CTAHR Student Works

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    Impacts of invasive Ilex aquifolium on Southern Vancouver Island soil chemistry & biodiversity
    (2025) Sloan Staggs Sink; Nancy Shackelford; Brooke Hayes
    English holly (Ilex aquifolium) is an invasive species found in parts of southwest Vancouver Island, where it thrives in moderately moist mesic soils. I. aquifolium seems to create “deadzones,” areas underneath the canopy where little to no vegetation grows. To explore the presence and potential causes of these deadzones, we investigated whether I. aquifolium canopies alter plant communities and soil chemistry properties in the invaded range on southern Vancouver Island. Soil and plant indicators were surveyed in Matheson Lake Regional Park and Metchosin Wilderness Park in the District of Metchosin beneath living I. aquifolium trees (n=5), treated trees (n=5), and controls (n=20). Plant surveys were used to assess the presence of these dead zones in the region, and soil surveys measuring key indicators such as soil organic carbon (SOC), and decomposition rates explored potential below-ground changes under I. aquifolium that might drive these zones. Findings show 16.5% to 20.8% less plant species richness under I. aquifolium canopies compared to controls and treated sites respectively. There was 9.65% to 17.25% less plant cover under living I. aquifolium trees compared to control and treated sites respectively. Decomposition rates in soil under living sites were 13.86% slower compared to treated sites and 7.26% slower to controls. SOC from living sites was higher compared to controls. Reduced decomposition rates under living and treated I. aquifolium may be one explanation for the formation of deadzones. Regional projections show a 0.2% cover of I. aquifolium could reach 50%+ cover after 35 years due to exponential growth patterns. I. aquifolium threatens soil health and biodiversity and could be considered a high priority invasive species, particularly in areas of new occurrence.
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    Responses of Soil Invertebrate and Bacterial Communities to the Removal of Nonnative Feral Pigs from a Hawaiian Tropical Montane Wet Forest
    (2018) Wehr, Nathaniel H.
    Feral pigs (Sus scrofa) are perhaps the most abundant, widespread, and economically significant large, introduced vertebrate across the Pacific Island region. This species has played a role in the degradation of native ecosystems and the extinction of multiple species of plants and animals on Pacific islands and has negative effects on both the ecotourism and agricultural industries. Despite numerous published studies on feral pigs in the Pacific Island region, some fundamental aspects of feral pig ecology remain poorly characterized, particularly belowground. To address these knowledge gaps, this thesis analyzed relationships between soil macroinvertebrates and microbes and feral pigs using nine sites located inside and outside of feral pig removal units representing a ~25-year environmentally-constrained chronosequence of removal in tropical montane wet forests in Hawai‘i. The results of these studies indicate that areas with active trampling by feral pigs correlate with lower abundance, biomass, and species richness of all soil macroinvertebrates. Comparatively, active rooting correlated with higher abundance and biomass of nonnative earthworms (Lumbricidae and Megascolicidae) and ground beetles (Carabidae). Further, my results indicate an overall increase in the net biodiversity of soil bacterial communities following feral pig removal, with biodiversity positively correlating to time since removal. Comparatively, environmental characteristics, including mean annual temperature and elevation, are better predictors of differences in functional and phylogenetic biodiversity among soil bacterial communities than feral pig removal. Collectively, these results indicate that the removal of feral pigs largely does not affect soil macroinvertebrates but does increase the diversity of bacterial communities, which could increase ecosystem stability.