Spaak, P. and Hoekstra, J.F. (1997) Fish predation on a Daphnia hybrid species complex: A factor explaining species coexistence? Limnology and Oceanography, 42, 753-762. ISSN 0024-3590.
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Recent studies on the life histories of Daphnia hybrids and their parental species have revealed that hybrids can combine an intermediate size with a relatively high reproductive rate, which might explain their success in many European lakes. Based on this information, we formulated the temporal hybrid superiority hypothesis to explain the coexistence of parental species and their hybrids. Our hypothesis states that in temporally changing environments, such as lakes, hybrids can coexist with their parental species because hybrid individuals have a higher fitness compared to the parental individuals during specific periods of the year. This is in contrast to the hypothesis that hybrids have a lower fitness than the parentals, which should be compensated by recurrent hybrid production. To test this hypothesis we studied population dynamics and seasonal changes in body size of two Daphnia species (D. galeata and D. cucullata) and their interspecific hybrid D. cucullata X D. galeata in Tjeukemeer, a shallow eutrophic lake in the Netherlands. In 1989, the year with the highest fish predation pressure, the larger D. galeata was first replaced by the smaller hybrid that was soon succeeded by the even smaller D. cucullata. In 1990 the Daphnia community was dominated by the hybrid and in 1991 by D. galeata. Deviations of the instantaneous rate of increase, averaged over the three species, showed a positive relationship with fish predation risk for D. cucullata, a negative relationship for D. galeata, and no relationship for the hybrid. This suggests a higher influence of fish predation on D. galeata. We conclude that the selective advantage of the hybrid over D. galeata under fish predation, expected from life-history experiments, is in agreement with our field data. [KEYWORDS: Shallow eutrophic lake; life-history variation; interspecific hybridization; fresh-water; body-size; tjeukemeer; zooplankton; cladocerans; growth; food]
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