BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.3. Ecosystems and Energy Flow

10.3.2. Energy Transfer: Food Chains and Trophic Levels

Organic molecules synthesized by autotrophs serve as a source of Nutrition (matter and energy) for heterotrophic animals. These animals, in turn, are eaten by other animals, thereby transferring energy through a series of organisms where each subsequent one feeds on the preceding one. Such a sequence is called a food chain, and each link in the chain corresponds to a specific trophic level (from the Greek trophē — food). The first trophic level is always occupied by autotrophs, termed producers (from the Latin producere — to produce). The second level consists of herbivores (phytophages¹), known as primary consumers (from the Latin consumere — to devour); the third level (e.g., carnivores) comprises secondary consumers, and so forth.

An ecosystem typically has 4–5 trophic levels and rarely more than 6. This is partly due to the fact that at each level, a portion of matter and energy is lost (incomplete consumption of food, Respiration of consumers, "natural" mortality of organisms, etc.); such losses are illustrated in Figs. 10.4 and 10.10 and discussed in more detail in Section 10.3.5. However, recent studies suggest that food chain length is also limited by other factors. The availability of preferred food and territorial behavior that reduces population density—and consequently the Abundance of higher-level consumers in a given habitat—may play a significant role.

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Fig. 10.4. Energy Flow and nutrient cycling in a typical food chain. Note that a two-way exchange is possible between carnivores and detritivores, as well as decomposers: detritivores feed on dead carnivores, while carnivores in some cases prey on live detritivores and decomposers. Herbivores are primary consumers; carnivores are secondary, tertiary, and higher-order consumers.

According to estimates, in some ecosystems up to 80% of primary production is not consumed by herbivores. Instead, dead plant material becomes food for detritus-feeding organisms (detritivores) or decomposers. Such chains are referred to as detrital food chains. Detrital food chains predominate, for example, in tropical rain forests.

Producers

Virtually all producers are photoautotrophs, i.e., green plants, Algae, and certain prokaryotes such as cyanobacteria (formerly called blue-green algae). The Role of chemoautotrophs on a biosphere scale is negligible. Microscopic algae and cyanobacteria, which make up the phytoplankton, are the primary producers in aquatic ecosystems. Conversely, the first trophic level of terrestrial ecosystems is dominated by large plants, such as trees in forests, grasses in savannas, steppes, fields, etc.

Primary Consumers

Primary consumers include heterotrophs that feed on producers, i.e., herbivorous animals, or phytophages. Some primary consumers do not eat plants directly but parasitize them. This is most clearly seen in parasites such as aphids, certain Fungi, and heterotrophic plants, such as the chlorophyll-free broomrape (Orobanche). Drawing a clear line between producers and consumers is not always easy: for instance, the semi-parasitic mistletoe grows on trees and draws their sap, yet simultaneously carries out Photosynthesis in its own green leaves.

On land, the main herbivores are insects, reptiles, birds, and mammals. In fresh and marine waters, these are typically small crustaceans (daphnia, barnacles, crab larvae, etc.) and bivalve Mollusks; most of them are filter feeders that strain out producers, as described in Section 8.2.1. Together with Protozoans, many of these form part of the zooplankton—a collection of microscopic drifting heterotrophs that feed on phytoplankton. The life of oceans and lakes depends almost entirely on planktonic organisms, which form the base of virtually all food chains in these ecosystems.

Secondary, Tertiary, and Higher-Order Consumers

Secondary consumers eat herbivores, meaning they are carnivores. Tertiary consumers and higher-order consumers are also carnivores.

These consumers can be divided into several ecological groups:

1) predators: catch and kill their prey;

2) scavengers: feed exclusively on carcasses that have begun to decompose;

3) parasites: feed on the Tissues or fluids of their hosts without killing them, at least immediately.

Here are two Examples of a photosynthesis-based food chain:

Plant (leaves) → Slug → Frog → Grass snake → Stoat

Plant (phloem sap) → Aphid → Ladybug → Spider → Starling → Hawk

Typically, the body size of predators increases with each transition to the next trophic level, while their population numbers decrease. In parasitic food chains, the reverse is true: consumers progressively become smaller and more numerous.

10.3. Provide examples of food chains for well-known major habitats, such as the sea, a lake, a forest, a meadow, etc.

Decomposers and detritivores (detrital food chains)

Upon death, an animal or plant becomes a source of energy and biogenic elements for detritivores, and they also feed on the Metabolic waste products of other species. Organisms that feed on such dead organic matter (DOM) are called decomposers and detritivores. For the ecosystem as a whole, METABOLISM/2.html">THE CONCEPT OF "waste" does not exist: through decomposers, ideally all chemical elements are recycled.

Typical decomposers are microorganisms—mainly fungi and Bacteria. They secrete digestive Enzymes onto the surrounding organic matter and then absorb the products of this external Digestion. Detritivores ingest small edible particles. This is the feeding mechanism of many small aquatic and terrestrial invertebrates, such as earthworms in the soil, sludge worms in estuaries, woodlice, as well as microscopic animals like springtails and oribatid mites. Methods for isolating and studying some of these are described in Section 11.2.

Detritivores may be consumed by carnivores, which in turn are eaten by higher-order consumers, and so on. Such a food chain is called a detrital food chain, in contrast to a grazing food chain, which starts with autotrophs. Two typical examples of a forest detrital food chain are as follows:

Leaf litter → Earthworm (Lumbricus spp.) → Common blackbird (Turdus merula) → Eurasian sparrowhawk (Accipiter nisus)

Animal carcass → Blowfly and its larvae (Calliphora vomitoria) → Common frog (Rana temporaria) → Grass snake (Natrix natrix)

The rate of organic decomposition depends on the substrate and climate. Urine, feces, and animal carcasses can be completely mineralized in a matter of weeks, whereas a fallen tree may take many years. The breakdown of wood (and plant residues in general) heavily relies on fungi that produce the enzyme cellulase, which hydrolyzes the Cellulose in Cell walls, thereby softening the dead substrate and facilitating colonization by detritivorous animals. Decomposition is fastest in warm, moist environments, such as tropical rainforests, and slows down under low temperatures and/or humidity conditions. The practical absence of litter in jungles and the low humus content of their soils, compared to the thick litter layer and high humus concentration in oak or beech forests, clearly illustrates this pattern. This factor is important to consider when managing Different types of forests (see Section 10.9.5).

¹ From the Greek phyt on meaning "plant," so strictly speaking, referring to organisms that feed on algae or cyanobacteria as phytophagous (herbivorous) is technically incorrect, though this terminology is traditionally used instead of the more precise terms "autotrophophages" or "producentophages" — Trans.