Application of harmonic radar technology to monitor tree snail dispersal

Abstract. Planned conservation efforts for tree snails of the endangered genus Achatinella, endemic to the island of O'ahu, Hawai'i, will include translocations among the remaining wild and captive‐bred populations. In order to establish optimal levels of artificial migration among neighboring groups of snails within fragmented populations, efforts to determine natural dispersal rates through direct observation were initiated. Capture–mark–recapture (CMR) efforts have proved inadequate for obtaining the requisite dispersal estimates, due to low recapture probabilities. In addition, snail dispersal beyond the boundaries of a finite CMR study site was indistinguishable from mortality. In the preliminary study reported here, both the low recapture probability and dispersal detection problems of past CMR efforts were addressed by using harmonic radar tracking. This approach yielded rough dispersal estimates that were unattainable using CMR alone by providing 100% recapture rates even beyond the normal survey area boundaries. Extensive snail movements within clusters of connected trees were frequently observed after tracking for merely a few hours, although movements between unconnected trees were rare and recorded only after monthly survey intervals. Just 11 out of 40 tracked snails made between‐tree movements (average distance of 4.94±1.52 m) during the entire 7‐month study, and provided the only data utilizable for inferring gene flow in and out of subpopulations. Meteorological data loggers were deployed when tracking began to look for an association between such snail movement and weather fluctuations. The resultant data indicate that increases in both wind gusts and humidity facilitate dispersal (R²=0.77, p‐value <0.001), and that passive wind dispersal alone may be responsible for many snail movements (R²=0.59, p‐value=0.0014). Despite having provided coarse estimates of short‐term dispersal and corresponding wind influences, the limitations of the radar method can be substantial.