One persistent explanation for plant species coexistence in tropical forests, the Janzen-Connell mechanism, postulates that density-dependent mortality mediated by natural plant enemies, such as fungal pathogens and insect herbivores, puts locally rare plant species at an advantage, preventing any one species from dominating. Evidence from vegetation plots suggests that this form of density-dependence plays a key role in the maintenance of plant diversity in the tropics.
Still, most theories explaining species coexistence emphasize on local processes, while one of the clearest and best-documented patterns in plant diversity is the strong correlation between diversity and humidity at regional to global scales. While differential plant drought sensitivity is an important determinant of changes in plant species composition along humidity gradients, alternative mechanisms are needed to explain positive diversity-humidity correlations.
To investigate how Janzen-Connell effects vary with humidity, we monitored seed arrival and seedling recruitment for 18 months at eight forest sites spanning a steep natural precipitation gradient across the Isthmus of Panama (1756 – 3203mm). In each of the 1ha forest plots, seeds were collected from fifty 1m2 seed traps every fortnight, and seventy-five 1m2 seedling plots that were treated with either a fungicide, an insecticide or used as a control (sprayed with equal amounts of water). All seeds and seedlings were identified by experts to the lowest taxonomical level, and seedling dynamics were monitored using four consecutive seedling censuses, every six months.
Results show that seedling recruitment was negatively density-dependent in control plots and that the strength of the negative density dependent effect increased along the precipitation gradient. This density-dependence was greatly reduced in the pesticide-treated plots. The insecticide treatment also largely eliminated the increase in diversity observed in control plots when comparing seedlings to seeds. The positive effects of insect herbivory on plant diversity increased significantly with rainfall.
Our results suggest a mechanistic explanation for the well-documented positive relationships between humidity and (i) the strength of negative density dependence and (ii) overall tropical forest plant diversity, where fungal pathogens, and especially insect herbivores operate as important mediators.