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Productivity, Consumers, and the Structure of a River Food Chain (Predation/Herbivory/Multltrophic Level Theory/Community Ecology/Predator-Prey Theory) J

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Productivity, Consumers, and the Structure of a River Food Chain (Predation/Herbivory/Multltrophic Level Theory/Community Ecology/Predator-Prey Theory) J Proc. Nati. Acad. Sci. USA Vol. 90, pp. 1384-1387, February 1993 Ecology Productivity, consumers, and the structure of a river food chain (predation/herbivory/multltrophic level theory/community ecology/predator-prey theory) J. TIMOTHY WOOTTON* AND MARY E. POWER Department of Integrative Biology, University of California, Berkeley, CA 94720 Communicated by Robert T. Paine, November 2, 1992 (receivedfor review June 10, 1992) ABSTRACT We tested models of food chain dynamics in METHODS experimentally manipulated channels within a natural river. As Tests of food chain models require the experimental manip- light levels increased, primary productivity and the biomass of ulation of productivity while holding food chain length and algae and primary predators increased, but the biomass of community membership constant (29, 30). We conducted grazers remained relatively constant. In the presence ofa fourth such a study in the South Fork Eel River on the Northern trophic level, algae and primary predators decreased, but California Coast Range Preserve (34°44' N, 123°39' W) in grazers increased. These results match predictions offood chain Mendocino County, CA. We subdivided sections of a rela- models based on classical predator-prey theory and suggest that tively homogeneous pool within the river to make 25 in- simple models of multitrophic level interactions are sometimes stream channels. We built channels (3.0 m long x 0.7 m wide sufficient to predict the responses of natural communities to x 1.5 m high) in blocks of five with frames of polyvinyl changes in environmental productivity and predators. chloride pipe and wood that supported side walls of heavy black plastic and end walls of 6.0-mm plastic mesh. Mesh Ecologists seek to describe the dynamics of natural systems ends permitted water exchange (current in pool < 0.3 cm s-1) and to predict responses of these systems to environmental and access for algae, invertebrates, and small, but not large change. Although ecological theory examining the dynamics (>30 mm long), fish. Thus, food webs assembling inside of species competing for a common resource at one trophic channels were limited to three trophic levels. Walls were level has been well developed (1-5), field experiments indi- buried in the natural stream bed (stones 2-10 cm in diameter). cate that interactions among species at different trophic We placed a set of four 7.5 cm x 7.5 cm ceramic floor tiles levels must be considered to predict the dynamics of natural at the upstream and downstream ends of each channel to communities (6-8). Models offood chains provide a first step serve as uniform sampling substrates for algae and benthic in developing a theory of multitrophic level communities and invertebrates. We also placed tiles in the two 3.0 m x 0.7 m represent an opportunity to link investigations at community areas flanking each block of channels to determine how and ecosystem levels. Therefore it is important to determine abundance patterns changed in areas accessible to a fourth how well food chain models can predict changes in the trophic level (fish > 30 mm in total length). dynamics of natural systems. Here we report results from a We manipulated productivity in the channels by using roofs field experiment designed to test basic food chain models by made of different materials: (i) clear plastic (mean photon examining the response of food chain structure to manipu- flux density of photosynthetically active radiation, X = 1342 lations of primary productivity and secondary predators in a ± 36 Amol of photons-m-2 s'1, n = 5), (ii) window screen northern California river community. (X = 912.2 ± 50.1 ,Urmolm-2.s-1), (iii) light shade cloth (X = Models of food chains are generally developed as exten- 582.7 ± 43.1 M.tmo m-2-s-), (iv) heavy shade cloth (X = 493.7 sions of basic predator-prey theory, which traditionally has ± 14.8 Lmol m-2 sl), and (v) black plastic (X = 1.94 ± 1.25 assumed that the rate of is a k.moklm-2_s'). Without roofs, light levels on the river bed consumption predators simple under 40-60 cm of water averaged 1514 ± 90 (n = 10) function of prey density (1, 2, 9-18). These models predict ,urmolm-2.s-. Each shade treatment was replicated five that community structure will be controlled by predators at times and was assigned to channels in a randomized block the top and by plant productivity at the bottom of the food design stratified to ensure that each treatment was repre- chain (9-18). In a food chain of a given length, increasing sented in each of the five positions possible within a block. productivity is expected to increase the abundance of pop- We first conducted an experiment to verify that the different ulations at the top trophic level and populations at alternate light regimes did not affect the foraging success of visual levels below it. Intervening trophic levels are not expected to predators by introducing three (48-58 mm) preweighed steel- increase in biomass with increases in productivity but expe- head (Oncorhynchus mykiss) into each ofthe 25 channels for 11 rience faster turnover as the trophic level above crops the days and determining if light affected fish growth and survivor- surplus productivity. For example, in a four-level food chain, ship. After removing all of the steelhead and measuring their increasing productivity should increase the abundance of wet weight, we initiated food web assembly experiments on 6 secondary predators (i.e., consumers of primary predators) July 1991. We censused the channels after 30 and 55 days by and herbivores but not primary predators (i.e., consumers of visually counting fish and predatory invertebrates within each herbivores) or producers. In a three-level food chain, how- channel, by enumerating all small invertebrates on the top and ever, increasing productivity should increase the abundance bottom of each tile, and by scraping algae from two tiles per of primary predators and producers but not herbivores. The channel for taxonomic identification and ash-free dry weight models also predict that when the top consumer in the food analysis in the laboratory. Invertebrate and fish abundances chain is reduced or removed, abundances should alternately were converted to biomass by collecting a sample of each taxa increase and decrease at sequentially lower trophic levels, and determining its average dry weight. producing a "trophic cascade" (12, 19-28). Fifty-eight days into the experiment, we measured primary productivity in three replicates of each light treatment and The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" *Present address: Department of Zoology, NJ-15, University of in accordance with 18 U.S.C. §1734 solely to indicate this fact. Washington, Seattle, WA 98195. 1384 Downloaded by guest on September 23, 2021 Ecology: Wootton and Power Proc. Natl. Acad. Sci. USA 90 (1993) 1385 21- ing light levels increased productivity (Fig. 1, linear regres- *E18 - . sion, P < 0.006) but did not affect survivorship (X2 = 6.18, P NE 15- > 0.1) or growth (ANCOVA with number of survivors, F15 = E 1.37, P > 0.25) of steelhead in the channels during the cm 12 - preliminary feeding experiment. At both sampling dates, the 0 M 9 - biomass of primary producers (comprised primarily by the E macroalgal chlorophyte Cladophora glomerata and its epi- a6 phytic diatom Epithemia sp.) correlated positively with light *is 3- (Fig. 2 A and B; both P < 0.0001). Likewise, the biomass of 01 small predators (mostly three-spined sticklebacks Gasteros- CL 0 250 500 75'0 10'00 1250 15'00 1750 teus aculeatus, juvenile California roach Hesperoleucas sym- metricus, damselfliesArchilestes californica, and dragonflies Light (gmol m -2S -) Aeshna californica) increased with light on both sampling FIG. 1. Relationship between manipulated light level and gross dates (Fig. 2 E and F; both P < 0.001). In contrast, herbivore productivity of algae in channels. Line of best fit: Productivity (mg abundance showed no significant trends with light-limited of 02 m-2.mink1) = 4.83 + 0.006 light (,amol of photons-m-2-s-1), r2 productivity (Fig. 2 C and D; both P > 0.15). Major herbi- = 0.377, n = 19. Light levels vary within treatments because of vores included mayfly nymphs (mostly Paraleptophlebia and differences in water depth. Centroptilum), caddisfly larvae (Gumaga, Mysticides, and four areas outside the channels by placing a tile from each site Lepidostoma), snails (Physella), freshwater limpets (Ferris- tested into a sealed glass container filled with partially sia), midge larvae (chironomidae), and water pennies (Eu- deoxygenated water (5.0 Mg of02/liter) and returning it to its brianix). The slopes ofthe relationships between biomass and channel for incubation. Using a portable oxygen meter, we light within each trophic level did not differ significantly first measured depletion of 02 from respiration in complete between the 30- and 55-day sample dates (ANCOVA testing darkness by wrapping the containers in aluminum foil for 1.5 interaction between sample date and light, all F16 < 2.55, P hr and then removed the foil to measure net productivity in > 0.1), suggesting convergence toward a stable pattern. The the channels after another 1.5 hr. mean biomass of primary predators and algae declined slightly but significantly between sample dates, however, RESULTS whereas the mean biomass of herbivores increased slightly Food chain models predicted the results of our productivity (Fig. 2, ANCOVA, all F47 > 4.47, P < 0.04), indicating that manipulations. As our experimental design intended, increas- the system was not strictly at equilibrium. The alternating 30 Days 55 Days 12 A m 12- B . co E10- : m E 10 8 I U a U 6 IP ?6 04 on Cb4 L & U-.
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