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Energy Flow and Ecosystems Analysis of guilds and that of functional groups are different approaches used to study shifts in feeding behavior and to complement trophic-level analysis Guilds are defined as assemblages of species or individual age classes that share a common source of energy at any given time (e.g., nectarfeeding birds and bees and insect-feeding birds and spiders) Functional groups are defined as being similar in their mode of feeding (e.g., filter feeders, shredders, and pursuit predators), but individuals may use a variety of different sources as resource availability shifts Thus, individuals of a single species may be distributed over several trophic levels and belong to different guilds and functional groups during each individual’s life span and reproductive period As nutritional requirements change and the availabilities of different types of food resources also change, individuals can often adapt to find different available sources of energy Analyses of similarities in these adaptations and the degree of overlapping functionality are used to understand the degree to which producer and consumer species are interdependent Numerous complex linkages (e.g., herbivory, predation, decomposition, parasitism, and mutualism) imply that few species are likely to be complete substitutes for other species As discussed later, some species may interact positively, negatively, or neutrally in association with other species Such complex relationships among species, especially under changing environmental conditions, complicate field experiments and make predictive models difficult to test fully Controls of Energy Flow in Food Webs A small increase in species richness can have a large effect on how energy flows through food webs The main features are the number of linkages among species and, especially, the type and strength of those linkages (Paine, 1969) For example, a simple trophic structure would be a linear series of three species (A–C) in a food chain with one species in each trophic level (Figure 2) Thus, a single species of plant is consumed by a single species of herbivore, which is consumed by a single predator species Although analysis is relatively definitive in these types of communities with few species, this simple food chain structure may preclude consideration of some questions A A B B C C of general concern, such as resiliency of the assemblage following a disturbance and species loss If one more plant species is added (D) to a simple community, then this slightly more species-rich food web provides some important additional dynamics in terms of alternative pathways for energy to flow Moreover, the herbivore (B) can switch from one food resource to another and this additional complexity increases generality and realism incrementally With two herbivore species (B and C) the food web is more complex and the predator (A) has a choice of prey resources Even with the same number of species, much more realism is added by considering the predator (A) to be an omnivore, and even more is added if the predator and one of the herbivores (C) are also cannibalistic These simple diagrams show how quickly the types and numbers of linkages (connectance) within food webs can change the dynamics of energy flow even with only a few species This last example is typical of some low-diversity stream food webs on isolated tropical islands currently under study and discussed later Ecosystem Analysis General rules regarding the relationships between energy flow and the control of food web complexity are currently incomplete In some habitats a complex relationship apparently does exist among the total annual amount (and seasonal distribution) of energy, the nutrient inputs to ecosystems, and the number of different species in a habitat In other habitats there is no evidence for a cause-and-effect relationship among the rate of energy flow, species growth and productivity, and the number of species in an ecosystem Other likely variables include evolutionary time and biogeographical distributions as well as the frequency and intensity of disturbances The particular species composition of a food web may also alter productivity Empirical evidence for predicting the importance of species-specific relationships is increasing, but methods for establishing which species regulate ecosystem functions remain controversial Currently, only a few studies have focused on the species-specific roles to determine which species have unique roles and how these roles shift as environmental conditions change A A D 241 B C D E B D C E Increasing trophic complexity Figure Energy flow through simple food chains and webs Small increases in species number lead to high trophic complexity as connectance (arrows) and cannibalism (looped arrows) increase The number and strength of connections among species are more important in regulating the flow of energy through food webs than simply the number of different species in food webs
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