Pathogenic interactions between fungi and plants facilitate plant species
coexistence and tropical rainforest diversity. Such interactions, however,
may be affected by forest fragmentation as fungi are susceptible to anthropo-
genic disturbance. To examine how fragmentation affects fungus-induced
seed and seedling mortality, we sowed seeds of six plant species in soils
collected from 21 forest fragments. We compared seedling establishment
in unmanipulated soils to soils treated with fungicides. Fungicides increased
germination of Toona ciliata seeds and decreased mortality of Syzygium
rubicundum and Olea dioica seedlings. The fungus-induced mortality of one
of these species, S. rubicundum, decreased with decreasing fragment
size, indicating that its interactions with pathogenic fungi may weaken as
fragments become smaller. We provide evidence that a potential diversity-
maintaining plant–fungus interaction weakens in small forest fragments
and suggest that such disruptions may have important long-term conse-
quences for plant diversity. However, we emphasize the need for further
research across rainforest plant communities to better understand the
future of diversity in fragmented rainforest landscapes.