Rates of premature fruit drop for 201 plant species on Barro Colorado Island, Panama

Pre-dispersal seed mortality caused by premature fruit drop is a
potentially important source of plant mortality, but one which has rarely
been studied in the context of tropical forest plants. Of particular
interest is premature fruit drop triggered by enemies, which – if
density-dependent – could contribute to species co-existence in tropical
forest plant communities.  We used a long-term (31 year) dataset
on seed and fruit fall obtained through weekly collections from a network
of seed traps in a lowland tropical forest (Barro Colorado Island, Panama)
to estimate the proportion of seeds prematurely abscised for 201 woody
plant species. To determine whether enemy attack might contribute to
premature fruit drop we tested whether plant species abscise more of their
fruit prematurely if they: (1) have attributes hypothesised to be
associated with high levels of enemy attack, and (2) are known to be
attacked by one enemy-group (insect seed predators). We also tested (3)
whether mean rates of premature fruit drop for plant species are
phylogenetically conserved. Overall rates of premature fruit drop were
high in the plant community. Across all species, 39% of seeds were
abscised before completing their development. Rates of premature seed
abscission varied considerably among species and could not be explained by
phylogeny. Premature seed abscission rates were higher in species which
are known to host pre-dispersal insect seed predators and species with
attributes that were hypothesised to make them more susceptible to attack
by pre-dispersal enemies, namely species which (1) have larger seeds, (2)
have a greater average height, (3) have temporally predictable fruiting
patterns, and (4) are more abundant at the study site. Synthesis.
Premature fruit drop is likely to be a major source of seed mortality for
many plant species on Barro Colorado Island. It is plausible that
pre-dispersal seed enemies, such as insect seed predators, contribute to
community-level patterns of premature fruit drop and have the potential to
mediate species co-existence through stabilising negative density
dependence. Our study suggests that the role of pre-dispersal enemies in
structuring tropical plant communities should be considered alongside the
more commonly studied post-dispersal seed and seedling enemies.