REGIONAL ENRICHMENT OF PREDACIOUS WATER BEETLES IN TEMPORARY PONDS ATOPPOSITE EAST-WEST ENDS OF THE PALEARCTIC

Regional and local species richness of temporary pond dytiscid water b eetles were compared among three regions within the Palearctic: (1) Sw eden in north west Europe, (2) Primorye and (3) Sakhalin Island in the Russian Far East. Both local and regional species richness were highe st in Sweden and lowest in Sakhalin. Regional species richness was cal culated from literature and collecting data for each region and for ne sted parts of regions. Local species richness was estimated from stand ardized net samples from fourteen or fifteen ponds in each region. Two different rarefaction techniques applied to the net-sample data confi rmed the observed interregional differences in species richness. Parti al least square regression showed that pond area, depth and temperatur e affected local species richness positively in each region: whereas i ncreasing shade and drought frequency had negative effects. Residuals from the regression analysis were positive in Sweden, negative in Sakh alin, and near zero in Primorye ponds. Consequently, the local species richness was related positively to regional species richness also whe n 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. d epth and temperature. The species' distributions among ponds displayed significantly nested patterns in Sweden and Sakhalin. However, specie s were significantly sorted along the pond area gradient only in Swede n. It is concluded that the observed interregional differences in loca l species richness are best explained by the accompanying gradient in regional species richness, lending support to the hypothesis of region al enrichment. Selected historical and ecological explanations for the observed differences in regional species richness are discussed.