Competition between native and exotic Daphnia: In situ experiments
The recently introduced exotic cladoceran Daphnia lumholtzi offers an excellent opportunity to study the interactions between exotic and native species in invaded communities. Lake surveys in Missouri have indicated a seasonal succession between native Daphnia and D. lumholtzi. In the current study, we examined competition between D. lumholtzi and the native Daphnia parvula by conducting seasonal in situ field experiments in 1.6 l enclosures. Competition was assessed by comparing the rates of increase (r) and birth rates (b) of each species when grown alone versus when grown together in these enclosures. At high densities, D. lumholtzi suppressed D. parvula rates of increase during the late summer and fall experiments, but did not appear to suppress D. parvula birth rates. The rates of increase of D. lumholtzi did not appear to be affected by the presence of D. parvula. The results of these experiments indicate that although competition between the two species occurs seasonally at high densities, the effects are asymmetrical. The lack of competitive effects on D. lumholtzi by D. parvula suggests that factors other than competition are involved in explaining the absence of D. lumholtzi in spring zooplankton assemblages.