Regional enrichment of predacious water beetles in temporary ponds at opposite east–west ends of the Palearctic
Regional and local species richness of temporary pond dytiscid water beetles were compared among three regions within the Palearctic: (1) Sweden in north west Europe, (2) Primorye and (3) Sakhalin Island in the Russian Far East. Both local and regional species richness were highest in Sweden and lowest in Sakhalin. Regional species richness was calculated from literature and collecting data for each region and for nested parts of regions. Local species richness was estimated from standardized net samples from fourteen or fifteen ponds in each region. Two different rarefaction techniques applied to the net-sample data confirmed the observed interregional differences in species richness. Partial least square regression showed that pond area, depth and temperature affected local species richness positively in each region, whereas increasing shade and drought frequency had negative effects. Residuals from the regression analysis were positive in Sweden, negative in Sakhalin, and near zero in Primorye ponds. Consequently, the local species richness was related positively to regional species richness also when compensated for differences in the local pond environment. This was verified when pond species richness of each region was correlated with principal component scores representing a combination of pond area, depth and temperature. The species’ distributions among ponds displayed significantly nested patterns in Sweden and Sakhalin. However, species were significantly sorted along the pond area gradient only in Sweden. It is concluded that the observed interregional differences in local species richness are best explained by the accompanying gradient in regional species richness, lending support to the hypothesis of regional enrichment. Selected historical and ecological explanations for the observed differences in regional species richness are discussed.
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