Impact of size-selective mortality on waterfleas (Daphnia)

Size‐selective predation shapes species and induces changes in life‐history traits. Exposure to sizeselective mortality will make being the size that is vulnerable dangerous and possibly fatal; thus, if small individuals are targeted, selection might favour faster growth to escape the vulnerable size range, whereas if large individuals are targeted, earlier maturation and/or slower grow are likely to evolve. Evolution caused by selecting large individuals has been shown for several fish species targeted by size‐selective fishing gear (i.e. fishery‐induced evolution). Also, introduced species may generate shifts in the prey’s life history. Here we used probabilistic maturation reaction norms (PMRNs) to look for evidence of introduction‐induced evolution in Daphnia pulex following an introduction of an alien predator (European perch, Perca fluviatilis) targeting large Daphnia to a lake that naturally hosted no planktivorous fish. We estimated PMRNs for clones collected before and after the introduction, and examined whether their PMRNs differed systematically. Introduction is a situation analogous to size‐selective fishing and was expected to lead to a shift in predation pressure favouring early maturing clones. Growth trajectories and size and age of maturation were determined for both pre‐ and post‐introduction clones. Studying one type of predation (natural) can often tell us something about the other (human‐induced)