False map, spiny softshell and smooth softshell turtle nest and nest-site habitat characteristics along the lower stretch of the Missouri National Recreation River in South Dakota.

摘要

Little is known about the ecology and reproductive habits of turtles in South Dakota. The spiny softshell (Apalone spinifera) and smooth softshell (A. mutica) are listed as species of concern in South Dakota and the false map turtle (Graptemys pseudographica ) is listed as state threatened. Information relating to habitat and nest site characteristics is needed to form sound management plans.;Surveys were conducted for turtle nests along the Missouri National Recreation River from just above Lewis and Clark Lake (river mile (RM) 835) to Ponca State Park (RM 753) in 2006 and 2007. Turtle nests were located by walking shorelines and sandbars while searching for tracks, scrapes and nesting turtles. Once located, each nest was identified to genus and recorded on global positioning system (GPS). Nest characteristics taken were number of eggs, egg size, depth to top egg in nest, and distance from water. To determine habitat variables female turtles were selecting for, I examined nest site, on-site location and off-site location land cover classification and habitat vegetation variables including type, height, and percent cover. Substrate composition and debris were also assessed.;Turtles were first observed nesting in 2006 on 6 June and nesting continued until 23 July 2006. During 2007 the first nest was found on 28 May and the last nest located was 28 June. In 2007, 230.5 hours were spent searching sandbars resulting in a detection rate of one nest every 5.5 hours. I found 17 false map and 45 softshell nests intact and excavated them for measurements.;The mean number of eggs in an Apalone sp. clutch was 14.61, mean egg diameter was 23.97 mm, and mean depth to the top egg was 9.09 cm. The mean number of eggs in a G. pseudogeographica nest was 10.5, mean egg width was 23.16 mm, mean length was 34.42 mm + 0.210 and the mean depth to the top egg was 9.95 cm. The mean straight line distance to water traveled by nesting Apalone sp. was 61.27 m and the mean straight line distance to water traveled by nesting G. pseudogeographica was 54.24 m. There was no difference in straight line distance to water between Apalone sp. and G. pseudogeographica (p = 0.552, t = 0.601, df = 36.47).;There was a difference between slopes of man-made sandbars and natural sandbars (p = 0.003, t = -3.699, df = 13.35). When the mean slope of a sandbar was compared to the mean straight line distance from nest to water on the same sandbar Apalone sp. had a R2 value of 0.1487 (n = 13) and G. pseudogeographica had an R 2 value of 0.2471 (n = 7).;In 2007, the depredation rate of monitored nests was 36%. There was no depredation of nests on the man-made sandbars. Straight line distance to the water and habitat characteristics at the nest site had no effect on nest depredation rates of located nests. I found 184 depredated nests (175 Apalone sp. and nine G. pseudogeographica).;Apalone sp. nested exclusively in bare sandy areas while G. pseudogeographica tolerated sparse vegetation around the nest site. A nest-site selection model was constructed using a matched-pair stepwise logistic regression at the off-site level for Apalone sp. using the habitat variables that had a difference (p ≤ 0.05) between the nest site and the off-site. The habitat variables used included: terrestrial herbaceous vegetation (p = 0.046), woody vegetation (p = 0.008), woody vegetation stem number (p = 0.020), mean vegetation height (p = 0.032), max vegetation height (p = 0.025) and leaf debris (p = 0.046). The final calculated model included max vegetation height and leaf debris (p = 0.007, McFadden Rho 2 = 0.123). A habitat nest-site selection model could not be constructed for Apalone sp. at the on-site level or for G. pseudogeographica at either the on-site or off-site level because there were no differences between habitat variables. Spatial analyses of the depredated nests at RM 804.5 suggest those turtle nests are clustered in bare open sand. Three Apalone sp. nests hatched before surveys were completed in 2007. Based on this data, an estimate incubation length for Apalone sp. turtles was 70--71 days. Vegetation surrounding the nest site may have decreased incubation length by causing an increase in nest cavity temperature.;Both Apalone sp. and G. pseudogeographica utilize natural and man-made sandbar habitat for nesting. However, natural nesting beaches are being reduced at a rapid pace, and man-made sandbars may not provide the most ideal conditions for hatchlings and adult turtles. Turtle populations along the Missouri National Recreation River are at risk for further decrease if management and conservation efforts are not focused on providing quality nesting habitat.