Impacts of Anthropogenic Structures on Predation of Painted Turtle (Chrysemys picta) Nests
Abstract
Anthropogenic factors can negatively impact wildlife populations, but deleterious effects may not be universal. We investigated the relationship between nest predation and spatial proximity to anthropogenic structures (campsites, trash bins, etc.) for 1375 painted turtle (Chrysemys picta) nests over 6 years for a population on the Mississippi River, Illinois. Although varying among years, the probability of nest predation increased with greater distance from anthropogenic structures over all years combined and did not differ between supplemental food attractant structures (e.g., fish cleaning table) and nonsupplemental food attractant structures (e.g., horseshoe pits).
Habitat alteration or other human activity can influence wildlife populations in multiple ways, including those that are spectacularly or subtly detrimental. For example, roads that bisect habitats can serve as a direct (e.g., animals struck by vehicles; Gibbs and Shriver 2002; Steen and Gibbs 2004; Aresco 2005) or indirect (e.g., enhanced predation of nests along habitat edges; Temple 1987) source of mortality in turtles. These human impacts also can vary substantially both temporally and spatially. For example, lightly traveled roads should have a low direct effect on wildlife mortality, but the impact should increase when traffic is heavy.
Human activity can also influence predator–prey interactions involving turtles, potentially in complex ways. For example, supplementing predators with human-derived food can enhance population size of predators and, subsequently, alter prey numbers (Vander Lee et al. 1999; Hamilton et al. 2002). In avian systems, some predators incidentally find nests while foraging for alternate food (Vickery et al. 1993). Thus, anthropogenic structures associated with supplemental food may attract predators and increase predation on nearby nontarget turtle nests. Despite the potential for anthropogenic factors to impact wildlife populations, the indirect effects of human-derived supplemental food have been poorly studied.
Turtle nests and their predators are a good system for examining spatiotemporal variation in the effects of anthropogenic structures on turtle nest predation. Nest-site choice is crucial for turtles (Refsnider and Janzen 2010), as mortality rates are highest during embryonic development (Ernst and Lovich 2009) and predation is responsible for the majority of nest mortality (Congdon et al. 1983; Schwanz et al. 2010). In addition, the location of a nest may increase offspring vulnerability to predation prior to (Kolbe and Janzen 2002a) and after (Kolbe and Janzen 2001) emergence. Thus, nest success is a key determinant of recruitment rates in many turtle populations, and high nest mortality has caused some turtle populations to decline (Gibbons 1968). Furthermore, inadvertently supplementing predators with food via garbage, bird feeders, and so on may reduce predation on turtle nests, yet other studies suggest that such supplemental foods can increase predation on nests (Cooper and Ginnett 2000). These conflicting results warrant further investigation, especially since turtles comprise a globally imperiled taxon (Buhlmann et al. 2009).
We evaluated 6 years of data to investigate the temporal and spatial effects of anthropogenic structures on predation of painted turtle (Chrysemys picta) nests at the Thomson Causeway Recreation Area (TCRA) in the Mississippi River near Thomson, Illinois (Kolbe and Janzen 2002b; Schwanz et al. 2010). Using this 450 × 900-m human-impacted island, we tested 3 hypotheses: 1) turtle nests located closer to anthropogenic structures generally suffer higher predation, 2) spatial patterns of predation on turtle nests change with annual intensity of nest predation, and 3) probability of nest predation increases with proximity only to anthropogenic structures associated with supplemental food.
Methods
The TCRA includes a recreational vehicle campground that contains a circular road, benches, campsites, trash bins, a fish-cleaning table, horseshoe pits, and toilet facilities. At least some of these anthropogenic structures at the TCRA may contain attractants, such as supplemental food, for nest predators, which are mainly raccoons (Procyon lotor) (Kolbe and Janzen 2002b). Because anthropogenic structures are situated within the nesting habitat of painted turtles, we compared predation between nests that were laid closer vs. farther from the anthropogenic structures identified previously.
The predation status of each nest was monitored at least every 3 days from oviposition until the end of the nesting season (mid-May to late June). Depredated nests were detected by observing broken eggshells outside the nest, clear excavation of the nest cavity, and absence of intact eggs in the nest. In mid-September of each year, all remaining nests were excavated and any eggs or hatchlings noted, and final predation status (depredated or intact) was determined. Intact nests at the end of the nesting season were determined in September to have been depredated if a conspicuous, empty hole was observed in the ground at the nest site.
The objective of this study was to determine whether nest predation varied across the nesting habitat as a function of distance from anthropogenic structures. In addition, we evaluated which types of anthropogenic structures influenced nest predation. If nest predation varies as a function of proximity to or type of anthropogenic structure, the data can be partitioned appropriately to further address questions related to spatial-dependent predation. Nest predation and nest location data over 6 years (1997, 1998, 2001–2003, 2005; total number of turtle nests = 1375) were used to test the hypothesis that probability of nest predation is greater near anthropogenic structures. Nest predation intensity has fluctuated significantly, ranging from as low as ca. 20% in 1997 to as high as ca. 96% in 2005. We compared patterns of nest predation between low (ca. 30%), medium (ca. 60%), and high (ca. 90%) years. Therefore, we chose to analyze data for years when overall nest predation was similar to these low, medium, and high rankings.
Logistic regression (SAS Institute 2008) was used to test whether nest predation increased or decreased with distance from an anthropogenic structure. The probability that a nest was depredated was modeled where the response variable was categorical (1 = depredated, 0 = intact) and the predictor variable (distance from closest anthropogenic structure) was continuous. We further tested the possible impact of supplemental food attractant structures (e.g., fish table, camper pad, trash bin) vs. nonsupplemental food attractant structures (e.g., road, benches, toilet facilities, horseshoe pits) on probability of nest predation, with the latter category serving as a control for any general impact of anthropogenic structure. Distance to the closest anthropogenic structure for each nest was scaled to the nearest meter to accommodate imprecision (e.g., a nest located 7.39 m from a trash can would be scaled to 7 m). The scaled measurements were then used to assess whether nests laid closer to such structures were more likely to be depredated. To evaluate the probability of nest predation as a continuous function of distance to closest anthropogenic structure, we used a cubic spline technique originally developed for visualizing natural selection (Schluter 1988; see also Kolbe and Janzen 2002b). Standard errors for the spline were calculated by bootstrapping the data 50 times.
Measures derived from the Akaike information criterion (AIC) were used to select the model that best described the data. Our model selection procedure helped us to identify which variables were important in influencing nest predation. All interactions and combinations of variables (anthropogenic structures and distance to closest anthropogenic structure) were run using logistic regression, and then each model was assessed using AIC, specifically QAIC (Burnham and Anderson 1998). The lowest QAIC value indicated the best model among the alternate models examining the data.
Results
Nests in closer proximity to all anthropogenic structures were more likely to experience predation in 2 of the 6 years (1998 and 2000) used in the analysis in comparison to nests farther from anthropogenic structures (Table 1). However, in 2003, nests laid farther from anthropogenic structures were more likely to experience predation. In other years (1997, 2001, 2005), all of which had especially high or low predation intensity (Table 1), we did not detect a pattern of nest predation in relation to distance from anthropogenic structures.
Over all 6 years, we found a significant, positive relationship between the probability of nest predation and nest distance from anthropogenic structures (Fig. 1; Table 2). The model with the lowest AIC value did not include type of structures separately or individually (e.g., benches, toilet facilities, camper pads, horseshoe pits, or road). Instead, this model included only distance to closest anthropogenic structure, regardless of type. In general, our model predicted that probability of nest predation should increase modestly but significantly with distance from an anthropogenic structure (e.g., 0.69 at 10-m distance vs. 0.59 at 1-m distance) (Fig. 1). This outcome contradicted the hypothesis that nests closer to anthropogenic structures should experience higher predation.
Citation: Chelonian Conservation and Biology 9, 1; 10.2744/CCB-0795.1
We explored this relationship in more detail by testing whether there was a significant difference between structure type and spatial probability of nest predation (Table 3). We found no evidence that nest predation depended on distance to any particular type of structure. Nonetheless, nests located relatively close to an anthropogenic structure had a higher probability of avoiding predators, further supporting the findings of the “reduced” model (Table 2).
We further tested the possible impact of supplemental food attractant structures vs. nonsupplemental food attractant structures on probability of nest predation (Table 4). In a model accounting for the food vs. nonfood nature of anthropogenic structures and nest distance from structure, we found that the nature of the structure was not significantly related to the probability of predation. Instead, as before, nests located farther from human-made structures had higher probabilities of predation (Fig. 1). Thus, at this field site, anthropogenic structures most likely to possess supplemental food did not affect the odds of predation on proximal turtle nests.
Discussion
Nest-site choice is critical for successful breeding in a wide range of oviparous taxa (Refsnider and Janzen 2010). Nest-site choice is important for turtles, not least because nest predation is naturally high in most populations. Of increasing relevance, anthropogenic activities could alter predator behavior, influencing nest success and thereby affecting the persistence and survival of declining turtle populations. In our study, we examined the effects of distance from anthropogenic structures on nest predation in a population of painted turtles. We showed that, in some years, turtle nests are subject to predation pressure that is inversely related to the proximity of nests to anthropogenic structures. In contrast, logistic regression and cubic spline analyses over all years combined revealed that probability of predation on turtle nests was positively correlated with distance from any anthropogenic structure, indicating that nests located closer to anthropogenic structures were less likely to encounter predation. Still, the annual differences observed in the logistic regression results suggest that more years of data are necessary to develop a better understanding of these processes. Even so, we found no significant evidence of a difference in spatial-dependent predation between different types of anthropogenic structures.
Field experiments to examine patterns of turtle nest predation provide another perspective on our observational findings. Experimental studies strongly implicate visual cues as a primary means by which raccoons locate turtle nests (Strickland et al. 2010), although such nest predators also possess excellent olfaction (Conover 2007). These sensory capabilities might cause nonhabituated raccoons to be deterred by anthropogenic structures, assuming they have a fear of humans or pets (e.g., dogs), even when supplemental food is associated with such structures. Numerous campers who visit the TCRA own dogs, which could cause turtle nests located farther from anthropogenic structures to suffer a higher probability of predation from raccoons.
Variation in predator–prey interactions as a function of anthropogenic activities may alter nest survival (Vander Lee et al. 1999; Bowen and Janzen 2008). Anthropogenic structures reduced recruitment in some (but not most) years in our study, yet generalizations regarding the negative impacts of anthropogenic structures should be made with caution (sensu Hamilton et al. 2002). Moreover, although predation on painted turtle nests at the TCRA is greater closer to ecological edges in some years (Kolbe and Janzen 2002b) and exhibits evidence of positive density dependence (Valenzuela and Janzen 2001), we did not explicitly account for those 2 variables in our study. Still, the lack of enhanced predation near anthropogenic “edges” where nest numbers are often higher (e.g., camper pads) suggests that habitat edges and nest density did not play important roles in our study. Regardless, studies of the behavior of nest predators at the TCRA would greatly enrich our understanding of the predation patterns that we detected.
Our results did not reveal a significant difference between the food attractant structures and the nonsupplemental food attractant structures in probability of predation on proximal turtle nests. Therefore, supplemental food attractant structures may not influence the foraging success of turtle nest predators at this particular study site. Alternatively, supplementing predators with food resources may be a way to reduce nest predation, at least in birds (Crabtree and Wolfe 1988; Vander Lee et al. 1999). Furthermore, Boag et al. (1984) and Miller and Hobbs (2000) suggested that predation risk on bird nests tended to decrease with lesser distance from anthropogenic trails in their study. In contrast, Miller et al. (1998) reported an increase in predation on bird nests located near anthropogenic trails.
Our study has broad implications for conservation of ground-nesting species because any anthropogenic activities that attract or deter additional predators could impact such species. For example, campers and visitors should avoid leaving trash or any other supplemental food items in such habitats, especially during the turtle nesting season, given that our results show that distance to anthropogenic structures could directly or indirectly influence nest predation rates. Indeed, human activities could be critical because persistence of this turtle population (and probably most) depends on substantial survival of nestlings (Schwanz et al. 2010). Thus, results from our research on a common, easily studied turtle may inform conservationists about strategies for protecting other species with similar nesting behaviors and life histories.
Future emphasis should be geared towards modeling nest survival times to see if nests that are laid closer to anthropogenic structures are depredated more quickly than nests that are laid farther away. This assessment can be done using survival times (e.g., proportional hazard analysis). Additional studies are necessary to quantify the long-term effects of anthropogenic structures on the population dynamics of turtles and their nest predators.
Cubic spline analysis of the probability of predation on nests of the painted turtle (Chrysemys picta) with increasing distance from anthropogenic structures. Results for all 6 years of this study combined are shown. Dashed lines bracketing the spline (solid line) represent standard errors calculated by bootstrapping the original data 50 times.
