Multispectral UAV-Based Monitoring of Cassytha Filiformis Invasion in Xisha Islands

Currently, numerous studies have reported that the invasion of Cassytha filiformis has affected both above and below ground communities, resulting in difficulties in the growth of original vegetation. Meanwhile, Cassytha filiformis was observed on the Xisha Islands in recent years which brings up the importance of monitoring its invasion to protect the biodiversity of the island. Nonetheless, to effectively monitor Cassytha filiformis at finer regional scales, there is a pressing need for centimeter-level resolution, a level of precision that current satellite sensors find challenging to attain in a consistent manner. Therefore, we adopted a DJI Phantom 4 unmanned aerial vehicle with five multispectral bands and centimeter-level spatial resolution to overcome this problem. An advanced deep learning network is employed to identify the invasion in Xisha Islands for three different time periods. Results show that the area of Cassytha filiformis on Bei Island increased from 211.8 m(2) in April 2020 to 458.6 m(2) in April 2021, and dropped to 112.8 m(2) in July 2021, while that on Ganquan Island changed from 1996.9 m(2) in April 2021 (dry season) to 1275.9 m(2) in July 2021 (wet season). By incorporating climatic indicators, we further found that Cassytha filiformis in both Bei Island and Ganquan Island favors dry climate and its large area invasion in 2021 was possibly caused by a drought event.