The niche-width variation hypothesis revisited: Microscale testing of the earthworm Bimastos syriacus (Rosa) and multispecies comparison

The niche-width variation hypothesis, which predicts positive correlation between niche width and genetic diversity, was tested in the earthworm Bimastos syriacus (Rosa). The earthworms were collected from the microclimatically warmer, drier, and more stressful south-facing (SFS) and opposite dense liveoak maquis north-facing (NFS) slopes at "Evolution Canyon", Lower Nahal Oren, Mt. Carmel, Israel. The opposite slopes of "Evolution Canyon", which are 100 m apart at the bottom, share the same geology and macroclimate but differ drastically in microclimate, flora and fauna. We predicted higher genetic diversity in earthworms living on SFS because they are exposed to climatically more heterogeneous and stressful conditions. Eighty-four specimens of earthworms were examined from SFS, NFS, and the valley bottom, each tested for 16 allozyme putative loci. Genetic diversity indices were higher on SFS as predicted. In a multispecies test involving 8 species (lichens, wild barley, earthworms, mollusks, diplopods and beetles) studied earlier we showed that the subpopulations living on SFS display significantly higher gene diversity than subpopulations from NFS. Our results indicate higher gene diversity in climatically more fluctuating and heterogeneous conditions and thus corroborate the prediction of the niche-width variation hypothesis.