Raw data from: Environmental variation structures reproduction and recruitment in long-lived mega-herbivores: Galapagos Giant Tortoises

Understanding drivers of vital rates is important for testing life history theory, predicting population dynamics, and guiding conservation. Migratory, long-lived animals are important to life history theory because they present an extreme among trade-offs in vital rates: delayed maturity, low fecundity, variable recruitment rates, and long generation times. Understanding how vital rates respond to environmental variation has been elusive for such species because of difficulties in studying wide-ranging animals over extended periods. Populations that live on elevation gradients can provide tractable study systems because considerable environmental change occurs over small geographic distances. Galapagos tortoises are an iconic example; they are long-lived, migrate seasonally, face multiple anthropogenic threats, and have cryptic early life history stages for which vital rates are unknown. From 2012–2021, we studied the reproductive ecology of two species of Galapagos tortoises along elevation gradients that coincided with substantial change in climate and vegetation productivity. Specifically, we 1) measured physical and reproductive condition of adult females, 2) tracked the movements of 33 adult females using GPS telemetry, and re-located them seasonally to measure condition, 3) recorded nest temperatures, clutch characteristics, and egg survival from 107 nests, and 4) used radio telemetry to re-locate 104 hatchlings to monitor growth, survival, and movements. We also monitored temperature, rainfall, and primary productivity from field and remotely sensed data along the elevation gradient. Adult females were either elevational migrants or year-round lowland residents. Migrants had higher body condition than residents, and body condition was positively correlated with fecundity. Nests occurred in the hottest, driest part of the tortoise's range, between 6–165m elevation. Clutch size increased with elevation, egg survival declined, and hatchling survival and growth were highest at intermediate elevations. Hatchlings dispersed rapidly between 100–750 m from their nests before becoming sedentary in ranges <0.05 ha. Environmental variability mediated by elevation influenced adult fecundity and juvenile recruitment. Predicted future climates will profoundly impact the relationships between elevation and vital rates of Galapagos tortoises and other species living on elevation gradients. Resilience will be maximized by ensuring connectivity of foraging and reproductive areas within current and possible future elevational ranges of these species.