Encyclopedia of biodiversity vol 3

FOOD WEBS Gary R Huxel and Gary A Polis University of California, Davis I II III IV V Introduction Types of Food Webs Omnivory and the Structure of Food Webs Patterns of Biomass and Energy in Food Webs Current Topics/Trends in Food Web Studies GLOSSARY community The most practical definition is a set of species that interact at a given location connectivity web This type of food web illustrates only feeding links without reference to strength of interaction or energy flow detrital shunts Energy and nutrients from the saprovore web reenter the plant herbivore predator food web when detritivores are eaten by predators that also eat plants, herbivores, or other predators donor control Consumer population growth is affected by their resources but consumers not affect the renewal rate of these resources and hence cannot depress their resources ecosystem A set of one or more communities and their abiotic environment energetic web This type of food web quantifies the amount of energy (or material) that flows across links joining species food or biomass pyramid A graphic representation of the energy or biomass relationships of a community, in which the total amount of biomass, or total amount of energy available, at each successive trophic level is proportional to the width of the pyramid at the appropriate height food chain A representation of the links between consumers and their resources, for example nutrients Ǟ plant Ǟ herbivore Ǟ carnivore In these representations, energy or material flows up the chain in a linear fashion In addition, a food chain can be a linear set of species within a food web food web A representation of feeding relationships in a community that includes all the links revealed by dietary analysis functional or interaction web This type of food web quantifies the strength of interaction between species linked using data from manipulative experiments recipient control Consumers substantially depress populations of their resources spatial subsidies Input from other habitats of organic carbon, nutrients, and prey or the movement of consumers These resources can influence greatly the energy, carbon, and nutrient budget of recipient habitats In general, nutrient inputs (nitrogen, phosphorus, and trace elements) increase primary productivity; detrital and prey inputs produce numerical responses in their consumers trophic level An abstract classification to describe subsets of species that acquire energetic resources in a similar way on a subset of species (e.g., top carnivores feed on primary carnivores which feed on herbivores which feed on primary producers) In natural systems, most species not feed strictly on the Encyclopedia of Biodiversity, Volume Copyright  2001 by Academic Press All rights of reproduction in any form reserved FOOD WEBS ‘‘trophic level’’ below them, making the trophic level concept a difficult term to assign operationally to species KNOWLEDGE OF FOOD WEB structure and dynamics is central to our understanding of almost all aspects of population and community ecology By their very nature of representing feeding relationships between species, food webs have the capacity to embody the rich complexity of natural systems In fact, most important interactions (e.g., competition, predation, and mutualism) cannot be isolated from a food web context I INTRODUCTION Food webs occupy a central position in community ecology Charles Darwin introduced the concept of an entangled bank in which he envisioned many kinds of species interdependent on each other in a complex manner governed by ‘‘laws acting around us.’’ In the simplest context, food webs incorporate the two factors that, a priori, one would consider most fundamental to the success of any one species: resources and enemies All species must acquire resources (food or nutrients) and suffer energy losses or mortality from predators (Fig 1) The abundance and success of any species is thus a product of these feeding interactions This inclusion of such ‘‘bottom-up’’ (productivity and resources) with ‘‘top-down’’ (consumption) factors largely determines the distribution and abundance of almost every species on the planet In particular, freshwater ecologists have enjoyed notable success by concurrently studying the interaction between these variable factors on the regulation of plant and animal abundance and thus the structure of freshwater communities This research shows the rich dynamical outcomes that can occur when predation and productivity vary and interact within a food web (Fig 2) Many important advances have arisen from analyses that concurrently incorporate more than one interac- FIGURE Food chain tion in a food web: keystone predation and herbivory, the intermediate predation and disturbance hypotheses, the size-efficiency hypothesis, trophic cascades, intraguild predation, apparent competition, and the recognition of the importance of indirect effects The outcome of virtually all interactions within a community can be modified, directly and indirectly, by other members of the food web This insight penetrates to all areas of community ecology For example, the results of experiments must be interpreted carefully for at least two reasons First, indirect effects, moderated by other species in the web, may exert large and sometimes contradictory effects to the direct effects of the manipulation Thus, under some food web configurations, removal of a predator may directly increase the level of its prey or may actually cause the prey to decrease because of indirect interactions Second, changes in species dynamics putatively caused by one factor may actually be a product of a second process II TYPES OF FOOD WEBS Food web research has grown at a tremendous rate and taken a diversity of forms Not surprisingly, ecologists have diverged in their methods, emphases, and approaches Nevertheless, trophic relationships in communities can be delineated in three basic ways Paine (1980) and Polis (1991) distinguished three types of food webs that evolved from ecological studies (Fig 3) The first is the classic food web, a schematic description of connectivity specifying feeding links Such connectivity webs simply demonstrate feeding relationships Examples of these are the early food webs of Forbes and Summerhayes and Elton (Fig 3) The second web type is also descriptive, quantifying the flow of energy and matter through the community These energetic webs quantify the flow of energy (and/or materials) between trophically connected species Examples of this type of food web include intertidal communities in Torch Bay, Alaska, and Cape Flattery, Washington (Paine, 1980) The third type use experiments to dissect communities to identify strong links and dynamically important species Such interaction or functional webs demonstrate the most important connections in an ecosystem (Fig 3) These food webs depict the importance of species in maintaining the integrity and stability of a community as reflected in its influence on the growth rates of other species They require experimental manipulations of the community (e.g., by removal or addition of particular species) In the following sections, we discuss the strengths and weaknesses of each ap- FOOD WEBS FIGURE Food web proach Of the three, only the last two have contributed substantially to our understanding of natural systems A Connectivity Webs Connectivity webs are representations of ‘‘who eats whom’’ without inference to the strength or type of interaction and energy flow (Fig 3) Early food webs were constructed for essentially two reasons: (i) to depict the interconnectivity of natural systems and (ii) to examine issues of ‘‘the balance of nature,’’ i.e., to analyze how harmony is maintained through complex predatory and competitive interactions within communities (Forbes, 1887) Such an approach was applied to agricultural systems to examine pests and possible food web manipulations to control pests As early as the 1880s, beetles were introduced into the United States to control agricultural pests Such control then benefited crop plants via an indirect interaction (predator pest prey crop) (following the success of Vedalia, a coccinellid beetle, in controlling cottony-cushion scale FOOD WEBS FIGURE Three conceptually and historically different approaches to depicting trophic relationships, illustrated for the same set of species The connectedness web (a) is based on observation, the energy flow web (b) on some measurement and literature values, and the functional web (c) on controlled manipulation Used with permission of Blackwell Scientific Publications in California in 1888, about 50 more coccinellids were introduced in the 1890s) The knowledge required to construct connectivity webs is straightforward: An approximate, qualitative knowledge of who eats whom is all that is necessary to produce a simple food web, whereas experimental manipulations or quantitative measurements are necessary to construct webs of interaction or energy flow Consequently, connectivity webs most frequently represent trophic interactions in communities and have received the most attention Hundreds of such webs slowly accumulated over a century They were useful to illustrate, in a totally nonquantitiative manner, the feeding interactions within a specific community Different scientists constructed webs of different diversity, complexity, and resolution, depending on their knowledge of the system and bias or understanding of particular groups For example, some may emphasize birds and lump all insects as one group Others will divide the insects into scores of groups and represent one or two bird species In the 1970s and 1980s, many theoretical and statistical studies were performed on connectivity webs cata- loged from the literature to determine similarities and natural patterns among them Empirical generalizations were abstracted from data of published connectivity webs These ‘‘natural patterns’’ largely agreed with predictions made by early food web models These models showed that food webs were constrained to be quite simple: Each species ate few species and had few predators; the total length of the number of links in a typical food chain was short, usually two or three; omnivory was very rare; and there were a few other patterns Early modelers argued that the congruence of patterns from the cataloged webs validated the predictions of their models They thus claimed that their Lotka–Volterra models were heuristic and represented processes that structure real communities For example, the addition of omnivory to model food webs causes webs to be unstable dynamically and exhibit relative low persistence (time before species are lost) Thus, these models make the prediction that omnivory should be relatively rare in those webs that persist in nature Comparison of omnivory in cataloged webs relative to its frequency based on chance shows that omnivory is statistically rare in real webs, as predicted by models The same FOOD WEBS FIGURE Food web showing aggregation within some trophic levels but not others (A) The dynamics of omnivory; (B) spatial subsidy; (C) detrital shunts general approach was used to validate other predictions of model webs, e.g., short chain lengths Thus, modelers soon ‘‘explained’’ these empirically derived patterns Although these studies, and the connectivity approach, make good food web diagrams, they are flawed to such a great a degree that today such analyses are viewed as providing little understanding of natural communities There are many reasons why this is so, of which only a few are mentioned here: Most vastly under-represent the species diversity in natural communities Most communities have hundreds to thousands of species, but these webs would represent Ͻ10–30 species on the average As a consequence, most connectivity webs have severe problems with ‘‘lumping’’ species and taxonomic biases Some trophic levels are distinguished by species (e.g., birds or fish), whereas other groups suffer a high degree of aggregation, e.g., all species of insect or annual plants are represented as one super-species—‘‘insects’’ or ‘‘plants’’ (Fig 4) Most species are highly omnivorous, feeding on many resources and prey that each have a distinct trophic history and are often at different trophic levels Because diet is very difficult to delineate, most connectivity webs greatly underrepresent the true nature of omnivory This poses several fundamental problems Connectivity webs typically only offer a static view of the world and webs are usually idealized representations that show all linkages that occur over large spatial and temporal scales Therefore, much of the important variability and changes due to local environ- mental conditions are lost However, studies that compare changes in connectivity over time and space and across environmental gradients (such as those by Mary Power and her group on the Eel River) can provide important insight into community structure and dynamics One can view connectivity webs as a first step in examining the interactions in communities (i.e., performing ‘‘natural history’’ studies), to be followed by quantification of the fluxes of energy and nutrients (as in energetic webs) B Energetic Webs Starting with the classic studies of Elton, Summerhayes, and Lindeman, food web studies turned toward quantifying flows of energy and nutrients in ecosystems and the biological processes that regulate these flows This approach is an alternative to connectivity webs to describe trophic connectedness within communities This ‘‘process-functional’’ approach explicitly incorporates producers, consumers, detritus, abiotic factors, flow out of a system, and the biogeochemical recycling of nutrients It views food webs as dynamic systems in time and space Such an approach necessitated analyzing energy and material fluxes in order to understand the behavior of ecosystems Thus, a typical analysis would quantify the amount of energy or matter as it travels along different pathways (e.g., plants Ǟ consumers Ǟ detritus Ǟ decomposers Ǟ soil) For example, the tracking of energy and DDT through a food web in a Long Island estuary enabled researchers to study bioaccumulation effects on top predators FOOD WEBS The use of energetic webs has provided a rich understanding of the natural world and allowed us to understand much about ecosystems Several important processes are included in energetic webs First, they quantify energy and material pathways and key species or processes that facilitate or impede such flows Second, they include an explicit recognition of the great importance of detritus, a subject virtually ignored in connectivity webs (10 to Ͼ90% of all primary productivity from different habitats immediately becomes ‘‘dead’’ organic detritus rather than being eaten by herbivores) Third, this approach recognized that a great amount of energy, nutrients, and prey originated outside the focal habitat, which is a key insight to understand natural communities Thus, energetic webs show how ecosystems function and which species dominate biomass and energy Beginning with Lindeman, researchers began to examine the efficiency of transfer from prey species to predator species It was found that energy transfer is generally inefficient with only about 5–15% of the energy of prey species being converted to energy of predators Peter Yodzis used this information to suggest that the length of food chains within a community would be set by the amount of energy entering into the base of the chain This argument was in opposition to Pimm and Lawton’s suggestion that food chain length is set by the resilience of the chain By resilience, Pimm and Lawton, using Lotka–Volterra models, meant the estimated time for model food chains to recover from some disturbance They argued that frequent disturbances (relative to growth rates of species) would result in shorter food chain lengths Furthermore, early studies examining the influence of primary productivity (thus, the amount of energy entering a food chain) did not support the hypothesis that food chain length was governed by energy transfer efficiency However, recent reexaminations of Pimm and Lawton’s work suggest that two factors influenced their results—density-dependent regulation of the basal trophic level and food chain structure (the lack of omnivory in their models) Moreover, recent studies of the role of energy efficiency have found that decreases in productivity result in shorter maximum food chains Thus, the relative role of resilience versus energy transfer in regulating the length of food chains is still debated One outcome of the argument for the role of energy transfer as the main governing factor of food chain length is a body of work that examines differences in energy efficiency among organisms For example, carnivores are found to have greater efficiency than herbivores Additionally, invertebrate ectotherms have greater efficiencies than vertebrate ectotherms, which in turn are more efficient than endotherms Yodzis and Innes used this information (and relative body sizes) to parameterize nonlinear predator–prey models In summary, the analysis of energy and matter flow is necessary and central to understanding the dynamics of populations and communities The success of a population is always strongly related to the energy and biomass available to it Consequently, it is difficult or impossible to understand the dynamics and structure of food webs and interacting populations without incorporating energy flow from below However, this energetic approach per se, although necessary, is not sufficient by itself to understand the dynamics of communities because energy flow and biomass production are functions of interactions among populations within the food web The transfer of energy and matter becomes complicated as they pass through the many consumers that populate community food webs For example, increasing the amount of nutrients to plants may increase the biomass of each consumer in the web or may just increase the biomass of a subset of consumers (e.g., only the plants, plants and herbivores, or only the herbivores), depending on the relationship between consumers and their resources Because of these considerations, pathways must be placed in the context of ‘‘functional’’ food webs to understand the dynamics of energy and material transfer C Functional or Interaction Webs Functional or interaction webs use experiments to determine the dynamics within a community Starting with Connell and Paine, empiricists began to use experiments to examine communities and food webs to discover which species or interaction most influenced population and community dynamics They manipulated species that natural history or energetic analyses suggested were important They used either ‘‘press’’ (continual) or ‘‘pulse’’ (singular) experiments to manipulate populations of single species and then followed the response of other species within the food web The philosophy of these studies was to simplify the complexity of natural systems with the assumption that many species and links between species were unimportant to dynamics Paine tested this assumption and found that indeed many links between species were weak (essentially zero) Experimental analyses of food webs are designed to identify species and feeding links that most influence population and community dynamics These alone are placed into an ‘‘interaction web’’ that, in theory, encom- FOOD WEBS passes all the elements that most influence the distribution and abundance of member species However, unlike connectivity webs, key species are identified through experiments rather than diet frequency or energy transfer The initial process of choosing certain species and interactions for experiments and excluding others is subjective, optimally based on strong intuition and a rich understanding of natural history As the researcher learns more, some elements are discarded and others are subject to further experimentation Eventually, the community is distilled into an interaction web, a subset including only species that dominate biomass and/or regulate the flow of energy and matter This approach has been used by experimental and theoretical ecologists to produce a rich understanding of the processes that most influence their communities They have been remarkably fruitful and have introduced many food web paradigms that go to the center of ecology, e.g., keystones species, the intermediate disturbance or predation hypothesis, the size-efficiency hypothesis, top-down and bottom-up control, trophic cascades, and apparent competition However, this approach is not without limitations Three major problems stand out First, many statistical shortcomings can beset experimental manipulation of food webs For example, replications are commonly difficult (time-consuming and expensive) and therefore experiments often lack the statistical power necessary to avoid type II statistical errors (significant biological differences exist among treatments but low sample size precludes their detection statistically) Second, the number of possible experiments is almost infinite Which ones should be conducted, and which species should be manipulated? The third and perhaps most troublesome problem is that experiments isolate a subset of species and links from the community food web, largely ignoring how manipulations interact with the remainder of the community Thus, unobserved indirect or higher order interactions may exert important effects on the dynamics of experimental species and, in theory, make the outcome of experiments indeterminate For example, predators are thought typically to suppress their prey However, if a predator is omnivorous, not only eating the prey but also consuming a more efficient predator on the same prey (i.e., it is an ‘‘intraguild predator’’), it may actually relax the predation load on their shared prey, thus increasing the shared prey’s abundance For example, guilds of biological control agents must be carefully structured because some species eat not only the host but also other predators/parasitoids and thus their presence decreases the number of control agents and increases target pest populations Many other cases exist in which consumers, via such intraguild predation, may indirectly facilitate its prey while concurrently exploiting it via direct consumption Another example of indirect effects mediated by other than studied ‘‘focal species’’ is shown by the interaction between Australian bell miners and their homopteran food (‘‘lerp’’) After these birds were removed experimentally, the insects first increased greatly in number and then vanished when other bird species invaded the now undefended miner territories Thus, the apparent effect of leaf miners on lerp insects (here, suppression or facilitation) depends on when the insects were surveyed Such complications have undoubtedly interfered with clear interpretation of many experiments The caveat is clear: Experiments can be indeterminate, producing contradictory, counterintuitive, or no results, depending on the relative strengths of the direct and indirect effects These problems can be anticipated and partially negated with the application of good intuition of the natural history of the system and important mechanisms Such intuition is a product of intimate empirical knowledge gained through observation and guided by a conceptual awareness of which interactions are potentially important Initially, this process is essential to design the appropriate experiments and identify which species and trophic links may be dynamically important At the end, experiments must be interpreted in a food web context to assess possible indirect and higher order effects Experimental results must be complemented with good descriptive, mechanistic, and comparative data to produce a deep understanding of the system This is one role for energetic and dietary data Experiments in the absence of natural history often not succeed and may mislead The important messages from this section are that the complex food webs of natural communities can be simplified and understood by isolating key species and links into ‘‘interaction webs,’’ experiments are absolutely necessary for this process, and experiments must be designed and interpreted with sound intuition based on natural history and theory III OMNIVORY AND THE STRUCTURE OF FOOD WEBS It is necessary to discuss feeding connections in more detail Empirical research and logic have shown that the vast majority of consumers on this planet are very FOOD WEBS omnivorous, feeding on many types of food throughout the entire food web This is not to say that all species are so catholic in their diets Specialists abound, e.g., many herbivores or parasites consume only specific plants or hosts However, these form a minority of consumers The ubiquity of omnivory carries many implications for our efforts to produce theory and models to understand how food webs operate in and shape natural systems Omnivory occurs ubiquitously when consumers eat prey from general classes of prey, such as arthropods, plankton, soil fauna, benthos, or fish The existence of multiple trophic types within these classes causes consumers to feed on species from many trophic levels For example, ‘‘arthropodivores’’ eat whatever properly sized arthropods are available (e.g., predaceous spiders and insects and insect parasitoids, herbivores, and detritivores) without pausing to discriminate among their prey according to trophic status For example, in the Coachella Valley desert delineated by Polis (1991) over 10 years of study, predaceous and parasitoid arthropods formed 41% of the diet of vertebrate and 51.5% of invertebrate arthropodivores, with the remainder of the diet being herbivorous and detritivorous prey Similarly, inspection of diet data of planktivores, piscivores, ‘‘insectivores,’’ carnivores, or benthic feeders reveals that such different channel omnivory is almost universal with the exception of those few taxa that specialize on a few species of prey Another important type of omnivory occurs when consumers eat whatever resources are available or abundant at a particular time or place, regardless of their trophic history When analyzed, the diet of a single species usually shows great differences through time (e.g., seasonally) and space (patches or habitats) Prey exhibit three general phenologies: pulsed (population eruptions lasting a few days or weeks), seasonal (present for 2–4 months), and annual (available throughout the year) Feeding on prey from all three phenologies produces diet changes over time for almost all non-specialist consumers Furthermore, many (most?) vertebrates opportunistically switch from plant to animal foods with season For example, granivorous birds, rodents, and ants primarily eat seeds but normally feed on the abundant ‘‘arthropods’’ (ϭ insects from all trophic levels and spiders) that appear during spring Alternately, many omnivorous, arthropodivorous, and carnivorous species consume significant quantities of seed or fruit In the Coachella Valley, 79% of 24 primary carnivores eat arthropods and/or plants; for example, coyotes eat mammals (herbivorous rabbits, rodents, and gophers; arthropodivorous antelope and ground squirrels; car- nivorous kit foxes and other coyotes), birds (including eggs and nestlings, e.g., carnivorous roadrunners; herbivorous doves and quails), snakes, lizards, and young tortoises as well as scorpions, insects, and fruit In New South Wales, 15 of 27 ant species are ‘‘unspecialized omnivores’’ eating nectar, seeds, plant parts, and a broad range of living and dead insects, worms, and crustacea Overall, it appears that most consumers eat whatever is available and whatever they can catch ‘‘Life history’’ omnivory describes the great range of foods eaten during growth and ontogeny by most species (the ‘‘age structure component’’ of dietary niche breadth) Such omnivory includes abrupt diet changes in species undergoing metamorphosis (e.g., many marine invertebrates, amphibians, and holometabolic insects) and gradual diet changes in ‘‘slowly growing species’’ (e.g., reptiles, fish, arachnids, and hemimetabolic insects) Changes at metamorphosis can be great; for example, 22% of the insect families in the Coachella Valley desert community undergo radical change in diet—larvae are predators or parasitoids and adults are herbivores Although not as dramatic, significant changes characterize slowly growing species so that differences in body size and resource use among age classes are often equivalent to or greater than 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 FOOD WEBS IV 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 A 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, 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 low-standing 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 are they fully counted Thus, considerations of food chain dynamics not become stranded in the immense complexity of real food webs On the other hand, GWH trophic levels are based on energy deriving from primary productivity Thus, ‘‘trophic level interactions 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 ... Philos Trans R Soc London Ser B 33 0, 2 93 30 4 Mitchell, A (1987) The Enchanted Canopy: Secrets from the Rainforest Roof Fontana/Collins, London 25 Moffett, M (19 93) The High Frontier—Exploring... beginning of the so-called ‘‘sixth extinction crisis’’ sensu Niles Eldridge of the American Museum of Natural History Amelioration of the impact of this crisis rests on a better knowledge of the... seriously Encyclopedia of Biodiversity, Volume Copyright  2001 by Academic Press All rights of reproduction in any form reserved 19 20 FOREST CANOPIES, ANIMAL DIVERSITY investigating canopy faunas of