Incidental nest predation in freshwater turtles: inter- and intraspecific differences in vulnerability are explained by relative crypsis

Abstract

There has long been interest in the influence of predators on prey populations, although most predator-prey studies have focused on prey species that are targets of directed predator searching. Conversely, few have addressed depredation that occurs after incidental encounters with predators. We tested two predictions stemming from the hypothesis that nest predation on two sympatric freshwater turtle species whose nests are differentially prone to opportunistic detection-painted turtles (Chrysemys picta) and snapping turtles (Chelydra serpentina)-is incidental: (1) predation rates should be density independent, and (2) individual predators should not alter their foraging behavior after encountering nests. After monitoring nest survival and predator behavior following nest depredation over 2 years, we confirmed that predation by raccoons (Procyon lotor), the primary nest predators in our study area, matched both predictions. Furthermore, cryptic C. picta nests were victimized with lower frequency than more detectable C. serpentina nests, and nests of both species were more vulnerable in human-modified areas where opportunistic nest discovery is facilitated. Despite apparently being incidental, predation on nests of both species was intensive (57% for painted turtles, 84% for snapping turtles), and most depredations occurred within 1 day of nest establishment. By implication, predation need not be directed to affect prey demography, and factors influencing prey crypsis are drivers of the impact of incidental predation on prey. Our results also imply that efforts to conserve imperiled turtle populations in human-modified landscapes should include restoration of undisturbed conditions that are less likely to expose nests to incidental predators.

Cues used by predators to detect freshwater turtle nests may persist late into incubation

“…We cannot discount the possibility that removal, processing, and reburial of eggs by researchers in the first 24 h affected depredation rates, especially at the start of incubation. although we present data for the first 72 h post-oviposition, we are aware they may not be indicative of depredation rates on nests from which eggs were not removed (Congdon et al 1983;Tinkle et al 1981;Christens and Bider 1987;Burke et al 2005;strickland et al 2010;geller 2012;Wirsing et al 2012).…”

“…Many studies of turtle nest depredation report that most depredation events occur within the first week post-oviposition (Tinkle et al 1981;Christens and Bider 1987;Congdon et al 1983Congdon et al , 1987Robinson and Bider 1988;Burke et al 2005;strickland et al 2010;geller 2012;Wirsing et al 2012;Holcomb and Carr 2013). For example, Congdon et al (1983) found that 87% of Blanding's Turtle (Emydoidea blandingii) nest depredation occurred within the first 5 days post-oviposition.…”

Investigation of cues used by predators to detect Snapping Turtle (Chelydraserpentina) nests

“…However, predation may continue throughout incubation and may even show a peak near the approximate time of hatching, indicating either that cues persist throughout incubation or that predators can use chemosensory, auditory, or other cues associated with hatching to find late-term nests (Snow 1982;Wirsing et al 2012;Mitchell et al 2014;Riley and Litzgus 2014).…”

“…High nest predation can substantially limit recruitment in some species and in extreme cases can lead to population declines. Nest predation is often higher in anthropogenically impacted environments because populations of "subsidized" predators, such as raccoons (Procyon lotor (L., 1758)) that scavenge from waste bins in urban or recreational areas, are not limited by the natural carrying capacity of the area (Boarman 1997;Wirsing et al 2012). Where predator populations become particularly large they can have substantial impacts, particularly on declining, threatened populations.…”

“…Unnaturally high nest predation rates in many areas (i.e., due to subsidized predators; Marchand et al 2002) can severely reduce recruitment into populations. Due to their size, Snapping Turtle nests may be more easily detected by predators than those of some other species (Wirsing et al 2012) and this species does not disguise its nest. The relative ease with which their nests can be mimicked presents an opportunity to test hypotheses about the cues that nest predators rely on to locate recently laid turtle nests.…”