Jennifer Williams

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Associate Professor

Research Interests

Population Ecology
Ecological and evolutionary processes

Relevant Thesis-Based Degree Programs

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Graduate Student Supervision

Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Effects of climate change on the habitat suitability of 4 relatively new invasive plant species in the Pacific Northwest (2022)

Invasive species are a substantial threat to biodiversity and ecosystem structure. This threat is exacerbated by the increasingly concerning and urgent outlook of predicted climate change, land cover change, and other human influences. Specifically, an increasing number of invasive plant species are spreading in the Pacific Northwest (PNW), an area of unique natural areas, economic value, and increasing human population. Predicting the potential habitat suitability for invasive plant species that are not yet established in the region is crucial for developing preventative management strategies. To this end, I developed habitat suitability models for four invasive plant species, two terrestrial species: Geranium lucidum and Pilosella officinarum; and two aquatic species: Butomus umbellatus and Pontederia crassipes. I initially considered 33 bioclimatic variables, 10 land cover types, and a human influence index as current model predictor variables with location records for each species drawn from the introduced range (North America). I projected each species’ current habitat suitability in the PNW region using ensemble modelling of six algorithms to 2050 and 2080, under 3 potential future climate scenarios. The majority of the coastal PNW is predicted to remain potential habitat for Geranium lucidum under all future climate scenarios, with some loss of habitat suitability in Oregon. In contrast, Pilosella officinarum, while currently suited to most inland regions of the PNW, is predicted to lose suitable habitat by 2050 under all climate scenarios, retaining high elevations as potential habitat. The suitable habitat for Butomus umbellatus in the PNW, which is currently moderately suitable, is not predicted to increase substantially in the future. Likewise, potential future habitat suitability remains relatively unchanged for Pontederia crasipes, which is currently highly suitable for inland waterways that do not experience freezing temperatures. Overall, the bioclimatic variables and human influence index were more important than land cover variables, suggesting that climate change and human activity are the determining factors for changes in future suitable habitat. My research provides a template to model other concerning species, assisting local land managers and practitioners to inform current and future management strategies and increasing the efficiency of allocating limited resources toward species with expanding ranges.

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Herbivorous insect and plant communities respond in complex ways to rainfall manipulation in an oak savanna grassland (2021)

Changes in precipitation due to climate change will have consequences for plant and herbivore communities alike. Multiple hypotheses exist to explain how changes in plant diversity and productivity may cause changes in herbivorous insect community composition. However, the effects of water availability on the relationships between plant and herbivore communities are less well understood. For my M.Sc. thesis, I used a long-term rainfall manipulation experiment in a remnant patch of Garry oak (Quercus garryana) savanna to examine how plant community composition and productivity have responded to changing levels of precipitation over a 5-year period. This highly endangered ecosystem is predicted to experience significantly wetter springs and drier summers by 2080. I also investigated plant-mediated, indirect effects of manipulated precipitation on the diversity and abundance of herbivores, drawing on multiple hypotheses that describe the relationships between the composition of plant and herbivore communities. I found that plant productivity increased significantly with increasing growing season soil moisture. Plant productivity was also influenced by plant diversity, with plant productivity increasing with increasing plant diversity but only in drier plots, highlighting a context dependent effect. Further, I found that the effects of soil moisture on the herbivore community were mediated by plant productivity. Herbivore abundance decreased with increasing plant productivity, refuting the more individuals hypothesis. This indirect effect of soil moisture was in addition to a significant negative effect of soil moisture directly on herbivore diversity. The results of this study show that drought will result in significant decreases in plant productivity in this ecosystem and both direct and indirect impacts on the composition of the herbivore community. This study provides valuable insight into the effects of climate change on a highly endangered ecosystem and how future changes in precipitation with climate change will effect plant and herbivore communities.

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Infraspecific life history variation in the introduced range of Cynoglossum officinale (2019)

All organisms must allocate resources to reproduce and survive. The study of intraspecific life history variation can provide insight into how different selective pressures, such as herbivory or climate, can favor different life history strategies and constrain others depending on which life stages are most vulnerable to the selective pressures. I examined how variation in herbivory and climate influences variation in the flowering size of plants and the occurrence of semelparity versus iteroparity at sites across part of the introduced range of an invasive plant, houndstongue (Cynoglossum officinale, Boraginaceae). Houndstongue is a short-lived semelparous perennial in its native range. In its introduced range, a previous study documented increased rates of iteroparity and a higher threshold flowering size compared to the native range. I hypothesized that the recent introduction of a specialist biocontrol root-boring insect (Mogulones crucifer) would decrease threshold flowering size, and reduce the proportion of iteroparous plants in the introduced range at sites with the weevil present. I surveyed 24 sites across the northwestern United States to quantify the frequency of semelparity versus iteroparity and to estimate abundance of M. crucifer. I found that the proportion of iteroparous plants varied across sites and that winter precipitation and weevil presence best explained this variation. Sites with greater precipitation and no weevils had a higher frequency of iteroparity. I used demographic data collected from six sites to estimate threshold flowering size. Sites with a higher attack rate by weevils had a lower threshold flowering size than those with a lower attack rate. The variation in frequency of flowering and threshold flowering size that I documented in North American houndstongue populations and the relationships between this variation and herbivory and climate provide evidence for how selective pressures shape the life histories of invasive plants.

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Across intermediate spatial scales, a specialist insect herbivore responds to climate and host plant size, not host density (2018)

Herbivory can have important consequences for plant-population dynamics, causing changes in population growth rate, abundance, local expansion, spread and even the evolution of life history and defense traits. Studies at large spatial scales such as latitudinal gradients spanning the equatorial region to the poles, have yielded broad generalizations in patterns of herbivory. At local population scales, the effects of herbivory are often context specific, limiting extrapolation. Less described in the literature is whether patterns of herbivory manifest in between these two disparate spatial scales. At intermediate, regional scales, my research indicates that patterns of herbivory that manifest in local populations are obscured. Within a regional study area, in the southern interior of British Columbia, I found Mecinus janthiniformis, a stem mining weevil, did not respond to host plant (Linaria dalmatica) density across sites, but rather to host size. At 36 of the 39 sites sampled, herbivores were found to distribute themselves according to host plant density within populations. The direction of this effect however varied by site, resulting in no pattern at the regional scale. In conjunction with earlier research, my results suggest plant populations experiencing herbivory in different spatial patterns (i.e. more herbivory in high or low density patches) may result in different outcomes for plant population spread and persistence over time.

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Mechanisms of coexistence between native and exotic plant species (2018)

One of the primary goals of ecology is to understand the processes that maintain biological diversity. The development of a modern coexistence theory has helped to advance this understanding by proposing a set of specific mechanisms that enable coexistence, specifically, species can coexist when niche differences between species are large enough to overcome fitness differences between species. Recent advances have used fully parameterized demographic studies to explain how traits, phenology, and evolutionary history contribute to niche and fitness differences, but there is a lack of empirical evidence of how competition and the environment interactively influence coexistence outcomes. Moreover, there is little empirical evidence of how indirect interactions between species mediate coexistence outcomes. Using an outdoor pot experiment and observational data from natural plant communities I explored the interactions between a pair of native (Plectritis congesta) and exotic (Valerianella locusta) co-occuring, annual plant species. With this system I answered the following four questions: (1) Can Plectritis and Valerianella coexist over the long run? (2) Does environmental variation change the intrinsic interaction between these species? (3) Does environmental variation enable coexistence by providing each species with an opportunity for positive low density growth rate in certain spaces or at certain times? (4) Do indirect interactions with pollinators destabilize or promote coexistence? The pot experiment predicted that Plectritis will exclude Valerianella over the long run. Although the coexistence outcome did not change between environmental treatments, the parameters used to calculate niche and fitness differences experienced significant changes. I did not find any evidence that Valerianella maintains abundances through variation in the environment in natural communities. Additionally, I did not find any evidence that niche differences are decreased through indirect interactions with pollinating insects. These species are still observed to co-occur at the site level and thus Plectritis may limit, but not totally eliminate, the abundance of Valerianella. Moreover, my experiment showed how vital rates and interaction coefficients depend on the environmental context, emphasizing that abundances are driven not only by competition and environment, but also through the interaction between competition and environment.

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Identifying the context dependencies of plant-herbivore interactions across a species' range (2016)

Spatial variation in abiotic and biotic factors creates local contexts that influence the intensity and outcome of plant-herbivore interactions. Previous studies have tried to represent the complexity of these context dependencies using latitudinal clines, but this approach has proven insufficient for many systems. Despite substantial variation in herbivore damage across populations, the relative contribution of local community factors to explaining this variation isless well known. I investigated plant-herbivore interactions across the entire range of Quercus garryana, specifically testing the relative importance of climate and population traits on herbivore communities, leaf traits, and the extent of damage caused by different insect feeding guilds. I performed similar analyses on trees grown in a common garden, allowing me to detect the relative importance of environmental and genetic contributions to leaf defense traits andsusceptibility to herbivory. Although I observed no variation in herbivore diversity among populations in the field, the abundance of herbivores declined with increasing latitude, while also responding to variation in population size, leaf traits, and climate. Leaf traits were also influenced by climate factors, but in addition varied with tree size and within the growing season. Differences in herbivore damage were best explained by long-term trends in spring precipitation, leaf traits, and population size, with no relationship to latitude. The relative importance of each of these factors depended on when the damage occurred as well as the insect feeding guild causing the damage. Conversely, the extent of damage in the common garden was constant across trees of different provenance,providing further support for the importance of climate in driving variation in herbivory. The findings of this study demonstrate the importance of climate, irrespective of latitude, on plant-herbivore interactions and in mediating the effects of other community factors. As such, they highlight the importance of conducting studies across diverse ecosystems and climate gradients. Only by understanding the underlying drivers of selection can we begin to drawgeneralizations and develop a predictive framework of plant-herbivore interactions to inform conservation and effective habitat management.

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Long-term effects of prairie restoration: community structure and native plant population dynamics 6 years after experimental management (2015)

Restoring degraded habitats with the goal of achieving long-term ecological complexity and stability is an essential component in combatting global declines in biodiversity. The main objectives of prairie restoration are to reduce the abundance of exotic species while enhancing native species richness and abundance, but it is often difficult to extend monitoring to evaluate these as long-term goals. Understanding how initial outcomes persist or change over time is essential for evaluating treatment efficacy. Additionally, observing how specific native populations persist and spread following restoration treatments can inform future decisions regarding seeding practices and management timelines. To assess the degree to which initial treatment effects continue after project completion, I revisited remnant patches of Pacific Northwest Garry oak savanna/prairie habitat 6 years after experimental restoration treatments were applied. I evaluated the composition and structure of the plant community at each site to determine if, and how, the effects of disturbance treatments and supplemental native seeding changed in the years following experimental management. I tracked the persistence of seeded species and measured spread of their populations as a metric to evaluate longer-term success, suitability of native species for restoration, and the ability of the habitat to support native plant populations. I found that plots that received supplemental seeding continued to exhibit higher native species richness than those left unseeded, and that both seeding and disturbance treatments could positively influence the long-term pattern of native species abundance. The initially-observed effects of disturbance treatments on reducing exotic grass abundance diminished after 6 years, but nevertheless these treatments significantly influenced the population trajectories of 4 out of 8 seeded species. There was spatial advance of most seeded species’ populations, as evidenced by occurrences in previously unoccupied plots. A case study of the seeded species, Plectritis congesta, allowed for estimation of the average rate of spread per generation and quantification of the long-term spatial influence of seeding efforts. The results from my extended monitoring confirm that seed limitation of native species and difficulties maintaining the reduction of exotic grasses continue to be major barriers to success in restoration of invaded prairies.

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