SOIL ORGANISMS - Soil Ecology - page 349

determined by the diversity and abundance of resources. However, this determination
is not simple and contradictory results may be obtained. In an English chestnut
( Castanea ) wood, a direct relationship was established between environmental diversity
as measured by the occurrence of 23 variables describing microhabitats and resources
and the diversity of species of Cryptostigmata (Acari) (Anderson, 1977) (Figure III.37).
In contrast, similar studies of a mixed-species wood near Paris (France) showed that
species richness may also be negatively correlated with the abundance (and hence
diversity) of microhabitats occurring in leaf litter (Garay, 1981b). In that situation, the
small standing crop biomass of litter resulted from rapid decomposition. The large flux
of assimilable organic matter and nutrients thereby made available may account for the
large observed diversity. Mechanisms of niche partitioning which operate in these
communities are discussed elsewhere (Section III.4.2.1.2).
Large-scale determination: the first link hypothesis
Many explanatory - and sometimes controversial - hypotheses have sought to explain
the outstanding species richness that occurs in many taxa in the humid tropics (see
e.g., Giller,1984; Solbrig, 1991). It seems likely that species richness is determined by
several factors which can act at two levels: (i) determination of the corporate niche
occupied by a community and (ii) separation into individual niches.
The main hypotheses that explain the possibly enlarged corporate niches in the humid
tropics compared with those of cooler and less humid environments refer to environmen-
tal favourableness, spatial heterogeneity, temporal variability and enhanced plant produc-
tion: these are considered to enlarge the resource base available to a given community.
However, these hypotheses have never directly addressed soil processes or considered
the effects of increased temperature. The latter is the one unquestioned difference that
occurs between tropical and extra-tropical environments,
In the 'first link' hypothesis, it is proposed that increased soil temperatures give roots
access to an enlarged nutrient resource base by increasing the efficiency of mutualistic asso-
ciations in the rhizosphere (see Chapter IV.3). An enlarged corporate niche due to increased
nutrient resources would result in a larger number of plant species once niche separation
has taken place. This would be the first link of a cascade process in which species richness
within the consumer and decomposer foodwebs would become larger than in colder areas,
irrespective of such biogeographic and historical determinants as Pleistocene refuges,
catastrophic events and the degree of ecosystem maturity (Odum, 1983) (Figure III.62).
This hypothesis originated from the changes noted in the structure of earthworm com-
munities along a thermo-latitudinal gradient extending from Iceland to Southern Venezuela
and Côte d'Ivoire (Lavelle, 1983c). The observed pattern was subsequently extrapolated to
plants, based on the similarities noted in the general functioning of the drilosphere and
rhizosphere systems (Lavelle, 1986).
Along this thermo-latitudinal gradient, earthworms become increasingly able to use
food resources of lower quality: in the coniferous forests of northern Europe, earthworms
only ingest litter (Figure III.44). In temperate-climate areas, part of the earthworm
community additionally ingests soil with a high organic content (taken in the rhizosphere
or from beneath the litter) or litter mixed with soil. In the latter case, labelling methods
have demonstrated that little if any soil organic matter is assimilated (Martin et al., 1992).