Spatial and temporal dynamics and effects of feral pigs (sus scrofa) on enterococci in soil and runoff of a forested Hawaiian watershed

Abstract

Feral pigs (Sus scrofa) are well distributed across Hawaiian watersheds and have adverse effects on native vegetation, soils, and fecal bacteria. While effects of feral pigs on TSS, runoff volume, and fecal bacteria have been documented previously, the spatial distribution and transport of fecal bacteria across watersheds is much less understood. This research investigated the effect of feral pigs on the fecal indicator bacteria (FIB) Enterococci (ENT) in soils and runoff in the Mānoa watershed by: (1) monitoring the activity of pigs with game cameras; (2) assessing the spatial and temporal distribution of ENT in soil and runoff; (3) determining the effect of feral pigs on runoff volume, total suspended solids (TSS) in runoff, and ENT in soil and runoff; and (4) identifying correlations among environmental variables and ENT. A pig activity index was created with data from game cameras at each of seven paired fenced/unfenced plots over the study period. Runoff and soil samples were collected quarterly from each of the seven-paired fenced/unfenced plots between January 2010 and May 2011. The spatial variability of soil ENT was assessed by collecting and analyzing 100 different soil samples in summer 2011 from across the upper-forested headwaters of the Mānoa watershed. Game cameras documented feral pigs at four of seven sites. High spatial and temporal variability was observed in runoff and soil ENT at the seven study sites. The mean ENT level in runoff for the study period was 4,060 MPN 100 mL-1, which greatly exceeded the EPA surface water standard of 33 MPN 100 mL-1. There was also a significant correlation between ENT in runoff and soil ENT, as well as between the pig activity index and soil ENT. Across the upper watershed, there was considerable spatial variability in soil ENT levels in summer 2011. The highest values were observed from sites adjacent to the Mānoa Cliffs and Lyon Arboretum trails. The lowest values were observed from sites adjacent to the Mānoa Falls trail. The Mānoa Falls trail gets much more human traffic than the other two trails which may partially explain these results. Spatial soil ENT values were not correlated with soil temperature, slope, or elevation, but had a weak correlation with soil series.