Food level effects on thecompetitive interactions of two co-occurring cladoceran zooplankton: Diaphanosoma brachyurum andDaphnia ambigua

Laboratory cohort life table experiments were conducted to determine food level effects
on the competitive abilities of two co-occurring cladoceran zooplankton. Diaphanosoma brachyurum and Daphnia ambigua raised on a low (0.2 mg C/l) and high (2 .0 mg C/l) resource level
indicated no significant species differences in the life history parameters; age at first reproduction,
brood size, or juvenile survivorship at either food level. However, D. brachyurum lived longer,
produced a greater number of broods and more young than did D. ambigua. Diaphanosoma
brachyurum had a higher realized rate of increase (r) at the low food level, whereas D. ambigua
showed a higher r when fed the high food level. These results suggest D. ambigua to be the competitively
dominant species when food is abundant and D. brachyurum to dominate when food
levels are sufficiently limited. Results of laboratory competition experiments followed the trends
indicated by life table experiments.