The study of the structure of feeding relationships among organisms in an ecosystem. Researchers focus on the interplay between feeding relationships and ecosystem attributes such as nutrient cycling, physical disturbance, or the rate of tissue production by plants and the accrual of detritus (dead organic material). Feeding or trophic relationships can be represented as a food web or as a food chain. Food webs depict trophic links between all species sampled in a habitat, whereas food chains simplify this complexity into linear arrays of interactions among trophic levels. Thus, trophic levels (for example, plants, herbivores, detritivores, and carnivores) are amalgamations of species that have similar feeding habits. (However, not all species consume prey on a single trophic level. Omnivores are species that feed on more than one trophic level.) See Ecology, Ecosystem, Food web
The three fundamental questions in the field of trophic ecology are: (1) What is the relationship between the length of food chains and plant biomass (the relative total amount of plants at the bottom of the food chain)? (2) How do resource supply to producers (plants) and resource demand by predators determine the relative abundance of organisms at each trophic level in a food chain? (3) How long are real food chains, and what factors limit food chain length?
A central theory in ecology is that “the world is green” because carnivores prevent herbivores from grazing green plant biomass to very low levels. Trophic structure (the number of trophic levels) determines trophic dynamics (as measured by the impact of herbivores on the abundance of plants). Indirect control of plant biomass by a top predator is called a trophic cascade. Cascades have been demonstrated to varying degrees in a wide variety of systems, including lakes, streams, subtidal kelp forests, coastal shrub habitats, and old fields. In all of these systems, the removal of a top predator has been shown to precipitate dramatic reductions in the abundance of species at lower trophic levels. Food chain theory predicts a green world when food chains have odd numbers of trophic levels, but a barren world (plants suppressed by herbivores) in systems with even numbers of trophic levels.
Although predators often have strong indirect effects on plant biomass as a result of trophic cascades, both predation (a top-down force) and resource supply to producers (a bottom-up force) play strong roles in the regulation of plant biomass. The supply of inorganic nutrients at the bottom of a food chain is an important determinant of the rate at which the plant trophic level produces tissue (primary production, or productivity). However, the degree to which nutrient supply enhances plant biomass accrual depends on two factors: (1) how many herbivores are present (which in turn depends on how many trophic levels there are in the system) and (2) the degree to which the herbivores can respond to increases in plant productivity and control plant biomass. The relative importance of top-down (demand) versus bottom-up (supply) forces is well illustrated by lake systems, in which the supply of phosphorus (bottom-up force) and the presence of piscivorous (fish-eating) fish (top-down force) have significant effects on the standing stock of phytoplankton, the plant trophic level in lake water columns. See Biological productivity, Biomass, Fresh-water ecosystem, Phytoplankton
Increases in productivity may act to lengthen food chains. However, food chain length may be limited by the efficiency at which members of each trophic level assimilate energy as it moves up the food chain; the resilience of the chain (measured as the inverse of the time required for all trophic levels to return to previous abunance levels after a disturbance); and the size of the ecosystem—small habitats are simply not large enough to support the home range or provide ample habitat for larger carnivorous species. See Ecological energetics, Systems ecology, Theoretical ecology