| | ||||
| | | | | |
| BACK TO PROJECT LIST | |||||
Dipteryx Seed Predator Project
Chart 1 Predator Percentages Chart 2 Seeds per Tree Secondary/Primary Chart 3 Percent Seeds Damaged
Katie Noonan
INTRODUCTION
Dipteryx panamensis is a large, emergent, neotropical tree that produces a large crop of woody seeds from December to March.
These large seeds are a food source for several animals in the tropical rainforest, including monkeys (Ateles geoffroyi, Cetus capucinus, Allouata palliata), bats, macaws (Ara spp.), agouti (Dasyprocta punctata), and squirrels (Sciurus spp.) (Baker et al., 1999). While monkeys and bats merely eat the surrounding fruit and leave the viable seed behind, agouti, macaws, and squirrels open the seed coat to eat the inside endosperm and embryo, leaving behind distinctive damage to the discarded seed coat. Agouti stockpile seeds for later use. Sometimes these seeds germinate and produce seedlings before the agouti eat them. The Great Green Macaw specializes on Dipteryx fruits and seeds, migrating long distances to locate and consume them (Stiles and Skutch, 1999).
The predator of each seed may then be identified according to the way in which the seed has been opened: agoutis chew a crescent-shaped hole into the side of the seed coat (Fig. 1). squirrels chew an irregular shape into the seed coat (Fig. 2); and macaws cleanly bite off the tips, leaving a straight line with no teeth marks (Fig. 3).
 |
 |
 |
| Fig. 1 Seed coat with agouti damage. | Fig. 2 Seed coat with squirrel damage. | Fig. 3 Seed coat with macaw damage. |
Furthermore, although agoutis may be an important predator of Dipteryx seeds, they also bury many seeds and therefore act as important dispersers.
In our small group project, we decided to investigate the proportion of Dipteryx seeds eaten by agouti, squirrel, and macaws in a primary forest compared to a secondary forest at La Selva Biological Research Station. Global change in land use patterns may result in the increase in secondary forest at the expense of primary forest habitats. In turn, this may change the numbers of seed predators and dispersers as the higher trophic level predators, which affect their populations also, change. Global climate change may also affect the amount of primary forest either directly or through increased human encroachment on existing old-growth forests. By examining the amounts of seed predation by different organisms in primary and secondary forests at La Selva, we hope to investigate indirectly how conditions for Dipteryx seed dispersal might change if the proportions of these habitats change.
HYPOTHESIS
We predict greater squirrel and agouti damage in secondary forests compared to primary forests because secondary forest provides (a) more cover for agoutis and squirrels (WWNFF CR Large Group Project, La Selva, 2001; Kricher, 1997), (b) less trees for macaws to perch and roost in, and (c) predators on agoutis such as ocelots and pumas may not move into disturbed areas near human activity.
MATERIALS
METHODS
A total of six trees were sampled: three from a primary forest and three from a secondary forest. Individual trees were chosen based on visibility and accessibility from the trail. Once a tree was identified, two 14-meter transects were extended in different directions from the base of the tree according to the following criteria:
Hog-nosed
viper on transect!!!
Using an 80cm rod as a guide, we sampled seeds within an 80cm wide plot along the transect. Group members shared responsibilities for collecting, classifying, and recording seed data. One group member collected seeds on one side of the transect followed by another group member to collect any missed seeds, and sometimes one group member would begin at one end and the other group member from the opposite end. Once seeds were identified they were tossed outside of the transect area. Seeds with no damage or damage, which could not be identified, were recorded as "others." If there was uncertainty, the members conferred with each other to identify the seed. The recorded results were used to construct bar graphs and tested for association with forest type using the Chi-square test.
RESULTS
We sampled 598 seeds and seed halves from the two forest habitats. The numbers of half seeds in each category was divided by 2 to avoid counting seeds twice and added to the number of whole seeds to make an adjusted total.
The numbers of seeds damaged by the different animals was associated significantly with forest type according to a Chi-square test (Chi-square = 24.91, 3 df, p< .005). However, the distribution of damage between trees within each habitat also differed significantly (p< .025).
| Click a thumbnail to see the full-sized version of that chart. | ||
 |
 |
 |
| Percent of Seed Damaged by Predator Type | Total Seeds per Tree-Secondary and Primary forests | Percent of Seeds Damaged |
The average number of seeds sampled per tree was 77 (range 47-126) for primary forest and 35 (22-41) for secondary forest .
The percentage of seeds with visible damage from seed predators also varied between habitats. 43% of seeds sampled in primary forest were damaged (range 12-68%), compared to 26% of seeds in secondary forest (range 11-36%) (Fig. 2).
The percentages of seeds damaged by category also differed (Fig. 3). Macaw and agouti damage was higher in primary forest. Squirrel damage was higher in secondary forest. Results varied greatly from tree to tree, and our sample size was so small that we did not attempt statistical test on the differences.
DISCUSSION
The results of our project indicate that the seed damage to Dipteryx seeds by different animals was significantly associated with forest type. However, the direction of the difference was opposite to our prediction. In our results, the primary forest had higher proportions of agouti and macaw damage to seeds. The evidence did not confirm our hypothesis.
There was considerable variability between trees in the same forest, as well as between forest types. This is consistent with the findings of other researchers. Baker et al. (1999) also report high variability in seed loss to agoutis, 9% to 74% of 322 seeds collected from 7 trees at La Selva. Baker et al. (1999) also report that research at Barro Colorado Island found that 56% of damage to Dipteryx seeds was by agoutis.
The high variability and small sample sizes do not allow us to draw conclusions about differences in types of seed predation in primary and secondary forests. If the study could be extended and controlled for the many other factors which might affect seed predation (age of trees, density of trees, number of seeds produced and timing) it might shed some light on the replacement and regeneration of Dipteryx. Changing patterns of land use might affect the community structure of forests, changing populations of the seed-dispersing animals and their predators. Reduction in seed damage by organisms such as agouti, monkeys and bats, which disperse the seeds, might indicate decline in populations of these important dispersers. Increase in damage by organisms which destroy seed without leaving some to germinate (e.g. the macaw) might indicate a decline in populations of these dispersed, long-lived trees. In our results, the primary forest had higher.
Our project was inconclusive. It suggests that this kind of data might be useful in tracking changes in plant and animal population, but that there are many factors we have not identified or explored which contribute to amounts of damage to seeds by different animals.