Sand temperatures for nesting sea turtles in the Caribbean: Implications for hatchling sex ratios in the face of climate change

Highlights

• Mean sand temperatures at nest depths were 31.0 °C, ranging from 29.1 °C to 33.3 °C

• Rainfall and spring tides had important cooling effects on incubation temperatures.

• Sex ratios of 3 species of sea turtle were female-biased during the past century.

• Rising air temperatures will exacerbate the female skew in future populations.

• Management strategies will be necessary to prevent localised extinction.

Abstract

A 200-year time series of incubation temperatures and primary sex ratios for green (Chelonia mydas), hawksbill (Eretmochelys imbricata) and leatherback (Dermochelys coriacea) sea turtles nesting in St. Eustatius (North East Caribbean) was created by combining sand temperature measurements with historical and current environmental data and climate projections. Rainfall and spring tides were important because they cooled the sand and lowered incubation temperatures. Mean annual sand temperatures are currently 31.0 °C (SD = 1.6) at the nesting beach but show seasonality, with lower temperatures (29.1–29.6 °C) during January–March and warmer temperatures (31.9–33.3 °C) in June–August. Results suggest that all three species have had female-biased hatchling production for the past decades with less than 15.5%, 36.0%, and 23.7% males produced every year for greens, hawksbills and leatherbacks respectively since the late nineteenth century. Global warming will exacerbate this female-skew. For example, projections indicate that only 2.4% of green turtle hatchlings will be males by 2030, 1.0% by 2060, and 0.4% by 2090. On the other hand, future changes to nesting phenology have the potential to mitigate the extent of feminisation. In the absence of such phenological changes, management strategies to artificially lower incubation temperatures by shading nests or relocating nest clutches to deeper depths may be the only way to prevent the localised extinction of these turtle populations.

Introduction

It is widely reported that climate change is having a profound impact on the functioning of ecosystems (Hoegh-Guldberg and Bruno, 2010). Species abundances and distributions are shifting due to the changing environment (Jones and Cheung, 2015, Poloczanska et al., 2013). The breeding phenology of migratory species is also changing in response to environmental change (e.g. McCarty, 2001, Parmesan et al., 1999, Walther, 2010). Furthermore, climate change-induced habitat loss puts species at risk of extinction when no alternative adequate habitat is available (Foden et al., 2013). The risk of climate change is particularly pronounced for marine ectotherms as they conform more closely to thermal tolerance limits than terrestrial ecotherms (Sunday et al., 2012) and, in particular, species whose biology and phenology are intimately linked to temperature (Somero, 2010). Of particular concern are species that exhibit-temperature-dependent sex determination. This applies to the majority of reptiles, including sea turtles. The concern of climate change is of particular importance to sea turtles considering that six out of the seven species are classified as critically endangered, endangered or vulnerable by the International Union for the Conservation of Nature (IUCN, 2014).

One of the conservation challenges for sea turtle ecologists is that climate change is impacting sea turtles in more than just one way. For instance, sea level rise may cause a loss of the beaches on which the females rely to nest (Fish et al., 2008, Fuentes et al., 2009a). Climate change models also predict an increase of rainfall and storm events at certain sites, which may be detrimental to nests as exposure to high levels of water negatively impacts the success of a nest (Patino-Martinez et al., 2014). Increasing air temperatures is also a concern for species that exhibit temperature-dependent sex determination and may lead to skewed sex ratios and local extinctions (Janzen, 1994). As such, increasing temperature was recently identified as one of the biggest threats to sea turtle populations' viabilities (Fuentes and Cinner, 2010).

Quantifying how warming temperatures will impact sea turtle populations across the globe is a conservation priority. A recent thread of research concerns the reconstruction of past sex ratios at important breeding sites based on historical air temperature data (e.g. Hays et al., 1999, Hays et al., 2003). These studies are important in informing how primary sex ratios fluctuated in the past. By using climate projections provided by the Intergovernmental Panel on Climate Change (IPCC) it is also possible to make projections on how the primary sex ratios may change in the future. This is of particular interest when trying to assess if a population is at risk of extinction in the near future due to climate change.

Current models used to estimate primary sex ratios are primarily based on air temperatures records alone (e.g. Hawkes et al., 2007, Hays et al., 1999, Hays et al., 2003). Such models are proven to be robust but having more variables may improve the model and make the reconstructions more accurate (Fuentes et al., 2009b). Working with the best available model is important when trying to accurately assess how global warming will affect the viability of sea turtle populations across the globe. The current study examines an array of environmental variables that may influence sand temperature of Zeelandia beach, St. Eustatius. Two hundred year time-series of incubation temperatures and primary sex ratios for the three species of turtles nesting at this site were then constructed. The results of this study inform on the extinction risks of these populations of sea turtles and help establish whether conservation efforts, such as shading or relocating nests, would be beneficial at this site.