Thaipotamon chulabhorn (Mealy Crab) is a freshwater crab, listed as ‘Least Concerned’ in the IUCN Red List of Threatened Species. Nonetheless, this crab is exclusively found in one small area of the Dun Lumpun Forest, Thailand, and currently faces high exposure to environmental changes. This study aims to investigate key dynamic factors that influence vulnerability to environmental changes of the Mealy Crab. The study was conducted between 15 March and 23 October 2016, covering the local summer until the end of the rainy season and being influenced by strong El Niño, thus suggesting drought episodes. This vulnerability assessment was carried out through crab population records from 1997 to 2016, field measurements/surveys, and interviews with experienced park rangers. The results revealed that drought is the major vulnerability factor that is threatening the population of the crabs and their livelihoods (7.96 out of 10), followed by an increased number of their natural enemies (7.41), variations in groundwater level (6.11), changes in groundwater quality (4.63), changes in forest soil (4.63) and human intervention (4.26). Since 1996, human intervention has been found to have a little direct impact on the crab population due to the restricted access to forest resources. The 2016 drought was accompanied by anomalously low rainfall and the early onset of the rainy season, contributing to an earlier and shorter mating period of the crabs. Nonetheless, a consistently increased number of the Mealy Crab population indicated that they are not significantly vulnerable to a larger number of their natural enemies, and even to the changes in groundwater and forest soil. In conclusion, the crab population was increasing along with changes in the habitat and climate and its growth cycle was unusual during the drought.