Population Viability of Sea Turtles in the Context of Global Warming

Abstract

Sea turtles present a model for the potential impacts of climate change on imperiled species, with projected warming generating concern about their persistence. Various sea turtle life-history traits are affected by temperature; most strikingly, warmer egg incubation temperatures cause female-biased sex ratios and higher embryo mortality. Predictions of sea turtle resilience to climate change are often focused on how resulting male limitation or reduced offspring production may affect populations. In the present article, by reviewing research on sea turtles, we provide an overview of how temperature impacts on incubating eggs may cascade through life history to ultimately affect population viability. We explore how sex-specific patterns in survival and breeding periodicity determine the differences among offspring, adult, and operational sex ratios. We then discuss the implications of skewed sex ratios for male-limited reproduction, consider the negative correlation between sex ratio skew and genetic diversity, and examine consequences for adaptive potential. Our synthesis underscores the importance of considering the effects of climate throughout the life history of any species. Lethal effects (e.g., embryo mortality) are relatively direct impacts, but sublethal effects at immature life-history stages may not alter population growth rates until cohorts reach reproductive maturity. This leaves a lag during which some species transition through several stages subject to distinct biological circumstances and climate impacts. These perspectives will help managers conceptualize the drivers of emergent population dynamics and identify existing knowledge gaps under different scenarios of predicted environmental change.

A morphologist, a modeler, and an endocrinologist consider sea turtle sex ratios in a changing climate. Some wine was involved

“…The consequences of skewed primary sex ratios at the population-level can be complex and depend on reproductive behavior and physiology of adults, as well as the age at sexual maturity (ASM), reproductive longevity of males vs. females (Maurer et al, 2021) and methods of assessment (e.g., Lasala et al, 2013;Patrício et al, 2019). While breeding frequency, nest number, and clutch size are commonly measured parameters for adult female sea turtles, adult male sea turtle behavior is far more difficult to study because they do not return to shore.…”

“…The relative benefits of such activities can be evaluated with population models that simulate the effects of variable primary sex ratios through time. Conservation strategy evaluation with population models has been used for sea turtles since the 1980s (e.g., reviewed by Heppell et al, 2003;Patrício et al, 2019Patrício et al, , 2021Maurer et al, 2021) and can help managers identify actions that are more or less likely to contribute to population persistence (Heppell and Crowder, 1998;Rout et al, 2013).…”

“…To explore the potential population-level effects of warming beach temperatures on the peninsular Florida loggerhead population under different assumptions about adult male behavior and reproductive success, we incorporated long-term primary sex ratio observations from nesting beach studies in southern Florida into a simple, age-structured simulation model with a variety of outputs, including nest and nesting female abundance, adult sex ratio, and effective population size (N e ) of adults and breeders. The last is an important response variable because highly female-skewed populations could continue to function while suffering inbreeding depression (Maurer et al, 2021). We then evaluated the benefits of increasing the frequency of strong male production years under various model assumptions to simulate mitigation strategies.…”

Multiple mating rescues offspring sex ratio but not productivity in a haplodiploid exposed to developmental heat stress

“…Alternatively, changes in OSR can be accompanied by changes in adult sex ratio (ASR) in species with thermal-dependent sex determination and in haplodiploids (Maurer et al, 2021;Mobley & Jones, 2009). Specifically, in haplodiploids, female sterility results in reduced offspring production but male sterility leads to a male-biased ASR, because fertilized eggs only generate (diploid) daughters, while (haploid) sons are generated from unfertilized eggs (Helle & Sabelis, 1985).…”

“…In this species, male, but not female, sexual maturation is delayed at high temperature, with the operational sex ratio of populations becoming more female-biased without observable changes on the adult sex ratio (Walsh et al, 2021). Alternatively, changes in OSR can be accompanied by changes in adult sex ratio (ASR) in species with thermal-dependent sex determination and in haplodiploids (Maurer et al, 2021; Mobley & Jones, 2009). Specifically, in haplodiploids, female sterility results in reduced offspring production but male sterility leads to a male-biased ASR, because fertilized eggs only generate (diploid) daughters, while (haploid) sons are generated from unfertilized eggs (Helle & Sabelis, 1985).…”