An understanding of the ecological factors thatdetermine energy flow in communities, such as food chain length, interaction strength, and con-nectedness, are also necessary to quantify c
Trang 1popula-(Hunter and Price 1992)
of bioenergetics of individuals, populations, communities, and ecosystems allows researchers tointegrate their findings across several levels of biological organization (Carlisle 2000)
Despite the importance of food webs and trophic interactions in basic ecology, ecotoxicologistshave not incorporated significant components of basic food web theory into investigations of contam-inant effects This reluctance is ironic because the concern about food chain transport of contaminants
in wildlife populations was at least partially responsible for much of the environmental legislation inthe early 1960s Reports of biomagnification of organochlorine pesticides and the subsequent effects
on birds of prey (Carson 1962) eventually resulted in the ban of organochlorine pesticides
One important exception to the general neglect of basic food web theory in ecotoxicology is theapplication of models to predict contaminant fate Contaminant transport models used in ecotoxico-logy are analogous to energy flow models derived from the ecological literature The application ofthese models for understanding fate and transport of contaminants in ecosystems will be described
inChapter 34 Quantifying the movement of contaminants through an ecosystem is only one ofseveral potential applications of food web theory An understanding of the ecological factors thatdetermine energy flow in communities, such as food chain length, interaction strength, and con-nectedness, are also necessary to quantify contaminant fate and effects For example, studies haveshown that trophic structure and food chain length regulate contaminant concentrations in top pred-ators (Bentzen et al 1996, Rasmussen et al 1990, Stemberger and Chen 1998, Wong et al 1997)
It is likely that other ecological processes, either directly or indirectly related to trophic ture, will play a role in determining contaminant transport Recent refinements in transport modelshave been primarily limited to quantifying the role of physicochemical characteristics that modify
struc-581
Trang 2contaminant bioavailability Further improvement of these models will require that gists develop a better understanding of the ecological factors that influence contaminant fate andtransport.
ecotoxicolo-Another potential contribution of basic feed web theory to ecotoxicology is the measurement
of food web structure and function as indicators of contaminant effects Although the relationshipbetween trophic structure and natural disturbance has been recognized for many years (Odum 1969,1985), there have been few attempts to determine how food webs respond to contaminants (Carlisle2000) There is some evidence that food chains are shorter in systems subjected to frequent dis-turbance, but the mechanistic explanation for this observation has not been determined Using foodweb structure and function as indicators of contaminant effects is appropriate for several reasons.Bioenergetic approaches at the level of individuals and populations have a long history in toxico-logy Growth is a common end point in many toxicological investigations that integrates numerousphysiological characteristics Energetic cost of contaminant exposure may be interpreted within thecontext of growth and metabolism For example, recent studies have combined measurements ofmetabolism, food consumption, and growth into an individual-based bioenergetic model to assesseffects of organochlorines (Beyers et al 1999a,b) Similar approaches could be used to measure theeffects of contaminants on flow of energy through a community Finally, because energy is a com-mon currency in all biological systems, understanding ecological effects of contaminant exposure oncommunities may help establish mechanistic linkages to lower (individuals, populations) and higher(ecosystems) levels of biological organization
27.2 BASIC PRINCIPLES OF FOOD WEB ECOLOGY
27.2.1 HISTORICALPERSPECTIVE OFFOODWEBECOLOGY
The strength of trophic interactions and the relationship between energy flow and community ture have been topics of considerable interest to ecologists for many years Charles Elton’s (1927)studies of feeding relationships in a tundra community and his representation of trophic levels as
struc-an energy pyramid (Figure 27.1a) focused the attention of ecologists on the importance of food
as a “burning question in animal society.” Subsequent representations of Elton’s trophic pyramidsincluded biomass and numbers of individuals as the fundamental components
Ecologists soon recognized that this simple depiction of energy flow treated all species within
a trophic level equally and, more importantly, did not account for microbial processes In tion, there was no attempt to quantify the movement of energy among trophic levels RaymondLindeman’s (1942) classic paper introduced the “trophic-dynamic” aspect of natural systems andrevolutionized the study of food webs On the basis of an extensive analysis of Cedar Bog Lake(MN), this work formalized the concept of energy flow through ecosystems and influenced a gener-ation of systems ecologists The study of ecology shifted from habitat associations and species lists
addi-to a more quantitative analysis of trophic relationships and energy flow Lindeman also recognizedthe inherent inefficiency of energy flow in ecological systems, setting the stage for a contentiousdebate concerning the importance of biotic and abiotic factors that limit the number of trophic levels
in communities
Lindemen’s box and arrow diagrams depicting energy flow and cycling of materials through acommunity were further refined by Eugene P Odum (1968) (Figure 27.1b), widely regarded as thefather of systems ecology The emergence of ecosystems ecology in the 1950s also highlighted philo-sophical differences between holistic and reductionist approaches While some ecologists felt thatunderstanding complex systems required sophisticated and quantitative analysis of all interactingcomponents, others felt that characteristics of ecosystems transcended those of individual com-ponents and could only be investigated by considering emergent properties Unfortunately, thesephilosophical differences between proponents of holism and reductionism still persist in ecologyand ecotoxicology today (Section 1.2 inChapter 1andBox 20.1in Chapter 20)
Trang 3Top predators Predators
Herbivores
Primary Producers
Top predators Predators
Herbivores Primary
FIGURE 27.1 Four different representations of trophic structure and food chains in the ecological literature.
(a) Eltonian trophic pyramid showing biomass at each trophic level (b) Box and arrow diagram showingenergy flow through a community (c) Descriptive food chain showing potential interactions among species.(d) Energetic or interaction food chain showing energy flux or strength of interactions (represented by thickness
of the lines) between dominant species in a community
27.2.2 DESCRIPTIVE, INTERACTIVE, ANDENERGETICFOODWEBS
Food webs depicted in the contemporary ecological literature fall into three general categories:descriptive, interactive, and energetic (Figure 27.1c,d) Descriptive food webs are probably the mostcommon and are produced by simply characterizing feeding habits of dominant species Descriptivefood webs are analogous to the use of presence–absence data in community monitoring becausethey provide no information on the relative importance of linkages among species In contrast,interaction and energetic food webs quantify the importance of linkages among species and energyflow Interaction food webs are constructed by manipulating the abundance of either predators orprey and measuring responses Interaction food webs have a long history in experimental ecologyand have been employed to identify keystone species (e.g., Paine 1980) The best examples ofinteraction food webs are from marine rocky intertidal habitats where experimental manipulation issimplified because of the low mobility of species and the essentially two-dimensional nature of thehabitat Energetic-based food webs are constructed by quantifying energy flow between species This
is generally accomplished by characterizing feeding habits and measuring secondary production ofdominant species in a community (Benke and Wallace 1997) Either interaction or energetic foodwebs would be appropriate for assessing contaminant effects; however, it is important to recognizethat both approaches are data intensive and require a significant amount of effort to develop.Because the strength of species interactions are not necessarily related to the amount of energyflow between trophic levels, bioenergetic and interaction approaches can yield different results
Trang 4For example, relatively little energy flows between kelp and sea urchins in marine ecosystems;however, as described in the following section, removal of sea urchins may have a large impact
on kelp populations and associated species Paine (1980) showed very different patterns resultedwhen marine food webs were based on connectedness, energy flow, or interaction strength Because
of potential differences between interaction and energetic food webs, these approaches may havedifferent applications in ecotoxicology If researchers are interested in modeling the movement ofcontaminants through a community, an energetic food web may be most appropriate However,
if the purpose of an investigation is to examine the direct and indirect effects of contaminants oncommunity structure, it may also be very important to know the strength of species interactions andconstruct an interaction food web
The strength of interactions within a food chain may also influence community stability; however,because of the lack of experimental studies and the different approaches employed by theoretical andempirical ecologists to measure interaction strength, the relationship between stability and energyflow is uncertain de Ruiter et al (1995) linked material flow descriptions with measures of interactionstrength to quantify the influences on stability of terrestrial food webs Their findings were consistentwith previous research that showed relatively small rates of energy flow in a community can havelarge effects on community stability Thus, predicting the effects of contaminant-induced alterations
on energy flow will not be straightforward because the functional role of a species in a communitymay not be directly related to its abundance or biomass
27.2.3 CONTEMPORARYQUESTIONS INFOODWEBECOLOGY
Most contemporary research in food web ecology has focused on two key topics: (1) identifyingfactors that limit the number of trophic levels; and (2) quantifying the strength of species interactions.One consistent observation in food web research is that most food webs are relatively short, averagingbetween 3 and 5 trophic levels in both aquatic and terrestrial habitats The length of food chainsand the number of trophic levels is assumed to be limited by the inefficient transfer of energy.Ecological systems conform to laws of thermodynamics, and the loss of energy from prey resources
to consumers limits the number of possible trophic levels On the basis of this argument, we wouldexpect shorter food webs in unproductive systems where resources are limited We also know thattop predators tend to occur in low numbers and are sparsely distributed compared to herbivores andother secondary consumers In an insightful essay on this topic, Colinvaux (1978) argued that therarity of large, fierce predators (e.g., tigers, great white sharks) in many ecosystems resulted fromthe inefficiency of energy transfer from lower trophic levels
Despite the intuitive appeal and broad theoretical support, few studies of food chains in ural communities have found consistent relationships between productivity and food chain length.Primary productivity may vary by orders of magnitude among communities, but the number oftrophic levels remains remarkably consistent Food chains are not necessarily shorter in unpro-ductive environments (e.g., arctic tundra) compared with productive environments (e.g., tropicalrainforests) Ricklefs (1990) estimated the average number of trophic levels based on net primaryproduction, ecological efficiency, and energy available to predators for a variety of communities(Table 27.1) In contrast to theoretical predictions, there was no consistent relationship between netprimary productivity and the estimated number of trophic levels
nat-Hairston, Smith, and Slobodkin’s (HSS) (1960) revolutionary paper offered an alternative ation for the relationship between productivity and food web structure According to the HSS model,species interactions (competition, predation) within and between trophic levels determined the struc-ture of food webs In a three trophic level system typical of many terrestrial communities, abundance
explan-of herbivores was controlled by predators, thus allowing primary producers to compete for resources.Support for this model in terrestrial food webs has been widespread, and predator control of herbi-vores is proposed as an explanation for the dominance of green plants in most terrestrial ecosystems
A general extension of this argument to other communities suggests that plants are controlled by
Trang 5TABLE 27.1
Estimated Number of Trophic Levels Based on Primary Production,
Energy Flux to Consumers, and Ecological Efficiencies
Community Net Primary Production (kcal/m 2 /year) Number of Trophic Levels
Source: Modified from Table 11.5 in Ricklefs (1990).
resources (nutrients, light, and space) in systems with an odd number of trophic levels and controlled
by herbivores in systems with an even number of trophic levels In an alternative synthesis of therelationship between energy flow and trophic structure, Hairston and Hairston (1993) observed thatthe mean number of trophic levels in pelagic (i.e., open water) systems (3.6) is significantly greaterthan in terrestrial systems (2.6) On the basis of the relative importance of competition among primaryproducers in pelagic and terrestrial systems, they provide a compelling argument for the hypothesisthat trophic structure determines food web energetics instead of visa versa
The hypothetical relationship between food chain length and community stability is also what tenuous Briand and Cohen (1987) reported that the average food chain length in food websfrom constant and fluctuating environments was 3.60 and 3.66, respectively Interestingly, theseresearchers reported that the complexity and dimensionality of a habitat had a greater influence
some-on food chain length than community stability In general, two-dimensisome-onal habitats (e.g., streambottoms, rocky intertidal zones) had shorter food chains than three-dimensional habitats (e.g., coralreefs, open ocean) Thus far, an adequate mechanistic explanation for this relationship has not beenprovided However, results are consistent with the observation that more complex habitats have agreater number of species
Experimental manipulations of food webs provide the most direct tests of the relationship betweentrophic structure, productivity, and disturbance Experiments conducted by Power and colleagues(Power 1990, Wootton et al 1996) extended the HSS model to aquatic ecosystems and demonstratedthe role of disturbance in regulating trophic structure As predicted by HSS, primary producers werelimited by resources (nutrients, space, and light) in communities with an odd number of trophiclevels, whereas communities with an even number of trophic levels were regulated by herbivores.Disturbance also played a prominent role by controlling abundance of grazers and shifting energy
to predatory fish These results indicate the need to advance from a single species perspective to
a community perspective when assessing the effects of disturbance (Wootton et al 1996) Moreimportantly, these results demonstrate that disturbance may alter trophic structure and energy flow
in food webs by removing key species
Food chain length and the number of trophic levels of a community may also influence resistanceand resilience stability Mathematical models predict that communities with longer food chainswill experience extreme population fluctuations, resulting in a greater probability of extinction oftop predators This hypothesis has important implications for the study of systems subjected toanthropogenic disturbance For example, we expect that effects of contaminants would be greater incommunities with greater trophic complexity and longer food chains
Food web interactions involving otters and sea urchins in kelp beds of western Alaska providesome insight into how disturbance can dramatically alter trophic structure (Estes et al 1998) Therole of otters as a keystone species in marine kelp beds is well established Otter predation on seaurchins, major consumers of early growth stages of kelp, maintains the structure of kelp forests
Trang 61972 1986 1989 1992 1995 0
100 200 300 400
0 5 10
Year
0 25 50 75 100
FIGURE 27.2 Effects of predation by killer whales on trophic structure of nearshore marine ecosystems in
western Alaska The figure depicts changes in otter abundance, sea urchin biomass, and effects on kelp densityfollowing increased predation by killer whales (Modified from Figure 1 in Estes et al (1998).)
Recovery of otter populations following protection from overhunting resulted in recovery of kelpforests along the Pacific Northwest coast However, a dramatic decline of sea otters over largeareas in western Alaska in the 1990s caused increased abundance of urchins and a correspondingdecline in kelp abundance (Figure 27.2) Surprisingly, increased sea otter mortality was attributed
to predation by killer whales, which shifted their foraging to coastal areas following reductions intheir preferred prey: seals and sea lions Estes et al (1998) speculated that reduced abundance ofseals and sea lions resulted from unexplained declines of forage fish stocks Thus, addition of atop predator (killer whales) to coastal Alaska converted this three trophic level system to a fourlevel system This spectacular example illustrates the connectance and interdependence of multipletrophic links and the interactions between oceanic and nearshore communities More importantly,this study demonstrates the difficulty predicting indirect effects of reduced prey abundance in naturalcommunities It is unlikely that researchers could have anticipated that declines in fish forage stocks inthe oceanic environment would cause a collapse of coastal kelp beds Similar “ecological surprises”(sensu Paine et al 1998) are likely to occur in systems where important predator or prey species areeliminated as a result of contaminants
Trang 70 5 10 15 20 25 30 35 40 0
0.2 0.4 0.6 0.8 1
Number of species
5 10 20 50 100 200 500
Number of predator species
(b) (a)
FIGURE 27.3 (a) Hypothetical relationship between connectance (number of interactions/number of possible
interactions) and the total number of species in a food web (upper panel) (b) Hypothetical relationship betweennumber of predator species and number of prey species (lower panel)
The other major generalizations regarding the structure of food webs are the relatively constantnumber of species interactions and the ratio of predators to prey Food web connectance, defined
as the observed number of trophic interactions divided by the total number of possible interactions,generally decreases with species richness (Pimm 1982) (Figure 27.3a) As a result, each species tends
to average about two trophic interactions, regardless of the number of species in the community.Similarly, the ratio of predator species to prey species in a community is relatively constant (generallybetween two and three prey species per predator species), regardless of the total number of species inthe community (Jeffries and Lawton 1985) (Figure 27.3b) Assuming that these theoretical predictionsare consistent among communities, connectance and the ratio of predators to prey may prove to beuseful endpoints for assessing effects of stressors on food web structure
27.2.4 TROPHICCASCADES
The trophic cascade hypothesis (Carpenter and Kitchell 1993) predicts that each trophic level in acommunity is influenced by trophic levels directly above (e.g., consumers) and directly below (e.g.,resources) According to this hypothesis, nutrients determine the potential productivity of a system,but deviations from this potential are owing to food web structure Thus, two conditions define atrophic cascade: (1) top-down control of community structure by predators; and (2) strong indirect
Trang 8effects of two or more links away from the top predator (Frank et al 2005) For example, increasedabundance of piscivorous fish in a lake can reduce abundance of zooplanktivorous fish, allowingabundance of zooplankton to increase The resulting increased grazing pressure by zooplankton ispredicted to reduce biomass of phytoplankton (see Chapter 20,Figure 20.1) Researchers conductinglarge-scale biomanipulation experiments in eutrophic lakes have taken advantage of these relation-ships and attempted to control primary productivity and eliminate algal blooms by introducing toppredators (Box 27.1).
Box 27.1 Biomanipulation Experimentsto Control Eutrophication
Experiments conducted in lakes have demonstrated the importance of trophic linkages and therelationship between food web structure and water quality Lakes provide an ideal habitat toexamine trophic interactions because they are well-defined, relatively closed systems and areamenable to experimental manipulation Biomanipulation experiments were initially motivated
by the observation that nutrients could account for only a portion of the variation in primaryproductivity among lakes, which often vary by an order of magnitude in systems with sim-ilar levels of nutrients (Carpenter and Kitchell 1993) Introduction of piscivorous fish to PeterLake, Wisconsin (USA) caused rapid reductions in abundance of zooplanktivorous fish and
an increase in herbivore (primarily Daphnia) body size These changes in food web structure
resulted in a 37% decrease in primary productivity and a dramatic increase in light penetration.Interestingly, herbivore body size was a better predictor of trophic effects on productivity thanabundance
The observation that primary productivity in lakes is influenced by food web structureprovided an opportunity to investigate the relationship between trophic structure and waterquality Despite dramatic improvements in control of point source inputs of nutrients over thepast several decades, noxious algal blooms are still a significant problem in many lakes Culturaleutrophication occurs in systems when grazing herbivores are unable to control abundance ofphytoplankton, especially blue-green algae If introduction of piscivorous fish can reduce pred-ation on herbivores by limiting abundance of zooplanktivorous fish, then grazing pressure onnoxious algae is expected to increase This idea was the impetus for a large-scale biomanipula-tion experiment conducted in Lake Mendota (WI) during the late 1980s (Kitchell 1992) As wasexpected, increased stocking of northern pike and walleye in Lake Mendota caused increasedabundance of large, grazing zooplankton However, because of a combination of unexpectedevents, including unusual weather patterns, greater runoff, and greater fishing pressure, theresults of this experiment were mixed Primary productivity did not respond throughout theexperiment as predicted, suggesting that food web interactions were not the sole determinant
of primary productivity in Lake Mendota However, results of this study and others conducted
by Kitchell and colleagues demonstrate that predation played a major role in structuring lowertrophic levels in lakes
These experiments highlight the close connection between trophic interactions and energyflow in lentic ecosystems It is important that ecotoxicologists recognize the significance of theseinteractions when characterizing food chain transport of contaminants in lake communities.Simple models of contaminant transport generally do not consider direct effects on trophicstructure or potential feedback between adjacent trophic levels In addition, food web manipu-lations conducted in lakes have generally not included a littoral or benthic component Becausesediments are a major sink for contaminants in most lentic systems, a complete understanding
of how trophic structure will influence contaminant transport requires that processes involvingsediments should also be considered
Trang 9Although there has been strong support for the trophic cascade hypothesis in lakes, the generality
of this hypothesis and the relative importance of top-down (predator control) and bottom-up (nutrientdriven) effects in other systems have been subjects of considerable debate An understanding of therelative importance of top-down versus bottom-up regulation is necessary to predict the consequences
of anthropogenic nutrient inputs into ecosystems and has important management implications Forexample, protecting top predators may be more important than nutrient control in systems regu-lated by top-down processes (Halpern et al 2006) Because much of the research documenting theimportance of top-down effects has been conducted in systems with relatively simple food websand low diversity, the significance of trophic cascades in complex and species-rich communitiesremains uncertain (Frank et al 2005) The removal of top predators from marine continental shelfecosystems has provided the best opportunity to test the generality of the trophic cascade hypothesis
at relatively large spatiotemporal scales Stock assessments of commercial fisheries over the past
50 years have shown significant reductions in biomass and size of top predators such as tuna andbillfish, but relatively minor effects on trophic structure (Sibert et al 2006) In contrast, removal ofcod from the northwest Atlantic Ocean resulted in dramatic effects on lower trophic levels and nutri-ent concentrations that were consistent with the trophic cascade hypothesis Halpern et al (2006)reported strong top-down control by top predators in 16 kelp forests located around the ChannelIslands, California Despite strong spatial gradients in chlorophyll a among sites, top-down controlaccounted for 7–10 times greater variability in abundance of lower and mid-level trophic levels thanprimary productivity These researchers noted that removal of top predators may convert ecosystemsfrom top-down to bottom-up control, making these systems more sensitive to nutrient enrichment.Although relatively strong support for the trophic cascade hypothesis has been obtained for someaquatic ecosystems, few studies have documented top-down effects in terrestrial environments.Strong (1992) argues that trophic cascades in lakes are an exception and generally restricted tospecies-poor habitats He suggests that terrestrial systems and more diverse aquatic communities aremore frequently characterized by “trophic trickles” rather than cascades Because predator control
is weaker and more diffuse in these species-rich communities, the effects of trophic interactions arebuffered More importantly, unlike aquatic systems where manipulative studies are common, thelack of experimental research in terrestrial habitats limits our ability to identify trophic cascades(Strong 1992) In one of the few experimental studies conducted with terrestrial communities tocharacterize trophic cascades, Salminen et al (2002) constructed food webs in laboratory microcosmsconsisting of three trophic levels (soil microbes, microbivorous-detritivorous worms, and predatorymites) Results showed strong top-down effects of predatory mites on trophic structure and that leadcontamination in soil disrupted these interactions Because some of the responses were an unexpectedoutcome of indirect effects of lead, these investigators urged caution when using traditional food webmodels to quantify contaminant effects Croll et al (2005) took advantage of a large-scale naturalexperiment to investigate the effects of top predators on plant biomass and community structure inthe Aleutian archipelago (Alaska) The introduction of arctic foxes to some islands greatly reducedabundance of seabirds, resulting in a two order of magnitude decline in guano Elimination of marine-derived nutrient subsidies to these islands had dramatic effects on plant biomass and communitycomposition
An important exception to the general absence of trophic cascades in terrestrial ecosystems isthe interaction between moose and wolves on Isle Royale reported inChapter 26 (McLaren andPeterson 1994) Results of long-term monitoring of wolves and moose have described a tightlycoupled predator–prey system Periods of low wolf and high moose numbers are correlated withintense grazing pressure on balsam fir, the primary forage of moose These results are especiallysignificant because they provide strong support for top-down control in a nonaquatic, three trophiclevel system However, it is important to note that because spatial boundaries are well defined andtrophic complexity is low, Isle Royale may represent a relatively unique situation
Quantifying the relative importance of consumer versus resource control in communities willrequire a more sophisticated understanding of population dynamics, species interactions, and the
Trang 10abiotic environment Resource enrichment experiments conducted in a terrestrial, detritus-basedfood chain showed strong bottom-up limitation of top predators (Chen and Wise 1999) Conversely,Stein et al (1995) reported that food webs in temperate reservoirs were regulated by complex weblikeinteractions rather than chainlike trophic cascades The lack of a zooplankton response to introducedpiscivorous fish (northern pike) and reduced abundance of planktivores were explained by poorfood quality for these grazers Brett and Goldman (1996) conducted a meta-analysis of 54 differentexperiments to test the generality of the trophic cascade hypothesis Meta-analysis is a powerful stat-istical approach for analyzing patterns and central tendencies of large datasets derived from multipleinvestigations Results of this analysis provided strong support for the trophic cascade hypothesis.However, a subsequent analysis of 11 mesocosm experiments showed no relationship between nutri-ent enrichment and the number of trophic levels (Brett and Goldman 1997) Another meta-analysis of
47 mesocosm experiments and 20 time-series studies conducted in marine habitats demonstrated theimportance of nitrogen enrichment and predation on pelagic food webs (Micheli 1999) As expec-ted, based on research conducted in freshwater systems, nutrient enrichment increased primaryproduction and addition of planktivorous fish reduced zooplankton abundance However, unlike pat-terns observed in lakes and streams, consumer–resource interactions did not cascade through othertrophic levels because of the weak interactions between grazers and phytoplankton As a result, it
is unlikely that biomanipulation of marine food chains would have the same effects on algal ductivity as those observed in lakes (Micheli 1999) Finally, the presence of trophic cascades mayalso influence the recovery of some aquatic ecosystems from anthropogenic disturbance Long-term(18 years) records of trophic structure in a hypereutrophic lake following reductions in total phos-phorus and organic matter showed that cascading influences of fish predators on zooplankton grazinghad much greater influence on recovery than changes in nutrient input (Jeppesen et al 1998)
pro-27.2.5 LIMITATIONS OFFOODWEBSTUDIES
Significant progress has been made in the development of food webs and the quantification of energyflow among trophic levels since the publication of Elton’s energy pyramids in 1927 Because trans-port of contaminants in a community is often intimately associated with the flow of energy, a betterunderstanding of trophic interactions will improve our ability to predict contaminant fate However,
as with any general ecological model, it is important to recognize the limitations and simplifyingassumptions of food webs Although grouping organisms into broad trophic categories has facilit-ated the development of mathematical models for estimating energy flow, this representation of foodwebs is greatly oversimplified In addition, most studies of food webs either ignore or minimize theimportance of omnivory, which may be the dominant mode of feeding for many species Relativelyfew consumers feed exclusively on resources from one trophic level Many consumers are opportun-istic generalists that feed on the most abundant, available, or energetically profitable food resources.Thus, pollution-induced alterations in prey communities may shift feeding habits of predators totolerant prey species with little impact on bioenergetics (Clements and Rees 1997)
Traditional representations of food webs often ignore the role of detritus, which is a majorcontributor of energy to many aquatic and terrestrial food chains Experiments conducted by Wallace
et al (1997) showed reduced biomass of most functional feeding groups when allochthonous detrituswas excluded from a headwater stream In addition, most characterizations of food webs are limited
to a single habitat, and often fail to consider energy flow between adjacent habitats Experimentsconducted by Nakano et al (1999) demonstrated the importance of terrestrial arthropods to trophicstructure of a small stream and the linkages between terrestrial and aquatic habitats Exclusion ofterrestrial arthropods shifted feeding habits of predatory fish to aquatic prey and caused significantchanges in energy flow and trophic structure
Food web studies are also limited by the general lack of information on interaction strengthamong species Knowing that a particular trophic interaction occurs in a community does not provideany indication of the strength of this interaction Thus, some assessment of interaction strength,