Local and geographical distributions for a tropical tree genus, Scaphium (Sterculiaceae) in the Far East - (Plant Ecology 148: 23–30, 2000)

Aug 27, 2026

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Tropical rain forests host many closely related sympatric species, making coexistence mechanisms central to biodiversity maintenance. This study compares local and geographical distributions of the six Scaphium species, an endemic tropical canopy tree genus from the Far Eastern tropics. Species differ by moisture and forest type: S. scaphigerum favors drier regions; S. longiflorum occurs in swamp forests; others occupy lowland and hill rain forests. In a 52-ha Lambir plot, elevation-linked soil variation produces habitat segregation, reducing direct competition via niche differentiation, while equilibrium forces may predominate for coexistence.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages8

Reading Time20 min

AccessFree to download

Introduction

Equilibrium vs non-equilibrium hypotheses in tropical rain forests

Niche differentiation and study-species selection

Study species and distributions

Local and geographical distributions of Scaphium species

Habitat segregation driven by elevation and soil variation

Plant Ecology 148: 23–30, 2000 . © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 23 Local and geographical distributions for a tropical tree genus, Scaphium (Sterculiaceae) in the Far East Toshihiro Yamada 1 ;4 , Akira Itoh 1 , Mamoru Kanzaki 1 ;5 , Takuo Yamakura 1 , Eizi Suzuki2 & Peter Shaw Ashton3 1 Department of Biology, Faculty of Science, Osaka City University, Osaka 558 -8585, Japan; 2Department of Earth Environmental Studies, Faculty of Science, Kagoshima University, Kagoshima 890 -0065, Japan; 3 Organismic and Evolutionary Biology, Harvard University, MA 02138, USA (Present addresses: 4 Faculty of Environmental and Symbiotic Sciences, Kumamoto Prefectural University, Kumamoto 862 -8502, Japan; 5 Tropical Forest Resources and Environments, Graduate School of Agriculture, Kyoto University, Kyoto 606 -8502, Japan) (e-mail: [tyamada@pu-kumamoto.ac.jp](tyamada@pu-kumamoto.ac.jp)) Received 8 February 1999; accepted in revised form 27 August 1999 Key words: Equilibrium force, Habitat segregation, Lambir, Soil, Tropical rain forest, Non-equilibrium force Abstract Tropical rain forests have an amazingly large number of closely related, sympatric species. How the sympatric species coexist is central to understanding the maintenance of high biodiversity in tropical rain forests. We compared local and geographical distributions among trees in Scaphium (Sterculiaceae), a tropical canopy tree genus. Scaphium is endemic to the Far Eastern tropics and comprises six species. Scaphium scaphigerum is distributed in drier regions than the other species’ geographical distribution ranges. Scaphium longiﬂorum is distributed swamp forests, whereas the others were distributed in lowland and hill tropical rain forests on undulating land. Scaphium borneense, S. longipetiolatum, and S. macropodum co-occurred in a 52-ha plot in Lambir, Sarawak and clearly showed an allopathic pattern of distribution related to elevation in it. In the plot, the elevational difference was correlated with soil variation. Consequently, the difference in edaphic condition promoted the habitat segregation of the species. Thus these ﬁve Scaphium species have divergent habitats at various spatial scales and coexist because they reduce direct competition by habitat niche differentiation. Although the non-equilibrium hypothesis for the coexistence of Scaphium species cannot be rejected categorically due to the lack of enough information about S. linearicarpum, the equilibrium force may play the predominant role which permits their coexistence. Introduction Many ecologists have been intrigued by the maintenance mechanism of woody plant species diversity in tropical rain forests and various theoretical hypotheses have been proposed. These hypotheses developed in two different directions, equilibrium and non-equilibrium hypotheses. The former postulate the existence of equilibrium forces which maintain a particular species mixture in a community. In an equilibrium community, each species occupies a different niche which results from and reduces direct competition (Whittaker 1975), and thus species composition is more or less stable according to environmental pre- dictability. The species-speciﬁc correlations between tree distribution and soil and/or topography in tropical rain forests have been reported by many scientists (Ashton 1969; 1988a; Lescure & Boulet 1985; Austin et al. 1972; Richards 1952; Rogstad 1990; Svenning 1999; etc.) are consistent with the existence of equilibrium force in tropical rain forests. On the contrary, the non-equilibrium hypothesis posits that equilibrium forces at work are weak. By this argument, species diversity is decreased through random local extinction in a stochastic dynamic system. However, species diversity can be maintained when the rates of local extinction are low and balanced by lo- 24 cal immigration and speciation. Therefore, the species composition of non-equilibrium communities is unsta

FAQs

What is the main research question about Scaphium coexistence?

How local and geographical distributions among Scaphium species relate to mechanisms that maintain high biodiversity in tropical rain forests, focusing on equilibrium vs non-equilibrium explanations.

How do the Scaphium species’ distributions differ across habitats?

S. scaphigerum is distributed in drier regions; S. longiflorum occurs in swamp forests; the remaining species are found in lowland and hill tropical rain forests on undulating land.

What role do elevation and soil variation play in the study plot?

Elevational differences correlate with soil variation in the Lambir 52-ha plot, and the edaphic conditions promote habitat segregation among co-occurring Scaphium species.

Aug 27, 2026

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