Competition Between Native and Exotic Daphnia

Date of Graduation

Summer 1998


Master of Science in Biology



Committee Chair

John Havel


The recently introduced exotic cladoceran zooplankter Daphnia lumholtzi offers an excellent opportunity to study the impact of an exotic species on invaded communities. Of particular concern are the possibilities that exotics may displace native taxa and alter trophic interactions. In situ field experiments, using replicated 1.6 liter Plexiglas and Nitex-mesh enclosures, were conducted to study competition between the exotic D. lumholtzi and the native species Daphnia parvula. Experiments were conducted in the early summer, late summer, and fall of 1997, and the winter of 1997-98. Competition was assessed by looking at the ending densities, population growth rates, egg ratios and birth rates of each species when alone, and when in combination. Dye tests indicated water exchange rates in the cylinders of 15-99% per hour. Chlorophyll-a in the enclosures was depressed by as much as 60% relative to that in the lake and, during the fall experiment, enclosures with Daphnia had significantly lower chlorophyll-a levels than blank enclosures. At high densities, D. lumholtzi suppressed D. parvula ending densities and population growth rates during the late summer and fall experiments. Suprisingly, there was no suppression of D. parvula birth rates in the presences of D. lumholtzi. Population growth rates of D. lumholtzi did not appear to be affected by the presence of D. parvula. D. lumholtzi grew poorly under both late summer and winter conditions. The results of these experiments indicate that, although competition between the two species occurs at high densities, the effects are asymmetrical and are not due to suppressed reproduction. The lack of competitive effects on D. lumholtzi by D. parvula suggests that other factors are involved in explaining the absence of D. lumholtzi in the spring zooplankton.

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© Jennifer Lynne Cacka