title: Dietary overlap between the non-native mosquitofish  (Gambusia affinis) and native common bully (Gobiomorphus cotidianus):  Evidence for interspecific competition?

authors: Ember, Flavian

abstract: Since their establishment in New Zealand in the 1930s, the globally invasive western mosquitofish (Gambusia affinis) has become widespread throughout lowland freshwaters, primarily in the North Island. Despite co-occurring in many ecosystems, interactions between native common bullies (Gobiomorphus cotidianus) and G. affinis are presently unknown. Like G. affinis, G. cotidianus are abundant in lowland waterways, tolerant of a wide range of environmental conditions and can consume similar prey. My research investigated the potential dietary overlap between these two species using a controlled mesocosm experiment and a stable isotope study of co-occurring G. affinis and G. cotidianus in natural lake habitats. 

Zooplankton were selected as the prey source for the mesocosm experiment. Twenty-one 100 L mesocosms were assigned either four G. cotidianus, four G. affinis, or were left as a control group (i.e., with no fish) and monitored for 14 days. Zooplankton community composition in the fish treatments was significantly different from the control tanks by Day 14 of the experiment yet did not differ significantly from one another. Both G. cotidianus and G. affinis reduced the abundances of larger crustacean zooplankton relative to the controls. This is likely to have reduced the effects of predation, competition, and physical interference by the crustaceans on rotifers, allowing rotifer species to reach numerical dominance in the treatment mesocosms by the end of the experiment. In contrast, zooplankton assemblages in the control mesocosms were dominated by cladocerans by Day 14, indicating that these had a competitive advantage in the absence of zooplanktivorous fish. 

Four Waikato lakes were selected as sites for the stable isotope study. Gambusia affinis, G. cotidianus, and basal food web taxa were collected from each lake during the austral winter of 2022. Samples were analysed for the stable carbon and nitrogen isotopes ¹³C and ¹⁵N, and results were compared to data published by Collier et al. (2018) from the same taxa and lakes during an austral summer. Mean δ¹³C values of G. affinis were significantly enriched compared to G. cotidianus at three lakes in both summer and winter, while the mean δ¹⁵N of G. affinis was significantly enriched at three lakes in summer and all lakes in winter. These differences were greatest in winter, even when the summer dataset was censored to match the winter fish length distribution. My results indicate that G. affinis and G. cotidianus are likely consuming different prey sources and that their diet is most similar during summer. Additionally, my results showed seasonal variation between lakes in the isotopic enrichment of both fish and basal food web taxa, suggesting food webs and trophic interactions may differ between sites. 

Overall, my thesis demonstrates the potential for interspecific competition between native G. cotidianus and non-native G. affinis in New Zealand through an example of zooplankton dietary overlap in a controlled experimental context. The stable isotope experiment provided mixed evidence for dietary overlap across both summer and winter in sympatric lake populations. Both experiments offer numerous future research opportunities to investigate the mechanisms behind the findings.

text: Dietary overlap between the non-native mosquitofish  (Gambusia affinis) and native common bully (Gobiomorphus cotidianus):  Evidence for interspecific competition? Since their establishment in New Zealand in the 1930s, the globally invasive western mosquitofish (Gambusia affinis) has become widespread throughout lowland freshwaters, primarily in the North Island. Despite co-occurring in many ecosystems, interactions between native common bullies (Gobiomorphus cotidianus) and G. affinis are presently unknown. Like G. affinis, G. cotidianus are abundant in lowland waterways, tolerant of a wide range of environmental conditions and can consume similar prey. My research investigated the potential dietary overlap between these two species using a controlled mesocosm experiment and a stable isotope study of co-occurring G. affinis and G. cotidianus in natural lake habitats. 

Zooplankton were selected as the prey source for the mesocosm experiment. Twenty-one 100 L mesocosms were assigned either four G. cotidianus, four G. affinis, or were left as a control group (i.e., with no fish) and monitored for 14 days. Zooplankton community composition in the fish treatments was significantly different from the control tanks by Day 14 of the experiment yet did not differ significantly from one another. Both G. cotidianus and G. affinis reduced the abundances of larger crustacean zooplankton relative to the controls. This is likely to have reduced the effects of predation, competition, and physical interference by the crustaceans on rotifers, allowing rotifer species to reach numerical dominance in the treatment mesocosms by the end of the experiment. In contrast, zooplankton assemblages in the control mesocosms were dominated by cladocerans by Day 14, indicating that these had a competitive advantage in the absence of zooplanktivorous fish. 

Four Waikato lakes were selected as sites for the stable isotope study. Gambusia affinis, G. cotidianus, and basal food web taxa were collected from each lake during the austral winter of 2022. Samples were analysed for the stable carbon and nitrogen isotopes ¹³C and ¹⁵N, and results were compared to data published by Collier et al. (2018) from the same taxa and lakes during an austral summer. Mean δ¹³C values of G. affinis were significantly enriched compared to G. cotidianus at three lakes in both summer and winter, while the mean δ¹⁵N of G. affinis was significantly enriched at three lakes in summer and all lakes in winter. These differences were greatest in winter, even when the summer dataset was censored to match the winter fish length distribution. My results indicate that G. affinis and G. cotidianus are likely consuming different prey sources and that their diet is most similar during summer. Additionally, my results showed seasonal variation between lakes in the isotopic enrichment of both fish and basal food web taxa, suggesting food webs and trophic interactions may differ between sites. 

Overall, my thesis demonstrates the potential for interspecific competition between native G. cotidianus and non-native G. affinis in New Zealand through an example of zooplankton dietary overlap in a controlled experimental context. The stable isotope experiment provided mixed evidence for dietary overlap across both summer and winter in sympatric lake populations. Both experiments offer numerous future research opportunities to investigate the mechanisms behind the findings.

year: 2023

title: Spatial signatures and mental maps: Using offenders’ activity locations to enhance geographic profiling for crime investigations

authors: Curtis-Ham, Sophie

subjects: Crime location choice

abstract: In crime investigations, geographic profiling involves inferring information about ‘whodunit’ from information about where and when the crime occurred. Such inferences are possible because people usually commit crime in places they know, rather than seeking opportunities elsewhere. The ‘spatial signature’ of a crime location thus reflects the ‘mental map’ of the offender: the locations they are familiar with from their everyday activities, such as where they live, work, go to school or visit family and friends. Previous empirical research suggests that people are more likely to commit crime near some activity locations in their mental map than others. However, these differences have not been accounted for in theory or in geographic profiling methods—particularly methods that infer the most likely suspect to have committed a crime, given the proximity of their activity locations to the crime. This thesis aimed to enhance geographic profiling by developing, validating, and applying a theoretical model that explains how people’s different activity locations influence their crime locations.

To achieve this aim, I first drew on existing theoretical and empirical literature to propose a theoretical model that identifies the attributes of people’s activity locations that influence whether they will commit crime nearby and the psychological mechanisms through which this influence occurs. Next, I tested hypotheses derived from the model to add to its empirical support, using New Zealand Police data on approximately 60,000 offenders who committed burglaries, robberies and extra-familial sex offences between 2009 and 2018. This national dataset included details of the offenders’ crime locations and their pre-crime activity locations such as home addresses, family members’ home addresses, schools, and the locations of prior crimes and other police interactions. I then applied the theoretical model in a new geographic profiling algorithm: the Geographic Profiling: Suspect Mapping and Ranking Technique (GP-SMART). For a given crime, GP-SMART predicts suspects’ probability of committing the crime given the proximity of their activity locations to the crime and the attributes of those activity locations that influence whether they will commit crime nearby—as described in the theoretical model—and prioritises the suspects accordingly. I tested GP-SMART’s accuracy by investigating how often it placed the actual offender among the top ranked suspects.
The theoretical model proposed that people are more likely to commit crime in locations where they have reliable knowledge that is relevant to the future crime. Attributes of people’s activities—such as their frequency and similarity to the future crime—affect the development of reliable and relevant knowledge. The empirical tests confirmed that offenders were generally more likely to commit crime closer to the kinds of activity locations that people visit more frequently (e.g., home versus family homes) or likely to impart more relevant knowledge about crime opportunities (e.g., prior crimes versus prior victim or witness location). They were also more likely to commit crime near prior activity locations that would generate both reliable and relevant knowledge, than near prior activity locations lower on either or both dimensions. By accounting for the theoretically salient attributes of suspects’ activity locations, GP-SMART ranked the offender at or near the top of the suspect list at rates exceeding chance and those produced by baseline methods—approximating existing algorithms—that use proximity alone.

This thesis makes significant theoretical, methodological and practical contributions. The theoretical model explicitly extends crime pattern theory, and the empirical studies add to the evidence base for this extension and core tenets of both the environmental criminology and investigative psychology theoretical approaches to geographic profiling. The police data included a wider range of activity locations than used in past crime location choice studies based on administrative data, and the findings highlight their utility for signalling offenders’ mental maps. Indeed, the size of the dataset required developing a novel method for sampling when using Discrete Spatial Choice Modelling—a major methodological contribution to the growing discrete crime location choice literature. But of most significance in light of the motivation for this thesis, the GP-SMART results demonstrate that enhancing geographic profiling by incorporating the attributes of people’s various activity locations that influence their crime locations, could help solve crime in practice.

text: Spatial signatures and mental maps: Using offenders’ activity locations to enhance geographic profiling for crime investigations In crime investigations, geographic profiling involves inferring information about ‘whodunit’ from information about where and when the crime occurred. Such inferences are possible because people usually commit crime in places they know, rather than seeking opportunities elsewhere. The ‘spatial signature’ of a crime location thus reflects the ‘mental map’ of the offender: the locations they are familiar with from their everyday activities, such as where they live, work, go to school or visit family and friends. Previous empirical research suggests that people are more likely to commit crime near some activity locations in their mental map than others. However, these differences have not been accounted for in theory or in geographic profiling methods—particularly methods that infer the most likely suspect to have committed a crime, given the proximity of their activity locations to the crime. This thesis aimed to enhance geographic profiling by developing, validating, and applying a theoretical model that explains how people’s different activity locations influence their crime locations.

To achieve this aim, I first drew on existing theoretical and empirical literature to propose a theoretical model that identifies the attributes of people’s activity locations that influence whether they will commit crime nearby and the psychological mechanisms through which this influence occurs. Next, I tested hypotheses derived from the model to add to its empirical support, using New Zealand Police data on approximately 60,000 offenders who committed burglaries, robberies and extra-familial sex offences between 2009 and 2018. This national dataset included details of the offenders’ crime locations and their pre-crime activity locations such as home addresses, family members’ home addresses, schools, and the locations of prior crimes and other police interactions. I then applied the theoretical model in a new geographic profiling algorithm: the Geographic Profiling: Suspect Mapping and Ranking Technique (GP-SMART). For a given crime, GP-SMART predicts suspects’ probability of committing the crime given the proximity of their activity locations to the crime and the attributes of those activity locations that influence whether they will commit crime nearby—as described in the theoretical model—and prioritises the suspects accordingly. I tested GP-SMART’s accuracy by investigating how often it placed the actual offender among the top ranked suspects.
The theoretical model proposed that people are more likely to commit crime in locations where they have reliable knowledge that is relevant to the future crime. Attributes of people’s activities—such as their frequency and similarity to the future crime—affect the development of reliable and relevant knowledge. The empirical tests confirmed that offenders were generally more likely to commit crime closer to the kinds of activity locations that people visit more frequently (e.g., home versus family homes) or likely to impart more relevant knowledge about crime opportunities (e.g., prior crimes versus prior victim or witness location). They were also more likely to commit crime near prior activity locations that would generate both reliable and relevant knowledge, than near prior activity locations lower on either or both dimensions. By accounting for the theoretically salient attributes of suspects’ activity locations, GP-SMART ranked the offender at or near the top of the suspect list at rates exceeding chance and those produced by baseline methods—approximating existing algorithms—that use proximity alone.

This thesis makes significant theoretical, methodological and practical contributions. The theoretical model explicitly extends crime pattern theory, and the empirical studies add to the evidence base for this extension and core tenets of both the environmental criminology and investigative psychology theoretical approaches to geographic profiling. The police data included a wider range of activity locations than used in past crime location choice studies based on administrative data, and the findings highlight their utility for signalling offenders’ mental maps. Indeed, the size of the dataset required developing a novel method for sampling when using Discrete Spatial Choice Modelling—a major methodological contribution to the growing discrete crime location choice literature. But of most significance in light of the motivation for this thesis, the GP-SMART results demonstrate that enhancing geographic profiling by incorporating the attributes of people’s various activity locations that influence their crime locations, could help solve crime in practice.

year: 2022

title: 'Closing the Gaps': From postcolonialism to Kaupapa Māori and beyond.

authors: Mahuika, Nepia

subjects: New Zealand

abstract: The article discusses New Zealand history, particularly examining Māori theories and interpretations of history. It considers postcolonial theory and Kaupapa Māori theory. The author comments on the government policy of "closing the gaps," referring to efforts to improve the conditions of underachieving New Zealand groups. He also reflects on Mātauranga, a Māori concept concerning knowledge. The history and oral tradition of the people of the Māori iwi, or social unit, of Ngati Porou is also discussed.

text: 'Closing the Gaps': From postcolonialism to Kaupapa Māori and beyond. The article discusses New Zealand history, particularly examining Māori theories and interpretations of history. It considers postcolonial theory and Kaupapa Māori theory. The author comments on the government policy of "closing the gaps," referring to efforts to improve the conditions of underachieving New Zealand groups. He also reflects on Mātauranga, a Māori concept concerning knowledge. The history and oral tradition of the people of the Māori iwi, or social unit, of Ngati Porou is also discussed.

year: 2011