Restoration of threatened species populations: tuatara rehabilitations and re-introductions
The role of scientific theory in encouraging greater efficiency and accountability for the restoration of threatened species populations is assessed for an endemic New Zealand reptile, the tuatara Sphenodon spp. The value of examining assumptions underlying concepts such as 'habitat requirements' theory and incorporating scientific principles into species recovery are discussed and perceived habitat needs for tuatara tested through experimental application. Species restoration in New Zealand, especially re-introductions, are typically undertaken as one-off, non-replicated, management exercises (trials). The lack of comparative controls for trials means that the reason for success or failure of management actions cannot be identified accurately and therefore, cannot be used to improve the probability of success for subsequent re-introductions. Trials also reinforce conservative re-introductions of species to habitats in which species are known to survive, because the risk of failure is inherently lower than re-introductions to dissimilar habitats where habitat suitability is unknown. An alternative approach is to plan management actions as experiments. Testing the full range of perceived habitat needs of species as experimental comparisons identifies the relative importance of tested factors (e.g. predators, refuges) for species recovery and identifies new management strategies (e.g. reduced level of predator control). Constructing testable hypotheses for the environmental factors thought to affect the success of restoration projects for tuatara identifies (among others) three factors: 1. absence of the introduced rat, the kiore (Rattus exulans) 2. presence of seabird colonies 3. presence of open canopy forest The threat posed by kiore to established tuatara populations was investigated by determining the existence and degree of food competition before and after an eradication program for kiore on offshore islands. Kiore successfully out-competed tuatara for favoured food items, but the degree of competition differed between forest types. Competition for food was greater in the early regenerating forest than the mature forest. These data support models which propose that kiore are but one of a number of historical and current environmental factors influencing the persistence of native fauna and that management tools other than eradication may enable restoration of fauna in the presence of kiore. To test the importance of two environmental factors, forest development and the availability of refuges, in determining the establishment of new populations of tuatara, a planned experimental reintroduction was conducted on Moutohora Island. Tuatara were released into sites where the forest was young with a closed canopy and older with a more open canopy, and in sites where refuges (seabird burrows) were distributed evenly at high densities and where burrows clumped with few burrows between patches. Vegetation age and burrow dispersion had no measurable effect on the survivorship or condition of tuatara. Although tuatara released in areas where burrows were clumped dispersed significantly further from their release points and continued to disperse away from release sites throughout the study, tuatara in all release sites were considered to still be reproductively viable 16 months after the re-introduction. Therefore, sites which support open and closed canopy forest, and seabird burrows at high and low densities should be considered as habitat options for future reintroductions of tuatara. Testing habitat needs as well-planned experiments offer more reliable information to guide future reintroductions than that generated by trial releases or releases to similar locations. Future reintroductions of tuatara and other wildlife should be designed as experiments to test declared mixes of habitat factors. This will accelerate species recovery by identifying important habitat prerequisites for re-introduction and management options for achieving these, thus refining criteria used for selecting new sites and increasing confidence, efficiency and accountability of subsequent management actions. An example of designing re-introductions as experiments is included as a plan for the re-introduction of tuatara to Tiritiri Matangi Island.
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