Abstract
This thesis investigated potential strategies that might advance landscape-scale eradication of invasive small mammals. Specifically, it explored (1) whether high-elevation landscape features of New Zealand’s South Island limit the movement of pest species and might function as barriers to reinvasion, and (2) if the strong seasonal winter conditions in high alpine environments cause species to modify their distributions, allowing them to be more efficiently targeted within contracted and/or predictable areas at certain times of the year. In relation to the second of these two potential strategies, two species were studied in closer detail: the European hedgehog, one of the most impactful predators in dryland ecosystems; and the European hare, a widely distributed herbivore of which the apparent persistence and space use in high alpine zones throughout winter is not well understood.
My research found strong evidence that both strategies would be effective. High-elevation landform barriers were found to severely limit the movements of range of invasive small mammal species, the extent of which varying between species with differing patterns of altitudinal distribution and habitat selection; and seasonal changes in the spatial ecology of certain pest species were evident, particularly for the Australian brushtail possum, which was not detected at high elevations throughout winter, and for the feral cat, which was not detected in high and mid elevations throughout winter. This suggests that the distributions of these two species seasonally contract in size and would mean that removal effort would only need cover the extent of their winter distributions to put all individuals at risk, increasing both the efficiency of removal; and increasing the efficacy of such programmes, as many removal techniques cannot be used in steep mountainous terrain found at high elevations. In contrast, I found that the stoat and European hare evidently persist in the high-elevations of this region throughout winter. Of the two species that were studied in closer detail: European hedgehogs that were captured in the alpine zone remained where they were in winter and entered hibernation rather than moving to lower elevations, so no opportunities to better target this species below the alpine zone were apparent. European hares, on the other hand, persisted in the high elevations but dramatically modified their space use when winter conditions were experienced and can evidently be better targeted at this time of the year by focussing removal effort in areas that they select for. Such areas include steeper slopes, northern and western aspects, more productive vegetation, and more solar irradiance.
These findings have a direct application in (a) assessing the extent to which pest control areas are defended from reinvasion from external source populations and the delineation of defensible areas; and (b) by increasing the efficiency of removal efforts by targeting species that predictably shift their spatial utilisation of their surroundings at certain times of the year.
Additionally, this study provided new insights into invasive small mammal ecology in the uplands of New Zealand’s eastern dryland zones. This included information on the altitudinal distribution and habitat selection of all invasive small mammal present in the eastern dryland zone of New Zealand’s Southern Alps, the home range size and resource selection of hares and hedgehogs, and juvenile dispersal distances, hibernation, and temperature-driven activity patterns of hedgehogs. All such information contributes to knowledge underpinning more effective removal of pest species.
The primary conclusion of this research is that upland environments present opportunities to increase the efficacy of removing pest species and defending sizable areas from reinvasion: advantages that can be found in very few other mainland landscapes.