Wetland hydrology and soils as components of Virginia bog turtle (Glyptemys muhlenbergii) habitat
Reptile populations are in decline worldwide, with turtle species showing some of the largest drops in population. The bog turtle (Glyptemys muhlenbergii) is considered one of the rarest North American turtle species, and this rarity is made more severe by anthropogenic factors. The wetland habitats that are used by bog turtles contain seepage areas and soil saturation that are characteristic of specific types of wetlands, suggesting that bog turtle rarity may in part be attributed to narrow habitat requirements. In this dissertation, I have sought to spatially and temporally characterize the hydrology and soils of wetlands that are used by bog turtles in an effort to determine how these factors are related to the species' habitat requirements, movement, and activity.
In Chapter 1, I evaluated hydrology over a continuous 28-month period using shallow groundwater wells in six wetland fens known to be used by bog turtles for breeding and six apparently similar, but unused, wetlands. The saturated surface area near wells was measured and correlated with depth to the water table. Overall, water tables remained high, with mean monthly depth to the water table for all 12 wetlands remaining > -35 cm (depth below surface datum is negative). Bog turtle breeding wetlands had significantly higher mean water tables and surface saturation than wetlands where no turtles were encountered, particularly during and after the two-year drought occurring in 2007 and 2008. Findings of Chapter 1 suggest that relatively small differences in water table hydrology can affect bog turtle biology and use of wetlands.
Bog turtles access soils and move through them to thermoregulate, find cover, and hibernate. Most wetlands used by bog turtles are also grazed by livestock that can modify soil strength. In Chapter 2, I identified dominant soil series and sampled surface soils from wetlands used by bog turtles and similar, but unused, wetlands. Samples were analyzed for organic carbon content and particle size distribution. Organic carbon content was greater in areas that were always wet (10%) than temporarily wet areas (5%). Somewhat higher organic carbon contents were present in wetlands that were used by bog turtles (8.8%) than wetlands where turtles were never encountered (5.7%). Soil textures were sandy loams and silt loams on all the study wetlands. Based on measurements of soil strength made with a static cone penetrometer, bog turtles selected wetland locations with low-strength soils. The mean and variability of soil strength were no different between grazed and ungrazed areas. The physical qualities of surface soils in bog turtle wetlands are dependent on consistently high water tables.
In Chapter 3, I described three field studies in which I deployed temperature loggers to measure and contrast ambient air and soil temperatures to turtle carapace temperatures during activity and hibernation. I used temperature signatures to evaluate the timing and cues of spring emergence and to recognize thermoregulatory activities during periods of turtle activity. Mean daily turtle temperatures (n=16 turtles) during the coldest portion of two winters ranged between 1.3°C and 6.1°C, with one turtle experiencing 14 continuous days at temperatures between -1°C and 0°C when ambient temperatures dipped below -10°C. Water tables remained within 10 cm below the soil surface throughout the winter, preventing freezing temperatures for shallow hibernating turtles. Soil temperatures at 10 cm depth were a primary cue for spring emergence. Daily mean summer turtle temperature (n=8) was 20.8°C. My findings indicated that the presence of water near the surface and the ability for turtles to submerge themselves in mud are important for thermoregulation.
In Chapter 4, I used radio telemetry to evaluate bog turtle activity (distance moved / hour), linear range, and the pathways used for dispersal. I also investigated bog turtle activity during sampling periods with either wet or dry hydrology. Mixed model analysis indicated that turtles were much less active between 18:30 and 09:30 relative to the daytime and that turtles were most active during times when hydrology was categorized as wet during 2008 when moderate to severe drought was the dominant condition. Sex was not a factor in turtle activity. Bog turtle paths during large movements (≥ 80 m) were mostly contained to areas within 80 m of USGS 7.5â quadrangle mapped streams. Turtles made large movements more frequently during dry conditions. Results suggested that drying conditions can stimulate bog turtles to either remain inactive in sparsely available saturation or to move long distances to find wetter conditions. Future conservation efforts should focus on allowing safe dispersal among habitats by reducing obstructions and risks to travel near streams. I n chapter 5, I used GIS-derived data to compare land cover, stream order, topographic wetness index inverse, presence of hydric soils, and presence of National Wetland Indicator (NWI) wetlands on bog turtle occupied wetlands (n=50) to the same variables on apparently unoccupied (n=48) wetlands or random areas (n=74) along streams. Occupied areas differed from random areas in having near zero values of the topographic wetness index inverse (indicating areas with low slopes and large upstream drainage areas that are more prevalent in wet portions of the landscape), the presence of > 50% low vegetation typical of non-forested agricultural areas, and presence of 3rd order streams. I used significant regression coefficients to create a GIS layer of high quality bog turtle habitat over the landscape, and tested this layer with bog turtle field survey data collected in 2009 independently of model building data. The resulting model has the potential to quickly rule out large portions of the landscape as potential bog turtle habitat.
Finally, in Chapter 6, I provided general recommendations for managing bog turtle habitats in Southwestern Virginia. Managing bog turtle wetlands must emphasize the maintenance of high water tables, while avoiding inundation. Maintaining connectivity among wetlands used by bog turtles is an important aspect to consider when developing bog turtle conservations plans associated with development and other land use changes. Educating landowners and enforcing existing wetland laws are imperative for effective bog turtle management in Southwestern Virginia.