Agriculture Reference
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Functional diversity ( i.e., the diversity of functions performed by the species of
a community) increases notably with temperature, unlike species diversity as measured
by standard mathematical indexes. This is clearly indicated by a lower niche overlap
due to a broader range of sizes among the species, more diverse patterns of vertical
distribution and the exploitation of a broader range of food-resources (Lavelle, 1983c;
Fragoso and Lavelle, 1987). This increase in functional diversity with temperature is
paralleled by a progressive change in the relative importance of individual ecological
categories, Epigeics are dominant in cold environments, anecics in temperate areas
and mesohumic endogeic species in humid tropical environments. Oligohumics are
restricted to the warmest soils of subequatorial humid savannas (Figure III.45) (see also
Chapter IV.4).
Spatial distribution
At the scale of an hectare, earthworm populations may be concentrated to varying degrees
within specific areas although intense aggregation is limited to epigeic populations
exploiting resources with heterogenous distribution patterns such as cow pats.
Differences in population densities have also been correlated with heterogeneity in such
soil properties as texture and microtopography (Fragoso and Lavelle, 1987; Phillipson et
al., 1976). Nonetheless, the variability of demographic processes may also determine
patchy distributions in metapopulations, irrespective of the heterogeneity of the soil.
Rossi and Lavelle (1998) found that all earthworm species in the Lamto (Côte d'Ivoire)
savannas had aggregated distributions, and that aggregation became increasingly intense as
species became smaller and had higher annual fecundities. Simulations of the population
dynamics of one of these species, the mesohumic species Millsonia anomala, have shown
that natural populations are actually composed of subpopulations with distinct dynam-
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