Consequences of the size structure of fish populations for their effects on a generalist avian predator |
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Authors: | Kloskowski Janusz |
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Institution: | (1) Department of Nature Conservation, Institute of Biology, M. Curie-Skłodowska University, Akademicka 19, 20-033 Lublin, Poland |
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Abstract: | Size-structured interspecific interactions can shift between predation and competition, depending on ontogenetic changes in
size relationships. I examined the effects of common carp (Cyprinus carpio), an omnivorous fish, on the reproductive success of the red-necked grebe (Podiceps grisegena), an avian gape-limited predator, along a fish size gradient created by stocking distinct age-cohorts in seminatural ponds.
Young-of-the-year (0+) carp were an essential food source for young grebes. Only adult birds were able to consume 1-year-old
(1+) fish, while 2-year-old (2+) fish attained a size refuge from grebes. Amphibian larvae were the principal alternative
prey to fish, followed by macroinvertebrates, but the abundance of both dramatically decreased along the carp size gradient.
Fledging success was 2.8 times greater in ponds with 0+ versus 1+ carp; in ponds with 1+ carp, chicks received on average
2.6–3 times less prey biomass from their parents, and over 1/3 of broods suffered total failure. Breeding birds avoided settling
on 2+ ponds. These results show that changes in prey fish size structure can account for shifts from positive trophic effects
on the avian predator to a negative impact on the predator’s alternative resources. However, competition did not fully explain
the decrease in grebe food resources in the presence of large fish, as carp and grebes overlapped little in diet. In experimental
cages, 1+ carp totally eliminated young larvae of amphibians palatable to fish. In field conditions, breeding adults of palatable
taxa avoided ponds with 1+ and older carp. Non-trophic interactions such as habitat selection by amphibians or macroinvertebrates
to avoid large fish may provide an indirect mechanism strengthening the adverse bottom-up effects of fish on birds. |
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