Understanding the distribution of introduced mammalian predators in an urban environment using monitoring tools and community trapping

Abstract

Introduced mammalian predators are one of the largest conservation threats to New Zealand native flora and fauna, and there is an increasing concern about their presence in urban environments, coupled with a recognition that cities present a unique opportunity for ecological restoration, due to the availability of a large number of volunteers and options for intensive management of green spaces and gardens. Predator control is an essential step towards the ecological restoration of urban environments, however, it requires an understanding of the factors influencing the distribution of these mammalian predators before successful control operations can be implemented. Few studies have investigated mammalian predators in urban environments, and there is little certainty about what drives their distribution in these environments. This thesis used simple mammal monitoring techniques and trapping data to investigate the distribution of mammalian predators within broad scale urban environments, with the aim of identifying drivers of their distribution.

Chew cards and tracking tunnels collected across three New Zealand cities were assessed for their efficacy as accurate monitoring devices in urban environments. In Chapter 2, monitoring devices were cross-checked between observers to assess the level of consistency in interpretation of chew and tracking marks. The consistency of chew card and tracking tunnel identifications was relatively high overall and were not substantially influenced by the city of identification, or the duration of card exposures. Monitoring devices were also assessed for their change in sensitivity between one and six-night exposures. Both devices were effective at detecting rats, however, tracking tunnels showed greater sensitivity and consistency in detecting mice and hedgehogs, whereas chew cards were better suited to the monitoring of possums. Neither device was particularly effective at detecting mustelids or cats.

In Chapter 3, mammalian predators were monitored across 24 monitoring lines in autumn, 2018, and results were compiled with spring 2017 and autumn 2018 data, pre-collected in two other cities, following the same procedures. There were distinct differences in the broad-scale habitat utilisation of rats, mice, hedgehogs, with possums being the only species to show a strong preference for urban forests. Only two of the tested microhabitat variables had an influence on species distributions. Detection of rats declined with increasing distance to the coast, and the increase in human population size was related to a significant increase in hedgehogs. There was a strong seasonal difference on the influence of local trap density and the detection of mammals. The increase in trap density within 25-50m radii was significantly related to a decrease in rat and hedgehog detections. Overall, there are substantial differences between the distributions of species in an urban environment.

Trapping is one of the main methods of predator control in New Zealand, and is already widespread within urban and suburban Wellington. In Chapter 4, I compiled trap data from 22 community trapping groups operating in residential and reserve areas in Wellington City. Residential groups (“backyard trappers”) used a high proportion of Victor and various rat and mouse traps, which was strongly linked to their high number of rat and mouse catches. Groups trapping in reserves used a high proportion of DOC 200, Victor and A24 traps, however, fewer hedgehogs were caught compared to residential areas. Catches were significantly influenced by various landscape variables. An increased distance of traps to streams led to significantly higher catches of rats, conversely, proximity to streams resulted in significantly higher catches of mice and hedgehogs. Although few catches of weasels were reported, traps closer to the coast and to forest fragments caught significantly more individuals.

The research in this thesis contributes to the small body of research conducted on mammalian predators within urban environments. The findings in this thesis can assist with the current and future predator management programmes, by highlighting areas of potential significance, particularly in Wellington.