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P.S. Ashton and J.V. LaFrankie
sists of two subgenera, Durio, with relatively few, bat-pollinated flowers and large
fruit, and Boschia with many, bee-pollinated flowers and small fruits. Species in
subgenus Durio appear to be uniformly in low population density in the forest,
whereas most Boschia are abundant in their preferred habitats (Ashton 1998b).
Durio and other forest tree species visited by the nectarivorons bat Eonycteris spe/aea
Dobson mass flower seasonally or supra-annually. The carrying capacity of the bat
will be limited by the reliability of the annual relay sequence of flowering trees.
The role of density dependence will not be resolved without much more, and
longer term, study.
2.4 Environmental Heterogeneity in Space
It is well established that biodiverse tropical forest varies in species composition in
relation to topography (Beccari 1904; Richards 1996; Ashton 1964; Austin et al.
1972; Hubbell and Foster 1990; Webb and Peart, submitted), and to soils (Richards
1996; Baillie et al. 1987; Ashton 1998b). Ashton (1977, in Terborgh 1994; Ashton
1998b) demonstrated a peaked distribution of species richness along a nutrient
gradient. The peak occurs towards the low end of the nutrient range. Species richness at high nutrient concentrations was inversely correlated with relative dominance (density) of a few fast growing canopy species. These results are consistent
with Tilman's (1982) resource competition hypothesis. Tilman predicted that plant
species richness will increase as two or more resources increase in concentration
from a low base, and also increasingly vary in space, because niche space and
opportunities for coexistence of nutrient specialists increase; but above a given threshold, he argued, one or a few species with high growth rates will outcompete others
for light and dominate the canopy, thereby suppressing species richness. This was
the case here.
Yet within any sector of a nutrient or topographic gradient the more biodiverse
rainforests carry many hundreds of tree species. Bearing in mind their longevity
and frequently limited dispersal range, it is hardly conceivable that such a rich
array can find and continue to occupy a complex spatial mosaic of multiple soil
resources. Hubbell (1995, 1997a, b) has argued that these species assemblages are
not in equilibri!lm but change over time due to random drift of species with limited
dispersal capacities. This hypothesis predicts lack of consistent rank orders of abundances within community types, and the preeminent influence of the predictions of
the theory of island biogeography. Only monitoring of marked individuals over
many decades can test whether this is so, but it is more likely that increasing concentrations of nutrients at first merely increase the number of edaphically equivalent species that can co-occur. This conclusion is consistent with the very narrow
range of maximum growth rates among species on the quite low nutrient soils
which carry the richest forests (Ashton and Hall 1992).
P.S. Ashton and J.V. LaFrankie
sists of two subgenera, Durio, with relatively few, bat-pollinated flowers and large
fruit, and Boschia with many, bee-pollinated flowers and small fruits. Species in
subgenus Durio appear to be uniformly in low population density in the forest,
whereas most Boschia are abundant in their preferred habitats (Ashton 1998b).
Durio and other forest tree species visited by the nectarivorons bat Eonycteris spe/aea
Dobson mass flower seasonally or supra-annually. The carrying capacity of the bat
will be limited by the reliability of the annual relay sequence of flowering trees.
The role of density dependence will not be resolved without much more, and
longer term, study.
2.4 Environmental Heterogeneity in Space
It is well established that biodiverse tropical forest varies in species composition in
relation to topography (Beccari 1904; Richards 1996; Ashton 1964; Austin et al.
1972; Hubbell and Foster 1990; Webb and Peart, submitted), and to soils (Richards
1996; Baillie et al. 1987; Ashton 1998b). Ashton (1977, in Terborgh 1994; Ashton
1998b) demonstrated a peaked distribution of species richness along a nutrient
gradient. The peak occurs towards the low end of the nutrient range. Species richness at high nutrient concentrations was inversely correlated with relative dominance (density) of a few fast growing canopy species. These results are consistent
with Tilman's (1982) resource competition hypothesis. Tilman predicted that plant
species richness will increase as two or more resources increase in concentration
from a low base, and also increasingly vary in space, because niche space and
opportunities for coexistence of nutrient specialists increase; but above a given threshold, he argued, one or a few species with high growth rates will outcompete others
for light and dominate the canopy, thereby suppressing species richness. This was
the case here.
Yet within any sector of a nutrient or topographic gradient the more biodiverse
rainforests carry many hundreds of tree species. Bearing in mind their longevity
and frequently limited dispersal range, it is hardly conceivable that such a rich
array can find and continue to occupy a complex spatial mosaic of multiple soil
resources. Hubbell (1995, 1997a, b) has argued that these species assemblages are
not in equilibri!lm but change over time due to random drift of species with limited
dispersal capacities. This hypothesis predicts lack of consistent rank orders of abundances within community types, and the preeminent influence of the predictions of
the theory of island biogeography. Only monitoring of marked individuals over
many decades can test whether this is so, but it is more likely that increasing concentrations of nutrients at first merely increase the number of edaphically equivalent species that can co-occur. This conclusion is consistent with the very narrow
range of maximum growth rates among species on the quite low nutrient soils
which carry the richest forests (Ashton and Hall 1992).