Fort Lewis Butterfly Survey Final Report

The Fort Lewis Military Installation contains the majority of the highest quality prairie and oak woodlands remaining in Washington State’s South Puget Sound region. Throughout the region, less than 3% of an estimated 150,000 pre-settlement acres remain (Crawford & Hall 1997). These rare prairie lands host four species of prairie-dependent butterflies that either are listed or are under consideration for rare species listing at the state and federal levels. Emergency federal listing under the Endangered Species Act has been petitioned for two of the butterflies, the Mardon Skipper (Polites mardon) and Taylor’s Checkerspot (Euphydryas editha taylori). The Puget Blue (Icaricia icarioides blackmorei) and Zerene Fritillary (Speyeria zerene) butterflies are currently candidates for listing at the state level.

The Nature Conservancy (TNC) undertook an extensive butterfly survey of 13 Fort Lewis prairies during the spring and summer of 2003 that targeted the above-mentioned species and also recorded all other species encountered. The goal of the survey was to determine the current distribution of prairie-dependent butterflies on Fort Lewis. Thirty-two of the 48 butterfly species observed on South Puget Sound prairies were observed on Fort Lewis during this survey. Many butterflies are located on various prairies throughout the installation, but the most abundant and diverse populations are found in protected Research Natural Areas (RNAs) (TA15, Johnson Prairie, and the Weir Prairies) and the surrounding edge of the Artillery Impact Area (AIA). The four target species were found almost exclusively in these areas.

The results of this survey expand the known range of three of the four target species. The Mardon Skipper was found to inhabit more of the AIA the previously recorded, while the Puget Blue and Zerene Fritillaries were each found on three prairies in addition to those they have recently been known to inhabit. Despite frequent and extensive surveys of historic habitat on Fort Lewis, the Taylor’s Checkerspot was observed only at its known location in the AIA.