Elsevier First Proof

CLGY 00558 a0005 Trophic Structure E Preisser, University of Massachusetts at Amherst, Amherst, MA, USA ª 2007 Elsevier B.V. All rights reserved. Introduction Further Reading Control of Trophic Structure Energy transfer from producers to higher Factors Affecting Control of Trophic Structure trophic levels s0005 Introduction emphasizes the role(s) played by nutrient limitation and energetic inputs to producers and the subsequent effi- p0005 Trophic structure is defined as the partitioning of biomass ciency of energy transfer between trophic levels in between trophic levels (subsets of an ecological commu- determining the biomass accumulation at each trophic nity that gather energy and nutrients in similar ways, that level. The second category, top-down control, empha- is, producers, carnivores). The forces controlling biomass sizes the importance of predation in producing patterns accumulation at each trophic level have been a central of biomass accumulation that are often at odds with those concern of ecology dating from the early twentieth-cen- predicted byPROOF energy inputs alone. While recognizing the tury work of Elton and Lindeman. While interspecific differences between these two factors, it is also important interactions such as omnivory and intraguild predation to emphasize that both bottom-up and top-down factors can make it difficult to assign many organisms to a single represent extremes along a continuum of importance for trophic level, several broadly defined trophic levels are regulatory control. While ecologists debate the extent nonetheless clearly distinguishable. Primary producers, to which bottom-up versus top-down control influence autotrophic organisms (primarily plants and algae) that trophic structure in particular ecosystems, there is a broad convert light or chemical energy into biomass, make up consensus that both need to be considered when consid- the basal trophic level. Primary consumers, generally ering community dynamics. referred to as herbivores, feed on primary producers. Their consumption of individual producers can range anywhere from a small fraction of the totalFIRST producer Bottom-Up Control s0015 biomass (caterpillars feeding on trees) to the entire organ- Bottom-up control of trophic structure means that the p0015 ism (fish feeding on algae). Secondary consumers kill production of biomass at each trophic level is a function and feed on heterotrophic organisms such as herbivores of energy input into the primary producer trophic level. and/or detritivores. Although secondary consumers are Biomass accumulated by producers then passes to higher referred to generally as predators, this trophic level trophic levels as a function of the between-level transfer includes parasites, parasitoids, and pathogens in addition ER efficiency. The resulting biomass pyramids are generally to carnivores. Secondary carnivores, or top predators, are characterized by abundant producer biomass and sharp organisms that eat carnivores. The most common exam- reductions in each higher trophic level. An important ples of this trophic level occur in aquatic systems, where exception to this pattern occurs in aquatic food webs, piscivorous fish such as tuna or pike eat smaller fish that where ‘inverted biomass pyramids’ can occur as a conse- feed on zooplankton (which, in turn, feed on phytoplank- quence of the extremely short generation time of ton). Finally, detritivores derive sustenance from dead unicellular producers relative to resident herbivores and organic matter emerging from each of the above trophic predators (see the section titled ‘Aquatic versus terrestrial levels. While relatively little attention is paid to this ecosystems’). trophic level, decomposers process a large fraction of net primary productivity (hereafter ‘NPP’) and are inte- Energy transfer to producers gral to nutrient cycling and ecosystem-level processes. s0020 ELSEVI Although a large amount of light energy is potentially p0020 available to producers, only a small fraction of the total is actually converted to producer biomass. Net photosyn- s0010 Control of Trophic Structure thetic efficiency (the percentage of available light energy that becomes biomass) in naturally occurring terrestrial p0010 Factors affecting the partitioning of biomass between and aquatic communities falls between 0.01 and 3%, with trophic levels can be divided into two broad categories. values approaching 10% for intensively managed agricul- The first of these categories, bottom-up control, tural systems. The resulting NPP is critically dependent 1 CLGY 00558 2 General Ecology | Trophic Structure on temperature and affected by water availability in ter- Bottom-up Top-down restrial systems and nutrient levels in aquatic systems. control control s0025 Energy transfer from producers to higher trophic levels biomass p0025 The overall transfer efficiency of energy between trophic Top predator levels is a function of three separate processes. The first of these, consumption efficiency, is the percentage of available productivity at a lower trophic level that is eaten by a biomass Predator higher trophic level. Grazers in temperate lakes, for example, remove nearly four times the fraction of primary productivity eaten by terrestrial grazers. The second process, the consumer’s assimilation efficiency, determines biomass what fraction of the biomass ingested by the consumer is Herbivore converted to energy. Finally, the consumer’s production efficiency determines the percentage of assimilated energy that yields new biomass. Taking all three pro- biomass cesses into account, the overall between-level transfer Producer efficiency ranges from 2 to 24%. PROOFTime Time p0030 System-wide patterns of consumer–resource transfer Figure 1 Left panel: Bottom-up control of a food chain. As f0005 efficiency are also affected by ecological stoichiometry, producer biomass (gray box) increases over time, all other the ‘match’ between the nutrient needs of consumers and trophic levels show correlated increases in biomass. Right panel: the nutrient supply of their resources. Transfer efficien- Top-down control of a food chain. As top predator biomass (gray box) increases over time, biomass in the trophic levels below cies are highest when consumers feed on resources whose either increase (herbivores) or decrease (predators and nutrient ratios are similar to their own, and decrease producers) in response. sharply when they feed on resources with dissimilar ratios. Consumer–resource nutrient ratios in aquatic systems are more closely matched than in terrestrial systems, The ‘green world’ hypothesis s0040 and in predator–herbivore versus herbivore–producer The concept of top-down control first gained widespread p0045 interactions. These facts have been invoked toFIRST explain attention as a result of the ‘green world’ hypothesis devel- why low transfer efficiencies are generally associated with oped by Hairston, Smith, and Slobodkin (hereafter ‘HSS’) in herbivore–producer interactions and occur in terrestrial 1960. In brief, HSS posited that the relative rarity of natural systems, while higher transfer efficiencies are character- disasters and obvious abundance of plant life implied that the istic of predator–prey interactions and occur in aquatic producer trophic level was generally limited by competition systems. for light, nutrients, space, and other resources. HSS further ER reasoned that the ‘green world’ around us is prima facie s0030 Patterns of biomass accumulation evidence that herbivores do not limit plant abundance; if p0035 As NPP and/or transfer efficiency increases, bottom-up they did, herbivores would be far more common and plants control predicts an increasing number of trophic levels as far less. Given that herbivores seem surrounded by more well as an increase in biomass at each trophic level. As food than they can eat, it seems unlikely that resource producer biomass changes over time, the effect ‘trickles competition limits them; HSS argued that predators are up’ to produce correlated changes in each of the higher responsible for suppressing herbivore abundance below the AU2 trophic levels (Figure 1, left panel). level at which they can regulate plant biomass. Predators, in turn, are often territorial and wide-ranging in their search for food; this implies that they are self-limited by competition for their herbivore prey. Finally, the fact that we are not sur- s0035 Top-DownELSEVI Control rounded by masses of decaying matter suggests that p0040 Top-down control means that predation by higher decomposers quickly and effectively exploit virtually all of trophic levels affect the accumulation of biomass at their food resources; as a result, this trophic level is likely lower trophic levels. Top-down control does not negate self-limited as well. While numerous researchers have sub- the importance of energy input into the basal trophic sequently identified potential flaws, limitations, and level; however, it suggests that biomass accumulation at inconsistencies in the HSS hypothesis, its simplicity, clarity, any one trophic level depends on the intensity of preda- and intuitive logic catalyzed research into the potentially far- tion from the above trophic level. reaching consequences of trophic interactions. CLGY 00558 General Ecology | Trophic Structure 3 A B Predator Top predator – (competition) + Herbivore – Predator Predator Producer (competition) (predation) Figure 3 A trophic cascade. Predators suppress herbivores f0015 (‘À’ arrow), which suppress producers

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