When do Janzen–Connell effects matter? A phylogenetic meta‐analysis of conspecific negative distance and density dependence experiments

The Janzen–Connell (J‐C) hypothesis suggests that specialised natural enemies cause distance‐ or density‐dependent mortality among host plants and is regarded as an important mechanism for species coexistence. However, there remains debate about whether this phenomenon is widespread and how variation is structured across taxa and life stages. We performed the largest meta‐analysis of experimental studies conducted under natural settings to date. We found little evidence of distance‐dependent or density‐dependent mortality when grouping all types of manipulations. Our analysis also reveals very large variation in response among species, with 38.5% of species even showing positive responses to manipulations. However, we found a strong signal of distance‐dependent mortality among seedlings but not seed experiments, which we attribute to (a) seedlings sharing susceptible tissues with adults (leaves, wood, roots), (b) seedling enemies having worse dispersal than seed enemies and (c) seedlings having fewer physical and chemical defences than seeds. Both density‐ and distance‐dependent mortality showed large variation within genera and families, suggesting that J‐C effects are not strongly phylogenetically conserved. There were no clear trends with latitude, rainfall or study duration. We conclude that J‐C effects may not be as pervasive as widely thought. Understanding the variation in J‐C effects provides opportunities for new discoveries that will refine our understanding of J‐C effects and its role in species coexistence.