SOIL ORGANISMS - Soil Ecology - page 348

sometimes result in an intense speciation. This effect of size on species richness has
already been assessed for above ground terrestrial animals by May (1978).
The larger diversity of arthropods as compared to non-arthropods ( e.g., Oligochaeta)
may possibly be interpreted as the effect of a greater niche specialisation due to their
external chitinised skeleton. As a result, adult size is fixed and little variation occurs
between individuals. The evolution of a chitinised exoskeleton has favoured the
development of efficient locomotory and protective structures and allowed the evolution
of a wide range of mouthpart types of fixed size and shape. Larval development permits
some plasticity in the niches used by a particular species, although still much less
than in non-arthropods. Conversely, during growth, Oligochaeta continually change their
ecological roles. For example, young anecic earthworms, often behave like epigeics and
progress deeper into the soil as they grow. Populations of a single earthworm species
may thus cover a relatively wide range of niches. At the end of the dry season, populations
of the tropical endogeic species Millsonia anomala comprise a majority of young indi-
viduals which are more resistant to drought and high temperatures than adults. In the
midst of the rainy season, populations show a preponderance of adults more highly
adapted to cool temperatures and a high soil moisture regime (Table III.13).

4.4.2.2 Biogeographical effects

For biogeographical reasons, certain taxa may be absent from environments in which
they could readily exist. This is not uncommon in large invertebrates and two examples
are the absence of the fungus-growing termites (Macrotermitinae) from Australia and
the Americas. In the latter environment, ants of the tribe Attini have been considered as
functional equivalents, even though the most prominent harvest is living leaves instead
of using dead plant material as a primary resource (Wheeler, 1907; Weber, 1972). A fur-
ther example is the striking differences between earthworm communities in tropical
forests: in Western Africa and Central America, communities in oxisols occurring in
regions with an average annual rainfall of 1600-2000 mm are dominated by endogeic
populations (Lavelle, 1978; Fragoso and Lavelle, 1987). Comparable forests in South
America have predominantly anecic populations (Nemeth, 1981; Lavelle and Pashanasi,
1989). In the former environment, most species belong to the family Megascolecidae which
has a relatively primitive pattern of organisation and comprises a majority of endogeic
species. In the Amazonian forest, most species belong to the family Glossoscolecidae, a
group that includes numerous true anecic species and in this respect is similar to the
Lumbricidae, the dominant earthworm family of most temperate regions of the world.

4.4.2.3 Environmental determinants

Species richness and diversity are highly dependent on variation in environmental
factors. This may range from the small scale at which -diversity is measured (a few
square metres to 1 ha), to the much wider continuum of a thermo-latitudinal gradient.

Smal scale determination
At the small scale at which communities operate, species richness and diversity are