Nonnative feral pigs (Sus scrofa) are recognized throughout the New World as one of the most destructive, introduced ecosystem engineers in the region. Conversely, soil macroinvertebrates are often given less prominence in conservation management despite their ability to invade and significantly alter both the structure and function of native ecosystems. The ecological relationships between feral pigs and soil macroinvertebrates remain largely undescribed despite soil macroinvertebrates serving as key protein sources for feral pigs. Bridging this knowledge gap is particularly important when considering the widespread invasions by both taxa. Our goal was to address this knowledge gap by analyzing the relationship between nonnative feral pig removal and soil macroinvertebrates using paired sites located inside and outside of five feral pig removal units representing a highly constrained ~25 year chronosequence of removal in tropical montane wet forests located on Hawai‘i Island. To discern changes over time following feral pig removal, soil macroinvertebrates were sampled for abundance and biomass across the chronosequence. Additionally, plots were established to specifically examine the impacts of active rooting, active trampling, and no recent feral pig activity despite the presence of pigs for comparison of specific behaviors and over time.
Results/Conclusions
In total, we uncovered 13 families of soil macroinvertebrates representing 11 orders. Across the chronosequence, the abundance and biomass of earthworms was negatively related to time since feral pig removal. Plots with active trampling showed decreases in all soil macroinvertebrate abundance and biomass. Conversely, plots with active rooting by feral pigs exhibited increases in both the abundance and biomass of earthworms and ground beetles. Combined with prior literature, our results suggest a positive feedback loop (i.e., a facilitative relationship) exists whereby the earthworms and feral pigs contribute to improved habitat conditions for each other. Evidence is most compelling in plots where feral pig rooting occurred because feral pigs consume earthworm biomass while rooting, which contradicts the higher biomass resultant at these plots. This study provides an excellent model for understanding facilitative relationships between nonnative species under the ‘invasional meltdown’ hypothesis. Furthermore, this study addresses the potential for a positive feedback loop between feral pigs and earthworms that, if true throughout their ranges, has important implications for the management of both taxa. This information is critical to habitat restoration because most prior studies addressing the relationship between earthworms and feral pigs have focused on agricultural sites rather than unmanaged natural habitats.