The distribution pattern of Hirneola auricula-judae in the Netherlands

The distribution pattern of Hirneola auricula-judae in the Netherlands is discussed. At first this pattern was thought to be mainly determined by the average daily minimum temperatures in winter; high population densities correlating with high winter temperatures. The curve which best fits the data is so steep, that it would mean the virtual exclusion of Hirneola auriculajudae from regions with winter temperatures only a few degrees lower than in the Netherlands. Nevertheless the species is reported as not uncommon in the northern U.S.A. and southern Canada, as well as in Central Europe. From the literature it appears that ‘H. auricula-judae’ in North America is not conspecific with the taxon so named in Europe, but specimens from Central Europe were found to be morphologically inseparable from specimens from Western Europe. Revaluation of the data on the distribution of the species in the Netherlands in relation to plant geographical districts showed that coexistent with the positive correlation of high population density with high winter temperatures is another one with alcaline soils. The corrected gradient of the correlation between population density and winter temperatures is less steep than originally calculated and consequently the relatively high population density in Central Europe is no longer inconsistent with the distribution pattern of the species in the Netherlands.