Too hot for sea turtles?

Sea turtles are particularly susceptible to climate change because their behaviour, physiology and life history are affected by environmental changes.

All life stages of sea turtles (e.g. hatchlings, juvenile and adult) can be affected by climatic processes. The more detectable impacts of climate change on sea turtles will occur during their interlude on land (during the laying and incubation of eggs and hatching) because there are clear and relatively straightforward effects of increased temperature, sea level rise and cyclonic activity on their nesting sites and nesting success. For example, predicted increases in sand temperatures will skew sea turtle population sex ratios towards predominantly females, decrease hatching success and alter the size of the hatchlings. Sea-level rise and cyclonic activity will cause loss and/or alteration of nesting beaches and egg mortality. A reduction of available nesting area will decrease the area available for nesting, potentially increasing nest infection and destruction of nests by turtles. Other predicted impacts from climate change include shifts in latitudinal ranges, alteration of reproductive periodicity, changes in hatchling dispersal and migration and indirect effects on food availability. Indeed, research investigating the impacts of climate change on the largest green turtle population in the world, the northern Great Barrier Reef (nGBR) green turtle population, predicts a complete feminisation of annual hatchling output by 2070 and a potential loss of up to 38% of available nesting area across the most important nesting sites for this population.

Ultimately, the risk that climate change poses to sea turtle populations will depend on their ability to adapt. Sea turtles have existed for hundreds of millions of years and during this time, they have survived dramatic climate fluctuations and changes in sea level. It is speculated that sea turtles have historically adapted to environmental changes by redistributing their nesting sites and nesting season, by developing new migratory routes and by changing their behaviour. The extent to which sea turtles will (or can) adapt either behaviorally or physiologically and how these responses may counteract impacts of climate change, remains to be seen. Despite sea turtles’ ability to cope with past climatic changes, their ability to do so again is uncertain. Current rates of climate change are much faster than historic rates and at present, they are being simultaneously affected by a variety of anthropogenic activities.

The uncertainty on whether and how sea turtles can adapt to climate change necessitates precautionary actions and adaptive management. A mix of different short-term and long-term approaches have been suggested including: 1) mitigating the threat by reducing global greenhouse emissions; 2) adaptively managing impacts from climate change to increase population persistence; and 3) employing actions that build biodiversity resilience, such as addressing current non-climate-related threats. Reducing emissions is perhaps the biggest challenge, but even immediate reductions will not stop the already apparent and unavoidable impacts of climate change but are still essential to ameliorate threats. Adaptive management is hindered by risks associated with implementing mitigation strategies (e.g. species relocations, manipulations or management actions that improve habitat) and a lack of understanding of how effective and feasible these strategies will be at reducing impacts at relevant temporal and spatial scales.

The majority of the suggested strategies, to date, focus on the nesting environment, as this is where most research on sea turtles occurs and baseline knowledge is strongest as well as where implementation and monitoring is logistically easiest. However, even on nesting beaches, the implementation of these strategies requires an understanding of the thermal profile at different nesting beaches, the current sex ratio of hatchlings entering the population and the proportion of males to females that are ready to mate at any one time for that population; information which is rarely available. Importantly, not many strategies have been suggested for mitigating in-water impacts from climate change or boosting resilience of foraging turtles, presumably because of a lack of data or an understanding of how these systems will be impacted.

Building biodiversity resilience, to date, has focused on reducing non-climatic threats under the rationale that large, healthy and stable populations will help maintain (1) genetic diversity, which can facilitate adaptation to variable conditions; (2) a wide geographic distribution, which can minimise the overall impacts of area-specific threats; and (3) a large breeding population, which can help absorb impacts through an increased ability to recover from population disturbance.

In this context, I conducted a survey with other sea turtle specialists, to explore factors that may influence the resilience of sea turtles to climate change, which indicated that persistence of nesting grounds themselves may also influence the resilience of sea turtle populations. This follows the rationale that optimal nesting areas are necessary for reproduction and therefore the entry of offspring into the population. It also provides buffer areas for sea turtles to redistribute the geographic locations of their current nesting grounds, if necessary, as an adaptive response to deal with environmental or land-use changes. This highlights the need to maintain and protect important nesting beaches and to identify and legally protect areas that will maintain suitable nesting environments in the future, even if they are not major nesting grounds today. This will be particularly difficult in areas where coastal development and beach alteration is widespread and continually expanding. Impacts from climate change are likely to interact with other anthropogenic threats, such as coastal development. Therefore, managers face the challenge of addressing the direct effects of climate change, as well as ongoing threats that sea turtles face throughout their geographic range. For logistical, financial and political reasons, natural resource agencies cannot address all of these drivers or “threats” simultaneously; priorities must be established. For this, there is a need to understand the relative impacts of current and future threats to the overall population dynamics and the variation of those impacts. This is particularly important for sea turtles, since each of their life stages has a different reproductive value (potential for contributing offspring to future generations), and therefore reductions to each life stage will impact population growth rates differently.

Risk and vulnerability assessments are increasingly being used to help prioritise management of species in the face of climate change and also to investigate the risk of not addressing evident threats. For example, I used a vulnerability assessment to identify which climatic process will cause the most impact on the terrestrial reproductive phase of the nGBR green turtle population and to explore how the vulnerability of this population to climate change will alter if the impacts of different climatic process are mitigated.

A green turtle, Chelonia mydas, at a foraging ground. Sea turtle can be affected at multiple life stages by climate change

Even with innovative decision-support tools, the implementation of management will likely be compromised without the necessary laws and policy. Existing national and international laws might need to be revisited and adapted to ensure that management can address emerging climate change threats. Arguably, many laws are ill-suited to climate change because of their static nature. Many were written to address specific types of threats at a time when climate change was not at the forefront of concerns and are consequently limited in a changing world. Legislative flexibility will be essential, particularly for emergency responses. But changes in legislation require scientific, political, and community support. Public awareness of the links between climate change, the potential impacts to sea turtles, and the need to take action can provide the momentum to do something about it. As iconic species, sea turtles could be used as flagships to promote understanding of the impacts of climate change on biodiversity, to build community support for conservation action and to provide incentives for effective management and support for research, conservation and changes in policy.

Ultimately, an integrated approach comprising several strategies will be needed. Most strategies will require community, government support and voluntary behavioural changes to minimise social and economic impacts and in many cases community consultation to improve the effectiveness and acceptance of new management arrangements. The best set of strategies will likely be site-specific and will depend on environmental, social, economic and cultural conditions at a particular location, yet also will be integrated at the appropriate regional scale. Importantly, targeted research to understand the adaptive capacity of marine turtles, the exposure and sensitivity of populations and key habitat to climatic processes, population-scale thresholds of concern, and synergistic impacts is necessary to help guide future efforts to manage sea turtles and enhance their adaptive capacity.

Suggested reading:
Fuentes MMPB, Pike DA, Dimatteo A & BP Wallace. 2013. Resilience of marine turtle regional management units to climate change. Global Change Biology, 19, 1399–1406.
Fuentes MMPB, Limpus CJ & M Hamann. 2011. Vulnerability of sea turtle nesting grounds to climate change. Global Change Biology, 17, 140-153.

Photograph: Mila Zinkova

This article is from issue


2012 Jun