Altitudinal distribution of the entire invasive small mammal guild in the eastern dryland zone of New Zealand’s Southern Alps

Abstract

New Zealand faces significant ecological problems caused by the introduction of a variety of invasive small mammal species. Many of these species originate from temperate to subarctic climates and occur across wide elevations in their native range, and so arrived predisposed to adapt to a variety of habitats and bioclimatic zones in their new environs, including the alpine zone. Almost all of New Zealand’s invasive small mammal species have been recorded in the country’s alpine zones, yet neither the altitudinal distribution nor the extent to which such species use high elevation areas has been clearly defined. We conducted extensive camera trap surveys in summer to autumn periods of 2019 and 2020 across an elevation range of 500–2250 m above sea level, and used detection rates and occupancy modelling to reveal the altitudinal distributions and habitat associations of all 10 invasive small mammals that occur in the dryland zone of the central South Island. We found altitudinal distributions varied greatly across species, and that while most exhibited decreasing detection rates and site occupancy probabilities with increasing elevation, some used the subalpine and alpine zones to a greater degree than adjacent lower elevations. There were clear habitat associations, as well as interspecific associations that helped to explain the altitudinal distribution of some species. Understanding how such factors influence the distribution of invasive small mammals has both broad implications for invasive species management, and direct applications in evaluating threats to native taxa, advancing management strategies, and benchmarking distributions in a changing climate.