5 November 2019

The Pigs and the Dirt

CTAHR researchers track soil community response to feral pig management

The Pigs and the Dirt

Feral pigs are a conundrum: nonnative and invasive throughout much of the world, they negatively impact ecosystems. Yet they are culturally important, and they serve as wild game for subsistence and recreational hunting. A common management approach for feral pigs in Hawai‘i is to fence areas of high conservation value, remove the resident animals, and keep them out while allowing them to roam the surrounding forest.

The impact of pigs’ removal on the structure and function of Hawai‘i’s native-dominated wet forests is a topic of interest to CTAHR affiliates Nathaniel Wehr (NREM), Creighton Litton (NREM), Christian Giardina (USFS, NREM), Steven Hess (USGS, NREM), Noa Lincoln (TPSS), and Nhu Nguyen (TPSS). They used a 25-year chronosequence of feral pig removal in Volcanoes National Park and Pu‘u Maka‘ala Natural Area Reserve to address the question, “How does feral pig removal affect soil bacterial and macroinvertebrate communities?” The answer is key to managing these forests, given the critical roles that soil microfauna and mesofauna have on forest ecosystems. 

The joint CTAHR effort has resulted in two publications: soil bacterial communities in Nature Scientific Reports and soil macroinvertebrates in Biological Invasions

The first paperreports that bacterial community diversity increased linearly over time following the removal of feral pigs, while functional diversity remained unchanged. This indicates feral pig removal increases soil bacterial redundancy, which likely promotes ecological resiliency of the soil. 

In contrast, the second paper shows that soil macroinvertebrate communities did not change following feral pig removal. However, earthworms and ground beetles were positively associated with sites rooted by feral pigs. Two possible hypotheses warrant investigation: the Worm-Farming Hypothesis suggests that changes to the soil from pig rooting, such as increased mixing of soil organic matter and reduced bulk density, will improve earthworm habitat; the Truffle-Worm Hypothesis suggests feral pigs are capable of intentionally seeking out habitats with higher availability of below-ground food resources. 

The studies highlight changes in soil communities associated with feral pig removal and help inform an increasingly common management scenario in Hawai‘i and beyond that attempts to balance the desire to eradicate feral pigs with the desire to maintain them for culture and recreation.

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