Daphnia body size and population dynamics under predation by invertebrate and fish predators in Lago Maggiore: an approach based on contribution analysis

Predation on Daphnia is size-specific: while zooplanktivorous fish select large, ovigerous females, carrying larger clutches, predation by invertebrates, particularly Cladocera, is generally regarded as acting mainly on young, small, non-ovigerous females. The two types of predators therefore produce different effects on the population of their prey: fish predation results in decreases in Daphnia fecundity, proportion of adults, and maximum body size, while predation by the invertebrate predators leads to decreases in the number and proportion of juvenile Daphnia. We investigated the effect of predation by three predators (one vertebrate, two invertebrates) on Daphnia. The study combines contribution analysis of Daphnia birth rate dynamics, with a body-size-oriented analysis of both predator and prey populations. Contribution analysis showed that during April-May, when Bythotrephes densities were low, changes in Daphnia birth rate were due to both changes in fecundity (ConF) and in the proportion of adults (ConA), with ConA being much larger (by absolute value) than ConF; such a pattern is expected under fish predation. Whereas during mid May to June, when Bythotrephes densities were high, changes in Daphnia birth rate were again mainly due to both ConF and ConA, but now the difference between these contributions was less pronounced. Although Bythotrephes predation produced similar effects on Daphnia population dynamics as fish predation, affecting both fecundity and the proportion of adults, it can be distinguished in two ways from fish predation. Firstly, the ConF-ConA pattern is more uniform under invertebrate predation than under fish predation and secondly, the mean body size of Daphnia increased continuously whereas the mean size of ovigerous females remained approximately constant except for the last 3 weeks of June, indicating a high mortality of juveniles during the whole period and an increased mortality of young females with eggs during the last 3 weeks. We conclude that both zooplanktivorous fish and invertebrate predators affect population dynamics of Daphnia in Lago Maggiore. However, the invertebrate predator Bythotrephes has the largest impact on Daphnia dynamics in June, both because of its high densities and its relatively large size at that time. Predation pressure exerted by Bythotrephes is high enough to reduce the Daphnia densities at the end of June to low values.