Grantee Research Project Results
Final Report: Evaluating Multiple Stressors in Loggerhead Sea Turtles: Developing A Two-Sex Spatially Explicit Model
EPA Grant Number: R829094Title: Evaluating Multiple Stressors in Loggerhead Sea Turtles: Developing A Two-Sex Spatially Explicit Model
Investigators: Wyneken, Jeanette , Crowder, Larry B. , Epperly, Sheryan
Institution: Florida Atlantic University - Boca Raton , National Marine Fisheries Service , Duke University
Current Institution: Florida Atlantic University - Boca Raton , Duke University , National Marine Fisheries Service
EPA Project Officer: Packard, Benjamin H
Project Period: November 15, 2001 through November 14, 2004 (Extended to November 14, 2005)
Project Amount: $349,421
RFA: Wildlife Risk Assessment (2001) RFA Text | Recipients Lists
Research Category: Biology/Life Sciences , Ecological Indicators/Assessment/Restoration , Aquatic Ecosystems
Objective:
Our broad project goal is to integrate the effects of multiple stressors into population models to evaluate and advance contemporary management options for species conservation. We study loggerhead sea turtles as a model species in which we integrate temporally and spatially specific data collected (and mined from existing data sources) on males and females to develop a two-sex model.
Summary/Accomplishments (Outputs/Outcomes):
Overview
During the past 3 years, we collected empirical data on spatial and temporal differences in loggerhead turtle sex ratios for the two largest subpopulations of loggerhead sea turtles in the Atlantic. We assessed predation risks in waters adjacent to nesting beach sites associated with the southern subpopulation and completed a spatially and temporally specific assessment of hatchling predation risks for the Florida population. We update our conceptual model to better reflect what we know about spatial differences in loggerhead turtle life histories. A study of initial loggerhead growth data in years 1–2 enabled us to update the durations (temporal exposures) of environmental stressors in our model. Through a series of collaborations, we have integrated the hatchling sex ratio and seasonal production into a two-sex population matrix model to test the effects of the different observed sex ratios as stressors on the population. We focused our final year’s efforts on parameterizing the conceptual model as a simulation model to address the long term effects of population stressors. We particularly addressed how highly skewed sex ratios, which are common in species with environmental sex determination, such as loggerhead sea turtles, are likely to stress the populations.
Almost by necessity, management alternatives address only a particular life stage, sex, habitat, or spatial location. Adding our results to assessments of the integrated population-level response and consideration of management trade-offs is introducing options for a new generation of population models. Our conceptual approach to wildlife risk assessment for loggerhead turtles now is serving as groundwork for examining leatherback turtle populations and may subsequently be extended to other migratory, long-lived species.
Our sex ratio data collection and predation risk assessments were influenced by extreme weather events (hurricanes). While the storms affected sampling, their impacts on hatchling numbers, predators, and sex ratios were real and thus the absence of hatchlings when they would be expected to be present was included in our data summaries and analyses. The main impacts were at Hutchinson Island, during the late season in 2004, where nest losses were associated with two hurricanes that made land fall directly over our field sites in September. The Sanibel/Captiva site was not used during August or September due to the impact of Hurricane Charlie earlier in the 2004 season. We were able to move our studies to an adjacent, nearly identical beach at Naples to complete sampling. At all sites, the numbers of late season hatchlings produced were limited by the storms. We completed sex ratio and predation assessments, where possible, with nests that were moved into the laboratory before the hurricanes made land fall but after the point when sex was determined.
Below, we outline the results of each component of the study.
Loggerhead Hatchling Survivorship and Predation Risks
Loggerhead hatchlings were followed as they migrated away from nesting beaches (Figure 1) to determine predation risks and losses to recruitment for the southern subpopulation. Due to extremely hazardous weather, we were not able to make such estimates for the northern subpopulation.
Figure 1. Sites of Loggerhead Hatchlings Predation Risk Study, Sampled During Early, Middle, and Late Subseasons
Hatchlings were followed by kayak as they swam offshore (the beginning of their pelagic life stage) from three beach sites: Boca Raton (N26°22΄ W080°04΄) and Hutchinson Island (N 27°20΄ W 080°13΄) on the east coast and Naples and Sanibel (N26°01΄24 W081°46΄) on the west coast of Florida. The predation rates were compared at early, middle, and late season at each of the sites, and seasons were compared across sites. Based on previous studies, an overall predation rate of 5 percent was expected.
Data collected during the summers of 2003 and 2004 showed no significant difference in trends. The overall rate of predation differed significantly from the expected rate of 5 percent on some of the beaches (Figure 2). The urban Boca Raton site had much higher predation than expected; the west coast site (Naples/Sanibel) had lower predation than expected. The percentage of turtles taken by predators at each location tended to increase as the season progressed and was significantly higher than expected in the late subseason at Boca Raton (Figure 3). Overall predation levels were lower at the Florida west coast sites and predation was relatively high only during the late subseason.
Figure 2. Predation Risk by Sampling Location. The asterisk indicates sites that differed significantly from one another and from the expected values.
Figure 3. Temporal and Spatial Differences in Predation Risk Show Slight to Significant Differences at Two Sites. We could not assess the east coast Hutchinson Island site because the beach and its nests were destroyed by hurricanes in September 2004. The asterisk indicates a subseason that differed significantly from the rest of the hatchling season at the Boca Raton site.
Sex Identification, Scaling Sex Production, and Sex Ratios
Our sampling design was to subsample 10 individuals for each of four nests on each beach, in each of three time periods (subseasons). We marked nests to estimate sex ratios across the season. Because nest frequency is approximately normally distributed, we targeted nests around the mean nesting date as well as the early and late standard deviation of the historical nesting distribution. So, early, mid, and late season nesting represented about the first, second, and third one-third of nesting. This provided three estimates of sex ratio representing three subseasons during the nesting season. In order to estimate the sex ratio of hatchlings produced over the whole season, we calculated the cumulative frequency distribution of total nests for each beach and overlaid the sex ratio estimates for the three dates sampled and interpolated sex ratios for dates between the three sampling dates. Using weekly hatchling production, we then assigned sex ratios for the nests in each subseason. We then calculated scaled sex ratios of each beach, region, and subpopulation.
We developed an accurate laparoscopic technique to identify the sex of individual posthatchling loggerhead sea turtles (Wyneken, et al., in revision). We used this technique to collect sex ratios by nest, subseason, and beach (see Figures 4 and 5).
Figure 4. Identifying Posthatchling Sex Laparoscopically
Figure 5. Sites Where Sex Ratios Were Identified and Nest Temperatures (the Environmental Cue That Drives Sex Determination in Sea Turtles) Were Monitored
In order to estimate the mean and standard deviation of the sex ratio for the two regional subpopulations, we weighted the mean of the sex ratio of hatchlings produced in each area. For example, the proportion of hatchlings produced in Georgia relative to other states representing northern loggerheads will vary from year to year, but the weighted average of Georgia, South Carolina, and North Carolina for each year should represent well the overall northern population sex ratio. These data were then included in our simulation model.
We compared the results to determine if sex ratio varied more by location or by year, recognizing we could not calculate a variance with just 2 years of data for each site. (Figure 6).
Figure 6. Graphic Comparisons of Sex Ratios for One Northern and Three Southern Sites’ Year to Year Differences and Site Differences in Sex Ratio
Modeling Report
Our original concept was to produce a spatially explicit population model that linked the northern and southern regional subpopulations. The motivation was based on the idea that males produced in the northern subpopulation mated with females in the south because the number of males produced in the south were few. The relevant literature suggested that the large southern subpopulation likely produced about 80–94 percent females, a sex ratio of between 4:1 and nearly 16:1. We estimated that the northern subpopulation would contribute the predominant proportion of males (with northern nests yielding approximately 65 percent male). If true, the northern turtles might well be critical to sustaining both subpopulations. Importantly, the status of the northern population, which has an order of magnitude fewer nests per year than the southern population, is declining at 2–3 percent per year. By contrast, the southern population is much larger and approximately stable.
We planned to develop linked matrix models of both the northern and southern subpopulations. Each model includes key life stages occupying different habitats that might be differentially occupied by turtles from different regional subpopulations. We have developed a conceptual model (Figure 7) that includes both sexes and allows for different spatial distributions of different subpopulations. Each habitat region could, in theory, represent different threats.
Figure 7. These Two-Sex Spatially Explicit Population Models Remain Separate for the Two Subpopulations. The two subpopulations are treated identically at present.
Our empirical sex ratio study yielded two very interesting results. First, the southern population can produce a substantial proportion of males (57–86% female, depending upon the beach in 2002), though 2003 produced a result more similar to the historical data (81–98% female). The year 2002 may have been anomalous, but it does show substantial capacity to produce males in the south in both years sampled. Second, hatchling output was not strongly male dominated in the north (averaging 75% female in 2002 and 47% female in 2003). The conclusion based on these data is that two assumptions critical to our motivation for linking the northern and southern subpopulation models may be unsupported. The southern population appears to produce far more males than previously thought and thus, southern males may not be limiting. The northern population may not be a major source of males for the regional population. For example, in 2002, the proportion of males produced in the south (32%) exceeded that produced in the north (25%) and the south produced about 10 times more hatchlings.
All this has led us to prioritize the development of independent models for the south and for the north rather than developing a linked model at this time. We retain the two-sex, spatially explicit approach, but our priorities have shifted toward updating the northern model and deriving parameters to populate a southern model. Thus, our new strategy involves updating for the northern population model (improved growth rate and adult survival estimates, from mark-recapture data, and updating hatchling sex ratios) as well as establishing new vital rate estimates for the southern population (i.e., growth and survival rates from in-water sampling programs—Indian River Lagoon—as well as adult survival rates from beach mark-recapture programs—Hutchinson Island).
We do not plan to abandon the idea of linking the regional matrix models, but the empirical data point to the value of updating and populating the regional models before we worry about the linkage issues. The genetic data suggest the regional subpopulations are linked, but infrequent matings could provide genetic homogeneity at the nuclear DNA level without having significant population dynamics consequences.
We next integrated our data into a stochastic simulation model (Figure 8A). We evaluated the effects on sex ratios on stable populations (e.g., South Florida, Figures 8A, B) and on a decreasing northern subpopulation coupled with an increasing southern population (Figure 8C). Based on models with the stable population assumption (Figures 8A, B), when more cohorts are included the oscillations in sex ratio decline but the average population sex ratio does not change. Even when we let the northern subpopulation decline in the model, there is very little effect on average sex ratio (Figure 8C). We expect this is because average sex ratio is dominated by that produced in the southern population.
A |
B | C |
Figure 8. (A) Consequences of Other Observed Hatchling Sex Ratios to Future Stages if Both Subpopulations are Stable; (B) Summarized by Stage Class; and (C) if the Northern Subpopulation Continues to Decline While the South Increases
We retrieved outstanding data from those sites that were unable to meet our deadlines because of storm damage to their facilities. In collaborations with a colleague and her graduate student, Jason Vaughan (formerly Dr. Wyneken’s technician on this project), we updated the population simulation matrix models for the northern and southern subpopulations, incorporating the sex ratio differences. The model demonstrated that the sex ratio of the northern subpopulation has little impact on the North Atlantic loggerhead population. However, because of its large size, the southern Florida subpopulation’s sex ratios can have far-reaching impacts on the population trends. Highly skewed sex ratios can ultimately lead to population decline in four to five generations.
We now are working with new data describing and clarifying vital rate estimates for the southern population, including growth and survival rates from in-water sampling programs—Indian River Lagoon and Hutchinson Island—as well as adult survival rates from beach mark-recapture programs. The remaining activities include completing data transfer from beach managers who promised historical data quantifying daily hatchling production, as well as working with mark-recapture data to include in our model.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 50 publications | 6 publications in selected types | All 2 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Alava JJ, Keller JM, Kucklick JR, Wyneken J, Crowder L, Scott GI. Loggerhead sea turtle (Caretta caretta) egg yolk concentrations of persistent organic pollutants and lipid increase during the last stage of embryonic development. Science of the Total Environment 2006;367(1):170-181. |
R829094 (Final) |
Exit Exit |
|
Wyneken J, Epperly SP, Crowder LB, Vaughan J, Blair Esper K. Determining sex in posthatchling loggerhead sea turtles using multiple gonadal and accessory duct characteristics. Herpetologica 2007;63(1):19-30. |
R829094 (Final) |
Exit |
Supplemental Keywords:
animal, reptiles, population biology, habitat degradation, indicators, temperature dependent sex determination, matrix population models, mark-recapture, aquatic, marine sciences, midAtlantic, zoology, toxics, sex, ecological effects,, RFA, Economic, Social, & Behavioral Science Research Program, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Toxicology, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, wildlife, Ecological Effects - Environmental Exposure & Risk, Monitoring/Modeling, Zoology, Environmental Statistics, Ecology and Ecosystems, Ecological Risk Assessment, Ecological Indicators, ecological exposure, risk assessment, predicting risk, spatial distribution, demographic, contaminants, demographic data, stressors, loggerhead sea turtles, multiple stressors, Wildlife Risk Assessment, wildlife populations, stress effects on wildlife populations, two-sex spatially explicit model, spatial demographic model, sensitive populationRelevant Websites:
http://www.fau.edu/webcast/ Exit
http://www.sefsc.noaa.gov/seaturtlecontractreports.jsp Exit
http://moray.ml.duke.edu/faculty/crowder/research/ Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.