Anton D. Tucker

Time-Geographic Density Estimation for Home Range Analysis

This research presents time-geographic density estimation (TGDE) as a new technique of animal home range analysis in geographic information science (GIS). TGDE combines methodologies of time geography and statistical density estimation to produce a continuous probability distribution of an objects spatial position over time. Once TGDE is applied to animal tracking data to create a density surface, home ranges and core areas can be delineated using specified contours of relative intensity (e.g., 95% or 50%). This article explores the use of TGDE for home range analysis using three data sets: a fixed-interval simulated data set and two variable-interval satellite tracks for a loggerhead sea turtle (Caretta caretta) corresponding to internesting and post-migration foraging periods. These applications are used to illustrate the influence of several parameters, including sample size, temporal sampling scheme, selected distance-weighted geoellipse function, and specified maximum velocity, on home range estimates. The results demonstrate how TGDE produces reasonable home range estimates even given irregular tracking data with wide temporal gaps. The advantages of TGDE as compared with traditional methods of home range estimation such as kernel density estimation are as follows: (1) intensities are not assigned to locations where the animal could not have been located given space and time constraints; (2) the density surface represents the actual uncertainty about an animals spatial position during unsampled time periods; (3) the amount of smoothing applied is objectively specified based on the animals movement velocity rather than arbitrarily chosen; and (4) uneven sampling intervals are easily accommodated since the density estimates are calculated based on the elapsed time between observed locations. In summary, TGDE is a useful method of home range estimation and shows promise for numerous applications to moving objects in GIS.

Determining origin in a migratory marine vertebrate: a novel method to integrate stable isotopes and satellite tracking

Stable isotope analysis is a useful tool to track animal movements in both terrestrial and marine environments. These intrinsic markers are assimilated through the diet and may exhibit spatial gradients as a result of biogeochemical processes at the base of the food web. In the marine environment, maps to predict the spatial distribution of stable isotopes are limited, and thus determining geographic origin has been reliant upon integrating satellite telemetry and stable isotope data. Migratory sea turtles regularly move between foraging and reproductive areas. Whereas most nesting populations can be easily accessed and regularly monitored, little is known about the demographic trends in foraging populations. The purpose of the present study was to examine migration patterns of loggerhead nesting aggregations in the Gulf of Mexico (GoM), where sea turtles have been historically understudied. Two methods of geographic assignment using stable isotope values in known-origin samples from satellite telemetry were compared: (1) a nominal approach through discriminant analysis and (2) a novel continuous-surface approach using bivariate carbon and nitrogen isoscapes (isotopic landscapes) developed for this study. Tissue samples for stable isotope analysis were obtained from 60 satellite-tracked individuals at five nesting beaches within the GoM. Both methodological approaches for assignment resulted in high accuracy of foraging area determination, though each has advantages and disadvantages. The nominal approach is more appropriate when defined boundaries are necessary, but up to 42% of the individuals could not be considered in this approach. All individuals can be included in the continuous-surface approach, and individual results can be aggregated to identify geographic hotspots of foraging area use, though the accuracy rate was lower than nominal assignment. The methodological validation provides a foundation for future sea turtle studies in the region to inexpensively determine geographic origin for large numbers of untracked individuals. Regular monitoring of sea turtle nesting aggregations with stable isotope sampling can be used to fill critical data gaps regarding habitat use and migration patterns. Probabilistic assignment to origin with isoscapes has not been previously used in the marine environment, but the methods presented here could also be applied to other migratory marine species.

Stable isotopic comparison between loggerhead sea turtle tissues

RATIONALE Stable isotope analysis has been used extensively to provide ecological information about diet and foraging location of many species. The difference in isotopic composition between animal tissue and its diet, or the diet-tissue discrimination factor, varies with tissue type. Therefore, direct comparisons between isotopic values of tissues are inaccurate without an appropriate conversion factor. We focus on the loggerhead sea turtle (Caretta caretta), for which a variety of tissues have been used to examine diet, habitat use, and migratory origin through stable isotope analysis. We calculated tissue-to-tissue conversions between two commonly sampled tissues. METHODS Epidermis and scute (the keratin covering on the carapace) were sampled from 33 adult loggerheads nesting at two beaches in Florida (Casey Key and Canaveral National Seashore). Carbon and nitrogen stable isotope ratios were measured in the epidermis and the youngest portion of the scute tissue, which reflect the isotopic composition of the diet and habitat over similar time periods of the order of several months. RESULTS Significant linear relationships were observed between the δ(13)C and δ(15)N values of these two tissues, indicating they can be converted reliably. CONCLUSIONS Whereas both epidermis and scute samples are commonly sampled from nesting sea turtles to study trophic ecology and habitat use, the data from these studies have not been comparable without reliable tissue-to-tissue conversions. The equations provided here allow isotopic datasets using the two tissues to be combined in previously published and subsequent studies of sea turtle foraging ecology and migratory movement. In addition, we recommend that future isotopic comparisons between tissues of any organism utilize linear regressions to calculate tissue-to-tissue conversions.

Euhirudinea from Australian Turtles (Chelodina burrungandjii and Emydura australis) of the Kimberley Plateau, Western Australia, Australia

Abstract Leeches were sampled from 2 turtle species, Chelodina burrungandjii and Emydura australis, from 9 rivers spanning the Kimberley Plateau of Western Australia, Australia. Placobdelloides octostriata was collected from both turtle taxa across the Kimberley Plateau, and Bogobdella diversa was collected from a single specimen of E. australis in the Fitzroy River, the westernmost catchment of the sample area. Relative prevalence of P. octostriata was influenced by host behavior, with a 10-fold difference in prevalence on the sit-wait predator C. burrungandjii (37.4–42.2%) in comparison with the more actively swimming E. australis (1.3–3.7%). The prevalence of parasitism was not substantially different between sites for each species. These are new host and locality records for Euhirudinea in Australia.

Comparative dietary ecology of turtles (Chelodina burrungandjii and Emydura victoriae) across the Kimberley Plateau, Western Australia, prior to the arrival of cane toads

Food webs in north-western Australian rivers exist in dynamic environments and will be influenced by land use practices, invasion of toxic cane toads (Rhinella marina) and the effects of climate change on river flows. Baseline studies are needed to understand aquatic food webs before these impacts. In the present study, we investigated the diets of two turtles (Emydura victoriae and Chelodina burrungandjii) in four upland rivers across a gradient of rainfall and land uses in the Kimberley Plateau of Western Australia. We captured turtles by snorkelling and recovered their prey by stomach lavage. We enumerated 2720 prey items from 390 E. victoriae samples and 308 prey items from 155 C. burrungandjii samples. Prey compositions distinguished E. victoriae as an omnivorous generalist relying on a diversity of animal and plant prey and C. burrungandjii as a piscivorous specialist, but with both species as likely predators of toxic cane toad eggs or tadpoles. Comparisons among the rivers showed variation in diets for both species that reflect differences in prey availability and location-specific food webs. Terrestrially based food sources were observed in 26% of E. victoriae samples and 3% of C. burrungandjii samples, which indicates the importance of the aquatic–terrestrial interface and land use practices within these rivers.

Comparing Diets of Kemp's Ridley Sea Turtles (Lepidochelys kempii) in Mangrove Estuaries of Southwest Florida

Abstract We quantified diets of Kemps Ridley Sea Turtles (Lepidochelys kempii) in Charlotte Harbor National Estuary, Florida, to identify possible ontogenetic variation in prey consumption, to examine the use of local prey, and to contrast the diets of conspecifics at other foraging areas. Dietary analysis was conducted by identifying prey remains recovered in 58 fecal samples from 53 turtles (24.2–63.7 cm midline straight carapace length). Turtles consumed seven prey categories: crustaceans, chelicerates, fish, sessile invertebrates, molluscs, plants/algae, and unidentified items. Spider crabs (Libinia sp.) are the dominant prey consumed in the mangrove estuary, occurring in 94.8% of fecal samples, accounting for 71.4% of dry mass, and as 76.3% in the Index of Relative Importance. We found no significant ontogenetic differences in prey composition between small (<40 cm) and large (≥40 cm) turtles, although crustaceans were more prevalent in diets of the larger turtles. The prey consumed in Charlotte Harbor differed significantly from a similar study of a nearby mangrove estuary in the Ten Thousand Islands. Sandy-Skinned Tunicates (Molgula occidentalis) were the predominant food item in the latter locale, and there were no significant ontogenetic differences in prey composition. A comparison of prey availability and use suggests that Kemps Ridley Sea Turtles ingested the most abundant prey in the Charlotte Harbor estuary. Geographic differences in diet may reflect localized differences in use of foraging habitat and available prey, but more studies are needed on the availability, use, and selection of both habitat and prey.

Changes of loggerhead turtle ( Caretta caretta ) dive behavior associated with tropical storm passage during the inter-nesting period

ABSTRACT To improve conservation strategies for threatened sea turtles, more knowledge on their ecology, behavior, and how they cope with severe and changing weather conditions is needed. Satellite and animal motion datalogging tags were used to study the inter-nesting behavior of two female loggerhead turtles in the Gulf of Mexico, which regularly has hurricanes and tropical storms during nesting season. We contrast the behavioral patterns and swimming energetics of these two turtles, the first tracked in calm weather and the second tracked before, during and after a tropical storm. Turtle 1 was highly active and swam at the surface or submerged 95% of the time during the entire inter-nesting period, with a high estimated specific oxygen consumption rate (0.95 ml min−1 kg−0.83). Turtle 2 was inactive for most of the first 9 days of the inter-nesting period, during which she rested at the bottom (80% of the time) with low estimated oxygen consumption (0.62 ml min−1 kg−0.83). Midway through the inter-nesting period, turtle 2 encountered a tropical storm and became highly active (swimming 88% of the time during and 95% after the storm). Her oxygen consumption increased significantly to 0.97 ml min−1 kg−0.83 during and 0.98 ml min−1 kg−0.83 after the storm. However, despite the tropical storm, turtle 2 returned to the nesting beach, where she successfully re-nested 75 m from her previous nest. Thus, the tropical storm had a minor effect on this females individual nesting success, even though the storm caused 90% loss nests at Casey Key. Summary: Data retrieved from motion dataloggers and satellite tags showed that a tropical storm can have a large effect on swimming energetics of a sea turtle, but has little effect on nesting.

Supplement 1. Raster format of loggerhead δ13C and δ15N isoscapes and associated standard deviation.

File List loggerhead_d13C_isoscape.txt (md5: 75d5be745e2018c1fbf639224e598736) loggerhead_d15N_isoscape.txt (md5: d76570667098245e4102e8380b588324) loggerhead_d13C_error.txt (md5: 7f8bb3cc91d34eb8ad033942bd2f93cf) loggerhead_d15N_error.txt (md5:995ce245c4e066001d6f8322e33038ef ) Description loggerhead_d13C_isoscape.txt – krigged surface of δ 13 C values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d15N_isoscape.txt – krigged surface of δ 15 N values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d13C_error.txt – standard deviation of the kriging prediction of δ 13 C values loggerhead_d15N_error.txt – standard deviation of the kriging prediction of δ 15 N values These four files are the δ 13 C and δ 15 N isoscapes and standard deviation rasters that are depicted in Fig. 1 of the main text. They are ASCII-formatted text files and contain a header to identify the structure of the file. They can be opened in ArcGIS by using the ASCII to Raster conversion tool or in R with the raster() function using the Raster package and then assigned a spatial reference within the program used. Resolution of each of the files is 0.0572 degrees.