Detritivorous fish indirectly reduce insect secondary production in a tropical river

Dominant animals can indirectly regulate population dynamics and energy flow for many other species in an ecosystem by altering habitat structure and resource availability. However, we know little about the degree to which other taxa can compensate for the loss of these dominant species. By removing animals and measuring responses of other animals as secondary production it is possible to assess how much other taxa can perform similar functions of these dominant taxa. Here we tested the response of aquatic insect secondary production to the loss of a dominant detritivorous fish Prochilodus mariae , in a tropical river, Rio Las Marias, Venezuela. Using an impermeable, plastic curtain, we excluded Prochilodus from one half of a 235‐m stream reach for 6 weeks. We measured insect production as biomass times empirically measured body mass‐specific growth rates for 8 common taxa constituting 59–74% of insect biomass. Removing Prochilodus increased the standing stock of benthic organic sediment. Biomass of the entire assemblage increased 1.7‐fold and insect production for 8 taxa tripled upon exclusion of Prochilodus . Two fast‐growing mayfly genera, Tricorythodes and Leptohyphes drove the increased secondary production. Despite low biomass of insects, growth rates were among the highest measured for freshwater insects, and these high growth rates in part caused high secondary production. Although insect production was high in the exclusion reach, insects did not compensate for the loss of Prochilodus in terms of consuming organic sediment showing that the capacity of detritivorous fish to process sediments is higher than that for aquatic insects in this tropical river.