Impacts of insect and microbe on plant species diversity and distribution

I am captivated by the phenomenon that most species in a community are rare and rare species are able to peacefully coexist with abundant species to make Earth a diverse planet. What mechanisms cause the rarity and commonness and why rare species can persist without being competitively excluded by common species?

In forest ecosystems, specialized nature enemies such as insect herbivore and fungal pathogens can aggregate in areas with high conspecific plant density, leading to elevated seedling mortality—a phenomenon known as negative density dependence. This suggests that when a plant species becomes abundant, enemy pressures increase, acting as a check to prevent it from excluding other species. Conversely, when a plant species becomes rare, enemy pressures decrease to avoid its extinction. Through this mechanism, nature enemies contribute to the coexistence of plant species.

Recent studies have revealed that not all plant species exhibit this enemy-driven negative density dependence. This raises new questions about the interspecific variation in the enemy-induced negative density dependence and its applications for species coexistence. To explore this question, I conducted a field survey of herbivory pressure of saplings in three stem-mapped forest plots in China.

Our findings indicated that herbivore pressures increased with conspecific plant density for rarer plant species, indicating the presence of density-dependent herbivory. On the other hand, common species experienced a negative relationship between herbivore pressure and conspecific density, potentially reflecting herbivore limitations (i.e., herbivores reducing conspecific plant density). This discovery implied that density-dependent herbivores may restrict species abundance, leading to species rarity, while herbivore limitations suppress the predominance of common species and prevent species exclusion.