DOI: 10.5061/dryad.tx95x6b24
title: Raw data from: Environmental variation structures reproduction and recruitment in long-lived mega-herbivores: Galapagos Giant Tortoises

description: 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
 &lt;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.
description: <em>Study
 site</em> The Galapagos Islands straddle
 the equator in the eastern Pacific ca. 1000km west of Ecuador. This
 volcanic archipelago consists of some 129 islands, including 13 large
 islands (over 1km<sup>2</sup>, Fig. 1), the oldest of which
 are ca. 4M and youngest &lt; 0.5M (Poulakakis et al. 2012). The
 climate is characterized by a hot wet season from January to May, and a
 cool dry season for the rest of the year (Trueman and d'Ozouville
 2010). However, during the “dry” season, persistent cloud cover results in
 humid upland conditions on the windward (southern) slopes of the larger
 islands (Colinvaux 1984). Vegetation patterns are driven by rainfall and
 substrate, which are largely determined by aspect, elevation, and lava
 flows. Our study
 focused on Santa Cruz Island (Fig. 1), which rises to 860m elevation with
 a surface area of 986km<sup>2</sup> (Snell et al. 1996), one
 of nine islands Giant tortoises are believed to have currently or
 previously occupied (Caccone et al. 2002). The island hosts the largest
 human population on the Galapagos Islands, estimated at  &gt;15000 in
 2010 (León and Salazar 2012). Most of the moist highland zones have been
 converted to agriculture and at least 86% of these zones are now degraded
 by either agriculture or associated invasive plant species (Trueman et al.
 2014, Laso et al. 2019). Tortoises on Santa Cruz occur in
 two separate areas on the island’s western and eastern flanks (Fig. 1).
 Western Santa Cruz tortoises, <em>Chelonoidis porteri</em>,
 are widely distributed from ca. 0–450m elevation along the southwestern
 flank of Santa Cruz (labelled “El Chato” in Fig. 1), and comprise several
 thousand individuals (MacFarland et al. 1974, Benitez-Capistros et al.
 2018, Tapia A et al. 2021). Eastern Santa Cruz tortoises, <em>C.
 donfaustoi</em>, occur on the east of the island (labelled “Cerro
 Fatal” in Fig. 2), between ca. 50–450m elevation (Fig. 2). Both species
 are of the “domed” morphotype, and they display strong size dimorphism,
 with females weighing up to ca. 120kg and males occasionally exceeding
 260kg (unpub. data). Both species are seasonal partial migrants along the
 elevation gradients, and females of each species use permanent nesting
 areas situated at different elevations with different environmental
 conditions that offer potential to test our hypotheses. Some of our
 datasets come only from western Santa Cruz tortoises because we began our
 study in El Chato, before expanding to the eastern Santa Cruz tortoise
 population. <em>Environmental data
 collection</em> Rainfall and shade temperature
 across the elevation gradient of each species were quantified via a series
 of weather stations located at 50m altitude increments from 50–400m in El
 Chato and 100–400 in Cerro Fatal. At each location, a 100cm x 12cm plastic
 tube sealed at the bottom was attached vertically to a stake with an
 unobstructed view of the sky. A layer of engine oil 5cm thick was poured
 into each tube to prevent evaporation of accumulated rain. At the
 beginning of each month, the height of the water plus oil column was
 measured, after which most of the water was syphoned out of the tube. The
 height of the water column was then re-measured and left to accumulate
 rainfall during the month. Shade temperature was measured every four hours
 with iButton thermochrons (iButtons, DS1922L, Maxim Integrated, San Jose,
 CA) housed in 5mm thick polystyrene cups fitted upside down to the
 underside of each rainfall accumulators. IButton data were downloaded
 monthly. To estimate vegetation productivity along the elevation gradient
 and also in nesting zones, we used the Normalized Difference Vegetation
 Index (NDVI) obtained from the MODIS (Moderate Resolution Imaging
 Spectroradiometer) product of the Terra satellite which has been shown to
 reliably capture variation in vegetation growth, productivity, and
 resources available to herbivores (Huete et al. 2002, Pettorelli et al.
 2005, Mueller and Fagan 2008). MODIS vegetation indices are provided at
 250m × 250m resolution every 16 days, yielding 23 composites per year. We
 calculated pixel values from raw data following methods described by
 Bastille-Rousseau et al. (2017). Pixel values averaged by month from
 0-400m along the elevation gradient within the range of tagged female
 tortoises were selected and projected onto a digital elevation model of
 Santa Cruz Island to assign elevation values to each pixel, and then
 binned at 25m intervals. NDVI within nesting zones and areas used by
 juveniles was obtained from values of pixels that touched minimum convex
 polygons formed from hatchling movement data collected in each zone (see
 below). <em>Female tortoise
 movements, physical and reproductive condition, and the timing of nesting
 </em> Between 2010–2021, custom-made
 GPS telemetry tags (e-obs GmbH, Munich, Germany) were fitted to a sample
 of adult female tortoises from both species<em> </em>(Blake et
 al. 2013, Blake et al. 2015, Bastille-Rousseau et al. 2016a,
 Bastille-Rousseau et al. 2016b). Tags were glued to the front of the
 carapace using non-toxic plumber’s epoxy (Fix-It Stick Epoxy Putty, Oatey,
 Cleveland, Ohio, USA). Tags collected fixes every hour and stored the data
 on a memory card that was periodically downloaded in the field using a
 wireless modem. Four times per year (Nov-Dec,
 Mar, Jun-Jul, Sep) between 2013–2015, we attempted to find all tagged
 female tortoises to determine their body condition and reproductive
 status. During these periods we also searched for non-tagged females
 dispersed along the elevation gradient and recorded the same metrics. For
 each adult female encountered we recorded mass to the nearest 0.5kg and
 curved carapace length to the nearest 0.5cm with a tape measure. Body
 Condition Index (BCI) was calculated as  BCI =
 m/L<sup>2.98</sup> where m= mass, and L=curved
 carapace length. The power coefficient was calculated based on the best
 fit for a large sample of length/mass measurements collected by Galapagos
 National Park Service (GNPS) rangers (Blake et al. 2015). We used a
 portable ultrasound machine (E.I. Medical Imaging Portable Ultrasound
 Ibex®) with a curvilinear 3.8 MHz transducer probe to scan adult female
 tortoises for the presence of eggs and developing follicles following
 methods of Robeck et al. (1990) and Casares et al. (1997). Complete counts
 of follicles and eggs were not possible, so we recorded presence/absence
 of follicles or eggs. <em>Monitoring of nesting
 zones and identification of nests</em> Studies of tortoise nesting were
 focused mostly in the three known nesting zones in El Chato, with
 supporting data from two zones in Cerro Fatal (Fig. 1). Nesting zones are
 distinguished by their open soil, evidence of existing and old nests and
 signs of intensive use by tortoises, and these concentration sites are
 well known to Galapagos National Park rangers. Mean elevation in the three
 El Chato nesting zones was 13m, 58m and 107m in the lower, middle, and
 upper zones respectively, and 90m and 165m for the lower and upper zones
 in Cerro Fatal. Nesting zones are separated by extensive areas of lava
 rock unsuitable for nesting. Between 2013–2016 from July to
 October, we searched for freshly made nests in the three El Chato nesting
 zones (n=98), and in 2016 for the two Cerro Fatal zones (n=12). We
 identified freshly constructed nests by signs of recent digging and the
 presence of moisture on the nest cap (tortoises often urinate and defecate
 in and on the nest site). On finding a fresh nest, we dug carefully under
 the hardened nest cap and removed it. We then removed all eggs,
 maintaining their orientation and marking each egg with a unique number
 ID. We recorded breaks and cracks in each egg, weighed them to the nearest
 gram and, with calipers, measured the diameter to the nearest millimeter.
 Eggs were then carefully put back into the nest in the same position in
 which they had been found. For a sample of 41 nests, we estimated the
 center of the egg chamber where we placed iButton thermochrons
 (temperature loggers, DS1922L, Maxim Integrated Products, San Jose, CA.)
 as we replaced the eggs. iButtons recorded temperature data every four
 hours. As we replaced the eggs back in their original location, we placed
 soil around each egg to secure it in its original configuration and packed
 surface soil to replicate the cap as closely as possible. It is the policy of the Galapagos
 National Park Service (GNPS) to protect tortoise nests in designated areas
 from feral pigs with wire mesh placed over nests and to open nests at the
 predicted end of incubation to extract hatchlings. This reduces the risk
 of entombment, but hatchlings are liberated before they would naturally
 emerge. This precluded us from reporting on natural incubation times and
 emergence dates, as well as mortality caused by entombment. Following GNP
 regulations, when incubation was estimated to be complete, we reopened the
 sample of nests. Hatchlings had either already exited the nest or had
 hatched but not emerged. When present, hatchings were measured to the
 nearest millimeter (linear length as opposed to CCL used for adults),
 weighed to the nearest gram, visually inspected for abnormalities, and
 then released at the nest. To quantify growth, survival, and movements, we
 selected a sample of hatchlings from randomly chosen nests to be fitted
 with VHF tags (RI-2B, Holohil Ltd. Carp, Ontario, Canada). Tags weighed 5g
 each; only tortoises that weighed &gt;60g were fitted with
 transmitters. The same attachment method as for adults was used, except
 the tags were placed at the rear of the carapace only. A survival analysis
 revealed no effect of transmitter mass relative to initial body mass on
 hatchling mortality (Table S10d). Thereafter, we identified the location
 of tagged hatchlings approximately bi-weekly, recording location obtained
 from a handheld GPS unit (Garmin GPSMAP 64, Olathe, KS., USA.), weight,
 and length. Hatchlings found dead were usually in an advanced state of
 decay and cause of death could not be determined.   All animal handling procedures
 followed the guidelines of the GNPS, the Max Planck Institute of Animal
 Behavior, and IACUC protocol #121202 of the State University of New York,
 College of Environmental Science and Forestry.