Warming conditions boost reproductive output for a northern gopher tortoise population

Ea Hunter,K Hanley,Kj Loope,Dc Rostal,Kk Drake,Kt Shoemaker,Dn Jones

doi:10.3354/esr01155

Abstract

The effects of climate change on at-risk species will depend on how life history processes respond to climate and whether the seasonal timing of local climate changes overlaps with species-specific windows of climate sensitivity. For long-lived, iteroparous species like gopher tortoises Gopherus polyphemus, climate likely has a greater influence on reproduction than on adult survival. Our objective was to estimate the timing, magnitude, and direction of climate-driven effects on gopher tortoise reproductive output using a 25 yr dataset collected in southeastern Georgia, USA, near the northern edge of the species’ range. We assessed the timing of climate effects on reproductive output (both probability of reproduction and clutch size) by fitting models with climate covariates (maximum temperature, precipitation, and temperature range) summarized at all possible time intervals (in 1 mo increments) within the 24 mo period prior to the summer census date. We then fit a final model of reproductive output as a function of the identified climate variables and time windows using a Bayesian mixture model. Probability of reproduction was positively correlated with the prior year’s April-May maximum temperature, and clutch size was positively correlated with the prior year’s June maximum temperature. April-May and June maximum temperatures have increased over the past 3 decades at the study site, which likely led to an increase in clutch size of approximately 1 egg (15% increase over a mean of 6.5 eggs). However, the net effect of climate change on gopher tortoise population dynamics will depend on whether there are opposing or reinforcing climate responses for other demographic rates.

Highlights

The effects of climate change on the demography and population persistence of at-risk species may depend on whether the seasonal timing of local cli-mate changes coincides with climate-sensitive physiological processes or life history stages
We found strong effects of climatic variability on the probability of reproduction and clutch size of gopher tortoises: the prior April−May maximum temperature positively affected probability of reproduction and the prior June maximum temperature positively affected clutch size
This timeline suggests that temperature plays a role in the process of whether to initiate reproduction and how much to invest in it

Summary

Introduction

The effects of climate change on the demography and population persistence of at-risk species may depend on whether the seasonal timing of local cli-mate changes coincides with climate-sensitive physiological processes or life history stages. Many reptiles have temperature-dependent sex determination, and warming conditions during the critical period for sex determination can have demographic consequences through skewed sex ratios for those species (Janzen 1994, Jensen et al 2018). For other processes, it is less clear how the timing of temperature and precipitation fluctuations could influence the outcome. Pre-reproductive processes such as gamete production are likely influenced by temperature: as ectotherms, warmer conditions may spur reptiles to acquire more resources, resulting in greater gamete production (Adolph & Porter 1993). Whether warming results in greater reproduction depends on whether seasonal warming and the timing of pre-reproductive processes coincide

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