Using Janzen–Connell to predict the consequences of defaunation and other disturbances of tropical forests

Background

The Janzen–Connell (J–C) model describes how seed dispersal and mortality shape tropical forest diversity. According to the model, seeds that fall close to their parent trees suffer high mortality due to host-specific predators, pathogens, and herbivores. In contrast, seeds dispersed farther away have a better chance of survival. Defaunation, particularly the loss of large-bodied seed dispersers, disrupts this balance by reducing seed dispersal distances. This study applies the J–C model to predict how defaunation and other disturbances affect tree recruitment and forest composition.

Goals and Methods

The study aims to assess how defaunation influences seed dispersal, seedling recruitment, and tree diversity through the lens of the Janzen–Connell (J–C) model. A key focus is on understanding how the interplay between seed dispersal and escape from natural enemies shapes tree recruitment. The researchers also investigate the extent to which hunting and habitat fragmentation disrupt seed dispersal by eliminating large frugivores, which play a crucial role in maintaining forest composition. To analyze these impacts, the study compares defaunated forests with intact forests in Amazonian Peru, examining shifts in forest structure and species diversity resulting from the loss of key animal species.

Conclusion

The findings confirm that defaunation leads to reduced seed dispersal, lower tree recruitment, and shifts in species composition. While some compensatory mechanisms exist (e.g., smaller frugivores replacing large ones), they are often insufficient to maintain pre-disturbance seed dispersal patterns. Consequently, large-seeded tree species decline, and wind-dispersed or small-seeded species become more dominant. These changes can have long-term consequences for forest structure and biodiversity.