Competition between a native freshwater crab and an invasive crayfish in tropical Southeast Asia

The freshwater crayfish, Cherax quadricarinatus, has been introduced across many tropical regions globally. This popular aquarium and aquaculture species, similar to many other non-native crayfish, is widely establish and can negatively affect native populations and ecosystems. Studies quantifying the impacts of C. quadricarinatus, however, are notably lacking. This study aims to address this paucity through investigations of potential interspecific competition in the tropical Southeast Asia. Owing to the lack of native crayfish within Sundaic Southeast Asia, we focus on the interactions between C. quadricarinatus and Parathelphusa maculata, a tropical lowland freshwater crab. The native crab is a functional equivalent freshwater decapod crustacean that is found in forest streams with the non-native crayfish, and interaction between the two species can possibly result in population level changes. As such, we quantify the factors (i.e. aggressiveness, size, and species combinations) that are associated competitive outcomes (for shelter) via ex situ experiments; and utilize in situ surveys to determine if these factors can influence the distribution of both species in streams where they co-occur. Results indicate that an asymmetry resulting from large C. quadricarinatus being able to effectively outcompete smaller P. maculata can explain the negative correlation between the size of the two species in situ. This is congruent with the relative distribution patterns of the two species, and suggests that healthy populations of P. maculata (i.e., with a wider range of sizes, including larger individuals) can prevent/slow the spread of the alien crayfish in recipient habitats. Findings from this study represent an important first step towards understanding and managing the impacts of this increasingly popular and worrisome tropical non-native species.