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    Plant invasion down under: exploring the below-ground impact of invasive plant species on soil properties and invertebrate communities in the Central Plateau of New Zealand
    (Springer Nature Switzerland AG, 2024-09-15) Pearson BM; Minor MA; Robertson AW; Clavijo McCormick AL
    The impacts of invasive plants on arthropod communities are often reported to be negative and have predominantly been explored aboveground, but there is a paucity of information regarding what happens belowground. To address this gap, we compared soil properties and soil fauna communities associated with two native plant species (Leptospermum scoparium—mānuka and Chionochloa rubra—red tussock) and two invasive species (non-N-fixing Calluna vulgaris—European heather and N-fixing Cytisus scoparius—Scotch broom) in the Central Plateau of New Zealand. We expected that (1) at individual plant level soil properties would be different under invasive and native plant species, with higher soil nutrient concentrations under invasive species, especially N-fixing broom; (2) total abundance of soil fauna would be higher under invasive plant species, as generally positive impact of invasive plants on soil invertebrates is indicated in the literature; (3) invasive plants, and especially N-fixing broom, will be associated with greater abundances of soil decomposer groups. We found that soil properties and soil fauna assemblages did not cluster by plant invasive status as initially predicted. At individual plant level, there was similarity in soil conditions between mānuka and broom, and between red tussock and heather. The invasive N-fixer (broom) had positive effects on soil N availability, with higher N pool and lower C/N ratio in soil under this species. There were no consistent differences in total soil fauna abundance between invasive and native plants. Broom and mānuka were associated with higher abundances of Collembola, Oligochaeta and Diplopoda; heather and red tussock had higher abundances of Hymenoptera and Hemiptera. Significantly more Oligochaeta and Collembola under broom matched the prediction of invasive plants (and especially N-fixing invasives) being associated with greater abundances of decomposers. However, another important decomposer group—oribatid mites—did not show the same tendency. These results evidence that simplified generalizations regarding the impacts of invasive plants are unlikely to be justified, since the ecological effects of plant invasions are complex and do not always follow the same pattern. Therefore, we need to take into consideration the ecological context and the traits of individual plant species and target organisms in an unbiased manner to fully understand the impacts of plant invasions.
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    Life history strategies of Tetranychus ludeni Zacher (Acari: Tetranychidae) with special reference to biological invasion : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Entomology at Massey University, Manawatū, New Zealand
    (Massey University, 2022) Zhou, Peng
    With the increase of worldwide trade and travel in recent decades, increasingly more arthropod species have become established outside their natural range of distribution, causing substantial ecological and economic impacts in novel habitats. Successful invaders may bear certain life history traits that can overcome various barriers such as mate and food shortage and inbreeding depression. Here, I investigated the life history strategies of a haplodiploid pest, Tetranychus ludeni Zacher (Acari: Tetranychidae), with special reference to its invasion success. It is native to Europe but now cosmopolitan. Virgin females laid larger eggs than mated females, giving rise to larger adults, and sons from virgin females produced more daughters at a higher rate than sons from mated females in their lifetime. Virgin females produced maximum number of sons in their early life to ensure subsequent mother-son mating but later saved resources to prolong longevity for potential future mating. Females maximised their resource allocation to egg production immediately after mating regardless of whether mating delay occurred to secure production of maximum number of both daughters and sons as early as possible. Mated females with mating delay increased proportion of daughters in offspring produced to compensate the loss of production of daughters during their virgin life. Neither mother-son mating nor sibling mating affected female reproductive output and longevity in any of the 11 successive inbred generations and neither sex showed inbreeding avoidance behaviour, suggesting that inbreeding has no negative impact on its invasion success at any points or generations. Mated females did not trade off their survival and lifetime reproductive output with dispersal. Long-distance dispersers invested more in dispersal in their early life while resident mites and short-distance dispersers invested more in reproduction during their early life, which may allow long-distance dispersers to explore the novel environment more effectively without compromising lifetime reproductive fitness. Older females with more mature eggs were more likely to disperse and move longer distances than younger ones with fewer eggs. Females increased dispersal probability and distance with the increase of population density. The synchronization of dispersal and reproduction and the positive density-dependent dispersal strategy may facilitate habitat colonization and invasion speed of T. ludeni. Findings from this study improve our understanding of the invasion mechanisms of T. ludeni and other haplodiploid species, providing knowledge for development of programmes for prediction and management of biological invasions.
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    Volatile organic compounds emitted by invasive and native plant species under invasion scenarios and their potential ecological roles : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand
    (Massey University, 2020) Effah, Evans
    Climate change, human migration, and global trade favour the spread of plant species beyond their natural ranges. Many of these plants become invasive, posing a risk to the persistence and survival of native species and the ecosystems they invade. In New Zealand, the European woody shrub Calluna vulgaris (heather) is the most widespread invasive weed on the Central Plateau of North Island. Like most exotic invasive plants, the chemical behaviour (i.e. chemical production and chemical mediated interactions) of heather in its invaded habitat is poorly understood. Moreover, despite the struggles of native plants to endure the stress induced by exotic weeds, no study has documented the chemical behaviour of native plant species in plant invasion scenarios. Volatile organic compounds (VOCs) are secondary plant metabolites that play a vital role in plant communication with other organisms and are highly responsive to biotic and abiotic stress. Therefore, measuring VOC emissions during plant invasion could provide valuable information about plant responses to the changing environment and their potential impacts on other community members. This thesis aimed to investigate VOCs emitted by the invasive weed heather and a New Zealand native plant Leptospermum scoparium (mānuka) under field conditions, while determining the environmental factors regulating their emissions and exploring their potential ecological impacts under lab and field conditions. Results from the field trials on the Central North Plateau showed variations in the volatile profiles of heather and mānuka growing at different sites, with both plants emitting lower amounts of VOCs at sites where other exotic invasive plants were present. This reduction in VOC emissions was mostly due to indirect changes in environmental factors, like soil properties, which were driven by the invasive weeds heather and Cytisus scoparius (Scotch broom; henceforth broom). This thesis also documents the chemical responses of heather to two major stresses encountered in New Zealand; 1) elevated solar ultraviolet radiation (UV) and 2) damage caused by its introduced specialist herbivore and biocontrol agent Lochmaea suturalis (heather beetle). Results from these trials demonstrate that high UV radiation reduced the volatile emissions of some compounds (mainly terpenoids) and that the impacts of herbivory by heather beetle on VOC emission depended on the developmental stage of the herbivore, plant phenology, and season. The ability of VOCs emitted from heather and broom to affect the germination and growth of mānuka was also tested in the lab, while the impact on arthropod communities were investigated at sites on the Central North Plateau. The results suggest that VOCs produced by invasive plants may have phytotoxic effects toward mānuka and may alter arthropod community structure. This thesis highlights the complexity of plant chemical communication under invasion scenarios and invites further exploration of the interactions between exotic invasive plants and native species to broaden our understanding of invasion ecology to support weed management, biocontrol, and conservation efforts.
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    Invasive alien species : a threat to sustainable livelihoods in the Pacific? : an assessment of the effects of Wasmannia auropunctata (little fire ant) and Achatina fulica (giant African snail) on rural livelihoods in the Solomon Islands : a dissertation presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Development Studies, Massey University, Manawatū, New Zealand
    (Massey University, 2016) Stronge, Dean
    Invasive alien species (IAS) are a global phenomenon and are recognised as a driver of environmental change which can affect the well-being of people in a multitude of ways. Despite this, the role of IAS in local livelihoods has received relatively little attention. Influencing all three of the sustainable development pillars (social, economic, environmental), IAS should be recognised as a significant development issue. But they are not. As such, IAS issues are new to many sectors and governments and therefore largely go unseen and un-actioned. Contemporary rural livelihoods in the Solomon Islands are heavily reliant on subsistence/semi-subsistence agriculture. Following a livelihoods’ framework developed for the Solomon Islands, this thesis explores the influence IAS have on rural livelihoods in this country. Using two qualitative case studies, Wasmannia auropunctata (little fire ant) and Achatina fulica (giant African snail), this study investigates how vulnerable/resilient rural livelihoods are to the effects of IAS and the implications IAS have for sustainable development in the Solomon Islands. The effects of IAS on rural livelihoods are complex and at times contradictory. W. auropunctata for the most part is not negatively affecting the dominant livelihood strategy (subsistence/semi-subsistence agriculture) practised in the Solomon Islands. While there are some social impacts associated with W. auropunctata, overall Solomon Island households can be considered resilient to this IAS. Achatina fulica is a different story. This species is negatively affecting the subsistence/semi-subsistence agricultural sector on which so many rural Solomon Island households depend. This has resulted in households implementing negative livelihood diversification measures as they fail to cope or adapt to the snails’ presence. Unlike for W. auropunctata, Solomon Island households have not demonstrated any resilience to A. fulica. Understanding how rural livelihoods are affected by various stressors and adverse events can help to design development policies and interventions geared towards building better lives for all people. This can only occur however, if the full range of shocks are recognised. To date, this is not the case for IAS, and as such, they are still a significant missing component of development policy.