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Behavior

Hawksbills in hot water? Temperature and Precipitation Impacts on Hawksbill Sea Turtle Nests

Article

Montero, N., dei Marcovaldi, M. A., Lopez–Mendilaharsu, M., Santos, A. S., Santos, A. J., & Fuentes, M. M. (2018). Warmer and wetter conditions will reduce offspring production of hawksbill turtles in Brazil under climate change. PloS one, 13(11), e0204188.

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0204188

Background

Climate change is already having effects on sea turtles. All seven species of sea turtle are dependent on temperature for habitat selection and physiology. The impacts of temperature and moisture on hatchling development have real long-term impacts on marine turtle populations. Temperature even determines if an embryo develops into a male or female within the egg – warmer sand temperatures lead to more females and cooler temperatures lead to more male hatchlings. As a result of being temperature dependent for growth, marine turtles have a narrow temperature range of 25-35°C (75-95°F) where eggs can successfully develop. Similarly, if a nest is too moist or too dry it can affect the how the eggs develop.

Concerns are growing among researchers about how climate change may be impacting the overall stability of marine turtle populations. Marine turtles are vulnerable to different environmental conditions and have different tolerances from species to species. How the hatchlings of each species are impacted by temperature, moisture, or other conditions still remains a mystery.

Figure 1. A map showing study sites in Brazil identified by black dots. (Montero et al., 2018)

The hawksbill sea turtle (Eretmochelys imbricate) is a mid-size marine turtle averaging over 100 lbs. These turtles tend to frequent tropical reefs where they feed on sponges, mollusks, and jellyfish. In this study, the researchers examined nesting beaches along the Brazilian coast. In Brazil, hawksbill turtles use five beaches in two main regions for nesting. Interestingly, of the hawksbills examined in Brazil, 42% were hybrids with loggerhead sea turtles (Caretta caretta) and 2% were hybrids with olive ridley sea turtles (Lepidochelys olivacea), the smallest species of sea turtle in the world. Between the two regions of Rio Grande do Norte and Bahia there are differences in temperature, nesting season, moisture, and nesting density. While Bahia has a nesting season from October – April and more nests overall, Rio Grande do Norte has a higher density of nests and a nesting season from November – May.

Study Goals

The goal of this study was to analyze how hawksbill sea turtle hatching success in Brazil is affected by changes in environmental conditions. Data was used from beaches in Bahia and Rio Grande do Norte, which were surveyed during the nesting season from 2005-2016. While there is some variability in sampling effort due to constraints between beaches, 5,017 hawksbill sea turtle nests were sampled in total during this time. Only nests that were not disturbed by predators or storms were included in this study. Data on climate was collected from weather stations located near the sampling sites.

Results

By 2100, hawksbill hatching success is expected to decrease by 11% as a result of increased temperatures and precipitation. In this article, the authors found the hatchlings were more likely to be fully developed and have higher emergence rates in cooler and wetter conditions. In fact, precipitation had a positive impact on hatchling success in all regions of Brazil. The authors hypothesized that wetter conditions may counter increasing sand temperatures. Between Bahia and Rio Grande do Norte there were differences in precipitation between nesting periods. Bahia has a higher hatching success rate than Rio Grande do Norte and is the primary hawksbill nesting location in the Southern Atlantic Ocean.

Figure 2. The beaches surveyed in Bahia and Rio Grande do Norte and the percentage of nests contributed to the regional total. (Montero et al., 2018)

Precipitation is only beneficial to an extent. An interesting aspect of development in eggs is that there needs to be oxygen and gas exchange through the shell for embryos to develop. Too much water can result in soil saturation; which, replaces the oxygen particles in the sand with water causeing embryos to suffocate. In Brazil, sand saturation is already having a negative impact on loggerhead sea turtle nests that occur during the wettest months. As the climate continues to change, precipitation is expected to increase; yet, hawksbills may have a nesting advantage. Hawksbill sea turtles tend to favor heavily vegetated areas farther from the surf than loggerheads. The preference for nesting habitat farther up the beach and a nesting season during the dry months may reduce the impacts of precipitation on hawksbill nests.

Figure 3. Average regional and individual beach hatching and emergence success with standard deviation. (Montero et al., 2018)

Something important worth considering is that 42% of hawksbill sea turtles were identified as hybrids with loggerheads. Loggerhead nests have a higher thermal tolerance than hawksbill turtles. While nesting success for hybrids was similar to the parent species, emergence success was lower. This could indicate that Brazilian beaches are a refugia or safe habitat for hybrid individuals in the face of climate change. However, this is speculation as the response of hybrid turtles to climate changes is mysterious.

Solar radiation was also an indicator of hatchling success. Interestingly, solar radiation seemed to have a negative influence on hatchling emergence. The negative relationship could be the result of increased air and sand temperatures from solar radiation pushing temperatures above the thermal threshold of the embryos.

Final Thoughts

Figure 4. Predicted conservative (light grey line) and extreme (black line) hatching success in Rio Grande Do Norte (a.) and Bahia (b.) in the future. (Montero et al., 2018)

Sea turtles have been around since the “Age of Reptiles” or the Mesozoic. Since first appearing in the fossil record, marine turtles have survived several mass extinctions. The authors even discuss how, during prior events, these species changed nesting grounds, nest depth, and nesting season. Interestingly, researchers have even begun to see range shifts in leatherback (Dermochelys coriacea) and loggerhead sea turtles due to warming waters. However, the rate of sea level rise and coastal development may be harming sea turtle resiliency by eliminating nesting habitats. For management and conservation purposes, priority should be placed on protecting nesting beaches, regulating coastal development, and maintaining nesting habitat to ensure marine turtle resiliency to climate change.

 

 

 

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