11. Patterns ofTree Species Diversity
163
and chemical defenses against herbivores, often leading to species-specific interdependencies (e.g. Coley 1983). Species diversity in plant communities has been variously attributed to environmental heterogeneity in space and in time, including
temporally differential influences on fecundity among species in competitive balance; to periodic climatic catastrophe and the stochastic mediators of extinction
rate; to rates of accumulation by speciation or immigration of competitively equivalent species; and to the favoring of rarer species by consumer pressure (modified
after Leigh 1982). None of these factors is likely to be universally preeminent in the
sustainment of species diversity. An examination of changes in their relative importance under a variety of physical conditions may assist in understanding why
species diversity is concentrated in certain habitats, and how it is maintained.
In this chapter, we will describe patterns of species diversity in the richest of all
biomes, lowland evergreen tropical rain forest, on global, regional and local scales
by means of a single life form, trees. This is possible for the first time thanks to
collaboration of scientists in many tropical countries with the Center for Tropical
Forest Science (CTFS) of the Smithsonian Tropical Research Institute (STRI) (Table
1): A network of large tree demographic plots in rain forests, censused according to
a uniform protocol and replicated internationally, is being laid out along gradients
of rainfall seasonality, soil fertility, canopy disturbance and insularity (Ashton
1998a). The plots generally represent samples of 50 ha, in which trees equal to or
greater than 1 cm diameter are censused. Although the network is global, only the
A.,ian component is near completion and we will concentrate on that. This is a
progress report: research is at an early stage in respect of several factors which may
enhance diversity in tropical forests.
Regional and local patterns of rain forest tree species diversity are now apparent
in Asia and globally (Table 2). There is a strong decline in tree species richness
correlated with the increasing length of the dry season, but no close relationship
with mean annual rainfall. This trend is nevertheless partially obscured by variation in species richness within the aseasonal wet tropics, the island forests of Sri
Lanka being exceptionally low and forests in edaphically heterogenous northwest
Borneo unusually high.
2 Causes of Diversity
2.1 Accumulation of Competitively Equivalent Species
Through Island Biogeographic Processes
Rates of immigration are influenced by degree of isolation of a forest area, rates of
extinction by its area; the immigration and extinction rates of an area balance to
create an equilibrium of species richness (MacArthur and Wilson 1969). As most
forests differ in area and isolation, the species richness of each individual forest
should be unique. This prediction may be obscured by the long life cycles of trees,
and their generally short dispersal distances in the tropics. There may therefore be
163
and chemical defenses against herbivores, often leading to species-specific interdependencies (e.g. Coley 1983). Species diversity in plant communities has been variously attributed to environmental heterogeneity in space and in time, including
temporally differential influences on fecundity among species in competitive balance; to periodic climatic catastrophe and the stochastic mediators of extinction
rate; to rates of accumulation by speciation or immigration of competitively equivalent species; and to the favoring of rarer species by consumer pressure (modified
after Leigh 1982). None of these factors is likely to be universally preeminent in the
sustainment of species diversity. An examination of changes in their relative importance under a variety of physical conditions may assist in understanding why
species diversity is concentrated in certain habitats, and how it is maintained.
In this chapter, we will describe patterns of species diversity in the richest of all
biomes, lowland evergreen tropical rain forest, on global, regional and local scales
by means of a single life form, trees. This is possible for the first time thanks to
collaboration of scientists in many tropical countries with the Center for Tropical
Forest Science (CTFS) of the Smithsonian Tropical Research Institute (STRI) (Table
1): A network of large tree demographic plots in rain forests, censused according to
a uniform protocol and replicated internationally, is being laid out along gradients
of rainfall seasonality, soil fertility, canopy disturbance and insularity (Ashton
1998a). The plots generally represent samples of 50 ha, in which trees equal to or
greater than 1 cm diameter are censused. Although the network is global, only the
A.,ian component is near completion and we will concentrate on that. This is a
progress report: research is at an early stage in respect of several factors which may
enhance diversity in tropical forests.
Regional and local patterns of rain forest tree species diversity are now apparent
in Asia and globally (Table 2). There is a strong decline in tree species richness
correlated with the increasing length of the dry season, but no close relationship
with mean annual rainfall. This trend is nevertheless partially obscured by variation in species richness within the aseasonal wet tropics, the island forests of Sri
Lanka being exceptionally low and forests in edaphically heterogenous northwest
Borneo unusually high.
2 Causes of Diversity
2.1 Accumulation of Competitively Equivalent Species
Through Island Biogeographic Processes
Rates of immigration are influenced by degree of isolation of a forest area, rates of
extinction by its area; the immigration and extinction rates of an area balance to
create an equilibrium of species richness (MacArthur and Wilson 1969). As most
forests differ in area and isolation, the species richness of each individual forest
should be unique. This prediction may be obscured by the long life cycles of trees,
and their generally short dispersal distances in the tropics. There may therefore be