506 Food Webs Detritus Little of the energy fixed by plants passes directly into the grazing food chain – herbivores eating plants and then eaten by carnivores Most of this primary productivity is uneaten by herbivores (median 480% on land, B50% in water) What happens to this dominant chunk of the world’s productivity? Is the detrital web a self–contained sink internally recycling energy and nutrients or a link that affects the population dynamics of the larger species? Uneaten plants (and animals) enter the detrital web, in which they are processed by microbes, fungi, and some animals Although some ecosystems are net accumulators of undigested biomass (e.g., carboniferous bogs and forests that supply today’s oil and gasoline), most ecosystems not accumulate plant biomass Rather, it is soon digested by detritivores, with nutrients and energy passing through ‘‘functional compartments’’ composed of diverse microbes and animals Several factors regulate the flow and availability of detritus to detritivores and then onto other consumers A major question is rather whether the detrital community is a sink that metabolizes most of this energy or a link that passes this energy up the food chain An unknown fraction of detrital energy and nutrients reenter grazing food chains when some detritivores are eaten by predators that also eat herbivores (e.g., a robin eats an earthworm) Such ‘‘detrital shunts’’ are common, interweaving energetics and dynamics of biophages and saprophages Bypassing herbivores, this linkage can affect herbivore regulation in a manner analogous to the spatial subsidies to consumers discussed previously Predator populations, subsidized by detritivorous prey, can increase and suppress other predators or herbivores The exact effect of detrital shunts depends on the relative benefits for each species and where detritus re-enters (to producers, herbivores, and intermediate or higher consumers) For example, nutrients from detritus greatly influence plant productivity; models show that a 10% reduction in detritus can cause a 50% reduction of plant biomass The dynamics of consumer control within the detrital web and those produced by infusion of detritivores into the grazing web are undoubtedly crucial to community structure and dynamics For example, detrital shunts to predators in the grazing chain can create the appearance of a simple linear trophic cascade, but with the difference that nutrition from detritivores sustains or elevates predators to levels sufficient to suppress herbivores Age Structure Effects in Food Webs Almost all species display complex life cycles, marked by moderate to radical changes in diet and habitat; such life histories fundamentally must affect every species with which they interact However, our understanding of how age- and stage-structured processes affect food webs and communities is embryonic Life history omnivory describes shifts in diet during development; often they are accompanied by ontogenetic changes in habitat Diet can change substantially either discontinuously (e.g., at metamorphosis) or slowly with growth Such life histories are widespread; an estimated 80% of all animal species undergo metamorphosis Changes in resource use can be dramatic (e.g., predaceous juveniles, plant-feeding adults in parasitoids and many other insects, and herbivorous tadpoles and predaceous frogs and toads), with prey size variation as great as three or four orders of magnitude Even among nonmetamorphic species, diets change greatly with age, with diet differences among age classes often more distinct than those among most species Thus many species can be classified as life-history omnivores, which are species that feed on different trophic levels over their various life stages Overall, complex life histories and age structure omnivory can exert diverse and profound effects on the dynamics of populations and food webs For example, they can either impede consumer control or amplify resource suppression via dynamics similar to those of spatial subsidy or detrital shunts The Roles of Nutrients and Stoichiometry Animals require both energy and a variety of ‘‘nutritional requisites’’ to grow, complete their life cycle, and reproduce Important nutrients include nitrogen, phosphorus, some trace elements, fatty acids, and vitamins Nitrogen is an integral component of many essential compounds: It is a major part of amino acids, the building blocks of proteins, including the enzymes that control virtually all cellular processes Other nitrogen compounds include nucleic acids and chlorophyll Phosphorus is used for adenosine triphosphate (ATP, the energy currency of all cells), nucleic acids (DNA and RNA), and phospholipids, particularly in cell membranes The availability of nutritional requisites constrains growth and reproduction in virtually every species Nitrogen and phosphorus are particularly important The ratio of carbon to nitrogen (C:N) in plants ranges from 10:1 to 30:1 in legumes and young green leaves to as high as 600:1 in some wood The C:N ratios in animals and microbes are much lower, ordinarily between 5:1 and 10:1 Such differences in C:N ratios between plants and their consumers lower the rate of decomposition by microbes There is an ample evidence that heterotrophs chronically lack adequate nitrogen to grow or reproduce optimally The importance of nutritional restriction is reinforced by the foraging literature that clearly shows that herbivores choose their foods based on nutrient as well as energy content In many cases, phosphorus availability constrains herbivore success The Redfield ratio describes the approximate stoichiometric mix (110 C:250 H:75 O:16 N:1 P) of elements found in marine systems In particular, the N:P ratio crucially determines productivity and species composition Thus, energy (CÀC bonds) and nitrogen could be abundant, but neither individuals nor populations grow maximally because phosphorus is insufficient Because phosphorus is essential to cell division (and thus reproduction), a high N:P ratio especially limits the growth of organisms that have high potential rmax, such as most herbivores and detritivores These organisms are key to the potential regulation of plant biomass (and ‘‘detritus.’’) Evidence suggests that high N:P ratios can impede trophic cascades For example, Daphnia, a key to many lake cascades, respond sufficiently rapidly to phytoplankton productivity to depress plant biomass In lakes with inadequate phosphorus, slower growing copepods replace Daphnia; these copepods not have the reproductive capacity to depress phytoplankton biomass