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

<p>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.</p>

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

<p>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.</p>

A quantitative approach for trap analysis between Al0.25Ga0.75N and GaN in high electron mobility transistors

The characteristics of traps between the Al0.25Ga0.75N barrier and the GaN channel layer in a high-electron-mobility-transistors (HEMTs) were investigated. The interface traps at the Al0.25Ga0.75N/GaN interface as well as the border traps were experimentally analyzed because the Al0.25Ga0.75N barrier layer functions as a dielectric owing to its high dielectric constant. The interface trap density Dit and border trap density Nbt were extracted from a long-channel field-effect transistor (FET), conventionally known as a FATFET structure, via frequency-dependent capacitance–voltage (C–V) and conductance–voltage (G–V) measurements. The minimum Dit value extracted by the conventional conductance method was 2.5 × 1012 cm−2·eV−1, which agreed well with the actual transistor subthreshold swing of around 142 mV·dec−1. The border trap density Nbt was also extracted from the frequency-dependent C–V characteristics using the distributed circuit model, and the extracted value was 1.5 × 1019 cm−3·eV−1. Low-frequency (1/f) noise measurement provided a clearer picture of the trapping–detrapping phenomena in the Al0.25Ga0.75N layer. The value of the border trap density extracted using the carrier-number-fluctuation (CNF) model was 1.3 × 1019 cm−3·eV−1, which is of a similar level to the extracted value from the distributed circuit model.

High Pressure Deuterium Passivation of Charge Trapping Layer for Nonvolatile Memory Applications

In this study, the deuterium passivation effect of silicon nitride (Si3N4) on data retention characteristics is investigated in a Metal-Nitride-Oxide-Silicon (MNOS) memory device. To focus on trap passivation in Si3N4 as a charge trapping layer, deuterium (D2) high pressure annealing (HPA) was applied after Si3N4 deposition. Flat band voltage shifts (ΔVFB) in data retention mode were compared by CV measurement after D2 HPA, which shows that the memory window decreases but charge loss in retention mode after program is suppressed. Trap energy distribution based on thermal activated retention model is extracted to compare the trap density of Si3N4. D2 HPA reduces the amount of trap densities in the band gap range of 1.06–1.18 eV. SIMS profiles are used to analyze the D2 profile in Si3N4. The results show that deuterium diffuses into the Si3N4 and exists up to the Si3N4-SiO2 interface region during post-annealing process, which seems to lower the trap density and improve the memory reliability.

Comparative Study on the Separate Extraction of Interface and Bulk Trap Densities in Indium Gallium Zinc Oxide Thin-Film Transistors Using Capacitance–Voltage and Current–Voltage Characteristics

The interface and bulk trap densities were separately extracted from self-aligned top-gate (SA-TG) coplanar indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) using the low-frequency capacitance–voltage (C–V) characteristics and space-charge-limited current (SCLC) under the flat-band condition. In the method based on the C–V curve, the energy distribution of the interface trap density was extracted using the low-frequency C–V characteristics, and that of the bulk trap density was obtained by subtracting the density of interface trap states from the total subgap density of states (DOS) at each energy level. In the SCLC-based method, the energy distribution of the bulk trap density was extracted using the SCLC under the flat-band condition at high drain-to-source voltages, and that of the interface trap density was obtained by subtracting the density of bulk trap components from the total subgap DOS at each energy level. In our experiments, the two characterization techniques provided very similar interface and bulk trap densities and showed that approximately 60% of the subgap states originate from the IGZO/SiO2 interface at the conduction band edge in the fabricated IGZO TFTs, although the two characterization techniques are based on different measurement data. The results of this study confirm the validity of the characterization techniques proposed to separately extract the interface and bulk trap densities in IGZO TFTs. Furthermore, these results show that it is important to reduce the density of interface trap states to improve the electrical performance and stability of fabricated SA-TG coplanar IGZO TFTs.