Understanding Food Webs - AP Biology

The energetic hypothesis states that the length of a food chain is limited by the inefficiency of energy transfer along the chain. Only ten percent of energy stored in organic matter at each trophic level is actually converted to organic matter at the next trophic level. This keeps trophic structures in check and limits the abundance of predatory organisms at the top of the trophic structure.

The cascade model, or top-down model, states that there is a unidirectional influence from higher to lower trophic levels. This model follows the idea that predation controls community organization. Predators limit the number of herbivores, and herbivores limit plants, and plants limit the amount of nutrients available in the soil.

Heterotrophs get energy from organic molecules. They are unable to fix carbon dioxide from the atmosphere into organic molecules, nor do they make oxygen as a byproduct of metabolism. They can consume autotrophs and heterotrophs (organic molecules) to get energy and carbon.

Primary production is defined as the synthesis of organic compounds from atmospheric carbon. The main process through which this synthesis occurs is photosynthesis. Primary production is characterized as the generation of biomass in autotrophs. On the other hand, secondary production is described as the generation of biomass in heterotrophs.

Secondary production is defined as the generation of biomass by heterotrophs, mainly through the transfer of organic compounds among trophic levels. Organisms that are primarily responsible for secondary production include animals, protists, and fungi.

In the food chain of most ecosystems, primary consumers are the organisms that consume primary producers, making them herbivores.

Secondary consumers consume primary consumers in the food chain (i.e. a snake eating grasshoppers).

Herbivores are organisms that eat plant material. They are considered as the primary consumers in an ecosystem. An example of an herbivore is a deer, which feeds on flowers and grass.

Decomposers are organisms that break down decaying organisms, thereby performing the process of decomposition. Common examples of decomposers include species of bacteria and fungi.

Carnivores are organisms that consume meat—animal tissue—as a part of their diet; therefore, on the food chain, carnivores may occupy the secondary, tertiary, or quaternary consumer levels.

Nutrient recycling specifically refers to the movement of materials within food webs and ecosystems that recycle them back into production. Nutrient recycling can be distinguished from biogeochemical cycles, like the water, nitrogen, and carbon cycles, which describe the exchange of matter through living and non-living components of the Earth.

The food web presents a graphical representation of the interconnected food chains within an ecosystem. In other words, food chains illustrate the trophic levels of a particular ecosystem.

Trophic level is the term used to a specific point on the food chain. An organism’s trophic level is the place they occupy in the food chain of a particular ecosystem.

Biodiversity refers to the different types of life present on Earth. This can refer to the variety of species in an ecosystem, the genetic variation across the planet, and ecosystem variation.

Both detritivores and decomposers play important roles in the food chain and energy exchanges within an ecosystem. Detritivores consume decomposing plant and animal tissue (i.e. vultures). Decomposers break down decomposing tissue using biochemical reactions rather than ingestion (i.e. fungi).

Omnivores are organisms that have a diet that includes of the ingestion of both plant and animal tissues. They have also been known to ingest bacteria and fungi.

Decomposition has been classified into the following three types: leaching, fragmentation, and chemical breakdown. Leaching is described as the flow of water through tissue, allowing it to be dissolved by the water solvent. Fragmentation is the physical breakdown of tissue to increase surface area exposure to microbes. Finally, chemical breakdown is defined as an enzymatic digestion of tissue.

The relationship that is occurring between the frog and the mite is known as commensalism because although the mite is benefitting from the frog, the frog is neither harmed or benefitting. Commensalism occurs when one organism benefits and the other is neither harmed or helped in any way. Parasitism benefits one organism at the expense of another. Mutualism benefits both organisms. Symbiosis is the general term for a specific interaction between species.

Commensalism is the type of relationship between two organisms in which one benefits while the other remains neutral. One such example of this is the relationship between whales and barnacles. The barnacles benefit, as they are able to gain mobility and feed off the current generated by movement of the whale. The whale, however, remains neutral; it gains no advantage or disadvantage from the presence of the barnacles.

Cohabitation relationships imply that both species remain neutral, while symbiotic and mutualistic relationships imply that both species gain benefit. In symbiosis, the two species depend on each other for survival. Parasitism implies benefit of one species, at the harm of the other.

Saprotrophs are decomposers that are capable of breaking down dead or dying organisms. Because of this, saprotrophs can obtain energy directly from any other organisms in an ecosystem.

Producers are autotrophs, and do not require organic input to create energy. Carnivores, herbivores, and omnivores are loose classifications of organisms based on diet. Carnivores typically feed on heterotrophs, while herbivores generally feed on autotrophs. Omnivores typically feed on both autotrophs and heterotrophs.