Sea turtles are threatened and endangered throughout their natural range due to various anthropogenic interferences, such as poaching and habitat destruction (World… c2016). Sea turtles provide many ecological benefits, such as acting as distributors of nutrients and sculptors of both terrestrial and marine substrate (Mancini et al. 2011). However, global climate change is threatening sea turtle populations’ ability to continue to provide these benefits (Mancini et al. 2011).
Beach habitat loss due to sea level rise is one aspect of global climate change that affects the nesting range of sea turtles (Fish et al. 008). Another aspect of climate change is the gradual warming of the atmosphere. Since sea turtles are reptiles who bury their eggs, their eggs require a steady temperature and moistness in the surrounding substrate for developmental purposes (Tomillo et al. 2012). According to Tomillo et al. (2012), variable conditions cause hatchling mortality to increase due to instability and an inability for the substrate to provide the ideal environment for the eggs. In addition, the temperature of the surrounding substrate causes the population dynamics of sea turtles to be skewed (Tomillo et al. 015).
The two elements of substrate conditions varying due to climate change are population dynamics and hatchling success. Sea turtles have temperature-dependent sex determination (TDS) which is determines offspring sex, with warmer nests generating females and cooler nests generating males (Hays et al. 2010). Females are produced in warmer, more stressful environments because it increases fecundity of future populations (Tomillo et al. 2015). However, according to Tomillo et al. 2015), TDS becomes ineffective under changes in the climate past the threshold range because natural selection may not act fast enough to perpetuate the genetic variation needed for warmer climates. The thermal threshold for sea turtle hatchlings can vary based on the species, but is 33°C on average (Fuentes and Cinner 2010).
Substrate temperature and moisture are also key to hatchling success. For example, developing embryos of olive ridley turtles (Lepidochelys olivacea) die at temperatures greater than 35°C which means the nest fails to produce any offspring (Hill et al. 015). Hatchlings also may die at the surface of the nest due to high temperatures during both the pre-emerging and emerging stages. (Hill et al. 2015). These two components of climate change are directly affecting threatened and endangered sea turtle populations. The purpose of this research is to understand the link between abiotic factors, such as substrate composition, and biotic factors, such as population dynamics and hatchling success, of sea turtle reproduction. Abiotic factors of sea turtle reproduction
Fuentes and Cinner (2010) did an analysis to determine which aspect of global climate change impacted sea turtle nesting the most. In this study, they found that the biggest threat to sea turtle reproductive success is sand temperature. Increased sand temperatures cause incubation periods to be shorter and the increased development rate also causes more oxygen to be consumed and more heat to be released, further raising the nest temperature (Chen et al. 2010). The three main impacts on substrate temperature are shading, watering, and depth (Hill et al. 015). According to Hays et al. (2001), sand albedo is the concept of substrate temperatures increasing as the proportion of solar radiation absorbed by the substrate increases. Because of this solar radiation, shading has shown to be more effective than watering the nests since the water would evaporate out of the surface layer drying out the nest (Hill et al. 2015). However, watering treatments are effective in increasing the water content of the sand, which is necessary for making the environment optimal for nest incubation (Hill et al. 2015).
Oxygen levels in nests also are dependent on substrate temperature (Chen et al. 2010). Oxygen diffusion occurs not only between the inside and outside of the developing egg but also between the nest and the external environment (Ackerman 1980). Oxygen is necessary for embryogenesis and hypoxia can delay development or cause mortality (Chen et al. 2010). Temperature and water concentration determine the gas diffusion rate of oxygen (Mota 2009). Biological response to abiotic factors Population dynamics, such as sex ratios, are determined by TDS in sea turtle hatchlings (Hays et al. 2010).
According to Charnov and Bull (1977), genotypic sex determination (GSD) is less beneficial than environmental sex determination, such as TDS, when environments are not stable and the sexes have a dissimilarity in reproductive fitness. Sex ratios in sea turtles are usually female-biased due to increased incubation periods, which can lead to intensified populations of females while male populations are less supported (Tomillo 2015). The transitional range of temperature (TRT) is where the offspring being produced is a mixture of both sexes, rather than all male or all female (Mrosovsky and Pieau 1991).
The TRT of sea turtles is about 1°C. Current global climate change predictions indicate that the air temperature will reach 1° to 4°C warmer than the current climate by the end of the 21st century. Since this rate of temperature increase would surpass the TRT of sea turtles, it is possible that all hatchlings produced will be female by the end of the 21st century (Tomillo 2015). Under climate change scenarios where gas emissions and concentrations increase, TDS has been shown to be ineffective in the sustainability of the population due to disproportional sex ratios (Figure 1, Tomillo 2015).
GSD populations were more negatively affected than TSD populations in low gas emission in both current mean temperatures and temperatures lower than the current mean temperatures (Figure 1). As gas concentration was increased, the differences in GSD and TDS were mitigated (Figure 1). All populations, regardless of temperature and sex determination method, were extirpated eliminated within 50 years in the high emission scenario (Figure 1).
This shows the importance of controlling climate change through the management of gas emissions as it has a detrimental effect on sea turtle population dynamics (Tomillo 2015). However, according to Hays et al. (2010), sex ratios may not be as in danger as other scientists believe. The operational sex ratio (OSR) is the number of males and females ready to mate (Kvarnemo and Ahensjo 1996). Male sea turtles return to the location of reproduction as much as 2. 6 times more often than females (Hays et al. 2010). This means that the OSR is less female-biased than the hatchling male-to-female ratio (Hays et al. 010). With the OSR at a potentially sustainable level, the decrease in reproductive output of sea turtle populations is less likely, since the likelihood of unfertilized clutches will be minimized (Hays et al. 2010). Water concentration can drastically affect hatchling success (Laloe et al. 2015). Water cool nests to bring the temperature of the nest down far enough to stimulate embryonic development and/or male production (Laloe et al. 2015).
However, too much water can cause the gas exchange to be limited causing mortality due to suffocation in the nest (Chen et al. 010). The concentration of water in nests is directly affected by variations in rainfall and temperature (Tomillo et al. 2012). For example, the El Nino Southern Oscillation causes higher hatchling mortality due to drier and warmer conditions in Costa Rica (Tomillo et al. 2012). It is predicted that hatchling survival will drop by 50 to 60% within the next 100 years in that region (Tomillo et al. 2012). However, due to global climate change, regions also facing drier and warmer conditions may face similar declines in hatchling survival rates. Management and Research
Methods of controlling substrate temperatures include nest shading, re-vegetation programs, tree planting, beach sprinklers, sand coloring, and habitat modi? cation (Fuentes and Cinner 2010). Shading has shown to be the most effective method of cooling nests, but can be used in combination with watering to achieve the optimal environment for sea turtle nests (Hill et al. 2015). Shade structures have to be a fabric or construct that allows rainfall to reach the nest or the sand temperature will actually be warmer around the nest than the environment receiving rainfall (Jourdan and Fuentes 2013).
Shade structures should be made of either mesh that is permeable to rain or vegetation with leaves that allow rainfall to reach the nest (Jourdan and Fuentes 2013). Watering the san via sprinklers is most effective when completed in shaded areas or at night while sprinkling during the day has shown to increase sand temperature (Jourdan and Fuentes 2013). However, caution must be taken if using both methods as it may be possible to cool the nest to shift the output from 100% female to 100% male (Hill et al. 2015).
This may be an ideal way to improve the OSR in the future if the OSR shifts to having too many females for too few male partners (Hill et al. 2015). The TRT should be used to obtain the ideal temperature when combing both shading and watering methods (Jourdan and Fuentes 2013). Climate controlled hatcheries are another alternative to irrigation and shading (Tomillo et al. 2015). Intentional modification of sea turtle population dynamics may be needed to combat the rising temperatures that are exceeding the TSD-effective temperature range (Tomillo et al. 2015).
However, more studies need to be done on the effects of targeting specific sex ratios as it may have negative impacts on the sea turtles’ natural reproductive strategies (Tomillo et al. 2015). Hatchling mortality due to incubation temperature also requires more research (Laloe et al. 2015). Increasing substrate temperatures has been linked to an increase in hatchling mortality, but most research has been focused on sex ratios (Laloe et al. 2015). Shading and irrigation techniques have been suggested, but studies need to be performed on clutches to understand their impacts on hatchling success and sex ratios (Hill et al. 015).
Figures Figure 1 Predictions of nesting females for TSD and GSD populations under various climate change scenarios over 200 years. Both current mean temperatures (a) and temperatures 0. 5°C lower than current conditions (b). The temperatures lower than current conditions (about 29. 9°C) were chosen to represent probable conditions of the early 20th century. Low, medium, and high concentrations were measured with corresponding temperature increases of 1. 0, 2. 0, and 3. 7°C from current mean temperatures.