Ecosystem-level evidence for top-down and bottom-up control of production in a grassland stream system

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.