Long distance migration of Galapagos tortoises: The importance of nesting and nest sites

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1 Long distance migration of Galapagos tortoises: The importance of nesting and nest sites Final Report to the National Geographic Society, Global Exploration Fund Grant number: 4012 By Stephen Blake, James Gibbs, Fredy Cabrera, Washington Tapia and Martin Wikelski October

2 Introduction This report aims to provide an account of activities undertaken, datasets collected and results obtained on the project Long distance migration of Galapagos tortoises: The importance of nesting and nest sites funded by the National Geographic Society s Global Exploration Fund (grant No. 4012). This project grew out of previous work on the movement ecology of Galapagos tortoises, which revealed that on Santa Cruz Island, in the heart of the Galapagos archipelago, giant tortoises undergo long distance seasonal migrations up and down the altitude gradient of the island (Blake et al. 2013). Previous NGS funded research, under the Committee for Research and Exploration (grant No. 8716), revealed that these migrations are important elements of the effectiveness of Galapagos tortoises as seed dispersers (Sadeghayobi et al. 2011). Blake et al demonstrated that a major factor driving the migration is food availability, the distribution of which varies seasonally with rainfall. Migrating adult tortoises spend the cool dry season in the humid highlands where forage is consistent but of low quality. During the rainy season, the arid lowlands green up and tortoises migrate downslope to exploit the flush of nutritious vegetation. Over subsequent months, lowland vegetation desiccates and tortoises return to higher altitudes. Another factor which could influence movement patterns of tortoises is nesting behavior, which has not yet been incorporated into our understanding of tortoise migration. Female Galapagos tortoises usually nest in lowland areas following cessation of the rains when temperature and food quality begin to decline (MacFarland et al. 1974), though on Isabela Island nests are also made as high as 1100m (Fowler De Neira and Roe 1984; Blake, pers obs.). Wide variability occurs in the timing of nesting, the duration of incubation and nesting success within and between populations, but little research has been undertaken on Galapagos tortoise reproduction in the wild. One hypothesis for the timing and location of nesting in Galapagos tortoises is that reproductive females reach optimal body condition following peak lowland vegetation productivity, and remain in the lowlands to nest before the energy expensive return to the highlands. Nesting patterns may also have evolved to maximize food availability, appropriate sex ratios or other positive survival/life history influences on hatchlings. A third non exclusive hypothesis is that adult female tortoises may be constrained to nest in the lowlands on some islands because it is only there that suitable soil conditions are found for incubation. To date, the drivers of nesting behavior in Galapagos tortoises and their roles in migration and reproductive success remain unknown. In order to embark on a research project to test these hypotheses we sought funding from NGS GEF to develop a pilot study that would allow us to develop and test methodologies and provide preliminary data on several aspects of tortoise reproduction and nesting biology. Specifically we sought to develop a program that would respond to the following four questions: 1. What are the physical characteristics of known nesting sites and are these characteristics present over the altitudinal range of environments that are accessible to tortoises? 2. What are the patterns of seasonal abundance and behavior of Galapagos tortoises at known nest aggregation sites and do nesting females display strong site fidelity? 3. How does the timing of nesting, incubation temperature, egg size, clutch size, hatching success and hatchling survival vary by nest aggregation site and environmental conditions?

3 4. Can we deduce patterns in sex ratio by incubation temperature among Galapagos tortoises from GNP breeding center records? Study site The Galapagos Islands are located in the Pacific Ocean some 1000km west of the coast of Ecuador. The Archipelago consists of 13 large islands and some 148 smaller islands and islets. On this project, our work was largely restricted to Santa Cruz Island, though some research was also undertaken on Espanola Island and Alcedo Volcano on Isabela Island (Figure 1). Combined the three islands represent much of the environmental diversity of the Galapagos archipelago. Espanola is flat and arid with low, but relatively intact vegetation communities, and is inhabited by saddlebacked tortoises (Figure 2) of the species Chelonoidis hoodensis, which are adapted to browse on tall woody vegetation and Opuntia cacti. Tortoises were hunted relentlessly by sailors in the 1800s, which coupled with the destruction of much of the island s vegetation by goats, led to the near extinction of tortoises by the 1970s, when just 14 individuals were found on the island. These animals were taken off the island and formed a captive breeding population. Today, almost all the tortoises currently on Espanola are the result of this captive breeding effort by the Galapagos National Park and Charles Darwin Foundation, and the reintroduced wild population is now self sustaining (Milinkovitch et al. 2013). Alcedo is an active volcano which rises to an elevation of ca. 1100m. Extensive arid lowlands give way to increasingly humid uplands toward the crater rim. The Alcedo tortoises were not targeted by whalers and sailors in previous centuries, and the population is largely intact. People have never settled on Alcedo and human impacts are minimal with one important exception. A dramatic increase in goat numbers in the 1980s and 1990s saw the volcano largely denuded of vegetation, which created a state of emergency for tortoises and other native wildlife. A successful goat eradication campaign in the early 2000s allowed vegetation to recover, though how closely current biological communities are to previous native vegetation is not clear. A single species of dome shelled giant tortoise (Chelonoidis vandenburghi) occurs on Alcedo (Figure 2). Santa Cruz contains the largest human population on Galapagos, numbering between 15 20,000 individuals, concentrated in the main urban areas of Puerto Ayora and Bella Vista. Land use is divided into three main categories: an urban zone, an agricultural zone and the Galapagos National Park. The agricultural zone occurs mostly on the upland slopes to the south and south west, strategically placed in the island s rain shadow by colonizing farmers in the early 1900s. The vegetation of the agricultural zone is almost completely transformed, heavily dominated by introduced species many of which are highly invasive (including blackberry, guayava, passion fruit, elephant grass and several other aggressive grasses) (Guezou et al. 2010). Natural vegetation occurs in several broad zones that follow the altitudinal gradient in rainfall. The littoral zone begins at the ocean s edge and is dominated by shrubs, small trees and salt tolerant herbs. This gives way to the arid zone which contains xeric vegetation such as cacti and arborescent and shrubby species, with annuals growing vigorously during periods of high rainfall. As elevation rises, the arid zone gives way to the transition zone, in which evergreen species are common. Grass abundance increases and large areas are covered in

4 mono dominant Bursera graveolens woodland, which is superseded by the Scalesia zone at higher elevations, dominated by trees in this endemic genus. Finally the humid zone is variable, and may be dominated by Miconia species or ferns and sedges (Wiggins and Porter 1971). Interestingly two species of giant tortoise occur on Santa Cruz in geographically separated populations to the southwest and southeast of the island. Each species appears have been founded as a result of different colonization events (Russello et al. 2005). Both species are domed shaped (Figure 2). Figure 1. Project study sites Alcedo Santa Cruz Espanola As part of our overall research program (not funded by this NGS grant), we have deployed over 80 GPS tags onto tortoises in all four populations in the three islands in an attempt to understand how environment influences movement patterns, particularly the evolution of migration. Figure 2 below shows an overview of the sites where GPS tags have been deployed, basic movements, and illustrates the different forms of tortoises included in the study.

5 Figure 2. Results from preliminary GPS tracking of Galapagos tortoises: (A) locations of study sites involving four species on three islands, (B) probable altitudinal and lateral migration on Alcedo, (C) partial altitudinal migration on Santa Cruz, (D) likely sedentary and/or nomadic movements in Espanola. Examples of generic tortoise morphology for each island are below.

6 Activities and Results For clarity of reporting, our results are organized to follow the pattern of questions posed in the original proposal to NGS. 1. What are the physical characteristics of known nesting sites and are these characteristics present over the altitudinal range of environments that are accessible to tortoises? Our activities were focused in three nest zones in the El Chato area of Santa Cruz (Figure 3). The Low zone is at a mean elevation of ca.10m, the Middle zone a t 65m, and High zone at 110m. The highest tortoise nest recorded in El Chato occurred at 145m. Figure 3. Location of El Chato nesting zones in relation to movement routes of GPS tagged adult tortoises. High Middle Low Soil temperature gradient We deployed I button thermochrons into soil in randomly chosen locations along the altitude gradient of Santa Crux from 0 400m elevation. I buttons were placed approximately every 50m of elevation, and were deployed at 20cm depth. The ibuttons were programme to collect a temperature datum every 3 hours. The ibuttons were dug up 6 months after deployment for data retrieval after which they were reburied at the original depth. Soil temperature varied considerably by altitude and season (Figure 4 a,b). Soil temperature across all sites from 2 400m generally increased or remained stable from January, when ibuttons were first deployed, until March and April, after which temperatures declined. This reflects air temperature on Galapagos during the transition from hot wet season (Jan Apr) to cool dry season. However there was substantial variation in the timing of peak temperatures, with the lowlands tending to peak earlier than higher elevations. For example from 2 50m, maximum temperature was reached at the end of January,

7 while at 350m and 400m, soil temperatures peaked in early March. By mid April, soil temperature at all elevations began declining. Information from the tortoise breeding center on Galapagos indicates that at mean incubation temperatures below 25 o C mortality increases dramatically (Marquez et al. 1999); our data show that soils above 350m never attain this temperature, and that soils at 166m and above only attained 25 o C for a short period between late January and May (Figure 4a). It is not likely that during any soils above 166m would have attained 25 o C prior to our deployment of ibuttons because the period January March is usually the hottest time on Galapagos. These data then support the notion that tortoise s choice of nesting sites along the altitude gradient are indeed constrained by soil temperature at elevations below ca. 166m in the El Chato region of Santa Cruz. The mean difference in soil temperature across the entire elevation gradient of the tortoise population (0 400m) varied between 3 7 o C (Figure 4b). Interestingly, the temperature differential between the lowest and highest nest sites (as approximated by 14m and 100m) reached over 3 o C in January, which is approaching the end of the incubation period. The mean temperature difference between 14m and 100m was 1.6 o C. We will require data over a full year to better explore the significance of this preliminary result, but it is interesting to note that at the captive breeding center, incubation is set at 28 o C and 29.5 o C, temperatures which result in mostly males and females respectively. On the basis of our preliminary temperature data, it is possible that the different El Chato nesting zones could produce very different sex ratios of hatchling tortoises, favoring males at high elevations and females in lower, warmer areas.

8 Figure 4. Soil temperature at 20cm depth by elevation at El Chato, Santa Cruz. a) at various elevations from 2 400m note red line at 25 o C indicates temperature below which egg mortality increases dramatically, b) temperature differentials between m and m. 400m is the upper limit of tortoise distribution on Santa Cruz and 100m is nearing the upper limit of nesting site locations (a) 27 Temperature (C) m 50m 100m 212m 350m 14m 60m 166m 258m 400m 15 1/5/2013 2/5/2013 3/5/2013 4/5/2013 5/5/2013 6/5/2013 7/5/2013 8/5/ (b) Temperature (C) m 400m 14m 100m 0 1/5/2013 2/5/2013 3/5/2013 4/5/2013 5/5/2013 6/5/2013 7/5/2013 8/5/2013

9 2. What are the patterns of seasonal abundance and behavior of Galapagos tortoises at known nest aggregation sites and do nesting females display strong site fidelity? Tortoise abundance in the nesting zones We designed a survey based on systematically placed point transects using Distance 6.0 software (Thomas et al. 2010). A total of 45 point transects were distributed in the three nesting zones (Figure 5). Our plan was to survey these point transects twice monthly, however problems with the health of our principal research technician, and the loss of a part time staff member meant that where were irregularities with this schedule early in the year. We hoped that this temporal resolution would provide precision on when males and females aggregated in and around the nesting zones. However low encounter rates of tortoises on the point transects mean that the method has low precision in abundance estimates and we were unable to calculate robust estimates of either encounter rate or density of tortoises by sex or size on a monthly basis. We have pooled the point transect encounter rates by season to provide an index of abundance (Figure 6) which suggests a decrease in adult tortoise abundance in the low and middle nest zone, but an increase in the high zone. We have also collected data on tortoise encounter rates by sex and size class on the walks between point transect locations, and we are confident that these data will boost our ability to detect trends in tortoise abundance over time. Figure 5. Locations of point transects used for bi weekly surveys of tortoise abundance in the three nesting zones

10 Figure 6. Encounter rate of adult tortoises in the three nesting zones by season Dry Wet Low Middle High Tortoise size in nesting zones Seasonal change in mean tortoise length encountered in the nesting zones was most extreme in the low zone mean length doubled from dry and wet season, reflecting an influx of females into this nesting zone (Figure 7), which is consistent with the seasonal migration patterns observed from GPS telemetry data. The same pattern, though less extreme was observed in the high zone, while no change in body size with season was detected in the middle zone. 3. How does the timing of nesting, incubation temperature, egg size, clutch size, hatching success and hatchling survival vary by nest aggregation site and environmental conditions? Nest temperature Soil temperature as quantified above does not necessarily reflect temperatures inside actual tortoise nests due to elaborate nest construction which could create a different microclimate compared to undisturbed soil. For this reason we deployed ibuttons in a selection of nests. We placed two ibuttons in each nest, putting one level with the deepest eggs, and a second ibutton level with the uppermost eggs. Unfortunately during this study there was little temporal overlap between nest and undisturbed soil monitoring (above) because our study started late in the incubation period. However, we currently have a larger sample of ibuttons in nests with which to compare to background soil temperature at the end of the current nesting season in early Nest temperature varied extensively with elevation and time of year (Figure 8). Mean daily nest temperatures in the highest nest zone were as low as 23.5 o C in early December to 28.5 o C in late January (Figure 8). Nests in the high nesting zone were significantly cooler than those in the middle zone, which maintained temperatures above the 25 o C high mortality cutoff. The mean temperature difference between nests in the middle and high nesting zones during the study period was 2.04 o C, with a trend

11 toward a lower temperature difference between nest sites as overall seasonal temperature increased (Figure 8b). Figure 7. Mean length of tortoises encountered on point transect surveys by zone and season

12 Figure 8. Temperature recorded inside four nests in the El Chato nest zones. a) Temperature recorded at the level of uppermost and lowermost eggs in (nests 1 and 8 occurred in the middle nest zone, with nests 63 and 64 in the high zone), b) mean temperature difference between nests in the middle and high zones Temperature (C) Nest 1 upper Nest 1 lower Nest 8 upper Nest 8 lower Nest 64 upper Nest 64 lower Nest 63 upper Nest 63 lower Nov21 Nov24 Nov27 Nov30 Dec3 Dec6 Dec9 Dec12 Dec15 Dec18 Dec21 Dec24 Dec27 Dec30 Jan2 Jan5 Jan8 Jan11 Jan14 Jan17 Jan20 Jan23 Jan26 Jan Mean temperature difference (C) of middle zone and high zone nests Nov21 Nov23 Nov25 Nov27 Nov29 Dec1 Dec3 Dec5 Dec7 Dec9 Dec11 Dec13 Dec15 Dec17 Dec19 Dec21 Dec23 Dec25 Dec27 Dec29 Dec31 Jan2 Jan4 Jan6

13 Location within a nest also led to rather different temperature regimes. While there was little difference in mean temperatures recorded by ibuttons placed level with the upper and lower layers of eggs, the variance in temperature of the upper layer was much larger than that at the lower layer (Figure 9). Due to heating from the sun, temperatures in the upper layer were higher than the lower from ca. 12:00hrs until 20:00 21:00hrs (Figure 9).It is unclear what effect the amount of variation in temperature regime may have on the development of incubating eggs. Figure 9. Mean temperature was consistent between upper and lower eggs, but the variance in temperature was considerably greater at the upper level 30 Nest 63 upper Nest 63 lower 29 Nest 1 upper Nest 1 lower 28 Temperature (C) Hour Nest fate Unfortunately, we were unable to obtain any natural data on incubation success this year due extensive damage to nests caused by pigs, and to a lesser extent by fire ants. A total of 42 tests were surveyed in El Chato December 2012, of which less than 30% survived. Some 57% of nests were confirmed as predated by feral pigs, and an additional 14% destroyed, most likely also by pigs or a combination of pigs, donkeys and fire ants (all of which are invasive species on Galapagos). Predation pressure seemed to increase as nest elevation decreased (Table 1), however a small overall number of nests found in the upper zone meant that sample size was too small to be reliable. Fire ants (Solenopsis geminata) were present in and around a large proportion of all nests, and were confirmed as predating on eggs from at least five nests.

14 This result is significant because it is generally considered that the tortoises in this population (the Tortoise Reserve of Santa Cruz) are among the most stable and least threatened of any Galapagos tortoise species. We have shown that less than 30% of nests survived through incubation. Indeed, onetwo months still remained after our survey but before the onset of most hatching and occlusion, and it is likely that even more nests were predated before occlusion. Based on these results we wrote a factsheet for the Galapagos National Park, in which we demonstrated unequivocally the serious impact pigs are having on Santa Cruz tortoises. Table 1. Nest condition by nest zone in the El Chato, December 2012 Nest condition Nesting area Intact Predated or Hatched Predated Total Low Middle High Total Percentage of all nests Figure 10. The distribution of nests and their fate in El Chato

15 Clutch characteristics Our ability to generate robust results on clutch characteristics during the grant period was compromised by the high rate of nest predation. However this year did provide preliminary data and allowed us to refine field methods for use in the future of and when pig predation can be better controlled. Many of the results below are simply first cut statistical analyses aimed at generating questions on pattern that could be latent in the data rather than for hypothesis testing. Clutch size Of the seven nests opened and surveyed, the mean number of eggs per clutch was 6.1 (SD: 4.0, range: 1 12). There appeared to be no relationship between elevation and clutch size. Egg size Forty three eggs were measured and weighed from 7 nests. Mean egg diameter was 59.6mm (range mm, SD: 5.6mm). Despite the small size, a fairly convincing exponential relationship was detected between egg diameter and elevation of the nest site (F (2,40) = 20.91, _P <0.001) (Figure 11), indicating that egg diameter is smallest in the lowest nest site, but quickly asymptotes in the middle and upper zones. We acknowledge that we have based this analysis on a very small dataset, and show the analysis as an illustration of potential analyses in the future. Figure 11. Egg size by elevation Egg Weight The mean weight of the 43 eggs sampled was 132.8g (range: , SD 17.58g). Elevation was a reasonable predictor of egg weight, and explained 30.1% of the variation in egg weight (F (2,40) = 10.03, P <0.001). To reiterate, we are not arguing that the relationship shown in the graph reflects reality, we simply use it as a demonstration of the kids of analyses we will perform when we have more extensive data at the end of the 2013 nesting and incubation season. Figure 12. Egg weight by elevation

16 Hatchling tortoise survival and movements Before embarking on a study of hatchling survival and movement using radio tracking, we first tested the suitability of VHF tags and attachment methods on infant tortoises in the Captive Breeding Centre of the Galapagos National Park. A number of papers have suggested a range of glues to attach tags to infant turtles and tortoises, ranging from epoxy, superglue and false nail adhesive (Mansfield et al. 2012). The current method of choice for some studies, nail glue, can be harmful to young fragile shell, and so tested attachment with superglue, which seemed to work well, was not difficult to remove, and showed no negative effects on infant tortoise scutes. The tags were larger than we had anticipated, and weighed between 5 8% of body weight, which though on the heavy side, was within acceptable limits for slow moving terrestrial tortoises. We had originally planned to fit VHF tags to 32 hatchling tortoises, however just six individuals were tagged during the reporting period. This was because of the catastrophic impact of feral pigs on nesting success described above. Therefore we currently have insufficient data to respond to our original question of differential survival of hatchlings by nest zone, but plan to deploy the remainder of the unused tags in early On February , two nests in the low nest zone were opened up by GNP rangers as part of their routine activities. We fitted VHF tags to three of the hatchlings found on each nest. A further six hatchlings were marked on the underside with nail varnish to begin a capture recapture study of survival. Such a small sample size was unlikely to yield any useful results, but was at least a start to what will become a long term study. In the first week on leaving the nest, all hatchlings had dispersed at least 50m from the nest (Figures 13 and 14). By week three, one tortoise had died, and of the remainder, two had moved over 100m from their nest. On weeks six and eight, two tortoises made rapid movements, which in the case of Moz, took the animal nearly 500m linear distance from the nest. Mathias followed suit in week 8 though only moved to ca. 270m from the nest. In week seven, a second tagged hatchling died. In week 15, the most widely dispersed tortoises all moved back toward their nests before moving away again in week 16. Moz moved a remarkable 300m linear distance in one week. It is difficult to describe how rugged the terrain is in the lowlands of Santa Cruz for these infant tortoises. Uneven lava is ubiquitous, and these infants must expend a prodigious amount of energy. Two of the hatchlings did not travel beyond 100m from their nests. No exploratory correlations were found between distance moved and weight change we cannot say whether tortoises that foraged further found better habitat, or whether remaining in relatively restricted area reduced energy expenditure.

17 It is important to note that this is the first time that the movements of hatchling Galapagos tortoises. This small dataset with just one year of data will provide publishable results on early dispersal of this species in what equate to the lost years in sea turtles (Carr 1952). Figure 13. a) hatchling tortoises leaving the nest, b) hatchling fitted with a radio transmitter, c) map of movements of six hatchlings over a 5 month period.

18 Figure 14. Dispersal distance of tagged hatchlings over 16 weeks since leaving the nest Distance from nest (m) Charlie Lucas Matias Moz Samuel Week since tagging Figure 15. Weight change in tagged hatchlings over time

19 4. Can we deduce patterns in sex ratio by incubation temperature among Galapagos tortoises from GNP breeding center records? Unfortunately, we were unable to make any progress toward answering this question. Rightly the Galapagos National Park has assumed primary responsibility for the analysis of the 40+ years of data they have collected from the Captive Breeding Center. Our team works closely with the park scientists and will be involved in this process, but we are not directly responsible for these analysis. Furthermore, the huge dataset is not clean, and requires considerable work to get it ready for analyses to begin. Publicity related to this project, and the Galapagos Tortoise Movement Ecology Programme more generally Movebank All of the hatchling movement data are available on the online animal movement database (Figure 16). We have set up a project page specifically for this study. We hope to work with NG Education (see below) to direct users of the NG Education web site to the movebank page, and generate curricula materials to help young people work through the data and discover their own relationships in the dataset. Figure 16. Hatchling tortoise movement data on Movebank NG Education The Galapagos Tortoise Movement Ecology Programme was recently chosen by NG Education as a feature in their Geostory, under the Tracking Animal Migration piece (Figure 17). The grant we are reporting on is not directly featured in this NGS application, however we hope to make the infant tortoise available through the web site. It is wonderful news that the tortoise program has this link to the NG Education team, since we already have a vibrant hands on education program for children on Galapagos (see Galapagos Tortoise Movement Ecology Programme Facebook page for more

20 information). We will be contacting the NG Education team to discuss how to become more integrated into their program. The reach of the NGS could propel our work into another realm of outreach opportunities. Figure 17. The NG Education web site featuring our program at the start of the application NGS upcoming film Next year, the program will be featured in a National Geographic Film for global distribution. Filming is due to occur in late October and early November, and will include footage of our team working with the hatchling tagged tortoises, as well as our outreach program and the rest of our tortoise related activities. This will be a wonderful opportunity to spread the word on our program through the strong collaboration with NGS media. Facebook Our program is displayed on our Facebook page, Galapagos Tortoise Movement Ecology Programme Scientific papers in preparation Two scientific papers will be prepared as a direct result of this research: 1. Losing the lost years: Preliminary data on the dispersal of hatchling Galapagos tortoises on Santa Cruz 2. Thermal constraints to nesting by Galapagos tortoises: Temperature profiles of soils and tortoise nests along an altitude gradient on Santa Cruz.

21 Conclusion This project was disappointing in its immediate technical outputs because we were unable to produce any conclusive results on the main research questions. This failure was due to 1) late start to the project relative to the nesting period of Galapagos tortoises on Santa Cruz, 2) the catastrophic loss of viable tortoise nests due to pig predation, 3) staff injury caused delays in implementation of fieldwork. However the project was set up as a pilot study in which we would try out new and novel research methods on a previously unstudied area of Galapagos tortoise biology, and in this regard, the one year project was a great success. We cannot respond to our original questions, but we now have the mechanisms in place to refine data collection to respond to these questions. In six months from now, we will have a full year of complementary data on adult tortoise distribution in and around nest sites, soil temperature and nest temperature profiles along the altitude gradient of interest to tortoise migration, many more VHF tagged hatchling tortoises to begin the study of survivorship and movement, from which at least two publications will come. Ironically and importantly, our failure to nail down our original scientific questions, allowed us to uncover and expose a serious conservation issue facing tortoises on Santa Cruz Island, namely pig predation of eggs and the destruction of nest sites on a large scale. We are collaborating with the park on harmonizing data collection by both park rangers and our research staff to monitor the situation in the future, and develop interim measures for the protection of nests, such as better caging around nests to prevent pig access. Ultimately however the only long term solution will be the eradication of pigs from the island. Along with the National Park activities our data and the future monitoring we will implement as a result of this project will help to generate the political will to implement this definitive solution. References Blake, S., C. B. Yackulic, F. Cabrera, W. Tapia, J. P. Gibbs, F. Kummeth, and M. Wikelski Vegetation dynamics drive segregation by body size in Galapagos tortoises migrating across altitudinal gradients. Journal of Animal Ecology 82: Carr, A Handbook of Turtles: The Turtles of the United States, Canada, and Baja California. Cornell University Press. Fowler De Neira, L. E. and J. H. Roe Emergence Success of Tortoise Nests and the Effect of Feral Burros on Nest Success on Volcan Alcedo, Galapagos Guezou, A., M. Trueman, C. E. Buddenhagen, S. Chamorro, A. M. Guerrero, P. Pozo, and R. Atkinson An Extensive Alien Plant Inventory from the Inhabited Areas of Galapagos. PLoS ONE 5. MacFarland, C. G., J. Villa, and B. Torro The Galapagos giant tortoises (Geochelone elephantopus) part 1: status of the surviving populations. Biological Conservation 6: