Ecosystem-level evidence for top-down and bottom-up control of production in a grassland stream system |
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Authors: | Alexander D Huryn |
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Institution: | (1) Department of Zoology, P.O. Box 56, University of Otago, Dunedin, New Zealand, NZ |
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Abstract: | Ecosystem-wide effects of introduced brown trout (Salmo trutta L.) and native river galaxias (Galaxiaseldoni McDowall) were studied by analysing ecosystem production budgets for two adjacent tributaries of a grassland stream-system
in the South Island of New Zealand. One tributary was inhabited by brown trout, the other by river galaxias. No other fish
species were present in either stream. The budget for the river galaxias stream indicated little top-down control of invertebrates
by fish predation (river galaxias consumed ∼18% of available prey production). A large proportion of annual net primary production
was required to support production by invertebrates (invertebrates consumed an average of ∼75% of available primary production),
and mean surplus primary production (i.e. not consumed) was not significantly different from zero. Primary and secondary production
were presumably mutually limiting in this system (i.e. controlled by simultaneous top-down and bottom-up mechanisms). In contrast,
the budget for the brown trout stream indicated extreme top-down control of invertebrate populations by fish predation; essentially
all invertebrate production (∼100%) was required to support trout production. Invertebrate production required only a minor
portion of annual net primary production (∼21%) and primary production was presumably controlled by mechanisms other than
grazing (e.g. sloughing, nutrient limitation). Predatory invertebrates had little quantitative effect on prey populations
in either stream. Recent experimental studies of invertebrate behaviour, fish behaviour, and food-web structure in New Zealand
streams with physically stable channels indicate that a trophic cascade should be observed in streams inhabited by brown trout,
in contrast to those inhabited by native fish. The results reported here provide ecosystem-level evidence supporting this
prediction.
Received: 10 March 1997 / Accepted: 12 December 1997 |
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Keywords: | Predation Food webs Trophic cascades Invertebrates Fish |
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