to inbreeding effects in box turtles by obstructing dispersal that would otherwise promote mating among unrelated conspecifics. A top priority for conserving these vulnerable turtles should be the preservation of intact tracts of habitat, restoring connectivity among sub-populations where possible, and preventing further fragmentation of occupied habitat by roads in order to promote gene flow and random mating.