Non-consumptive Effects of Native Deer on Introduced Earthworm Abundance

Chronic ungulate herbivory impacts are well documented, consistently showing changes in plant community dynamics. In contrast, indirect ungulate effects on soil biota and processes are less well understood and idiosyncratic. Evidence suggests that increased deer abundance in northeastern North American forests may facilitate invasions by non-native earthworms and non-native plants through indirect non-consumptive processes. We sampled earthworm abundance using paired open and fenced plots (experimentally excluding deer) from 2008 to 2011 at 12 sites at West Point, NY and in 2013 at 21 additional sites across four states that varied in exclosure size and age since establishment. Fencing decreased earthworm abundance at West Point and in regional surveys. At West Point, negative effects of fencing on earthworm abundance decreased with soil pH and were stronger at sites dominated by native than non-native understory vegetation. Sites dominated by native vegetation had more acidic soils and lower earthworm abundance compared to sites dominated by non-native vegetation. In the regional survey, negative effects of fencing on earthworm abundance increased with time since fences were established, but effects were not affected by exclosure size or site location. We show unforeseen indirect effects of deer exclusion on earthworm populations. Results illustrate the need to account for complex interactive effects among co-occurring stressors, such as deer, earthworms, and non-native plants. Failures to account for these interactions will result in hidden treatments, will complicate interpretation of ecological experiments, and will create difficulties in designing appropriate management strategies aimed at reducing stressor effects.