Remote sensing of intertidal habitats predicts West Indian topsnail population expansion but reveals scale-dependent bias

Abstract

High-resolution imagery is lacking for much of the West Indies, impeding accurate intertidal habitat assessments and conservation planning. The West Indian topsnail, which inhabits rocky shores, is an important, regional fishery. It was overfished to extinction in Bermuda but reintroduced in 1982. In this study, we estimate potential population size through habitat mapping using high-resolution imagery and ground-based survey data. We also test the effects of image resolution and map scale on intertidal habitat assessments. The coastline of Bermuda was mapped as a linear feature (1:500) using high-resolution imagery. Topsnail population size was predicted using length of preferred habitat and population density. With the comprehensive map as ground-truth, effects of scale were assessed in two ways: supervised classification of low-resolution imagery and progressive map scale coarsening (Douglas-Peuker simplification). Bermuda’s coastline is 296 km at this map scale, 50 % of which is rocky shore. Topsnail population could expand significantly if all preferred habitat is occupied. However, image resolution and map scale drastically affect mapping robustness. Unsurprisingly, automated classifiers poorly distinguished narrow intertidal habitats. More disturbingly, coarsening map scale differentially affected habitats. Fine-scale mapping enabled by high-resolution imagery is vital for intertidal conservation planning. Limitations of low-resolution imagery and scale-dependent biases are pertinent beyond intertidal habitats. Numerous predominantly linear habitats may be especially sensitive to sea-level rise and other effects of climate change, so careful consideration of the effects of scale on habitat assessments and the use of high-resolution imagery are strongly recommended.