05.02 Food Webs and Energy Transfer

Food chain

A direct line where a secondary consumer eats a primary consumer that eats a producer. Usually, several food chains overlap and interact to create a food web.

Food Web

A food web can be a complex place. Consumers can be primary, secondary, tertiary, or even quaternary, depending on how many steps energy goes through to get to them. A food web would not be complete without some decomposers—organisms that break down dead, decaying organic matter. These decomposers return important nutrients to the soil or substrate.

Decomposer (Saprotroph)

Energy source: eating dead organisms or waste products Example: fungi, some bacteria, some insects

Secondary Consumer (Heterotroph; carnivore)

Energy source: eating primary consumers Example: Seals, tuna, octopuses

Primary Consumer (Heterotroph; herbivore)

Energy source: eating primary producers Example: small fish, zooplankton, manatees

Primary Producer (Autotroph)

Energy source: photosynthesis or chemosynthesis Example: plants, algae, some bacteria.

Producers

Every ecosystem needs producers. In most cases, producers use photosynthesis to transform energy from the sun. A few use chemosynthesis, getting energy from chemical compounds instead of the sun. Either way, they create the organic molecules that are the basis of food webs everywhere. Producers are also called autotrophs. "Auto" means self, and "troph" means nutrients, so these organisms make their own food.

Consumers

In an ecosystem, consumers don't always act the way they are supposed to. A polar bear usually eats larger fish that are considered secondary consumers, making the bear a tertiary consumer. But would the bear eat smaller fish that are primary consumers? If it was hungry and had the opportunity, sure! Plenty of consumers (including humans) are also omnivores, eating both producers and other consumers. The levels of consumers in a food chain or food web can represent typical actions, but the consumers do not always abide by the rules. Consumers are also called heterotrophs. "Hetero" means other, so these organisms get their food from other sources.

Primary Consumers

Organisms that feed directly on producers. These are herbivores that eat the producers.

Primary Producers

Primary producers have the ability to produce organic molecules using an energy source, usually the sun. These are organisms that produce organic matter using inorganic compounds and an energy source.

Secondary Consumers

Secondary consumers are the carnivores that eat the primary consumers.

10 Percent Energy Rule

The production efficiency varies but is often estimated at 10 percent, meaning that only about 10 percent of the energy absorbed is used for growth. This is often referred to as the 10 percent energy rule. In food chains, the predator only gains about 10 percent of the prey's energy. So a fish that nibbles on some kelp, for example, only gets about 10 percent of the original energy that the kelp absorbed. If a bigger fish swims by and eats that little fish, it will only get one tenth of that 10 percent, or 1 percent of the original energy from the sun. This pyramid effect leads to lower populations of predators at upper trophic levels.

Decomposers

These are a type of consumer that feed off of dead, decaying plants and animals. Their role is important in the food web, but they are hard to spot because many of them are so small, like bacteria and fungi. Decomposers help recover and recycle nutrients from the decaying matter and return it to the soil, air, and water. Decomposers are also called saprotrophs. "Sapro" means detritus, which is decayed matter. Saprotrophs are a type of heterotroph, since the matter they consume is coming from other organisms.