Abstract
Nonnative, invasive feral pigs (Sus scrofa) modify habitats by disturbing soils and vegetation, which can alter biogeochemical processes. Soil microbial communities drive nutrient cycling and therefore also play important roles in shaping ecosystem structure and function, but the responses of soil microbes to nonnative ungulate removal remains poorly studied. We examined changes in the soil bacterial community over a ~25 year chronosequence of feral pig removal in tropical montane wet forests on the Island of Hawai’i. We extracted bacterial eDNA from soil samples collected inside and outside of ungulate exclosures along this chronosequence and sequenced the eDNA using the Illumina platform. We found that ungulate removal increased diversity of soil bacteria, with diversity scores positively correlated with time since removal. While functional and phylogenetic diversity were not significantly different between pig present and pig removed soils, soil bulk density, which decreases following the removal of feral pigs, was a useful predictor of dissimilarity among sites and correlated to changes in functional diversity. Additionally, increases in soil porosity, potassium, and calcium were correlated to increases in functional diversity. Finally, sites with greater mean annual temperatures were shown to have higher scores of both functional and phylogenetic diversity. As such, we conclude that feral pigs influence overall bacterial community diversity directly while influencing functional diversity indirectly through alterations to soil structure and nutrients. Comparatively, phylogenetic differences between communities are better explained by mean annual temperature as a climatic predictor of community dissimilarity.
Highlights
Soil microbes play critical roles in water retention and purification, nutrient cycling, primary production, soil formation and processing, and carbon sequestration, with individual taxa capable of contributing either highly specialized or generalist roles[1]
Feral pig activity has been directly associated with increased soil microbial activity attributed to potential increases of soil N availability associated with feces and urine[6,35] and indirectly via increased availability of soil organic matter[36]
Our initial assessment of the effects of environmental variables on bacterial community dissimilarity indicated that mean annual temperature (MAT), elevation, soil bulk density (SBD), soil porosity, soil potassium (K), soil calcium (Ca), and the sum of stem density of large trees, small trees and shrubs, and tree ferns were all potential predictors of variation among bacterial communities at each of the nine sites
Summary
Soil microbes play critical roles in water retention and purification, nutrient cycling, primary production, soil formation and processing, and carbon sequestration, with individual taxa capable of contributing either highly specialized or generalist roles[1]. Numerous factors including pH, soil water content[2,3], cation exchange capacity[3], and soil organic matter content[4] have been shown to influence the composition, structure, and function of soil microbial communities. Given that many soil microbes are highly sensitive to these localized environmental factors, the soil microbiome has been associated with both above- and belowground plant and animal communities[1,2,5,6,7]. Resultant changes include decreased diversity and abundance of plants[17,18], fungi[19], and wildlife[20,21] as well as being associated with the increased presence of specific bacteria in watersheds[22,23,24]. The presence of feral pigs has been associated with faster microbial community recovery following rapid unexpected changes in the soil environment, such as the application of fumigants[37]
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