Effects of vertebrate seed dispersers, insect seed predators, and pathogens in seed survival at the pre-dispersal stage of several tropical woody plants

Background/Question/Methods The importance of vertebrates, insects, and pathogens in plant reproduction, particularly during fruit development and seed dispersal, is well known, however their absolute and relative effects in multiple coexisting tropical plant species has not been explicitly determined. There have been few manipulative experiments at the pre-dispersal stage due to the difficulty of studying processes occurring in the tree canopy. In this study, we quantify the total and relative effects of vertebrate seed dispersers, insect seed predators, and fungal pathogens on seed survival and seed removal at the predispersal stage within and among multiple woody plant species in Central Panama. The main effects of each of these organisms at the predispersal stage are investigated through experimental exclusion within tree canopies using vertebrate exclosures, insecticides, and fungicides from a total of 20 canopy species in two different forests accessed with canopy cranes. Seed removal is measured through counts of marked fruit on branches; removal of immature fruit is assumed to result in seed mortality, whereas removal of mature fruit results in seed dispersal. Predispersal seed mortality due to different agents is quantified through comparisons of treatments and controls in seed viability (assessed via germination trials in a growing house).

Results/Conclusions Removal of insect seed predators significantly increased seed survival compared to controls and other treatments. Seed removal was low overall for species in one forest, where vertebrate seed disperser abundance has been reduced by habitat fragmentation and hunting. Few studies to date have experimentally determined the combined effects of vertebrate seed dispersers, insect seed predators, and pathogens at the pre-dispersal stage. Determining which of these organisms has the greatest effect on plant communities is important in determining the mechanisms controlling plant population dynamics and how plant communities may respond to environmental change. Vertebrate seed dispersal, insect seed predation, and fungal diseases are expected to change in the face of increasing anthropogenic pressures on ecosystems, including hunting, deforestation, and global warming.