Agriculture Reference
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have been underestimated, due to their small size.
Niche partitioning
Communities of Acari tend to form 'loosely-defined species assemblages' (Anderson
and Hall, 1977). Despite the apparent lack of a simple association with environmental
factors (see
e.g.,
Ghilarov, 1977), a few general rules have been observed (Vannier, 1985):
(i) some clear differences exist in feeding regimes, mainly related to size (Luxton,
1972; Anderson,1977) (Figure III.35);
(ii) some populations are distributed in aggregations that have little spatial overlap: in
samples of 20 containing at least one out of three potentially competing Oribatid
species, only 8 % had three and 14-19 % had two (Cancela da Fonseca, 1987). Similarly,
Stanton and Tepedino (1977) found that small patches of litter (100 g dry weight)
deposited on the surface of temperate and tropical soils had, after three weeks, been
colonised by a similar number of species (11 to 14) forming assemblages with equiva-
lent diversities irrespective of the overall diversity which was greater in the tropical
environment. In a
Pinetum-Vaccinietum myrtilli typicum
association, Blaszak
et al.,
(1977) observed a clear difference of species assemblages in patches with the lowest
vegetation stratum dominated by either mosses or
Vaccinium,
Topographic effects were
also reported;
(iii) in patches of equal size, species diversity is directly related to those of micro-
habitats and food resources present (Anderson and Hall, 1977) (Figure III.37).
In some cases, associations of species with specific environmental conditions have
been identified. However, many species are ubiquitous and generalisation of such
findings is difficult (Wallwork, 1967).
