346 N. F. Angeli and L. A. Fitzgerald
(4.3 ha; 400 m offshore) because mongooses were absent there (Dodd, 1980; Thomas & Joglar, 1996). Two successful introductions indicate St Croix ground
lizards are relatively easy to establish in new areas. A pop- ulation was established on Ruth Island (9.7 ha; 430 m off- shore), a dredge-spoil island, with only nine founders from Protestant Cay in 1989. A translocation of 57 founders from Green Cay to Buck Island (69 ha; 2.6 km offshore) in 2008 was successful (Treglia & Fitzgerald, 2011; Fitzgerald et al., 2015). An estimated total of c. 3,000 individuals now occur on these four islands, with .2,000 on Buck Island. Green Cay, Protestant Cay and Ruth Island each harbour 100–400 individuals (Fitzgerald et al., 2015;Angelietal., 2018). Allofthese islands are small(totalarea 85 ha), leaving the lizard populations vulnerable to stochastic events such as hurricanes, predator invasion and habitat degradation.
Methods
Analysing landscape change We compiled data on protected areas, the predicted distri- bution of mongooses and land-cover types, and predicted the carrying capacity for lizards in each 30 m2 cell in a grid covering the entire island of St Croix. We digitized the land-cover map from 1750 (Hopkins, 1989) using the Georeferencer plugin tool in QGIS 2.14.0 (Open Source Geospatial Foundation, Beaverton, USA). Maps from 1950 showed that at that time 97% of land was urban or agricul- tural pasture (Ward et al., 2000). We obtained land-cover data for 2016 from Landsat 2016 (Angeli et al., 2018). Be- cause the maps from 1750 and 2016 showed landscapes with a heterogeneous mix of land-cover types that were not dominated by agriculture and urban development such as in 1950,we compared the area occupied by different land-cover types in 1750 and 2016 using a paired Welch’s t test.
Estimating lizard carrying capacity
We created a map layer for St Croix ground lizard carrying capacity using the results of a mechanistic binomial mixture model developed for Buck Island (Angeli et al., 2018), which accounted for latent abundance and low detection of the species. In that model, we found significant associations of lizard abundance with areas identified by operative temper- ature modelsasoptimal forthe species’ thermoregulation (see model description in Supplementary Material 1, and de- scription of the full modelling process in Angeli et al., 2018 and Fitzgerald et al., 2015). We scored the final data set for mean lizard abundance per grid cell as: 1–6 lizards = not suitable, 7–12 lizards = potentially suitable, 13–20 lizards = suitable, and 21–25 =most suitable (Supplementary Fig. 1).
These estimates were summed to obtain an estimate of lizard habitatAacross all areas (Table 1) for the presently dispersing population and the future population (Supplementary Fig. 2).
Prioritizing suitable sites for reintroduction
We considered protected areas on both public and private land for reintroduction. We prioritized potential reintro- duction areas by considering the total area of the site and suitable habitat for St Croix ground lizards in the site. We also gave higher priority to sites where the threat of preda- tion by mongooses was mitigated through management. Mongoose management was in place at some sites (e.g. Sandy Point); at other sites there was capacity for land managers to control mongoose populations. Capacity to controlmongooses was assessed based on re-
ports from staff engaged in the trapping of mongooses to protect nesting marine turtles (Angeli, 2017). We also took into account the threat of predation at individual sites based on the distribution of mongooses on St Croix. Mongooses are not evenly distributed across St Croix; their population density varies. We used a species distribution model created by Gould (2007) to visualize overlap between priority rein- troduction areas and mongoose presence. Mongooses were absent from 7.96 km2 (3.65%) of St Croix, most notably in the south of the island where the carrying capacity of St Croix ground lizards was thus predicted to be greater.
Results
Landscape change In 1750, 42% of St Croix was forested,,11% was devoted to pasture or agriculture and ,5% was urban. According to the historic map, the remaining 42%consisted of woodland, shrubland, and edge forest. The 2016 map showed 32% pas- ture and agriculture, 31% forest cover and 12% urban areas. Succession of fallow fields to shrubland, woodland and edge forest comprised the remaining 25% of land cover in 2016. We found no statistical differences in land-cover area be- tween the 1750 and 2016 maps (Fig. 1; t = 0.013,df = 4.26, P=0.989). The model of mongoose occurrence on St Croix showed numerous areas where mongooses are pre- dicted to be absent that overlapped with priority areas for reintroduction of the St Croix ground lizard (Hoagland et al., 1989; Gould et al., 2007; Fig. 2).
Lizard carrying capacity
Themodels estimated that StCroix could potentially support 142,421 lizards across 1,169 ha. We identified 19 protected
areas, with 2.4−100% suitable habitat (Table 1, Fig. 2). Pop- ulation models estimated 21,469 St Croix ground lizards
Oryx, 2021, 55(3), 344–351 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319001091
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