Disturbance intensity overwhelms propagule pressure and litter resource in controlling the success of Pontoscolex corethrurus invasion in the tropics

The mechanisms by which exotic earthworms invade new habitats are largely unknown. Previous research concluded that factors including habitat disturbance, propagule pressure and variation in the litter resource may significantly drive the establishment of exotic earthworms, however, these studies did not conduct manipulative experiments to compare the effects of these three factors. To fill this knowledge gap, we established a moderate disturbance experiment and a severe disturbance experiment in a tropical natural forest (Xishuangbanna, China) with the exotic earthworm Pontoscolex corethrurus. The former mesocosm experiment manipulated disturbance (no disturbance vs moderate disturbance) and propagule pressure as the independent factors, while the latter mesocosm experiment manipulated propagule pressure and litter resource (difference in quality and diversity) as the independent factors that affect the establishment of P. corethrurus. In the moderate disturbance experiment: disturbance and propagule pressure did not affect the final population size of P. corethrurus, i.e., populations in all treatments declined to the same size at the end of the experiment. In the severe disturbance experiment: the population size of P. corethrurus increased in all treatments, with larger population size observed under the treatments with higher propagule pressure; litter treatments with a higher C/N ratio slightly increased the population biomass. In conclusion, our results showed that disturbance (severe vs no or moderate disturbance) overwhelmed the effects of propagule pressure and litter resource on the establishment of P. corethrurus. We suggest that the protection of tropical natural forests from severe disturbance, e.g., slash-and-burn deforestation, is the key means to prevent earthworm invasion.