Rainforests harbor remarkable plant species richness and understanding the source of rainforest diversity is a long-standing question in plant-community ecology. The Janzen-Connell hypothesis proposes that rainforest diversity could be maintained by the selective inhibition of seedlings that germinate near conspecific neighbors. Because common species have a higher probability of geminating near conspecifics, this mechanism stops common species from coming to dominate a community, whilst simultaneously providing rare species with an advantage and preventing their local extinction. 

Recent observations of experimentally manipulated plant communities have implicated microorganisms, specifically fungi, as agents that selectively inhibit the recruitment of common species. However, the microbial communities themselves remain largely unexplored.We report the first investigation into microbial communities associated with rainforest tree species and identify patterns in bacterial, fungal and oomycete community structure. The microbial community data was collated with extensive (spanning fifty years) plant community data collected from this study site to establish whether microbial communities have a role in the maintenance of rainforest plant diversity.

We demonstrate preliminary evidence that both bacterial and fungal factions of the community may have a role to play in the maintenance of rainforest tree diversity. We also demonstrate that major abiotic drivers of community structure act differently upon fungal and bacterial factions. Microorganisms likely to be responsible for the selective inhibition of common species leading to the maintenance of rainforest diversity are currently being identified through next gen sequencing.

Understanding how rainforests can stably support high levels species diversity, and the effect of changing environmental factors upon these communities, is central to our ability to conserve these ecosystems. The question of how diversity is maintained may also contribute to our ability to approach conservation at the level of ecosystems and help abate the flood of extinctions that are occurring at every trophic level.