Extracting useful data from imperfect monitoring schemes: endangered butterflies at San Bruno Mountain, San Mateo County, California (1982-2000) and implications for habitat management.
Managers surveyed for sensitive butterfly species in the San Bruno Mountain Habitat Conservation Plan area between 1982 and 2000 using an opportunistic "wandering transect" method. To extract as much valuable information as possible from the data collected by this method we analyzed patterns of surveys and butterfly presence and absence within 250 m square cells gridded across the area within a Geographic Information System. While estimates of butterfly abundance were not possible, the data could be tested for trends in butterfly occupancy. For those cells surveyed during at least 10 years, no trends in the total number of occupied cells was evident for either Callippe silverspot or mission blue butterfly. There were cells, however, that showed positive or negative trends (P<0.20) in occupancy for each species (Callippe silverspot: 14 positive, 15 negative, 6 cells occupied all years; mission blue butterfly: 40 positive, 40 negative, 2 cells occupied all years). We conclude that for the period 1982-2000 the population of each species was stable in overall total distribution, but indicate geographic areas of concern for each, specifically the edges of the northeast ridge for Callippe silverspot butterfly and the northwest of the study area for mission blue butterfly. Vegetation composition analysis using orthophotography with field corroboration indicates that those areas with declines in occupancy for these species experienced native coastal scrub succession and a corresponding loss in grassland butterfly habitat, while positive trending and stable cells had stable grassland proportions. Habitat managers at San Bruno Mountain should therefore incorporate programs for protecting grassland butterfly habitat not only from invasive weeds but also from succession to native coastal scrub. This approach illustrates the feasibility of using occupancy as an indicator to track butterfly status in a protected area even when suboptimal data collection methods are used, but the difficulties of using these data also reinforces the need for managers to devise monitoring schemes appropriate for their objectives before implementing them.