Exploitative competition in differently sized Daphnia species:: A mechanistic explanation

The concept of the threshold food concentration that allows metabolic maintenance of an animal (C-0) has been used as an analogy to Tilman's R* (equilibrium resource requirements) to explain competitive abilities of cladoceran species. Until now, however, theoretical analysis has been hampered because C-0 was measured in experiments where cladoceran growth was determined in response to fixed, low food concentrations, not as a result of the cladocerans' grazing activity. C-0 is an indirect estimate from nonequilibrium conditions. It still needs to be shown that growing Daphnia are efficient enough to suppress resource concentrations to levels equivalent to their C-0. We designed flow-through experiments to establish equilibrium conditions between algal inflow and cladoceran population biomass with three differently sized Daphnia species. The equilibrium food concentration resulting from the daphniids' grazing activity was labeled C*, as it can be used to predict competitive abilities in analogy to Tilman's R*. Equilibrium was reached in each experiment. Daphnia grew to carrying capacity without fluctuations in numbers or biomass, and biomass remained constant after this level was reached. Mortality was compensated for by somatic growth or, in the case of the small species D. ambigua, by production of offspring. When the carrying capacity was reached, algal concentrations remained constant at C*. As predicted from previous growth experiments, larger species suppressed algal concentrations to lower levels, i.e., they were predicted to have higher competitive ability. This prediction was tested in a mixed-species experiment with D. pulicaria and the smaller D. galeata. That experiment clearly demonstrated the mechanism of exploitative competition. In the beginning, the smaller D. galeata performed better, but as soon as the food concentration fell below D. galeata's individually determined C*, the smaller species suffered mortality. At this point, D. pulicaria had not yet reached its C*. It continued to grow and replaced D. galeata. Numerical values of D. pulicaria's C* were slightly higher than the independently determined, corresponding C-0 values. This was to be expected as C-0 applies to zero mortality. However, the rank order among species was identical for both characteristics. We propose that both C* and C-0 have strong predictive power for mechanistic models of competitive interactions.