Agriculture Reference
In-Depth Information
4.3.1.4
Community structure
Species richness and diversity
Earthworm communities rarely comprise more than 8-10 species and there are no
marked changes in diversity along latitudinal gradients. Such a low species richness is
considered to result from the large functional plasticity that occurs within populations.
The ecological roles of individuals within particular populations may be quite diverse,
due to the important size differences between young and adult worms, and the adapt-
ability of populations to variable environments (Table III.13). During the dry season in
the savannas of Côte d'Ivoire, populations of the mesohumic endogeic species Millsonia
anomala are dominated by young individuals that are better adapted than adults to the
hot and dry conditions prevailing; during the rainy season, the environment becomes
progressively more suitable for adults which are favoured by cooler temperatures and a
better water supply (Lavelle, 1978). Adults live deeper in the soil than the young, ingest
less soil per unit of fresh weight and have their optimum activity in moister soils.
At a geographical scale, ß diversity increases from the coldest to the warmest
climates. This appears to result from two processes:
(i) the effect of past glaciation. This largely destroyed the fauna of the northern
hemisphere above latitudes of 40-50°N (Michaelsen, 1903); endemic species were
destroyed and only a few species with high capacities for colonization re-invaded
the area following the retreat of the glaciers;
(ii) the exploitation of marginal niches by earthworms in the humid tropics.
The considerable radiation that has taken place among the earthworms of the humid
tropics has led to the development of highly diverse adaptations and the selection of
such peculiar species as specialist arboreals, deep-dwelling oligohumic geophages
and even carnivorous species, as in the African genus Agastrodrilus (Lavelle, 1983b;
Fragoso and Lavelle, 1987). This may be due to: (a) the greater efficiency of the mutu-
alistic system of digestion at high temperatures, which permits earthworms to live
on particularly poor food resources, and (b) the longer time during which radiation,
adaptation and selection have operated in the stable tropical environment.
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