A technique used by conservationists to help preserve sea turtle populations by shading their nests might actually do more harm than good if not properly monitored, according to a new study by veteran environmental science researcher James R. Spotila, PhD, a professor emeritus at Drexel University.
Sea turtles nest on tropical and subtropical beaches, where developmental success of egg clutches depends on the temperature of the next in which they’re growing. Higher nest temperatures can increase embryo and hatchling mortalities and has been linked to the production of more female hatchlings than male. Nest shading – a technique where conservationists build small shade structures over clutches — has been used on some beaches to reduce nest temperatures and thereby increase the number of overall hatchlings, while reducing female-biased sex ratios. But according to the study, coauthored by Spotila, in Biological Conservation, this widely used conservation tool could be causing a long-term decline in the overall number of nesting females and thereby reducing the total population size.
Researchers modeled short- and long-term effects of reducing mean nest temperatures on a leatherback turtle (Dermochelys coriacea) population, for which the effect of temperature on sex ratios and emergence success (# hatchlings emerged/ # eggs) is well established.
To test the effectiveness of nest shading, researchers simulated mean nest temperature reductions of -0.5 C, -1 C, -1.5 C and -2 C in relation to current mean (30.4 C) and projected population responses over 100 years. Ultimately, researchers found the popular conservation tool should be applied less liberally.
“Nest cooling may only be needed under critically low hatchling production and extremely biased female sex ratios that we only found at +2 C,” said Spotila. “If nest shading is to be used, it should be applied strategically to optimize hatchling production with natural sex ratios to achieve both short-term conservation goals and long-term population sustainability.”
Amid climate change threats, it’s important to closely consider how temperatures may change over the next 100 years, according to the researchers.
The team simulated the effect of increasing mean nest temperatures under climate change scenarios to determine if more frequent shading could be necessary if temperatures passed a certain threshold. “Nest shading may only be needed to conserve sea turtles under extreme female biases that could result in non-fertilized eggs and that are accompanied by high embryo and hatchling mortality as we found in the highest future mean nest temperature increases, but not in the scenarios of low and intermediate temperature increases,” said Spotila.
“Nest shading could be an effective way to increase the number of hatchlings produced, but it must be used with caution,” Spotila said. “For example, future managers could shade nests after the thermosensitive period had passed (i.e., during the last third of the incubation period) – in this way, high nest temperatures could potentially be mitigated, reducing at least mortality occurring in late developmental stages, without altering sex ratios.”
The researchers caution that before such refined interventions are implemented more research should be conducted on the effects of nest temperatures during embryonic development. In addition, climate change may affect sea turtle populations and species differently, with thermal variability even occurring among nearby nesting beaches. Evaluating the effects of nest temperatures on the particular population in need of conservation will be critical — and ecological risks should be considered before any human intervention.
“Ultimately, our results will help guide future management actions for sea turtle populations and could inform management of other species with temperature-dependent sex determination,” said Spotila.
“As climate gets warmer, climate mitigation strategies to protect sea turtle nests will likely be more used,” he said. “Analyzing the impact of any conservation actions involving human intervention, as well as that of climate change on sea turtles on the long-term will be critical for adequately conserving them into the future.”
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