A trophic cascade with Chaoborus : population dynamics of ex-ephippial generations of Daphnia

Abstract

Changes in population dynamics and competitive outcome between Daphnia pulex and D. rosea in response to severe predation by Chaoborus larvae were investigated in a field experiment. In a nearly fishless lake with unusually high densities of Chaoborus flavicans and C. obscuripes, the two coexisting Daphnia species developed morphological defense structures to a different degree. An enclosure-exclosure setup was applied to investigate the population dynamics of both species in presence and absence of the predators. The field study followed the population growth of freshly hatched juveniles of D. pulex and D. rosea as they naturally emerged from ephippia in the lake's sediment during spring. Both Daphnia species have a large body size and, in enclosures with Chaoborus only, developed neckteeth and elongated tail spines as inducible defenses. Nevertheless, Chaoborus accounted for high mortality rates in ephippial neonates and strongly reduced the chance of individuals to reach maturity, especially in D. rosea, the dominant species in enclosures without Chaoborus. It is concluded that, in the first generation, when otherwise large and morphologically defended Daphnia are synchronized by ephippial origin to a population of only juveniles, a trophic cascade with Chaoborus may become highly effective.