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500 Food Webs validate other predictions of model webs, for example, 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 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 species are vastly under-represented in food webs or lumped in taxonomic or functional groups Most communities have hundreds to thousands of species, but these webs would represent o10–30 species on 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, for example, all species of insect or annual plants are represented as one super-species – ‘‘insects’’ or ‘‘plants’’ (Figure 4) Most species are highly omnivorous, feeding on many resources and prey so that each has a distinct trophic history and are often at different trophic levels Because diet is very difficult to delineate, most connectivity webs greatly under-represent 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 environmental 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) Top predators Final intermediatelevel consumers Series of intermediatelevel consumers Consumers (herbivores or detritivores) (b) 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 - consumers - detritus - decomposers - soil) For example, the tracking of energy and dichlorodiphenyltrichloroethane (DDT) through a food web in a Long Island estuary enabled researchers to study bioaccumulation effects on top predators 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 490% of all primary productivity from different habitats immediately becomes ‘‘dead’’ organic detritus rather than being eaten by herbivores) Third, this approach recognized a great amount of energy, nutrients, and prey that 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 Yodzis (1989) used this information to suggest that the length of food chains within a Top predator Top predator Primary consumers Primary consumers Primary producers Detritus Detritus Plants or detritus (a) Energetic Webs Nutrients Primary producers Less productive habitat More productive habitat Top predators Predators Intermediatelevel predators Intermediatelevel predators Consumers (herbivores) Consumers (detritivores) Primary producers 70 − >90% Detritus Nutrients Tissue from all consumers (c) Figure Food web showing aggregation within some trophic levels but not others (a) The dynamics of omnivory; (b) spatial subsidy; (c) detrital shunts
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