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Weber S.B.,University of Exeter | Broderick A.C.,University of Exeter | Groothuis T.G.G.,University of Groningen | Ellick J.,Ascension Island Turtle Group | And 2 more authors.
Proceedings of the Royal Society B: Biological Sciences

The effect of climatewarming on the reproductive success of ectothermic animals is currently a subject ofmajor conservation concern.However, formany threatenedspecies,we still knowsurprisingly little about the extent of naturally occurring adaptive variation in heat-tolerance. Here, we show that the thermal tolerances of green turtle (Chelonia mydas) embryos in a single, island-breeding population have diverged in response to the contrasting incubation temperatures of nesting beaches just a few kilometres apart. In natural nests and in a common-garden rearing experiment, the offspring of females nesting on a naturally hot (black sand) beach survived better and grewlarger at hot incubation temperatures comparedwith the offspring of females nesting on a cooler (pale sand) beach nearby. These differences were owing to shallower thermal reaction norms in the hot beach population, rather than shifts in thermal optima, and could not be explained by egg-mediated maternal effects.Our results suggest that marine turtle nesting behaviour can drive adaptive differentiation at remarkably fine spatial scales, and have important implications for how we define conservation units for protection. In particular, previous studies may have underestimated the extent of adaptive structuring in marine turtle populations that may significantly affect their capacity to respond to environmental change. © 2011 The Royal Society. Source

Weber N.,University of Exeter | Weber S.B.,University of Exeter | Godley B.J.,University of Exeter | Ellick J.,Ascension Island Turtle Group | And 2 more authors.
Biological Conservation

Accurate estimates of abundance are fundamental to the conservation of threatened species, but are often difficult to obtain directly. Population size assessments of marine turtles are often based on counts of nests, which are then related to abundance using the mean number of clutches laid by individuals within a season. Due to low re-encounter probabilities, clutch frequency has proven difficult to estimate reliably, particularly for large populations that make a major contribution to global stock assessments. We use a combination of VHF radio-telemetry and Argos-linked Fastloc™ GPS devices to improve clutch frequency estimates for one of the world's largest green turtle rookeries at Ascension Island. Females fitted with VHF tags at the start of the season (. n=. 40) were re-encountered with a probability of 85% and laid a minimum average of 5.8 clutches. Three of these turtles were fitted with VHF and GPS devices and using the data collected by the latter, were found to lay an average of 6.3 clutches. GPS-telemetry detected emergences observed using radio-telemetry, and confirmed that some radio-tagged turtles laid again after their last observed emergence. Correcting for missed nesting events yielded a mean clutch frequency of 6.3, more than doubling the previous estimate of 3.0 for this population. Applying this revised assessment to annual nest counts reduces the estimated size of this population by 52%. Conventional tagging approaches may considerably underestimate annual fecundity of turtles, resulting in inflated population size estimates. We call for urgent reassessment of baseline abundance values for regionally important populations. © 2013 Elsevier Ltd. Source

Weber S.B.,Ascension Island Government | Weber S.B.,University of Exeter | Weber N.,Ascension Island Government | Weber N.,University of Exeter | And 7 more authors.
Biodiversity and Conservation

Although many species of marine mega-vertebrates are threatened as a result of human activity, some populations are showing promising signs of recovery following decades of protection. In this study, we report on the status of the South Atlantic’s largest green turtle (Chelonia mydas) nesting aggregation at Ascension Island, 70 years after legal protection and the cessation of commercial turtle harvesting that decimated the population. Using a monitoring dataset spanning 36 years, we modelled long-term trends in nesting activity at both a rookery level and across individual nesting beaches and beach clusters. Since monitoring began in 1977, the average number of green turtle clutches deposited annually at Ascension Island has increased sixfold, from approximately 3,700 to 23,700; a trend that has been accompanied by a significant decrease in the average size of nesting females. Interestingly, however, rates of increase in nesting activity have varied dramatically among nesting beaches, ranging from 0.4 to 6 % growth per annum. More than 97 % of this variation could be explained by distance from the main human settlement of Georgetown—the historic centre of turtle harvesting—with beaches closer to Georgetown experiencing the most rapid growth. More rapid population growth close to human centres seems counterintuitive, but may reflect the more intensive depletion of these accessible, local stocks during the harvesting era. Overall, the Ascension Island green turtle population appears to be recovering strongly, mirroring positive trends for this species across many parts of its geographic range. While not a cause for complacency, these trends are encouraging and demonstrate the capacity of marine megafauna to rebound when anthropogenic pressures are alleviated through conservation action. © 2014, Springer Science+Business Media Dordrecht. Source

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