Large frugivores are considered to be important seed dispersers for many tropical plant species. Their roles as seed dispersers are not well known in Southeast Asia, where degraded landscapes typically lack these animals. Interactions between 259 (65 families) vertebrate-dispersed fruits and frugivorous animals (including 7 species of bulbul, 1 species of pigeon, 4 species of hornbill, 2 species of squirrel, 3 species of civet, 2 species of gibbon, 1 species of macaque, 2 species of bear, 2 species of deer, and 1 species of elephant) were studied for 3 years in a tropical seasonal forest in Khao Yai National Park, Thailand. The purpose was to examine the dietary overlaps among the large frugivores and the characteristics of fruits they consumed. Most fruit species are eaten by various kinds of frugivores; no close relationship between a particular fruit and a frugivore was found. The number of frugivore groups that served a given plant species was negatively correlated with seed size. Additionally, the fruit/seed diameters consumed by bulbuls were significantly smaller than consumed by the other nine groups. These trends of fruit characteristics were consistent with those observed elsewhere in Southeast Asia: small fruits and large, soft fruits with many small seeds are consumed by a wide spectrum of frugivores while larger fruits with a single large seed are consumed by relatively few potential dispersers. Importantly, these large, single-seed fruits are not consumed by the small frugivores that thrive in small forest fragments and degraded areas in Southeast Asia. To insure the natural seed dispersal process in the forest, an evaluation of all frugivore groups in the forest is urgently needed in Southeast Asia.
We investigated the seed dispersal of Aglaia spectabilis, a large-seeded tree species in a moist evergreen forest of Khao Yai National Park in Thailand. Although one-to-one relationships between frugivores and plants are very unlikely, large-seeded plants having to rely on few large frugivores and therefore on limited disperser assemblages, might be vulnerable to extinction. We assessed both the frugivore assemblages foraging on arillate seeds of Aglaia spectabilis and dispersing them and the seed predator assemblages, thereby covering dispersal as well as the post-dispersal aspects such as seed predation. Our results showed that frugivores dispersing seeds were a rather limited set of four hornbill and one pigeon species, whereas two squirrel species were not dispersers, but dropped the seeds on the ground. Three mammal species were identified as seed predators on the forest floor. Heavy seed predation by mammals together with high seed removal rates, short visiting times and regurgitation of intact seeds by mainly hornbills lead us to the conclusion that hornbills show high effectiveness in dispersal of this tree species.
This chapter investigates whether geographically separated plant communities are inhabited by frugivores with distinct frequency distribution for body size with respect to mean seed size or mean seed elongation. Data on the shapes and sizes of seeds consumed and dispersed by frugivores in neotropical and palaeotropical rain forest: the Guianas (Guyane-also known as French Guiana; Suriname-formerly known as Dutch Guiana); Central Africa (Ivory Coast, Gabon, Cameroon, Uganda); Southern Thailand; and the wet tropics of Australia in Queensland, were compiled. The relationship between seed width and length were analysed, and the mean size and shape of seeds across continents for whole communities and within groups of plants dispersed by particular guilds of animals was compared. Results indicated that the differences in the mean size of seeds in plants from four rain forests on different continents reflected the mean size of the animals handling them. However, the relationship was a tenuous one and, when examined in more detail, the expected or straightforward patterns broke down. Large seeds from African sample were elongate relative to smaller one, though few constraints were expected on the shape of large seeds. Large seeds in Thailand were also relatively elongate and showed strong selective pressure to become elongate a seed size increased. However, Australia, with the smallest mean disperser size, had seeds that did not show a change in shape as a function of seed size, nor any selective pressure to do so. In contrast, seeds from the Guianas, with the second largest mean disperser weight, had large seeds that were rounder than small ones and demonstrated strong selection for this to occur. Clearly, different animal behaviour, such as carrying and dispersing seeds rather than swallowing them, can exert opposing selection on seed size and shape.
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