12
Functional Differentiation and
Positive Feedback Enhancing
Plant Biodiversity
TAKASHI KOHYAMA 1 ,2, EIZI SUZUKI 3 , SHIN-ICHIRO AIBA 3 , AND TATSUYUKI SEIN0 1
I Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810,
Japan
2 Harvard University Herbaria, Cambridge, MA 02138, USA
3 Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan
Abstract
Ecosystems are under the control of negative feedback due to resource competition,
and it is difficult to explain the coexistence of many species in an ecosystem. By
contrast, evolutionary history suggests that positive feedback between living organisms and environments contributes to increasing biodiversity. This paper presents a
conceptual framework to interface these feedbacks, taking forests and tree communities as an example. One of the prevailing global patterns of biodiversity is the
latitudinal gradient of tree species diversity in forests. A tenfold difference exists in
species diversity between tropical lowland forests and either tropical high-altitude
forests or temperate forests. We examined tree census data from permanent plots
across various forest types in eastern A<;ia. Tree species diversity increased exponentially along a geographic gradient while ecosystem measures such as biomass,
biomass turnover rate and asymptotic canopy height increased only linearly. Examination of a size-structure-based dynamic model of tree populations suggests
that these ecosystem measures multiplicatively contribute to the extreme species
diversity in tropical lowland rain forests. The same model also shows that any
singular species with higher resource-use efficiency replaces all coexisting species
under the constraint of functional tradeoff. Such replacement brings about increasing efficiency of ecosystems in resource exploitation, and in turn presents a greater
opportunity for species coexistence. The non-linear relationship between whole
Key words. asymptotic size, biomass, coexistence, ecosystem, feedback, forest, forest architecture hypothesis, functional differentiation, size-structure-based model, species diversity, recruitment capacity, resource gradient, tree community, tropical rain forests, turnover
rate
179
Functional Differentiation and
Positive Feedback Enhancing
Plant Biodiversity
TAKASHI KOHYAMA 1 ,2, EIZI SUZUKI 3 , SHIN-ICHIRO AIBA 3 , AND TATSUYUKI SEIN0 1
I Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810,
Japan
2 Harvard University Herbaria, Cambridge, MA 02138, USA
3 Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan
Abstract
Ecosystems are under the control of negative feedback due to resource competition,
and it is difficult to explain the coexistence of many species in an ecosystem. By
contrast, evolutionary history suggests that positive feedback between living organisms and environments contributes to increasing biodiversity. This paper presents a
conceptual framework to interface these feedbacks, taking forests and tree communities as an example. One of the prevailing global patterns of biodiversity is the
latitudinal gradient of tree species diversity in forests. A tenfold difference exists in
species diversity between tropical lowland forests and either tropical high-altitude
forests or temperate forests. We examined tree census data from permanent plots
across various forest types in eastern A<;ia. Tree species diversity increased exponentially along a geographic gradient while ecosystem measures such as biomass,
biomass turnover rate and asymptotic canopy height increased only linearly. Examination of a size-structure-based dynamic model of tree populations suggests
that these ecosystem measures multiplicatively contribute to the extreme species
diversity in tropical lowland rain forests. The same model also shows that any
singular species with higher resource-use efficiency replaces all coexisting species
under the constraint of functional tradeoff. Such replacement brings about increasing efficiency of ecosystems in resource exploitation, and in turn presents a greater
opportunity for species coexistence. The non-linear relationship between whole
Key words. asymptotic size, biomass, coexistence, ecosystem, feedback, forest, forest architecture hypothesis, functional differentiation, size-structure-based model, species diversity, recruitment capacity, resource gradient, tree community, tropical rain forests, turnover
rate
179