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.

Description of methods used for collection/generation of data

Since 1987, weekly censuses of seed rain have been conducted within the 50-ha plot as part of a research project coordinated by S. J. Wright. A first set of seed traps (n = 200) was established in 1987 and has been continuously monitored since then. These seed traps are located at 13.5 m intervals on alternating sides of and 4-10 m from 2.7 km of pre-existing trails within the 50-ha plot. Another 50 traps were added in naturally occurring tree fall gaps within the 50-ha plot between 2002 and 2004 and have been monitored since. A final set of 200 traps were established 2 m from each of the 200 original traps in 2011 and were censused for 23 months. Each trap consists of a 0.8 m tall PVC frame and 1 mm mesh covering 0.5 m2 of forest floor. In weekly censuses, all fruits and seeds encountered in each trap are identified to species, counted, and categorised as mature or immature. In some species, late flowers included flowers with swollen ovules. These late flowers were categorised separately and excluded from this study. Fruits labelled as ‘immature’ were dropped several weeks to months later. The distinction between immature and mature fruits was based on the examination of endocarps: If the endocarp of seeds within a fruit is filled with material that is solid (rather than hollow or filled with liquid or soft substances), the fruit was considered mature. Our species-specific estimates of rates of premature seed abscission are based on the relative frequencies of mature and immature fruits. Since we did not conduct systematic germination trials across the full range of species, we cannot rule out the possibility that some of the seeds scored as immature would be able to germinate. We are still confident that in the vast majority of cases, the seeds inside fruits scored as immature are indeed inviable (given the timing at which the bulk of premature fruit drop happens in many species) and that premature fruit drop typically translates to seed mortality in the studied plant community. 

Methods for processing the data

In this study, we used information on fruits and seeds from woody plant species (trees, shrubs, and lianas) recorded in the traps during the period 1988 to 2018 (31 full calendar years). All analyses and data preparation were conducted in R v 4.0.5. Counts of fruit in the traps were multiplied by species-specific average seed-to-fruit ratios (S. J. Wright; unpublished data) and added to the number of seeds. For each species, the weekly counts (n = 1615 censuses) of mature and immature seeds were then summed across all seed traps. This was done separately for each full calendar year in the dataset. Since 90% of species fruit only once during a calendar year, and since few species fruit in the December-January period, this approach will typically generate data on overall fruit drop rates for individual fruiting events. Not all species were encountered in every single year, but in total there were 5,295 unique species × year combinations in the original dataset. Only species × year combinations with fewer than 10 fruits or individual seeds were analysed for the publication. Species names were matched to a static copy of The World Flora Online (http://www.worldfloraonline.org) v.2021.01 valid at 2021-12-09 using the function "WFO.match" from the R package {WorldFlora}. The function "WFO.one" was then used to find one unique matching name for each submitted name. using the argument 'priority = "Accepted"', it first limits candidates to accepted names, with a possible second step of eliminating accepted names that are synonyms. Note that the species with the 4-letter ID code GUA1 is listed as a synonym of Guarea guidonia in The World Flora Online database. This is incorrect. The former Guarea grandifolia (GUA1) and Guarea guidonia (GUA2) are distinct species. Pennington (1981) provided the name Guarea megantha for GUA1, which we adopt here.

fruit_drop.csv

Number of variables: 11

Number of cases/rows: 5295

Variable List: