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6 March 2024 A Gis-Based Local Species Distribution Model and Population Monitoring of the Endangered Mission Blue Butterfly, Icaricia icarioides missionensis (Lycaenidae), at the San Francisco Peninsula Watershed
Richard A. Arnold, Robert B. Jensen
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The environmental or habitat attributes that help to explain an endangered species' occurrence, geographic range and abundance are essential for identifying effective conservation measures to protect and maintain the species. Using binary logistic regression and geographic information system (GIS) techniques, a local species distribution model (LSDM) was devised for the endangered Mission Blue butterfly, Icaricia icarioides missionensis, at the 9,316 ha San Francisco Peninsula Watershed located in San Mateo County, CA. Due to the butterfly's close association with its larval foodplants, three perennial Lupinus (Fabaceae) taxa, these foodplants were used as a surrogate to predict previously undiscovered locations where not only the foodplants but also the Mission Blue might be found. Three environmental attributes, vegetation, soil, and geology types were identified using digital GIS data layers for 22 known foodplant locations from the northern portion of the watershed. A single logistic regression analysis determined specific vegetation, soil, and geology types that were the best predictors for occurrence of the butterfly's larval foodplants. Lupinus were strongly associated (p <0.05) with seven vegetation types (California annual grassland, Baccharis pilularis-Ceanothus thyrsiflorus scrub, B. pilularis-Nassella pulchra, B. pilularis-non-native grassland, B. pilularis-Toxicodendron diversilobium scrub, disturbed, plus roads and shoulders), six soil types (Barnabe-Candlestick complex, Candlestick-Krom-Buriburi complex, Candlestick variant loam, Fagan loam, and Miramar coarse sandy loam), and eight geology types (artificial fill, Greenstone, Sandstone, sheared rock, Granitic rocks of Montara Mountain, alluvial fan and fluvial deposits, Santa Clara formation, and Serpentinite). The GIS analyses identified 1,823 target areas, measuring 2,149 ha or 25.2% of uplands scattered throughout the watershed, which shared the same combinations of the environmental attributes that characterize known foodplant locations. Furthermore, the top target areas are combinations of the aforementioned vegetation, soil, and geology types that the logistic regression analyses confirmed were the best predictors of potential foodplant occurrence. Although the top 10 combinations of these environmental attributes included only 9.5% (205 ha) of the entire target area they included 73.6% of the total foodplant area. Using information from our initial findings and adding to it in subsequent years, our field surveys ultimately identified 119 new foodplant locations with Mission Blue also observed at 61 (51.3%). Overall, Mission Blue adults were observed at 71 (50.4%) of the 141 total foodplant locations in the watershed. All 141 foodplant locations lie within the target areas. The collective area of lupine was a mere 3.45 ha, which represents < 0.04% of uplands and 1.6% of the target areas at the watershed. Even though all portions of the watershed were surveyed, due to the scarcity of Lupinus throughout the entire watershed and small area of the vast majority of lupine patches, the LSDM proved to be extremely helpful in identifying where to focus our field surveys to search for new foodplant locations. In addition, transect counts conducted throughout the entire adult flight season were used to estimate population numbers of the butterfly along a 5.3 km stretch of an unpaved service road in the northern portion of the watershed. On this stretch of service road the estimated annual adult generation size ranged from 94 to 289 ( = 192.8 ± 65.7) butterflies over a 12-year period.

Richard A. Arnold and Robert B. Jensen "A Gis-Based Local Species Distribution Model and Population Monitoring of the Endangered Mission Blue Butterfly, Icaricia icarioides missionensis (Lycaenidae), at the San Francisco Peninsula Watershed," The Journal of the Lepidopterists' Society 78(1), 1-15, (6 March 2024).
Received: 16 May 2023; Accepted: 15 September 2023; Published: 6 March 2024

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endangered species
generation sizes
habitat suitability model
transect counts
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