Food Webs

The network of interactions depicted in a food web provides the most complete yet succinct visual summary of a biological community. The study of food webs has a long history in ecology, incorporating at least two major lines of inquiry: one emphasizing how species are inextricably linked through their interactions with one another, hence focusing on population and community dynamics; the other concerned with the flux of organic matter and energy. Most examples can be categorized very roughly as connectance food webs, which attempt to identify all possible linkages; energy flux food webs, which quantify organic matter flow along a limited number of major pathways; and trophic-interaction food webs, which emphasize population processes and species interactions. As the field continues to mature we should expect to see increasing effort to merge the patterns depicted in food web structure with the processes of energy flow and species interactions.

An impressively detailed food web for the Broadstone Stream in southern England provides an excellent example of a connectivity food web (Figure 10.9). All species are of equal importance in a connectance web and all lines are of equal weight, because the web is constructed from diet presence-absence data. This food web is amongst the most detailed on record and includes all of the benthic community including macrofauna, meiofauna, protozoa, and algae (Schmid-Araya et al. 2002, Woodward et al. 2005). It includes 131 consumer species supported by eight basal resources and three additional food sources such as eggs, for a total of 842 links. The meiofauna comprised 70% of the species present, demonstrating the need to include small-bodied organisms in the analysis. Food web structure varied seasonally due to

FIGURE 10.9 Connectivity food web for the invertebrate community in Broadstone Stream (Autumn 1996). Numbers represent species and food items: Protozoa: 1-10. Turbellaria: 11. Rotifera: 12-37. Nematoda: 38-41. Oligochaeta: 42-48. Tardigrada: 49 and 50. Acari: 51-56. Insecta: Odonata: 57 Cordulegaster boltonii; Plecoptera: 58 Leuctra nigra, 59 Leuctra hippopus, 60 Leuctra fusca, 61 Nemurella pictetii, 62 Siphonoperla torrentium, 63 Plecoptera larvulae, 64 Leuctra nigra adult; Trichoptera: 65 Plectrocnemia conspersa, 66 Potamophylax cingulatus, 67 Adicella reducta; Megaloptera: 68 Sialis fuliginosa; Coleoptera: 69 Platambus maculatus, 70 Helodidae Gen. sp., 71 Elmidae sp.; Diptera: Ceratopogonidae 72 Bezzia sp.; Tipulidae: 73 Limonia sp., 74 Limonia modesta, 75 Dicranota sp., 76 Pedicia sp., 77 Limnophila sp., 78 Hexatoma sp., 79 Limoniinae Gen. sp.; 80 Rhypholophus sp.; Chironomidae: 81 Macropelopia nebulosa, 82 Trissopelopia longimana, 83 Zavrelymyia barbatipes, 84 Conchapelopia viator, 85 Apsectrotanypus trifascipennis, 86 Zavrelymyia sp. 2, 87 Paramerina sp., 88 Krenope-lopia sp., 89 Pentaneura sp., 90 Natarsia sp., 91 Prodiamesa olivacea, 92 Brillia modesta, 93 Heterotrissocladius marcidus, 94 Heterotanytarsus sp., 95 Eukieferiella sp., 96 Georthocladius luteicornis, 97 Corynoneura lobata, 98 Chirononomus/Einfeldia sp., 99 Polypedilum albicorne, 100 Micropsectra bidentata, 101 Mectriocnemus sp. Adult; Simuliidae: 102 Simulium sp. Crustacea: 0stracoda:103; Cladocera:104-105; Copepoda: Cyclopoida: 106-111; Harpacticoida: 112-116; algae and plant material: 118-122. Various: 123 Plecoptera eggs, 124 Turbellaria eggs, 125 Rotifera eggs, 126 fine organic matter (FPOM), 127 coarse organic matter (CPOM), 128 Leptothrix ochracea. (Reproduced from Schmid-Araya et al. 2002.)

changes in species richness, resulting in temporal changes in the proportion of species at the top and the base of the food web. Despite its complexity, relatively simple patterns in food web structure could be found in relation to body size. Meiofaunal and macrofaunal subwebs were effectively two compartments because large prey were invulnerable to small predators and large predators were not effective in consuming very small prey (Woodward et al. 2005).

Measurement of energy flux provides a quantitative assessment of the strength of linkages along each pathway. In Figure 10.10 we see a less-detailed food web, but one that quantifies organic matter pathways by converting information from gut analyses into annual ingestion rates for caddisfly larvae dwelling on snag habitat in the Ogeechee River, Georgia (Benke and

Wallace 1997). The pathways from amorphous detritus and diatoms to several filter-feeding caddis larvae were particularly strong, but Hydropsyche rossi derived substantially more energy from consuming animal prey than did the other filter feeders. Hall et al. (2000) constructed food webs based on the analysis of organic matter flow for a reference stream in the southern Appalachians and a stream where leaf litter was excluded. Their analysis included nearly 90% of invertebrate production, but <30% of total links, suggesting that not all feeding links need be identified for this approach to be successful. The main basal sources in the reference stream were leaf detritus, bacterial carbon (C), and animal prey, with each contributing 25-30% of the energy supply. A few pathways accounted for most organic matter flow,

Amorphous Diatoms Vascular plant Fungi Filamentous detritus detritus algae

Ingestion values (g nY2 yr"1) ■ » immiiii^-

Amorphous Diatoms Vascular plant Fungi Filamentous detritus detritus algae

Ingestion values (g nY2 yr"1) ■ » immiiii^-

FIGURE 10.10 Energy flow food web for caddisfly larvae in the Ogeechee River, Georgia. The thickness of the arrow represents the ingestion fluxes. Abbreviations: Ephemerop = Ephemeroptera, Lepto = Leptoceridae, Lim-neph = Limnephilidae, M. carolina = Macrostemum carolina. (Reproduced from Benke and Wallace 1997.)

and largest flows were associated with detriti-vores due to their low assimilation efficiency. In the litter exclusion stream, flows to predators were reduced, a few pathways dominated, and consumption rates per biomass were higher, indicating strong interactions with the remaining common taxa.

Although gut content analysis remains a widely used approach to the estimation of energy fluxes, despite the difficulty of identifying bits of soft-bodied prey and amorphous detritus, analysis of the natural abundance of stable isotopes in basal resources and in consumers is providing important new insights into energy flows and trophic position (Peterson and Fry 1987). The ratio of the naturally occurring 13C isotope of C relative to the more abundant 12C, expressed as S13C, is a useful diet tracer because C is the main elemental component of organic matter, it passes through food chains with little alteration of its isotopic composition, and S13C values often differ between major groups of primary producers such as benthic algae, macrophytes, and terrestrial plants. In addition, the S13C of consumer tissues directly reflects the mixture of prey resources consumed and assimilated over previous weeks, months, or even years depending on the tissue studied (Finlay 2001). The application of stable isotope methods has led to key insights that would have been difficult to achieve using prior methods. For example, despite the abundance of detritus from macrophytes and floodplain forests in large tropical rivers such as the Amazon and Orinoco, S13C analyses of consumers indicate a minor contribution of this material to animal food webs. Instead, phytoplankton and periphy-ton are the critical energy sources for most fish species (Araujo-Lima et al. 1986, Hamilton et al. 1992).

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