Discipline: Ecology Environmental and Earth Sciences
Macy Ricketts - Montana Tech of the University of Montana
Post-dispersal seed predation is a key factor in the spacial distribution of plants. Studies performed by the Ecology of Bird Loss project based in Guam and the Northern Mariana Islands indicate that seed predation plays a role in the germination rates of seedlings within gaps of the forest canopy. In the Mariana Islands, the island of Guam provides a unique opportunity to study seed predation within an ecosystem that lacks an apex predator, as birds have been nearly extinct on the islands since the introduction of the brown tree snake. Studies have also shown that birds in the Mariana Islands serve as primary seed dispersers, and that Guam, lacking seed dispersers, has increased gaps in the canopy of its karst limestone forests. As such, in this study, a comparative study of seed predation between the islands of Guam and Saipan was performed, in which seed removal in gaps and closed canopies within the karst limestone forests were compared. Bare seeds were used instead of fruits in order to simulate post-dispersal of seeds via gut passage of birds in the best way possible.
This study’s purpose is to demonstrate what happens to seeds that are dispersed by birds in both an ecosystem with birds (Saipan) and one without (Guam). Four sites on Guam and five sites on Saipan were selected for study based on existing Ecology of Bird Loss study areas in native forest. Choice of tree seed species used in this study was selected based on the availability of sufﬁcient numbers of seeds, but was constant between Saipan and Guam. Species used in this experiment were Aglaia mariannensis, Aidia cochinchinensis, Morinda citrifolia and Carica papaya. Seeds from these species were separated from fruits and allowed to dry overnight. Two aluminum trays were placed at each gap and closed canopy site. Trays consisted of a plastic lid and one side cut out. Two different treatments were applied to these trays. The first consisted of an open side, and the second had the same opening covered with a ½” wire mesh. One gap/closed canopy pair per site contained a ‘control’ treatment which consisted of a tray with an opening covered with a small wire screen. Five gap and five closed canopy areas were selected at each site for study, and there was a total of 110 trays deployed on each island. Each gap/canopy pair was visited and trays were observed daily for three days. Seeds were noted as ‘removed’ if they were missing or if they appeared gnawed. The study showed very little seed removal at all, and little difference in seed removal between gaps and closed canopies. For Carica, seed removal on Guam was greater in the gaps than in closed canopies. For Aidia, results suggest that species removing seeds differ between caps and closed canopies between islands. Future research would explore seed removal based on seed size and further identify major classes of seed predators in the hopes of predicting tree species survivability on Guam and in the Mariana Islands.
Funder Acknowledgement(s): I thank M. Naputi for assistance in the field, as well as N. Muchoney and E. McCann for their assistance. Funding was provided by a NSF-REU grant awarded to Dr. Haldre Rogers, assistant professor at the University of Iowa, Ames, IA.
Faculty Advisor: Haldre S. Rogers,