The Janzen-Connell hypothesis is one of the most important hypotheses in ecology, but is mostly tested indirectly without accounting for the underlying plant-associated organisms, or only for highly host-specific organisms. Advances in massive sequencing allow to sample, for example, the fungi communities associated with different plant species, and spatial analysis can reveal spatial patterns in the number of organisms (species) shared by plant species and their neighbours. We show how combining these tools provides new perspectives for testing the Janzen Connell hypothesis. We illustrate our approach using a natural experiment in two fully-mapped Mediterranean forest plots, where the dominant dry- and fleshy-fruited species have distinctly different seed deposition patterns, leading to contrasting expectation about the emerging spatial structures. Our analysis confirmed these expectations and provided deep insights into how the neighbourhood load of plant-associated fungi and herbivorous insects changes during plant ontogeny and how seed dispersal mechanisms modulates Janzen-Connell effects.

Plant species bearing extrafloral nectaries (EFNs) may indirectly influence other plant species by sharing protective ants, potentially altering plant herbivory levels. However, the propagation of indirect effects in this type of mutualism has seldom been investigated. We investigated indirect effects via ant sharing among twenty-one EFN-bearing plant species of the tribe Bignonieae distributed on 28 plots in the central Brazilian Amazon. Using an ecological network index that quantifies potential indirect effects among plant species via ant sharing, we distinguished plant species that could most strongly affect and most strongly be affected by other species via shared dominant and subordinate ant species. These Bignonieae species differ markedly in attractiveness to ants, and we investigated how these differences influenced the indirect effects between plant species. We also tested whether plant species with a higher potential to influence ant visitation to other plants experienced less herbivory due to attraction of more aggressive/dominant ants. We found that the most attractive plant species (here termed the promoter species) had the highest potential to indirectly affect ant visitation to less attractive plant species (here termed the receptor species) in the community, mainly via changes in attraction of dominant ant species. However, the potential indirect effects among plants did not translate into herbivory patterns. We also found that ant attendance and herbivory did not differ among promoter species, their neighbours, and non-neighbouring plants. We conclude that, unlike patterns found in studies investigating indirect effects in other mutualisms involving plants (e.g., pollination), the consequences of indirect effects among plant species are limited in this ant-plant network. This pattern could be explained by generally low herbivory levels and the small foraging areas of shared ant defenders. Indirect interactions and their effects between plant species that share protective ants appear in this system to confer limited costs and benefits.