Abstract
An ability to adapt to a diversity of habitats, coupled with a strong ability to disperse across land and sea, have enabled rats to devastate global biodiversity. In New Zealand, the introduction of three rat species – kiore Rattus exulans, Norway rat R. norvegicus, and ship rat R. rattus – has contributed to the decline and extinction of a host of indigenous fauna. A primary focus for conservation work in New Zealand over the past few decades has been managing wild populations of invasive mammalian predators, including rats. The aim of this thesis was to address key ecological and spatial knowledge gaps to aide the expansion of two pest management operations in the South Island: Te Manahuna Aoraki, an eradication project in the tussock dryland environment of the Mackenzie Basin, Canterbury (targeting Norway rats); and the Forest & Bird control operation in the beech forests of the Makarora Valley, central Otago (targeting ship rats).
In the Mackenzie Basin, I conducted a radio-tracking study of male (n = 2) and female (n = 2) Norway rats to characterise the ranging behaviour around a cluster of man-made ponds. The resulting estimates determined that home ranges varied from 3.03–5.44 ha in area, and 369.2–615.2 m in length. There did not appear to be any significant differences in ranging behaviour between the two sexes. These results were consistent with estimates obtained from individuals on offshore islands. This is believed to be the first home range study of a mainland, non-commensal population of Norway rats in New Zealand.
Using n = 47 DNA samples collected from rats through the Mackenzie Basin and upper Waitaki Valley, I also conduced a microsatellite analysis using eleven loci to reveal potential eradication units. Little genetic distance and high relatedness were found between the three populations sampled from the upper reaches of the Basin. This suggests that there may be a degree of connectivity between populations, and that they cannot be divided into separate eradication units.
The final study, taking place in the Makarora Valley, examined the relationship between ship rat captures, and microhabitat factors. Surveys measured vegetation and geological variables at a fine scale (20 m) around n = 96 trap sites. Corrected trap indices (CTI) were calculated from historical trap records dating from 2017 –2020. Selected factors were then inputted into a negative binomial generalized linear model, and stepwise AIC selection was undertaken to determine the best-fitting variables. The final model suggests that the density of mature beech may have a negative effect on CTI, while the presence of fruit-bearing understorey plants (round-leaf coprosma, horopito, wineberry) may be positively associated with CTI. This information can help optimise the future placement of traps to maximise numbers of rats caught.
Overall, the studies presented in this thesis will enable each respective operation to make informed decisions about the expansion of their work to eradicate or control wild rats. Furthermore, this knowledge may also assist future operations in other mainland areas.