Individual study: Translocation and habitat use of St. Lucia whiptail lizards Cnemidophorus vanzoi to Praslin Island, St. Lucia
Dickenson H.C., Fa J.E. & Lenton S.M. (2001) Microhabitat use by a translocated population of St. Lucia whiptail lizards (Cnemidophorus vanzoi). Animal Conservation, 4, 143-156
Animal translocation is sometimes used as a management tool in an attempt to enhance the chances of species survival. Understanding microhabitat use of translocated species may be a crucial step towards assessing their adaptation to the new environment.
Prior to this study, the St. Lucia whiptail lizard Cnemidophorus vanzoi was confined to two islands: Maria Major and Maria Minor, in the Lesser Antilles archipelago of the West Indies. The total world population at this time was estimated at around 900 individuals. To reduce the risk of extinction and enhance population size, this species was translocated to a nearby island. Microhabitat use was determined three years after translocation to evaluate the success of this action.
Translocation: In 1995, 42 St. Lucia whiptail lizard were translocated from Maria Major Island (10.2 ha) to Praslin Island - a 1.1 ha island uninhabited by people. Prior to translocation, feral goats and black rats Rattus rattus were eradicated in 1991 and 1993 respectively. The translocation was carried out by the Jersey Wildlife Preservation Trust.
The first translocation of 25 lizards were predated by a single Indian mongoose Herpestes auropunctatus which once detected, was destroyed. A further 17 lizards were subsequently introduced. Small numbers of rats were also found after the initial erradication, so measures were taken to trap these. All lizards were released in a mixed woodland area as this was assumed to be the most suitable habitat based on observations from their native islands.
Lizard abundance and distribution: In 1997-1998, (three years post-release), lizards were counted along nine north-south oriented transects, spaced 15 m apart. The location of each lizard sighting was marked and each individual was sexed. The total population size was estimated using DISTANCE software, which enables calculation of probability of detection within a defined area, thus allowing population size to be estimated from the number of sightings. Lizards were monitored both during a wet season (October-December 1997) and during a dry season (January-March 1998).
Vegetation analysis: Five broad habitat types were distinguished from aerial photographs taken on the 4 October 1997. Ground-truthing was undertaken by a vegetation census. Detailed vegetation characteristics were recorded by collecting and identifying plants within 40 quadrats placed at 10 m intervals along each of the line transects used for lizard sampling. Quadrat size varied according to habitat - in woodland and shrub 5 m² quadrats were used, whilst in grass habitats 1.5 m² quadrats were employed. Percentage cover of rocks, bare soil, leaf litter and coarse woody debris in each quadrat was also recorded. Five broad habitat types were distinguished. These corresponded closely to the five habitat types distinguished from the aerial photographs:
i) mixed woodland dominated by milkwood Tabernaemontana citrifolia,
ii) Manchineel woodland dominated by manchineeel Hippomane mancinella,
iii) Shrub habitat, characterised by lantana Lantana camera,
iv) Mixed grassland dominated by blue porterweed Stachytarpeta jamaicensis
v) Bunchgrass, dominated by Schizacyrium micostachym.
Lizard abundance, density and habitat use: Three years after translocation, the population size of St. Lucia whiptail lizards on Praslin Island had increased from 42 (the original founder population) to 145 (± 23).
During the wet season the highest density of lizards was observed in shrub habitat followed by mixed woodland, with the least favoured habitat being bunchgrass. During the dry season, the highest densities were again observed in shrub habitat, the second most favoured habitat was Manchineel woodland and the least favoured was bare rock. Overall, shrub was the most favoured habitat and bare rock was the least favoured. The average density of lizards in all five habitats in both seasons and overall is given in Table 1 (attached).
Conclusions: The goal of the translocation was achieved as the population of St. Lucia whiptail lizards grew quickly, more than doubling in size every year such that Praslin now support 14% of the global population. The authors suggest that there is strong evidence that thermoregulation is the primary driving force governing habitat use, as exposed habitats are avoided, particularly during the dry season. The habitats themselves undergo seasonal changes and this is reflected in lizard use. There is a significant increase in exposure in mixed-woodland during the dry season and it is thus avoided, with higher densities of lizards observed in Manchineel woodland instead. This provides further evidence that the major constraint on habitat use is the ability of lizard to thermally regulate effectively within the habitat. Other potential drivers of habitat use were not evaluated in this study.
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