Assessing the Ecological Consequences of Domestic Pig Grazing on the Understory Vegetation of an Oak Woodland
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This study addresses the effects of domestic pig (Sus scrofa domestica) grazing on understory vegetation of an oak woodland adjacent to a hazelnut orchard. Oregon white oaks (Quercus garryana) are associated with filbertworms (Cydia laiferreana), a native lepidopteran pest that uses the acorns as its primary food source during its larval stage. Filbertworms also affect agriculture, infecting hazelnuts and rendering the nuts inedible and unmarketable. Oregon white oak habitats are already under constant threat from land use change due to urbanization and agriculture, and this conflict exacerbates the problem. Pig grazing of infected acorns after they drop in the fall could offer a disincentive of removing oak habitats. It is common for hazelnut farmers to spray pesticides intensively or remove oak stands entirely to mitigate against pests. Pig grazing may (1) be an effective organic alternative and a potentially profitable scenario that could enhance the sustainability of agricultural practices while (2) promoting conservation of a rare ecosystem and (3) facilitating pig farming. Even with this potential win-win-win scenario, any management activity in an ecosystem could impose unintended consequences and introducing an uprooting mammal can be risky. I tested whether grazing domestic pigs in Oregon white oak stands has any negative effects on understory vegetative cover. I hypothesized that the pigs will (1) reduce percent cover of the herbaceous understory, (2) increase percent cover of bare ground, and (3) eliminate certain preferred herbaceous species from the site. I also hypothesize that (4) more accessible/less obstructed areas within the plots will have even lower levels of herbaceous/litter percent cover and even higher levels of bare ground percent cover. I visually estimated percent cover of bare ground and herbaceous/litter in quadrats along three transects using a Before-after Control-impact (BACI) design before and after grazing treatments in grazed and un-grazed woodland plots. Domestic pig grazing showed no significant effects on the percent cover of herbaceous/litter and bare ground. There was no loss of specific herbaceous species/litter. This suggests domestic pigs pose a relatively low threat to the understory vegetation of a native oak woodland when regulated through minimal time in the area. Pigs can then provide the win-win-win scenario without harming the understory of an already stressed habitat.