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Abstract

We used Daphnia as a model to study the effect of predators on clonal and taxon coexistence within the same habitat. Different Daphnia species and their hybrids coexist in many lakes worldwide. We studied the potential influence of planktivorous fish on the maintenance of these species assemblages using 20 Daphnia clones, belonging to five hybrid and parental taxa of the Daphnia galeata-hyalina-cucullata complex originating from the Plu beta see (Northern Germany). With these clones, two competition experiments were conducted in two large-scale indoor mesocosms, the Plankton Towers in Plon, Germany. We varied the presence of fish and kairomones. Using allozyme electrophoresis we observed Diel Vertical Migration (DVM) of individual clones. Without fish, two clones belonging to the largest taxa (D. galeata and D. galeata X hyalina) became numerically dominant after three weeks. The mere presence of fish kairomones led to a significant decrease in the niche overlap between the five taxa. With fish present in the towers relative densities of smaller D. cucullata X galeata clones were highest. We observed that under different predation intensities different taxa showed the highest instantaneous rate of increase. Without fish and fish kairomones this rate was the highest for D. galeata, when fish kairomones were present D. galeata X hyalina had the highest relative r. With increasing fish predation D. cucullata and D. cucullata x galeata had the highest relative r, respectively, showing that changing predation pressure facilitates the co-occurrence of taxa in this species complex.