Michelle Tseng

The increased occurrence of heatwaves and microplastic pollution may be decreasing the productivity of aquatic ecosystems. In response to changing environmental conditions, phytoplankton adjust their lipid profile and consequently alter their nutritional quality, which has implications for the productivity of their consumers. Elevated temperatures reduce the nutritional quality of phytoplankton, largely by decreasing their polyunsaturated fatty acid (PUFA) content, but it is unclear whether heatwaves elicit a similar or stronger response. The effect of microplastics, and the combination of heatwaves and microplastics, on the nutritional quality of phytoplankton is similarly unknown. To address these knowledge gaps, I conducted a laboratory experiment to examine the effects of constant warming, heatwave, and microplastics on the population growth, the lipidome, and the proteome of a freshwater green microalga, Tetradesmus obliquus. I also investigated their ability to recover once returned to ambient conditions. Although I applied the same total amount of heat in the constant warming and heatwave treatments, I saw clear differences in the lipid profile. The lipidomic response under heatwave involved a transient induction of more numerous and acute changes, whereas changes accumulated over time with constant warming. Extraplastidial membrane lipids and PUFA significantly decreased under both heat treatments, but neutral lipids decreased only under heatwave. Interestingly, plastidial lipids increased in all treatments, suggesting a protective response within photosynthetic organelles. The proteomic response followed a similar pattern, with a stronger but transient response under peak heatwave temperature, and an increase in differentially expressed proteins over time under constant warming. I also found that the co-occurrence of microplastics did not induce a unique lipidomic response under heatwave, but did so in the constant warming group. Upon return to ambient conditions, T. obliquus showed signs of recovery from both treatments, particularly with MP present, but maintained a slightly different lipid profile one-week post-heating. My findings suggest that future global change may have significant consequences for freshwater algal nutritional quality and thus the productivity of aquatic food webs. Further investigation into these effects at the community level and in multi-stressor scenarios is needed to better predict broader ecological responses to our rapidly changing environment.

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Urban forests, the trees that are planted and managed within built-up areas, provide valuable services to humans and wildlife, including lowering air temperature, carbon sequestration, improved quality of life for humans, and habitat and resources for insect, birds, and other animals. However, tree species planted within these forests are often selected for aesthetics or their ability to tolerate urban stressors, and are dominated by introduced tree species. Whether the selected trees can support urban biodiversity has largely been ignored. Trees are host to important plant-dependent organisms such as leaf-feeding insects and epiphytic microbes. These organisms are important for nutrient cycling, pest control, and tree health. Limited previous research suggests that tree provenance (i.e. native or introduced) might affect the diversity of insects and microbes supported by specific trees. Native trees are hypothesized to be a more palatable resource for phytophagous insects (and hence support a higher diversity and abundance of insects) but evidence linking tree provenance to microbial diversity or community composition is mixed. This thesis tests the hypothesis that native and introduced trees support unique communities of insects and microbes. In 2023 we sampled insect communities throughout the growing season from 80 trees (16 tree species) on the University of British Columbia Vancouver campus. We also sampled the leaf bacterial community once from each tree. We found the same overall abundance of insects on native and introduced trees, but native trees supported higher insect richness. Functional diversity and insect community composition also differed between native and introduced trees. Tree provenance did not affect microbial richness or community composition. Insect and microbial community composition were also unique between the coniferous and deciduous hosts. Overall, our findings indicate that native trees support higher insect diversity than their introduced counterparts, and suggest the continued replacement of native trees with introduced species may exacerbate the continued loss of insects worldwide. We recommend prioritizing native trees to maximize wildlife habitat and resource provisioning in urban areas.

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Despite habitat loss contributing to worldwide declines in biodiversity, urbanized areas can harbour a surprising amount of pollinator diversity. Cities contain a wide array of floral resources from human interventions, and a growing public interest in urban pollinator conservation has resulted in a notable increase of intentionally managed habitat. Urban green spaces like residential yards, community gardens, and parks can attract a variety of species through providing plants for nectar, pollen, nesting habitat, and larval herbivory. While many garden characteristics are known to affect pollinator species richness and abundance, few studies examine the gardener’s influence on the garden. Human decisions fundamentally shape urban environments and, in this case, gardeners have direct control over garden plant composition. By partnering with the David Suzuki Foundation’s Butterflyway Project based in Metropolitan Vancouver, Canada, this study investigated 20 pollinator gardens on how gardener decision-making, barriers, effort, and budget affected garden plant communities, and in turn how the richness and geographic origin of flowering plants affected pollinator species richness and visitation. In particular, gardeners are interested in whether there are any especially attractive plants to maintain for pollinators. Results indicate that plant richness is the only significant factor in explaining pollinator visitation and richness. Neither the proportion of garden species that are native plants nor floral area had any effect on pollinator measures. However, the relationship between proportion native plant richness and pollinator richness differed among sampling periods. There was overwhelming positive support for planting ecologically-functional plant species, in spite of the wide range of native and non-native, ornamental plants found in gardens. No gardener preferences, barriers, or key effort and budget variables correlated with garden plant species richness. Gardening preferences and barriers varied between solo and community gardens. There were 19 highly attractive pollinator plants in this system, including those such as Canada goldenrod (Solidago canadensis), Douglas aster (Symphyotrichum subspicatum), globe thistle (Echinops ritro), and giant onion (Allium giganteum).

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Climate change is resulting in ongoing temperature warming and increased frequencies of heatwaves. In aquatic ecosystems, temperature not only affects the distribution and abundance of phytoplankton, but warming may also be reducing the quantities of key nutrients produced by these important primary producers. However, thus far most research in this field has examined the effects of warming temperatures on nutrient production in individual species of phytoplankton. To characterize how warming affects phytoplankton-based nutrients at the community scale, we subjected naturally occurring phytoplankton assemblages to three temperature treatments (ambient, warming, heat wave) in a seven-week laboratory experiment. We used community-wide fatty acid composition and stoichiometric indicators (C:N, C:P and N:P ratios) as our measures of phytoplankton nutritional quality. By the end of the experiment, there was no effect of temperature on phytoplankton community composition. Phytoplankton communities from the heatwave treatment had decreased concentrations of C, N and P, but neither the heatwave nor warming affected community-wide C:N, C:P and N:P ratios. Both warming and the heatwave reduced phytoplankton polyunsaturated fatty acid (PUFA) content, but as the heatwave subsided, PUFA quantities in this treatment approached those found in the ambient temperature treatment. We then fed warmed phytoplankton communities to naturally-collected zooplankton assemblages and found that the PUFA composition of the zooplankton communities closely reflected that of their food source. Our results suggest that 1) temperature warming has negative effects on phytoplankton community nutritional quality, 2) these responses are not caused by broad-scale shifts in phytoplankton taxonomy, and 3) phytoplankton PUFA levels appear to closely track water temperature. Furthermore, we provide evidence that zooplankton communities experience indirect effects of temperature warming through nutritional shifts in their phytoplankton resource. Overall, this study improves our understanding of the types of phytoplankton nutrients that are affected by warming, how quickly these nutrients can respond to temperature change, and the down-stream effects of phytoplankton-based nutrients on zooplankton consumers.

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The field of climate change impacts on ecology primarily focuses on the measures of abundance and distribution of individuals to assess organism response, but the measure of organism quality is not often applied to assessing organism response within food webs. The abiotic factor of temperature has known effects on organism quality, but it is unclear how the biotic factor of predator presence impacts organism quality. The goal of my thesis was to investigate (a) the combined effects of warming and predator presence on prey nutrient quality, and (b) how temperature affects organism quality and the nutritional needs of consumers. To address the combined effects of warming and predator presence on one measure of prey quality - organism body size - I conducted a meta-analysis on 14 papers that tested both warming and predator presence on prey body size in aquatic systems. Across all studies, I found no net effect of warming on body size, a large increase in prey body size with predator presence, and an additive effect of the two factors combined. I then conducted a laboratory experiment using the primary producer Scenedesmus obliquus, the primary consumer Daphnia pulex, and the secondary consumer Chaoborus americanus to investigate temperature mediated changes in algal quality and consumer nutritional needs (measured using the fatty acid profile of algae that affected D. pulex population size and C. americanus growth rates). Overall, we observed changes in S. obliquus quality with temperature and mild cascading effects of these changes on D. pulex and C. americanus. Further investigation is needed into the effects of warming and predator presence on other nutrients (such as carbohydrates and proteins that may respond differently to temperature), and if the relationship between body size and quality holds true for all organisms. Overall, my thesis provides insight into how predator presence can have a stronger effect on organism body size than warming and suggests greater care must be taken when interpreting the results of studies that assess the effects of temperature on organism body size in the absence of biotic factors.

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