How insects and microbes shape plant diversity and distributions

Tue, 18 Apr 2023 00:00:00 +0000

Most plant communities contain a few common species and many rare ones. I am interested in why dominant plants do not exclude their rarer neighbours, and how interactions with insects and microbes help determine which species become common, which remain rare and which can persist together.

One explanation is that a plant’s natural enemies catch up with it as it becomes locally abundant. Herbivores and pathogens can accumulate around dense populations of the same host, reducing plant performance and creating opportunities for other species. This process, known as conspecific negative density dependence, can stabilize coexistence. However, its strength varies greatly among plant species, suggesting that natural enemies do more than impose the same density-dependent cost on every host.

Herbivores and the persistence of rare species

In ongoing work, I examine how density-dependent herbivory differs between rare and common plants. Across tropical, subtropical and temperate forest plots in China, I measured leaf damage on 195,196 leaves from 3,479 saplings representing 60 species. To estimate the area removed by herbivores consistently, I used species-specific deep-learning models to reconstruct damaged leaves and manually validated the results.

For rarer species, herbivore damage increased as the density of nearby conspecific plants increased. A similar pattern occurred when neighbours were close relatives. These relationships weakened as species became more abundant and could reverse among common species. The results suggest that rare species receive relative release from herbivores when they occur at low densities, while common species may remain constrained even when locally sparse. This contrast offers a possible explanation for why density dependence is often stronger for rare than for common species and how herbivores can help stabilize forest diversity.

Microbial feedbacks above and below ground

The same coexistence question extends below ground. In shade-house and field experiments in a subtropical forest, we found that rare tree species grew better in soils associated with distantly related neighbours, but performed poorly in soils associated with their own species or close relatives. Common species showed a different response. These phylogenetically structured plant–soil feedbacks help explain how rare species can persist when surrounded by diverse, distantly related neighbours.

In a complementary greenhouse experiment, we manipulated leaf and soil microbial communities across plant communities containing different numbers of species. Leaf microbial inoculation reduced productivity most strongly in low-diversity communities, producing a positive relationship between plant diversity and productivity. Soil inoculation counteracted this effect at low diversity, partly through changes in arbuscular mycorrhizal fungi. The study shows that the benefits of plant diversity depend on interactions between aboveground and belowground microbiota, rather than on either group alone. Read the study in Plant and Soil.

Together, these studies show that insects and microbes are not uniformly harmful or beneficial. Their effects depend on host density, evolutionary relatedness and community diversity. By linking leaf damage and microbial feedbacks to plant performance, this work aims to explain how local biotic interactions scale up to shape species coexistence, community productivity and the distribution of plant diversity.

Biodiversity–ecosystem functioning | Zihui Wang

How insects and microbes shape plant diversity and distributions

Herbivores and the persistence of rare species

Microbial feedbacks above and below ground