Invasive earthworm damage predicts occupancy of a threatened forest fern: Implications for conservation and management

Adequate detection and monitoring of threatened, endangered, and sensitive species presents a challenge to forest managers seeking to balance management activities with conservation and forest health. This is especially true for rare, cryptic plant species that are difficult to detect, like goblin fern (Botrychium mormo), which is small and does not emerge from the duff layer of the rich hardwood forests it inhabits every year, even when present. Imperfect detection of this species makes it difficult to monitor, because lack of plants detected at a specific site does not necessarily indicate that the species has been extirpated there. In this study, 80 historic locations of B. mormo were surveyed for occurrence over three consecutive years to assess probability of occupancy and environmental factors expected to impact occupancy, including earthworm damage and canopy closure, while accounting for detectability. We found that probability of occurrence is most strongly related to earthworm damage and were able to identify levels of earthworm damage at which the species is more likely to remain present or be extirpated. These results suggest that use of a simple metric for quantifying ecological impact of earthworm damage can be used during monitoring to assess the likelihood that B. mormo is still present. With this information, forest managers can prioritize sites for habitat preservation and better shape policy and management decisions to protect and enhance habitat for this species. In addition, our study demonstrates the utility of occupancy modeling for management and conservation of rare and elusive plant species.