Data for: Latitudinal gradients in the species diversity of Japanese earthworms

Although a latitudinal gradient of species diversity is known for diverse
animal groups, few detailed studies on this topic exist for soil animals.
Our study aimed to clarify the formation process of latitudinal gradients
in the species diversity of soil animals in terms of dispersal and
evolutionary processes using terrestrial earthworms. We used 4074
earthworms collected from 131 sites in the Japanese archipelago between
31.7°N and 45.1°N. Because morphological classification was difficult for
these earthworms, especially for juveniles, we employed a DNA-based method
for species delimitation. We used the DNA data for phylogenetic diversity
indices and population genetic analysis in subsequent community-ecological
analyses. We analysed latitudinal changes in local and regional species
diversity and the replacement and nested structures among species
assemblages using phylogenetic diversity indices and examined whether
environmental factors and the ecological traits of earthworms related to
dispersal ability contribute to geographic diversity patterns. Our
earthworm samples comprised 113 megascolecid and nine lumbricid molecular
operational taxonomic units (MOTUs). The species assemblage of
Megascolecidae presented higher γ-, β- and α-diversity at lower latitudes
affected by temperature, precipitation and snow depth. Overall β-diversity
was greater at lower latitudes, reflecting greater spatial turnover due to
greater γ-diversity with more local species at lower latitudes. In
contrast, relatively high nestedness was observed at highest latitudes,
where γ-diversity and overall β-diversity were lowest and spatial turnover
was minimal, suggesting that high-latitude species assemblages were formed
through the range expansion of a subset of species from lower latitudes.
Such range expansion was likely facilitated for potentially
parthenogenetic species. In contrast, Lumbricidae presented greater
γ-diversity at higher latitudes and a converse nested structure from high
to low latitudes. Our study demonstrated that DNA-based species
delimitation is necessary to understand the exact geographic diversity
pattern and its formation process for organisms whose morphological
classification is difficult. The contradistinctive species-assemblage
patterns between related earthworm groups might be considered to have
resulted from their low dispersal ability, different biogeographical
histories, and characteristic topography in the Japanese Archipelago,
which covers a latitudinally wide range.