Food Webs differences among most biological species Life history omnivory expands the diet of species throughout the entire animal kingdom with the exception of taxa that use the same food species throughout their lives (e.g., some herbivores) and those with exceptional parental investment (e.g., birds and mammals) so the young not forage for themselves ‘‘Incidental omnivory’’ occurs when consumers eat foods in which other consumers live Thus, scavengers and detritivores not only eat carrion or organic matter but also the trophically complex array of microbes and macroorganisms that live within these foods Frugivores and granivores commonly eat insects associated with fruits and seeds Predators eat not only their prey but also the array of parasites living within the prey In each case, consumers automatically feed on at least two trophic levels These types of omnivory are widespread and common Their ubiquity poses many questions First, how does omnivory affect food web structure? Most obviously, it increases complexity and connectivity Second, can we ignore omnivory in the analyses of food webs? By its very nature, omnivory causes consumers to have a great number of links, each of which may be numerically unimportant in the diet For many reasons delineated later, we cannot arbitrarily ignore apparently minor diet links if we hope to understand dynamics Patterns of Biomass and Energy in Food Webs Primary productivity is among the most fundamental biological processes on the planet, transferring the energy locked in light and various inorganic molecules into forms useful to sustain producers and the diversity of consumers What factors control primary productivity and regulate its distribution among plants, animals, and microbes? How changes in primary productivity work their way through a food web to alter the abundance and biomass of herbivores to predators and detritivores? As discussed later, such key questions are best assessed using a food web approach However, considerable controversy exists regarding the exact way that food web structure influences community and ecosystem dynamics Trophic Levels, Green Worlds, and Exploitative Ecosystems Ecological research has amply demonstrated that food webs in nature contain hundreds to thousands of species, reticulately connected via multiple links of various strength to species in the autotroph and saprophagous channels and in the same and different habitats; omnivorous, age-structured consumers are common Nevertheless, much food web theory still relies on the idealization of trophic levels connected in a single linear chain (plant herbivore carnivore) Here, we evaluate this simplification and some of its implications In particular, we focus on two grand theories whereby food webs are considered to be central to community organization The trophic level ideal in a simple linear food chain has had great appeal Trophodynamics sought to explain the height of the trophic pyramid by reference to a progressive attenuation of energy passing up trophic levels, envisioned as distinct and functionally homogeneous sets of green plants, 503 herbivores, primary carnivores, and, sometimes, secondary carnivores This is a bottom–up community theory based on the thermodynamics of energy transfer In counterpoint, Hairston, Smith, and Slobodkin’s green-world hypothesis (GWH; Hairston et al., 1960) is primarily a top–down theory, with abundance at each level set, directly or indirectly, by consumers at the top of the chain Thus, carnivores suppress herbivores, which releases green plants to flourish These and earlier theoretical studies attempted to simplify food webs greatly to find generalities among them GWH reduced complex webs to food chains in which species were pigeonholed into specific trophic levels This allowed for predictions on how higher trophic levels (e.g., predators) influenced the dynamics of lower trophic levels (e.g., primary producers) Oksanen et al.’s (1981) exploitation ecosystem hypothesis (EEH) generalizes GWH to fewer or more than three trophic levels Trophic cascades are examples of food chains that behave approximately according to EEH Trophodynamics and EEH each rely on the integrity of trophic levels and the existence of a single, albeit different, overwhelming mechanism that imposes structure on ecosystems EEH proposes a conceptual framework of ‘‘exploitation ecosystems’’ in which strong consumption leads to alternation of high and low biomass between successive levels Even numbers of ‘‘effective’’ trophic levels (two or four levels) produce a low-standing crop of plants because the herbivore population (level 2) flourishes Odd numbers (one or three levels) result in the opposite effect: Herbivores are suppressed and plants well Proponents of EEH differ on subsidiary points, the first being the role of bottom–up effects in which primary productivity sets the number of effective levels The most productive systems support secondary carnivores and therefore have four levels and lowstanding crops of plants Low-productivity systems (e.g., tundra) support only one effective level – plants More productive habitats (e.g., forests) have three Productivity is never high enough to support more than three effective levels on land or four in water Other studies argue that physical differences between habitats, by affecting plant competition and consumer foraging, cause three levels on land and four in water EEH definitions of trophic levels are distinctive and adopt the convention that trophic levels occur only if consumers significantly control the dynamics or biomass of their food species Without top–down control, consumers not comprise an effective trophic level regardless of biomass or number of species involved Supporters of EEH have noted that only when grazers regulate plants are grazers counted (as a trophic level), and only when predators regulate grazers they are fully counted Thus, considerations of food chain dynamics not become stranded in the immense complexity of real food webs Whereas, GWH trophic levels are based on energy deriving from primary productivity Thus, ‘‘trophic level interactions y weight particular links in the food web for their energetic significance.’’ A trophic level is ‘‘a group of organisms acquiring a considerable majority of its energy from the adjacent level nearer the abiotic source.’’ Despite these differences, both EEH and GWH theory argue that variability in the number of trophic levels exerts profound consequences on community structure and dynamics Considerable controversy exists as to the validity of GWH and EEH The consensus has swung against these grand