SEED PREDATORS, DRUPE PREDATION RATES AND THEIR EFFECTS ON THE REGENERATION DYNAMICS OF Olea welwitschii IN KAKAMEGA FOREST, KENYA
H. M. TSINGALIA *
Department of Biological Sciences, Moi University, Box 3900, Eldoret, Kenya
*Author to whom correspondence should be addressed.
Careful scrutiny of data collected during the forest inventory in the 1960s on the undergrowth reveals absence of seedlings of Olea welwitschii. This study sought to determine why Olea welwitschii does not regenerate inside the forest and to examine the roles of predispersal and post-dispersal drupes and seed predation intensities as precipitators of lack of regeneration. Seed/drupe predation experiments were conducted at two levels. The first level examined pre-dispersal seed and drupe predation. Rectangular seed/drupe traps made out of meshed fabric with a collecting surface of 0.25 m2were randomly placed to cover all the four quadrants under the canopy of five randomly selected Olea fruiting trees. The second level examined post-dispersal predation by sampling seeds and drupes under the crown of each of the randomly selected fruiting trees by dividing it into four quadrants. Within each quadrant, five 1m2 quadrats were established on the ground in a random fashion to cover at least 10-30 per cent of the total canopy area. Frequency- and distance- dependent seed/drupe predation were examined using 30 (l m2) quadrats with equal numbers of seeds and drupes and inspected every 12 hours for one month to determine differences in seed and drupe predation rates. Effects of density and distance were tested by setting up 50 m long transects with the base of the fruiting trees as the starting point. Two transects per tree in two different directions, north/south and east/west at five fruiting trees were established. Along each transect, eleven sampling stations were established at a five metre interval. A trapping grid was established under the crown of five fruiting Olea adults and away from the crown to determine differences in small mammal predator density. Results of predispersal drupes predation was derived from the pooled sample of all drupes collected. A total of 1386 drupes were collected. Of these, three (3) per cent had insect holes. A closer examination of size and weight of the drupes that were attacked revealed that smaller drupes (70%) were attacked more than larger ones (30%). Overall, there were more small drupes falling on the forest floor (89 per cent) than large ones (11 per cent). Drupe and seed density was a decreasing function of distance from the parent trees ad distance from the parent trees had a significant effect on drupes and seed predation rates. Regardless of density, seeds were removed at higher rates under the parent crowns, while drupes were ignored by the predators. One species of rodents, Praomys jacksoni was trapped. The majority of the captures were under fruiting Olea adults (3.71 animals per trapping night) rather than farther away from the adult trees (0.61 animals per trapping night; Z=8.724, p<0.01). A total of 18,000 seeds were examined for pathogen attacks and only 10 per cent of the intact seeds showed sign of insect/rodent attack. Drupes density greatly influenced fungal infection rates (ᵡ2=55.867, p<0.001). Results clearly suggest that density is not an important factor in the predation of Olea drupes and drupes. The failure to find greater predation rates for drupes in large clumps suggest that either large clumps are no easier to find than small clumps for predators or these predators remove all the drupes they find regardless of clump size. Predation risk of drupes and seed appears to be influenced by patchiness in the activity patterns of drupes eaters.
Keywords: Seed predation, drupe predators, Kakamega, density-dependency, tropical forests, Olea welwitschii