The tri-spine horseshoe crab, Tachypleus tridentatus, is a threatened species that inhabits coastal areas from South to East Asia. A Conservation management system is urgently required for managing its nursery habitats, i.e., intertidal flats, especially in Japan. Habitat suitability maps are useful in drafting conservation plans; however, they have rarely been prepared for juvenile T. tridentatus. In this study, we examined the possibility of constructing robust habitat suitability models (HSMs) for juveniles based on topographical data acquired using unmanned aerial vehicles and the Structure from Motion (UAV-SfM) technique. The distribution data of the juveniles in the Tsuyazaki and Imazu intertidal flats from 2017 to 2019 were determined. The data were divided into a training dataset for HSM construction and three test datasets for model evaluation. High accuracy digital surface models were built for each region using the UAV-SfM technique. Normalized elevation was assessed by converting the topographical models that consider the tidal range in each region, and the slope was calculated based on these models. Using the training data, HSMs of the juveniles were constructed with normalized elevation and slope as the predictor variables. The HSMs were evaluated using the test data. The results showed that HSMs exhibited acceptable discrimination performance for each region. Habitat suitability maps were built for the juveniles in each region, and the suitable areas were estimated to be approximately 6.1 ha of the total 19.5 ha in Tuyazaki, and 3.7 ha of the total 7.9 ha area in Imazu. In conclusion, our findings support the usefulness of the UAV-SfM technique in constructing HSMs for juvenile T. tridentatus. The monitoring of suitable habitat areas for the juveniles using the UAV-SfM technique is expected to reduce survey costs, as it can be conducted with fewer investigators over vast intertidal zones within a short period of time.
Abstract:Intensive rainfall and frequent inundation have become a serious problem in urban areas all over the world. Climate change and heat island effect may be the cause of the phenomena. Widespread impervious pavement/surface of the ground makes things worse. In order to promote an effective river basin management in urban areas and reduce runoff, a registration system called "Safety Plan for 100mm/h-Rainfall" ("100mm/h Anshin Plan" in Japanese), a scheme for preventing and mitigating inundation caused by extremely heavy, short-term rainfall (such as 100mm/h-rainfall) was established in April 2013 by the central government in Japan. This study carried out a questionnaire survey to examine how municipalities effectively utilize the registration scheme for their watershed management. As a result, it is found that there are municipalities who have started/revised subsidizing installation of private rainwater retention/infiltration facilities in association with the registration system; however, municipalities in general are not so active in promoting runoff reduction by subsidizing private facilities. In addition, in the plans emphasizing public works for runoff reduction, public involvement is not so active, whereas in the plans devised with relatively new committees of watershed management, public involvement as well as private retention activities tend to be active. Based on the results, prospects of how a safety plan should be utilized in an urban watershed are discussed and examined from practicality's point of view.
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