title: Fish introductions and light modulate food web fluxes in tropical streams: a whole-ecosystem experimental approach.
abstract: Decades of ecological study have demonstrated the importance of top-down and bottom-up controls on food webs, yet few studies within this context have quantified the magnitude of energy and material fluxes at the whole-ecosystem scale. We examined top-down and bottom-up effects on food web fluxes using a field experiment that manipulated the presence of a consumer, the Trinidadian guppy Poecilia reticulata, and the production of basal resources by thinning the riparian forest canopy to increase incident light. To gauge the effects of these reach-scale manipulations on food web fluxes, we used a nitrogen (15N) stable isotope tracer to compare basal resource treatments (thinned canopy vs. control) and consumer treatments (guppy introduction vs. control). The thinned canopy stream had higher primary production than the natural canopy control, leading to increased N fluxes to invertebrates that feed on benthic biofilms (grazers), fine benthic organic matter (collector-gatherers), and organic particles suspended in the water column (filter feeders). Stream reaches with guppies also had higher primary productivity and higher N fluxes to grazers and filter feeders. In contrast, N fluxes to collector-gatherers were reduced in guppy introduction reaches relative to upstream controls. N fluxes to leaf-shredding invertebrates, predatory invertebrates, and the other fish species present (Hart's killifish, Anablepsoides hartii) did not differ across light or guppy treatments, suggesting that effects on detritus-based linkages and upper trophic levels were not as strong. Effect sizes of guppy and canopy treatments on N flux rates were similar for most taxa, though guppy effects were the strongest for filter feeding invertebrates while canopy effects were the strongest for collector-gatherer invertebrates. Combined, these results extend previous knowledge about top-down and bottom-up controls on ecosystems by providing experimental, reach-scale evidence that both pathways can act simultaneously and have equally strong influence on nutrient fluxes from inorganic pools through primary consumers.
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.|Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources, East Lansing, Michigan, 48824, USA.
authors.forename: Sarah M
authors.identifier: 0000-0001-5503-7386
authors.initials: SM
authors.lastname: Collins
authors.affiliation: School of Natural Resources, University of Nebraska-Lincoln, Hardin Hall Room 403, Lincoln, Nebraska, 68583, USA.
authors.forename: Steven A
authors.initials: SA
authors.lastname: Thomas
authors.affiliation: School of Natural Resources, University of Nebraska-Lincoln, Hardin Hall Room 403, Lincoln, Nebraska, 68583, USA.|Department of Ecology, State University of Rio de Janeiro, 524 Rua São Francisco Xavier, Rio de Janeiro, Rio de Janeiro, 20550-013, Brazil.
authors.forename: Thomas
authors.initials: T
authors.lastname: Heatherly
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.
authors.forename: Keeley L
authors.initials: KL
authors.lastname: MacNeill
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.|Institute of Biology, Federal University of Bahia, Ondina Campus, Rua Barão de Geremoabo, 147, Salvador, Bahia, 40170-290, Brazil.
authors.forename: Antoine O H C
authors.initials: AO
authors.lastname: Leduc
authors.affiliation: CNRS UMR 7618, Institute of Ecology and Environmental Sciences Paris (iEES), Université Pierre et Marie Curie, 7 Quai Saint Bernard, Paris, 75252, France.|Deptartment of Biological and Environmental Sciences, University of Jyväskylä, PO Box 35, Jyväskylä, 40014, Finland.
authors.forename: Andrés
authors.initials: A
authors.lastname: López-Sepulcre
authors.affiliation: Biology Department, College of William and Mary, Williamsburg, Virginia, 23187, USA.
authors.forename: Bradley A
authors.initials: BA
authors.lastname: Lamphere
authors.affiliation: Department of Biology, University of Victoria, PO Box 1700 STN CSC, Victoria, British Columbia, V8W 2Y2, Canada.
authors.forename: Rana W
authors.initials: RW
authors.lastname: El-Sabaawi
authors.affiliation: Department of Biology, University of California Riverside, 209 University Laboratory Building, Riverside, California, 92521, USA.
authors.forename: David N
authors.initials: DN
authors.lastname: Reznick
authors.affiliation: Odum School of Ecology, University of Georgia, Athens, Georgia, 30602, USA.
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Pringle
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Flecker
mesh_terms: D000818:Animals; D018533:Biomass; D005399:Fishes / Q000502:physiology*; D020387:Food Chain; D008027:Light; D011157:Population Dynamics; D045483:Rivers; D014300:Trinidad and Tobago; D014329:Tropical Climate; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water
keywords: benthic macroinvertebrates, primary production, Trinidad guppy, Neotropics, 15N; nitrogen flux; reach-scale experiment; stable isotope tracers; stream food web; top-down and bottom-up effects; trophic linkages