A test of three alternative pathways for consumer regulation of primary productivity

The pathways linking consumer effects to primary productivity (PPR) are likely to vary among taxa because of species-specific trophic and functional differences. Thus, it is necessary to understand the dynamics of consumer–PPR interactions so that effects of species loss on ecosystem function can be addressed from a mechanistic approach. In this study, I used three fish taxa (orangethroat darter, Etheostoma spectabile; western mosquitofish, Gambusia affinis; and bullhead minnow, Pimephales vigilax) as model consumers with different trophic and functional characteristics to test alternative mechanisms for consumer regulation of PPR (i.e., trophic cascade, terrestrial nutrient translocation, and sedimentary nutrient translocation). Experiments were conducted in stream mesocosms fitted with a combination of fish and terrestrial insect barriers to address relative importance of consumer-driven top-down and bottom-up control of PPR. A predatory invertivore, orangethroat darter, increased PPR through an apparent trophic cascade by localized reduction of benthic grazing invertebrate densities (i.e., top-down). A surface feeding insectivore, western mosquitofish, consumed terrestrial insects on the stream surface, increasing PPR by enhancing allochthonous nutrients in the mesocosms (i.e., bottom-up). A benthic omnivore, bullhead minnow, consumed benthic food items, resulting in increased PPR by enhancing availability of autochthonous nutrients via translocation of sedimentary nutrients (i.e., bottom-up). However, under specific environmental contexts, this species also consumed terrestrial invertebrates, potentially affecting PPR through terrestrial nutrient translocation as well. In this study, the trophic and functional characteristics of different species resulted in alternative pathways that increased PPR, suggesting that in natural ecosystems multiple consumer-driven pathways may be influencing PPR simultaneously and could potentially be important for temporal persistence of ecosystem function in changing environments.