Microhabitat preferences of two sympatric Scaphium species in a 50 ha Plot in Pasoh

A remarkable feature of tropical woody biodiversity is the presence of a large number of closely related, sympatric species in some genera. Such species are most likely to have relatively similar niches, sharing the same ecological and physiological heritage via their common ancestral lineage. How these sympatric species coexist is a central to understanding of the maintenance of high biodiversity in tropical rain forests. The mode of the coexistence can be divided into equilibrium and non-equilibrium. The equilibrium coexistence poses that each species occupies a different niche, which results from and reduces direct competition. For the equilibrium coexistence of plant species, the diversification of a microhabitat in relation to topography and shading conditions is among the most important. On the other hand, the non-equilibrium postulates that equilibrium forces at work are weak and species coexist by means of a stochastic non-equilibrium factor. To detect how sympatric species coexist in a tropical rain forest, we compared tree forms and local spatial distribution patterns between two sympatric Scaphium species (Sterculiaceae) of a tropical canopy tree, S. macropodum and S. linearicarpum using data obtained from a 50 ha plot in Pasoh Forest Reserve (Pasoh FR). The total numbers of S. linearicarpum and S. macropodum larger than 1 cm in DBH (Diameter at Breast Height) in the plot were 148 and 726, respectively. Frequency distributions of DBH followed L-shaped distributions in both cases, suggesting that they have near-equilibrium population structures. Both species were distributed unevenly and were aggregated in some clumps in the plot. Furthermore, their distributions were strongly associated with topographic variables. Their habitat preferences in relation to topography were similar each other; they were rare on valleys and flat areas and abundant on slopes, although the range of S. macropodum’s distribution is larger than that of S. linearicarpum. An index of spatial association shows a positive spatial association between two species. These results suggest that habitat niche segregation in relation to topography does not exist between these species and therefore they must be under a severe direct competition for occupying a restricted area where is suitable for their regeneration. For the equilibrium coexist of these species, differentiation in regeneration niches associated with gap regimes is required. But the shading conditions of saplings and pole size trees do not differ between these species. Thus the regeneration niche differentiation appears unlikely to exist between them. Consequently, we can hypothesize that a stochastic non-equilibrium factor is important for the coexistence of these two Scaphium species.

Toshihiro Yamada, Toshinori Okuda, & N. Manokaran
Pasoh: Ecology of a Lowland Rain Forest in Southeast Asia
Tohsinori Okuda, N. Manokaran, Yoosuke Matsumoto, Kaoru Niiyama, Sean C. Thomas, & Peter S. Ashton
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