Species Vulnerability & Decline
Unique and threatened ecosystems are one of the IPCC’s Five Reasons for Concern (IPCC, 2014). The outlook for leatherback turtles is bleak. Leatherbacks have experienced up to 78 percent population decline from 1985-2011 (Tapilatu et al., 2013), with the Pacific population declining 95 percent since the 1980s (Oskin, 2013). Sea turtles are heavily affected by climate change because of their wide range of habitats (Jino et al., 2018; Rickles, 2018). Marine turtles rely on both coastal and marine ecosystems so are vulnerable to rapid changes on both of these fronts. Their vast migration areas could make them more adaptable to different climates, but they are not adapting fast enough for climate change.
It is unclear whether marine turtles will be able to adapt either behaviorally or physiologically to altered incubation conditions to counter feminization associated with warmer temperatures. As a group, they are long-lived and late-maturing which means evolution will be slow (Hillebrand et al., 2017). Leatherbacks have evolved with a climate changing at a much slower rate than projections suggest for the next 100 years of global environmental change (Hawkes et al., 2009).
Though they are estimated to have existed for around 100 million years, Leatherbacks are listed as critically endangered and are predicted to be extinct in the next fifteen to twenty years (Oskin, 2013; Tapilatu, 2013; Gates, 2013).
Marine turtles occupy a wide range of different habitat types throughout their lifetime, but their primary habitat is oceans. Oceans are one of the most vulnerable regions to climate change as they absorb more than 90 percent of the extra heat associated with carbon emissions ( IPCC, 2014; USGCRP, 2018). The oceans regulate the global climate system by absorbing and redistributing heat and carbon dioxide (USGCRP, 2018). Anthropogenic carbon emissions cause ocean warming, acidification, and deoxygenation (USGCRP, 2018).
Ocean Warming, Acidification, and Deoxygenation
Warming of the world’s oceans affects sea levels, ocean circulation, stratification, productivity, and, ultimately, entire ecosystems (Hillebrand et al., 2017; USGCRP, 2018).
- Combined changes in air and ocean temperatures alter ocean currents and wind patterns, which affect primary producers (USGCRP, 2018).
- Thermohaline circulation patterns are expected to change in intensity and direction due to changes in temperature and freshwater ice melt at the poles (Hawkes et al., 2014).
- Increased carbon dioxide levels in the atmosphere are also causing a decline in ocean oxygen concentrations (USGCRP, 2018). Deoxygenation is partially due to reduced oxygen solubility: warm water holds less oxygen (USGCRP, 2018).
- Ecosystem changes related to temperature and stratification further influence oxygen levels by affecting photosynthesis and respiration (USGCRP, 2018).
Warming and ocean acidification will disrupt entire marine ecosystems. Differences in how species respond to physical conditions could cause increased abundance, species decline, or change in species location or range (Esteban et al., 2018; USGCRP, 2018).
- Species may be exposed to new predators, competitors, and diseases that they have not adapted for (Hillebrand et al., 2018; USGCRP, 2018).
- Species are adapting to their changing climates through individual characteristics or behavioral modifications, timing of biological events, and changes in geographic range (USGCRP, 2018).
- Species range has moved northward, and warmer conditions in the spring has shifted the timing of biological events earlier in the year (USGCRP, 2018). Marine communities have shifted up to 17.4 miles per decade in the span of just 100 years (USGCRP, 2018).
- Local and global extinctions may occur when climate change outpaces the capacity of species to adapt (USGCRP, 2018); biodiversity will decrease (Hillebrand et al., 2018).
Since marine turtles use ocean currents as roadways for migration, they are found in great numbers where ocean currents meet. According to Hawkes et al., marine turtles may cross entire ocean basins with the assistance of major oceanic surface currents during their development (2009). When they first enter the sea, hatchlings have been shown to swim in directions that should draw them into local surface currents, facilitating their movements across ocean basins, and influencing the distribution of juvenile turtles (Hawkes et al., 2009). Changes in ocean circulation patterns have the potential to affect dispersal of juveniles as hatchlings may no longer able to rely on passive transport in surface currents for migration (Hawkes et al., 2014). So far, there is not evidence of a direct link between ocean deoxygenation or acidification and its effect on the health of marine turtle populations.
Tropical Pacific circulation patterns have already been attributed to climate change, with disruption of upwellings which are essential for jellyfish. The main prey of leatherback turtles are jellyfish, which are known to respond sensitively to changes in climate, peaking in abundance earlier in the year (Hawkes et al., 2009). The availability of food in the ocean may affect how often females can return to breed and, therefore, govern lifetime reproductive success (Hawkes et al., 2009). There is not yet direct evidence of turtles being impacted by changes in ocean currents, but as this hazard increases, it will affect the migration and feeding patterns of sea turtles. However, the influence of rising temperatures is evident. According to Hawkes et al., leatherback turtle nests are now being recorded at their most northerly in a decade of monitoring (2009). The range of leatherback turtles has moved 300 km north in the last 17 years.
Coastal environments also influence the wellbeing of marine turtles. Climate change threatens coasts through increased temperatures, storm surges, sea level rise and coastal flooding (IPCC, 2014).
- Global mean surface temperature could increase 5°C by 2100 under RCP8.5, a higher emissions scenario (IPCC, 2014).
- An increase in extreme weather events in the most severe categories, such as hurricanes or typhoons, could also occur with changes in the global climate, which may cause significant loss of beaches and erosion of shorelines (Hawkes et al., 2009). Climate change related risks from extreme events, such as heat waves, heavy precipitation and coastal flooding, are already moderate; with 1°C additional warming, risks are high (IPCC, 2014). Risks associated with some types of extreme events (e.g., extreme heat) increase progressively with further warming (IPCC, 2014).
- Advancing glacial melt of arctic and antarctic ice sheets will continue to contribute to rising sea level, which could be up to 1.0 meter by 2100 under a higher emissions scenario (IPCC, 2014). Coastal systems and low-lying areas will increasingly experience submergence, flooding and erosion due to sea level rise (IPCC, 2014).
After hatching, leatherback turtles live at sea except when they return to coasts to nest. Female leatherback turtles nest every 1-3 years and will nest 7-10 times in a season, about every 10 days. The female turtles haul themselves out of the ocean and onto shore at night, excavate a hole several feet deep with their back flippers, lay around 100 eggs, bury and camouflage the nest, and then return to the ocean. The process takes about two hours.
Reproduction & Feminization
Increasing land temperatures have significant implications for reproduction. Temperature is of profound importance as an environmental factor for marine turtles, affecting features of their life history from hatchling sex determination to adult distribution (Hawkes et al., 2009). Warming temperatures threaten long-term reproductive success of the leatherback population, with temperature increase associated with a greater prevalence in feminizing beaches (Patricio et al., 2018; Patino-Martinez et al., 2011; Tapilatu et al., 2013; Hawkes et al., 2009). The average temperature of the eggs during incubation determines the sex of the turtles. As a result, climate change is causing feminization of the species.
Marine turtle clutches are sensitive to temperature changes and typically incubate successfully only between 26 and 36°C, with temperatures at the high end of the range becoming females and the lower end becoming males (Hawkes et al., 2014).
- 50% of either sex produced at the ‘pivotal temperature,’ between 28 to 31°C (Hawkes et al., 2009).
- A mixture of sexes is produced within a transitional range of temperatures but temperatures on either end of the spectrum will result only males or females.
- Incubation duration is inversely correlated with incubation temperature, so warmer nests incubate on the beach for a shorter period.
85 percent of leatherback turtles are now born female (Treadway, 2018); this number will increase as nesting sites get hotter. While specific thresholds are not available for leatherbacks, it has been estimated for loggerhead turtles that a 2°C increase will lead to a 99.86 percent female hatching rate (Rickles, 2018). Easter Australia Green Sea Turtles are now 99 percent female (Welch, 2018). Models estimate that Caribbean Colombian leatherback sea turtles currently produce approximately 92% female hatchlings (Patino-Martinez et al., 2011). These models predict that complete feminization could occur as soon as the next decade.
Additional Reproductive Threats
Other effects of climate change, such as extreme weather events, precipitation, and sea level rise also have potential to affect marine turtle populations (Rivas et al., 2018; Hawkes et al., 2009).
- Increase in the frequency and severity of extreme weather events threatens nests and nesting beaches. According to Hawkes et al., beach erosion caused by increased storm frequency and intensity reduces suitable nesting locations (2009). Storms also leave debris such as tree limbs on beaches, which can prevent turtles from coming on shore to nest since their large size makes them opportunistic in selecting a nesting beach (Hawkes et al., 2014).
- Sea level rise leads to shoreline erosion and even total loss of beaches. This is especially important considering female turtles travel great distances, expending valuable energy, to nest in the areas where they hatched (Patino-Martinez et al, 2014; Hawkes et al., 2014). When sea level rise causes flooding or inundation of beaches, sea turtle eggs are washed away or swamped (Rickles, 2018).
- Sea level rise will lead to an increase in fortification of coastal areas and hard structures to protect human settlements. This ‘protection’ reduces total sandy beach availability and leads to a loss of beaches while blocking beach access, disorientating turtles, or rendering sand inappropriate for nesting (Hawkes et al., 2014).
Adaptation & Conservation
The number of different threats to Leatherbacks makes it increasingly difficult to find management techniques to combat these issues (Rickles, 2018). Since reproduction is a major concern for the species, and the open ocean is difficult to manage, adaptation efforts are focused on coastal habitats: nesting beaches (Rickles, 2018).
- On popular nesting beaches in Trinidad, sea turtle poachers have become conservationists and ecotourism guides, leading beach clean-ups and educating tourists during nesting season. Jino et al. found that community-based conservation efforts could increase nesting populations (2018).
- Dutton et al. also found that aggressive beach protection and egg relocation efforts in the Caribbean were likely to increase the nesting population (2005). While this is promising for conservation, there is no evidence it reduces feminization.
- García–Grajales et al. similarly found that shading nests increases hatching success rate, but their study showed 96.3 percent of the juvenile Leatherback turtles were female (2019).
- However, Esteban et al. more optimistically estimate that relocation and shading could shift ratios from 97–100% female to 60–90% female for very little cost (2018).
One challenge is that global conservation efforts often fall onto small coastal communities. A case study in Grande Riviere, Trinidad, a major nesting site, found that locals had limited scientific knowledge of climate change and felt that adaptation was the state’s responsibility, despite the community being very dependent on natural resources including leatherback turtle ecotoursim (Mycoo & Gobin, 2013).
The identification of existing male-producing beaches will critically important. Given the flexibility of Leatherback turtles in nest selection relative to other marine turtles, it is more likely that they will quickly adapt by nesting further from the equator before the species is extinct. Relocation of eggs to cooler beaches could support this process. Reducing anthropogenic stressors, including mitigating climate change, is necessary for the fate of Leatherbacks and other species (Hillebrand et al., 2018).
About the Author
Allison Pilcher graduated from St. Lawrence University in 2019 with a combined major in Environmental Studies and Sociology, and minors in Government and Caribbean and Latin American Studies. Her interest in Leatherback sea turtles began after witnessing Leatherbacks nest while studying in Trinidad. This web page was produced for Dr. Jon Rosales' Adaptation to Climate Change course in the spring of 2019.
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Banner Image: Jordan Beard, Wikimedia Commons