Fungal spillover influences plant competitive ability and affects species’ coexistence

Tropical forests are the most biodiverse ecosystems in the world and are critical to global ecosystem functions. Anthropogenic activities have increased forest fragmentation, which could introduce new biotic interactions into tropical forests via spillover, yet little is known about how spillover influences tropical tree diversity. Existing spillover theory has mainly focused on transmission dynamics rather than plant species coexistence. We developed a theoretical spatial model consisting of one predator (fungi) interacting with two prey (plant) populations and introduced a new fungal pathogen via diffusion to mimic spillover. We aimed to understand how spillover pathogen host-specificity, spillover intensity, and plant competitive abilities alter plant coexistence outcomes. Results show that spillover, contingent on plant species’ competitive ability, can either promote or disrupt coexistence. While spillover disrupts plant existence at the forest edge, generalist spillover acts as an equalizing mechanism, enabling plants with different competitive abilities to coexist. In contrast, specialized spillover acts as a stabilizing mechanism when targeting the strong plant competitor, eventually reversing plant apparent competitive abilities. These effects are heightened with wind. Our model provides insight on the influences of fungal pathogen spillover on plant community composition and diversity when the effects of fungal spillover in field are unknown.