Effects of Deer Feeders, Habitat and Sensory Cues on ...

Am. Midl. Nat. 147:123–134

Effects of Deer Feeders, Habitat and Sensory Cues on Predation Rates on Artificial Turtle Nests ALISON M. HAMILTON1,2

AND

ADAM H. FREEDMAN

Department of Wildlife Ecology and Conservation, IFAS, University of Florida, P.O. Box 110430, Gainesville 32611 AND

RICHARD FRANZ Florida Museum of Natural History, University of Florida, P.O. Box 117800, Gainesville 32611 ABSTRACT.—We used artificial nests modeled after those of Trachemys scripta, a wide-ranging freshwater turtle species, to evaluate the effects of deer feeders, habitat type and visual and olfactory cues on nest predation in northern Florida. Nests were placed at lake sites with and without deer feeders, and in three habitat types: road, edge and forest. Overall nest mortality due to predators was high (89%). Nest survival was 5.5 times higher at lakes without deer feeders than at those with feeders. Among habitat types, survival was highest at road nests (23%), while survival rates at forest (4%) and edge nests (6%) were lower than that at road nests. No significant difference in survival was detected with respect to the presence of visual or olfactory cues. Our results suggest that deer feeders reduce recruitment in freshwater turtle populations, and that generalizations regarding the negative impacts of roads should be made cautiously, in a taxon and site-specific fashion.

INTRODUCTION Survivorship during the egg stage is extremely low in chelonians. Although amount of water (Ragotzkie, 1959; Gemmell, 1970; Ewert, 1979; Jackson and Walker, 1997), temperature (Breckenridge, 1960) or encroachment of vegetation into the nest cavity (Turkowski, 1972) are responsible for some mortality during the egg stage, predation is the primary cause of nest failure for turtles (Ewert, 1979; Tinkle et al., 1981). Rates of nest predation are generally high, varying among populations and species. Predation of Emydoidea blandingii and Chelydra serpentina nests in Michigan was 67% and 70%, respectively (Congdon et al., 1983; Congdon et al., 1987) and 94% of Chelydra serpentina nests in New York were destroyed by predators (Petokas and Alexander, 1980). In some years for particular populations, predation levels reach 100% (Congdon et al., 1983; Congdon et al., 1987; Landers et al., 1980). Therefore, life histories of many chelonians reflect the challenge of coping with high early mortality ( Jackson, 1988). The production of large, and frequently multiple clutches within a season, is a reproductive adaptation consistent with long-standing coevolutionary relationships with nest predators. Nest predators are thought to be important in structuring turtle populations ( Jackson, 1988), and turtles may exhibit nesting behavior that minimizes nest predation. Several species of chelonians void their bladders during nesting. This behavior has been suggested to supplement nest moisture (Legler, 1954; Jackson and Walker, 1997), loosen the soil to facilitate nest construction (Carr, 1952), serve an antifungal purpose, deter predators, serve as a cement to make excavation of the nest difficult or simply may be a physi1 2

Present address: Department of Biology, Box 9019, University of North Dakota, Grand Forks, 58202 Corresponding author: e-mail: alison�[email protected]

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ological response to egg laying with no adaptive function. Legler (1954) suggested that the odor associated with this fluid may enable predators to locate nests. The underlying mechanism for this behavior is unclear, and the potentially negative effect on hatching success has not been tested. As some predators such as raccoons (Procyon lotor) rely on olfactory cues to locate prey items (Whelan et al., 1994), it is possible that the odor associated with turtle bladder water may attract potential predators, as has been found for the scent associated with defecation in some species of waterfowl (Clark and Wobeser, 1997). Nest placement may also influence predation rates, but previous studies have inconsistent findings. Predation rates at avian nests may be elevated along roads and ecological edges (Gates and Gysel, 1978; Ambuel and Temple, 1983; Wilcove et al., 1986; Rudnicky and Hunter, 1993). Likewise, survival rates of turtle nests, susceptible to many of the same predators as avian nests, may also vary depending on nest placement (Legler, 1954; Gemmell, 1970; Christens and Bider, 1987; Temple, 1987). Artificial nests are used commonly as a surrogate for real avian nests to evaluate variation in predation intensity between habitat types, or with proximity to habitat edges (Burkey, 1993; DeGraaf et al., 1999). Although rates of depredation upon artificial nests may not reflect those for natural nests (Major and Kendal, 1996; Butler and Rotella, 1998), artificial nests are useful for identifying ecological patterns and relative effects of different habitat management practices on nest survival (Burkey, 1993; Vander Haegen and DeGraaf, 1996; Vander Lee et al., 1999; Wilson, 1998; Cooper and Ginnett, 2000). Because turtle nests are difficult to locate, use of artificial nests facilitates experimental designs involving large sample sizes and equal numbers of nests within each treatment. Such experimental approaches are useful in answering questions surrounding the nesting behavior in turtles, such as the role of an olfactory or visual cue on predation rates. Densities of mammalian nest predators vary at multiple spatial scales. Dijak and Thompson (2000) found a greater abundance of raccoons and opossums in agricultural landscapes with high stream density than in forested landscapes with low stream density; greater abundance of raccoons with latitude was hypothesized to be due to an increase in corn-producing cropland with latitude. At the local scale, raccoons are more abundant along agricultural edges and riparian corridors than in the forest interior (Dijak and Thompson, 2000), preferring habitats comprised of a mixture of cropland and forest (Pedlar et al., 1997). As in agricultural landscapes, the higher density of raccoons in suburban areas may be due to increased availability of anthropogenic foods (Hoffmann and Gottschang, 1977). If increased food supply affects local predator density, areas with supplemental feeding would be expected to have higher rates of nest predation, if such feeding did not shift predator diets away from nests. In contrast, the presence of deer feeders lowered survival of artificial nests that mimicked natural nests of ground-nesting birds (Cooper and Ginnett, 2000). However, no study has looked at the impacts of feeders on nest success of freshwater turtles. As feeders were present in parts of our study site, and because they were situated within turtle nesting habitat, we used artificial nests to compare rates of nest depredation between areas containing feeders and those that did not. We also used artificial nests to determine whether predation rates at turtle nests differed among road, forest and edge habitats. Female turtles of medium to large-bodied species often nest in areas with little ground cover and full sun exposure (Petokas and Alexander, 1980; Schwarzkopf and Brooks, 1987; Plummer et al., 1994) such as the habitat found on or along roads. For many species of turtles, hatchling gender is determined by nest temperature during the incubation period (Bull and Vogt, 1979; Wilhoft et al., 1979; Janzen, 1994), and use of open areas (such as sandy roads) may facilitate shorter incubation time (Packard et al., 1987) and influence hatchling sex ratio. However, such nesting behavior

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may be detrimental if densities of predators are greater along roads or if predators travel roads while searching for prey. Researchers have speculated that roads act as movement corridors for some predators (May and Norton, 1996), elevating predation levels above those in the forest interior or in a roadless ecosystem. We hypothesized that turtle nesting behavior is a tradeoff between use of open sunny areas optimal for incubation and increased potential for nest destruction by elevated densities of predators searching for nests along roads. Therefore, we predicted that predation rates of road nests would be greater than those of edge or forest nests in this study. We also evaluated the effect of olfactory (turtle bladder water) and visual (sand marker) cues on predation frequency. Because some predators are thought to respond to olfactory and/or visual cues, we predicted predation rates would be higher at nests treated with bladder water and/or sand markers than those without. Due to potential increases in predator density in areas with deer feeders, we predicted that predation rates would be higher for nests in areas with feeders than in those without. METHODS

AND

MATERIALS

Study site.—This study was conducted during July and August 1999 at White Oak Plantation and Conservation Center, a 7000 acre area located in northeastern Florida (Nassau County) and southeastern Georgia (Camden County). This property is a mosaic of high and lower intensity use areas. Experiments were conducted at four lakes on the property, all with documented Trachemys scripta populations (Franz et al., 1999). Lakes included in the study varied in size and the level of human activity in adjacent areas. Supplemental deer feeders filled daily with corn are located near White Oak Market (WOM) and Lodge Lake (LL), whereas Spare Lake (SL) and Golf Course Lake (GCL) do not have adjacent feeders. In addition to the target species (white-tailed deer, Odocoileus virginianus, and fox squirrels, Sciurus niger), American crows (Corvus brachyrhynchos) and raccoons were frequently observed visiting these feeders. Potential predators of turtle nests recorded at the site during this study and previous research included American crow, raccoon, Virginia opossum (Didelphis virginiana), red fox (Vulpes fulva), grey fox (Urocyon cineroargenteus), skunks (Mephitis mephitis), house cat (Felis silvestris), dog (Canis familiaris), feral pig (Sus scrofa), black racer (Coluber constrictor), yellow rat snake (Elaphe obsoleta) and scarlet kingsnake (Lampropeltis triangulum) (Franz et al., 1999). Artificial nests.—Nest depth, clutch mass and egg size of our artificial nests mimicked nests of the yellow-bellied slider, Trachemys scripta, a common turtle species on the property. This species is ubiquitous throughout the southeastern United States, and is found in most freshwater habitats within its range, including both pristine and man-made habitats (Ernst et al., 1994). Artificial nests were approximately 10 cm deep, and contained 11 eggs, roughly equivalent to 107 g, the average clutch mass of Trachemys scripta in northern peninsular Florida ( Jackson, 1988). Bobwhite quail (Colinus virginianus) eggs were used, as it was not possible to collect a large enough number of slider eggs before the onset of the study, and bobwhite quail eggs reasonably approximate the size and shape of slider turtle eggs. Use of appropriate-sized eggs is necessary to effectively sample the entire predator community (Roper, 1992; DeGraaf and Maier, 1996; DeGraaf et al., 1999), as eggs larger than those of the target species may be less vulnerable to predation, leading to an underestimation of true predation rates and predator species diversity. Rubber gloves were worn while digging nests and handling eggs to reduce human scent. Nests were dug with a small shovel during daylight hours during the last week of July 1999. Placement of eggs during this time is consistent with diurnal nesting in this species

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TABLE 1.—Experimental design showing number of nests for each lake, treatment and habitat type

Habitat

Nest treatment

Lodge lake

Spare lake

White Oak market

Golf course lake

Road �Scent �Scent �Scent �Scent

� � � �

Sand Sand Sand Sand

4 3 4 4

5 5 5 5

2 2 2 2

1 1 1 1

�Scent �Scent �Scent �Scent

� � � �

Sand Sand Sand Sand

4 4 4 4

5 5 5 5

2 2 2 2

1 1 1 1

�Scent �Scent �Scent �Scent

� � � �

Sand Sand Sand Sand

4 5 4 4 48

5 5 5 5 60

2 2 2 2 24

1 1 1 1 12

Edge

Forest

Total

Total

47 12 11 12 12 48 12 12 12 12 49 12 13 12 12 144

(Iverson, 1977) and timing of the end of the nesting season of Trachemys scripta in northern Florida ( Jackson, 1988). One hundred and forty-four nests were set out in three habitats: road (on or immediately along the side of a road), edge (along or within 1 m of the forest edge) or forest (in the forest interior, at least 10 m from the road). All roads in the study were dirt roads receiving light use (�2 vehicles/d). Flagging was placed within approximately 5 m of the nest to enable relocation. Each nest was assigned to one of four treatments: no visual (sand) or olfactory (bladder water) cue, both visual (sand) and olfactory (bladder water) cues, a visual (sand) cue but no olfactory (bladder water) cue and no visual (sand) cue but an olfactory (bladder water) cue. All habitat types and all nest treatments were assigned to each lake type (Table 1). Nests receiving an olfactory cue had 15 cc of bladder water emptied with a syringe directly on the top of the eggs. As it was not possible to collect enough bladder water from female Trachemys scripta before the onset of the study, bladder water was collected from captive female Aldabra tortoises (Geochelone gigantea), another species known to void its bladder while egg laying. Bladder water was used within 24 h of collection. Visual cues consisted of a circle of sand, approximately 3 m in diameter, smoothed over the top of the nest. Visual cues were not intended to mimic those which would have been present at a natural nest site, but were intended to enable classification of predators from tracks and evaluate whether predation rates at nests with visual cues would increase over time as visually oriented predators learned to associate cues with artificial nests. Artificial nests were located �30 m from each other in an attempt to reduce spatial dependence. Nests were checked daily. An attempt was made to approach the nest along a different path each time to minimize human scent trails associated with the nest. Although human visitation or use of flagging have been found to reduce survivorship of avian nests, these factors do not appear to influence survivorship of turtle nests (Burger, 1977; Tinkle et al., 1981; Tuberville and Burke, 1994). Each nest was monitored for 21 d, which is less than the approximately 66-d incubation period for Trachemys scripta in northern Florida ( Jackson, 1988). A nest was considered depredated if the nest cavity was opened, even if all eggs

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remained intact. Nests not depredated during the study were opened after 21 d to determine if they remained intact or had suffered an unobserved depredation event during the study. No predation events were missed. The relative importance of various predator types is rarely known (Anglestam, 1986). To compare relative abundance of predators among lakes and habitat types, track stations were placed at each lake and monitored for 4 d. Stations were circles of sand roughly 3 m in diameter, baited with three quail eggs placed at the center. When the eggs were removed the track station was rebaited and the sand was replaced to enable detection of tracks. Equal numbers of track station nights (number of stations � number of nights) were recorded in each habitat type. Number of track station nights differed between lakes with respect to lake size and the number of artificial nests placed at each lake and were as follows: GCL (12), WOML (24), LL (48) and SL (60). Analysis.—We estimated nest survival using the Kaplan-Meier, product-limit method (Cox and Oakes, 1984; Pollock et al., 1989), which has been used to estimate survival for artificial nests (Vander Lee et al., 1999; Cooper and Ginnett, 2000) and various vertebrate species (Beringer et al., 1998; Thogmartin and Johnson, 1999). Kaplan-Meier allows individuals to be removed from (censored) the analysis part way through the time period of interest (e.g., due to radio collar failure, research-related mortality, etc.), while still providing data up until the point of ‘‘censoring.’’ Farnsworth et al. (2000) found that the Kaplan-Meier and Mayfield methods provided similar results. We evaluated S(t) as survival for day t after nest deposition, for all nests within a treatment, for t � 1–21. This approach is justified as most turtle nests are depredated within 1–2 d (Legler, 1954; Burger, 1977; Tinkle et al., 1981; Congdon et al., 1983), implying that rates of nest predation are a function of time since nesting rather than extrinsic biological or environmental factors. We compared survival functions between groups with log-rank tests (Cox and Oakes, 1984; Pollock et al., 1989). Log-rank tests compute a chi-square statistic based on the sum of observed—expected events (i.e., instances of predation) for each time period, testing the null hypothesis that the �2 groups being compared have the same survival curve. By treating all time periods equally, such tests place more weight on later time periods containing fewer at risk individuals. This approach is appropriate, as we were more interested in the realized differences in survival that could impact turtle populations. We also used an approximately normal Z-test to compare rates of survival to day 21 between survival curves (Pollock et al., 1989). The Z-statistic and variance estimate are as follows: Z�

Sˆ 1 (t 21 ) � Sˆ 2 (t 21 ) �var Sˆ 1 (t 21 ) � var Sˆ 2 (t 21 )

2 [1 � S(t)] ˆ ˆ [S(t)] ˆ var(S[t]) � . r(t)

The values of Sˆ and var Sˆ are the estimates of survival until day 21 and the variances for the two curves, respectively, while r(t) represents the number of individuals at risk at time t. Survival estimates are cumulative and are, therefore, based upon all nests tracked from day 1. Since variances increase when few nests remain, these tests are conservative. For analyses using multiple pairwise comparisons, Dunn-Sidak correction factors (Sokal and Rohlf, 1995) were used to determine significance levels. These were � � 0.009 and 0.017, for nest treatment and habitat analyses, respectively. RESULTS Predation rates at artificial nests were high, with 125 nests (88.0%) destroyed by predators during the study period. Only 13 nests survived the three-week study period. Two road nests were excluded entirely from the analysis because road crews resanded roads shortly after

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FIG. 1.—Kaplan-Meier estimates of daily survival for artificial turtle nests (a) at lakes with and without supplemental feeders, (b) in road, forest and edge habitat types and (c) for different treatments with olfactory and visual cues

nests were placed. Four additional road nests were censored before the end of the monitoring period due to road maintenance activities that likely masked visual and olfactory cues. When survivorship of nests located near deer feeders (LL and WOML) was compared with survivorship at lakes lacking supplemental feeders GCL and SL) a significant difference was detected (Fig. 1; �2 � 8.017, P � 0.005; Z � 3.102, P � 0.001), with 5.5 times higher survivorship at lakes without supplemental feeders (with feeders, 0.029 � 0.020 SD; without feeders, 0.185 � 0.046 SD). For different nest treatments (presence of olfactory or visual cues), no significant difference was detected among survival curves and rates (for all comparisons �2 � 6.077, P � 0.014; Z � 1.720, P � 0.043; Fig. 1). Nest survival varied with respect to habitat type (Fig. 1). Nests placed along or on roads and those along habitat edges had significantly different survival (�2 � 6.47, P � 0.011; Z � 2.287, P � 0.011), with 2.7 times higher survivorship of road nests (0.233 � 0.068 SD) than for nests along edges (0.063 � 0.030 SD). Survival of nests along roads was 4.7 times greater than for those in the forest interior (0.041 � 0.028 SD); the difference between survival rates was significant (Z � 2.607, P � 0.005), although that between survival functions was not significant (�2 � 4.82, P � 0.034). Artificial nests placed in edge and forest habitats did not significantly differ in their survivorship (�2 � 0.476, P � 0.490; Z � 0.524, P � 0.300). Each night predators removed eggs from the majority of track stations. However, the removal rate was 17% higher for track stations at lakes with feeders (92% of track station

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TABLE 2.—Predation rates and predator identity recorded at track stations. Identity is recorded as percent of stations at which that predator type was recorded

Lake type

Percent track station nights predated

Armadillos

Corvids

Meso-mammalsa

Feeders No feeders

91.7 75.0

3.0 26.0

18.2 37.0

47.0 42.6

a Meso-mammals such as opossums and raccoons. Armadillos are considered separately from opossums and raccoons because (1) their tracks are distinctive and (2) they were not seen using supplemental corn feeders during this study

nights) than lakes without feeders (75%). The composition of tracks recorded at the track stations varied between sites with and without supplemental feeders (Table 2). Rates of egg removal were consistent among habitat types (85% of track station nights at edge and forest locations and 83% of road locations). However, identity of tracks left at track stations varied between habitat types (Table 3). Opossums and raccoons were the most frequent visitors to track stations at sites with feeders (Table 2). For track stations placed at lakes without supplemental feeders, the tracks left suggest that armadillos (Dasypus novemcinctus), avian predators, opossums and raccoons are all significant components of the predator community (Table 2). DISCUSSION Predation rates detected at artificial nests during this study were high, but within the range reported for loss of natural nests in other studies. Predation rates observed during this study may have been elevated due to the influence of human observers, as it has been suggested that predators may follow human scent trails to locate nests, although other researchers found nest visits by investigators had no influence on nest success (Gottfried and Thompson, 1978). Nonetheless, predation rates detected in this study may not represent actual rates of predation of natural nests at this site, as previous researchers have found both elevated (King et al., 1999) and reduced (Davison and Bollinger, 2000) predation rates at artificial avian nests compared to those detected at natural nests. Although the use of artificial nests to evaluate absolute predation rates has been cautioned against, use of this method to compare relative rates among areas within a study site is appropriate. Therefore, it is likely the patterns of elevated predation rates at lakes with supplemental feeders are real, as experimental treatments, use of flagging, and investigator presence did not differ among sites. TABLE 3.—Predation rates and predator identity recorded at track stations. Identity is recorded as percent of stations at which that predator type was recorded

Habitat type

Percent track station nights predated

Armadillos

Corvids

Meso-mammalsa

Edge Road Forest

85.4 83.3 85.4

17.1 2.5 19.5

19.5 40.0 19.5

43.9 42.5 46.3

a Meso-mammals such as opossums and raccoons. Armadillos are considered separately from opossums and raccoons because 1) their tracks are distinctive and 2) they were not seen using supplemental corn feeders during this study

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Artificial nests attracted predators with demonstrated previous experience robbing emydid nests. Some emydids in Florida (Pseudemys floridana and P. concinna) dig a three-hole nest (Franz, 1986; Jackson and Walker, 1997), with the majority of eggs deposited in the main nest cavity, but some eggs placed in two accessory ‘‘satellite’’ holes. Artificial nests constructed in our study consisted only of a main nest cavity. In several instances, when artificial nests were robbed and all the eggs were removed, the predator dug holes on each side of the main cavity, searching for the satellite holes and the ‘‘remainder’’ of the clutch. This suggests that our artificial nests were recognized as turtle nests by at least some members of the predator community. Differences were detected between the predator communities of lakes with feeders (WOM and LL) and those without (SL and GCL). The greater number of detections of opossums and raccoons at lakes with feeders the higher rate of predation at track stations at lakes with feeders and multiple sightings of groups of raccoons at deer feeders during the course of conducting field work support the conclusion that the presence of supplemental feeders are inflating the numbers of predators, such as raccoon and opossums, found at these sites. Dijak and Thompson (2000) hypothesized that variation in abundance of mammalian predators in Missouri was due to a functional response to habitat conditions or food availability. Our data are consistent with this hypothesis, and with the general observation that large aggregations of raccoons occur around concentrated food sources (Melvin Sunquist, pers. comm.). Additionally, supplemental feeding may increase productivity, survival and population size of predators in the long term, even when aimed at reducing nest predation (Clark et al., 1996). Regardless of the means by which deer feeders elevate the abundance of nest predators, the higher predator abundance detected at lakes with deer feeders probably had a negative effect on nesting success of turtles and birds on our study site. Further studies are necessary to quantify the long-term effects of supplemental deer feeders on the population dynamics of nesting species and their nest predators. Although supplemental food has been shown to decrease skunk depredation of duck eggs (Crabtree and Wolfe, 1988), this appears to be, at least in part, due to a foraging shift toward the roadside edge where supplemental food was placed and, therefore, away from the core of the nesting area. Similarly, Vander Lee et al. (1999) found that supplemental prey reduced depredation of artificial nests when prey were placed at the edges of nest plots where predators would encounter it before entering the nesting areas. Clearly, the spatial orientation of supplemental feeding with respect to nesting habitat will influence the effects of feeding on nest survival. Even so, the studies by Crabtree and Wolfe (1988) and Vander Lee et al. (1999) were of short duration and, therefore, could not detect the effects of a numerical response of predator populations to supplemental feeding. Nests with olfactory cues (turtle bladder water) were predicted to suffer higher mortality than nests without. Additionally, the presence of a visual cue was predicted to lower survivorship over time as some diurnal predators that are visually oriented (such as crows) may locate these nests with increased frequency if they learn to recognize the visual cues. No differences in predation rates were detected among nest treatments. This has two possible implications- turtle bladder water may not be an olfactory cue, or the Aldabra tortoise bladder water used in this experiment did not accurately mimic the olfactory cues found at a natural nest in northern Florida. Experiments using Trachemys scripta bladder water may have different results, if this olfactory cue is recognized by the local predator community. We are unable to conclude whether bladder water may attract nest predators, as suggested by Legler (1954). Although visual cues did not influence nest survival, we did find some evidence suggesting that predators at this site learned to associate our visual cues with the presence of eggs. As

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part of this study, track stations were set out without eggs. These stations were identical to those used to monitor the predator community except that eggs were not placed in the center. Of these track stations, 56% showed evidence of ‘‘predation’’ events (tracks or scratches) and many of them were excavated, presumably during efforts to locate the nonexistent nests. Opossum and raccoon tracks were ubiquitous in all habitat types, which is consistent with previous observations that raccoons are frequently the major predators at nests of most turtle species (Wilhoft et al., 1979; Franz, 1986; Jackson and Walker, 1997). We also found armadillo tracks at many depredated forest and edge track stations. Armadillos have been implicated as nest predators in a Panamanian population of slider turtles (Moll and Legler, 1971), but were not documented to rob nests of another emydid species in north Florida ( Jackson and Walker, 1997). Although armadillos visited track stations during our study, we do not know if they depredate natural turtle nests, as armadillos are opportunistic foragers and will exploit any abundant food resource. Our results suggest that there may be an advantage to nesting along roads, as survival of road nests was higher than that of nests in other habitat types. These results are surprising, as other researchers have suggested the use of roads by foraging midsized nest predators such as opossums, raccoons, foxes (Small and Hunter, 1988; May and Norton, 1996) and have found elevated predation rates at avian nests along roads (Burkey, 1993). A large body of literature almost unanimously supports the notion that nest success declines with increasing proximity to an edge, and is often particularly elevated along roads (Legler, 1954; Gemmell, 1970; Christens and Bider, 1987; Temple, 1987; Linck et al., 1989; Burkey, 1993; Paton, 1994; Jackson and Walker, 1997). In contrast, we found that nest predation rates were lowest along roads, which, by definition, are associated with habitat edges. These results serve to emphasize that generalizations concerning the relationship between nest predation and edge effects should be made cautiously, depending both on the taxa of concern, and other habitat features that may influence predator densities and nest success. Our study site appears to contain high predator densities, and the beneficial effect of roads may only occur when other habitats that might otherwise serve as nest refugia are saturated with nest predators. Furthermore, benefits accruing in the form of elevated nesting success along roads may be more than compensated for, if high predator densities associated with habitat fragmentation by road construction and/or habitat conversion, depress survival rates of other age classes important to population growth. Acknowledgments.—We are grateful to John Lukas, General Consul Director, and the staff of the White Oak Conservation Center for facilitating and supporting our research on the herpetofauna of northern Florida. We thank C. McGowan and P. Rider for assistance with field work. Our research was funded through an internship program sponsored by the Howard Gilman Foundation and the White Oak Conservation Center, Yulee, Florida. We thank C. Austin, T. Squire and two anonymous reviewers for helpful comments that improved this manuscript.

LITERATURE CITED AMBUEL, B. AND S. A. TEMPLE. 1983. Area-dependent changes in the bird communities and vegetation of southern Wisconsin forests. Ecology, 64:1057–1068. ANGLESTAM, P. 1986. Predation on ground-nesting birds’ nests in relation to predator densities and habitat edge. Oikos, 47:365–373. BERINGER, J., S. G. SEIBERT, S. REAGAN, A. J. BRODY, M. R. PELTON AND L. D. VANGILDER. 1998. The influence of a small sanctuary on survival rates of black bears in North Carolina. J. Wildl. Manage., 62:727–734. BRECKENRIDGE, W. J. 1960. A spiny soft-shelled turtle nest study. Herpetologica, 16:284–285.

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ACCEPTED 14 AUGUST 2001