The development of habitat suitability models for fiddler crabs residing in subtropical tidal flats

Fiddler crabs are deposit feeders that play critical roles as ecological engineers in tidal flats. Recently, anthropogenic disturbances have led to fiddler crab habitat loss and/or degradation; therefore, we developed a mathematically derived habitat suitability index (HSI) model to aid in habitat assessments of fiddler crabs. Based on what is known about the life histories of fiddler crabs, the variables of an HSI model included submersion time, interstitial salinity, and soil 1) grain size, 2) organic matter content, and 3) heat capacity. We acquired density data from four fiddler crab species-Xeruca formosensis, Tubuca arcuata, Austruca lactea, and Gelasimus borealis-at five sites on the western coast of Taiwan to develop the HSI models. Significant positive correlations existed between the HSI output and fiddler crab density for all target species except for those X. formosensis inhabiting the topsoil layer (0-20 cm). For the deep burrowers X. formosensis, however, a positive linear relationship was documented when only those data from the 40-60 cm layers were considered and if the habitat was a sand-covered mudflat. Mapping for each species was conducted using the HSI models with the highest reliability, and four classification levels were established for the habitat suitability of each species at one of the study sites. Based on this field test of a fiddler crab HSI model, we have developed a platform for the proactive conservation of fiddler crabs inhabiting subtropical tidal flats.