Coexistence of similar zooplankton species by differential adaptation to reproduction and escape in an environment with fluctuating food and enemy densities : II. Field data analysis of Daphnia.

1. In the Klostersee in southern Federal Republic of Germany, Daphnia hyalina (H) and D. cucullata (C) have coexisted for at least 50 years. in 1971/72 they exhibited seasonal density fluctuations which are in accord with predictions from a model based on alternative adaptations of two competitors to natality (food utilization) and mortality (escape from enemies) in an environment that fluctuates with respect to food and intensity of predation. 2. In general, both species fluctuate in a similar fashion. Birth (b), death (m) and growth rates (r) are correlated between both species. Furthermore, in either species birth and death rates are positively correlated. But the birth and death rates of H are about 1.7 times as large as those of C, resulting in stronger density fluctuations of H. This fact, in combination with the seasonal course of the b/m-ratio produces a marked seasonal segregation of the abundances of both species, with H dominating in early summer and C dominating in autumn and winter. 3. In both species, the fluctuations of the growth rate are mainly determined by fluctuations of the death and not the birth rate. 4. Birth and death rates are much higher in the warm season than in the cold season, but births dominate in the cold, deaths in the warm season. Evidence is presented that mortality is mainly due to predation by fish. 5. The differences of natality and mortality which cause the seasonal segregation between the two species, exist independent of seasonal temperature fluctuations. But, there is an additional temperature effect accentuating the differences. 6. A partial correlation analysis shows that by measuring the density of one species and lake temperature, the density difference between both species and, hence, the density of the second species can be predicted with good precision. 7. The importance of the observed temporal segregation and other factors for the stabilized coexistence of both species is discussed.