Balancing the good and the bad: Assessing the positive and negative effects of alien species on native plant demography

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2022

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Abstract

Alien species are considered one of the primary threats to native plant populations and their control is often prominent among proposed management actions. While negative alien effects are well documented, there are also many ways that alien species can have positive effects on native plant populations that may actually contribute to their persistence. Moreover, the effect of alien species on native plants can change in magnitude and direction over varying abiotic conditions. The success of native plant populations is determined by a mix of ecological and genetic factors. Alien (and native) species and abiotic conditions could also drive selection of plant traits. In order to understand the drivers of native plant population success in the face of changing climate and increasing prevalence of alien species, it is vital to understand the relationship between genotype, phenotype, and fitness of native plants. In chapter one, I quantified the effects of neighboring alien and native plants on all demographic rates in a population of the Hawaiian endemic plant Schiedea globosa, performing biannual censuses for 4 years to encompass relatively harsh and as well as benign seasons and years. The effects of alien neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien neighbors benefit S. globosa plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effects of heterospecific native neighbors were less often positive, indicating fundamentally different effects of native and alien neighbors on the demography of the focal native. These mixed effects highlight the need to consider potential benefits of alien species in the management of threatened native plants and that those benefits may be altered by changing abiotic conditions. In chapter two, I constructed population models for multiple Schiedea species across populations and years, using demographic rate regressions driven by the effects of alien and native neighbors, integrating the mixed effects of alien and native species on demographic rates of populations to project the net effect on population growth of native populations. The effects of alien and heterospecific native plant neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien and native neighbors benefit native plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effect of alien and heterospecific native neighbors on population growth was generally positive-- the mixed, but largely positive, net effects of alien and native neighbors on population growth highlight the need to consider potential benefits of alien, as well as native, species in the management of at-risk native plant populations, and that those benefits may be altered by changing abiotic conditions, as indicated by differing effects across (and within) years and populations. In chapter 3, I used paternal half-sibship pairs to measure the heritability of morphological traits under field conditions of the critically endangered Schiedea adamantis, which were found to be heritable in prior studies in greenhouse conditions, in reintroduced populations. I also performed a selection analysis, regressing fitness components against traits of outplants that I hypothesized might influence response to climate and alien and native neighbors to study the relationship between genotype, phenotype, and fitness of plants in restoration outplantings and assessing potential for evolutionary rescue. I found no significant heritability of any of the morphological traits. I did find evidence of selection, as leaf shape, area, and whole plant morphology had significant effects on fitness components (growth, survival, and reproduction), and significant interaction effects showing traits influenced fitness components differently at different levels of shade. Together, these results suggest that while variation in traits benefit individual plants in differing field conditions, these outplantings may not have the ability to respond to selection through evolution.

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Loomis, Alexander (2022). Balancing the good and the bad: Assessing the positive and negative effects of alien species on native plant demography. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/25240.

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