Population dynamics of the Little Grebe Tachybaptus ruficollis: increase, decrease or cycles?

By using a time series of more than 30 years, we describe the population dynamics of three grebe species occurring in a pond area of north-eastern Bavaria (Tachybaptus ruficollis, Podiceps nigricollis, P. cristatus): (1). The Little Grebe showed no consistent increase or decrease in its population counts across years. However, autocorrelation and spectral analysis of population counts suggested a population cycle with a length of 17 years. Population numbers of the Little Grebe were correlated to temperatures in January of the preceding Winter. Using the residuals from this regression in spectral analysis none of the possible cycles passed significance after a Bonferroni correction. (2) Compared to the other two grebe species, the Black-necked Grebe showed the most pronounced fluctuations between years. The time series appears to be random. (3) The Great Crested Grebe showed an increase of population numbers since 1985. This increase is explained by a change in the management system of the ponds. Presently fish farmers put noncommercial fish species into special ponds which increases the food availability.