Multi-scale habitat characterization of West Maui Amastra spp. via vegetation surveys and UAV imagery

Abstract

Amastridae, the only endemic Hawaiian land snail (kāhuli) family, once comprised over 300 species, but only ~22 are known today. These molluscs experience extremely high extinction risk via habitat loss, invasive species, and climate change, accentuating the urgency for addressing Hawaiian ground snail knowledge gaps. This research focuses on Amastra species from West Maui, where recent surveys by the Maui Nui Snail Extinction Prevention Program have identified some of the last, wild populations occurring at a landscape scale. This project characterizes the habitat of Amastra species using two spatial scales: local-scale vegetation surveys and landscape-scale unmanned aerial vehicle or UAV imagery, the latter is the first time UAV imagery has been used for Hawaiʻi terrestrial invertebrate conservation. We hypothesized that Amastra-present areas will exhibit distinct vegetation compared to unoccupied areas and that vegetation patterns will be consistent between local-scale surveys and landscape-scale imagery. For vegetation surveys, we found a statistically higher percent cover of Freycinetia arborea (ʻieʻie) in Amastra-present versus absent areas, confirming a documented habitat association. Additionally, Amastra-absent areas had statistically higher percent cover of all-ferns, which are known to alter calcium availability – a significant macronutrient for land snails. For UAV imagery, we created Structure-from-Motion 2D and 3D outputs, and classified vegetation for F. arborea and all-ferns cover. A novel Amastra Habitat Index (AHI = estimated F. arborea percent cover - estimated all-ferns percent cover) was developed and applied to the entire study area. Amastra Habitat Index values were statistically higher in Amastra-present versus absent areas, relating and expanding local-scale vegetation patterns. Multi-scale habitat characterization where Amastra spp. persist will inform future surveys and identify potential predator proof exclosures (kīpuka kāhuli), thereby advancing native Hawaiian land snail conservation.