AP Biology 8.1-8.2: Responses to Environment and Energy Flow

Trophic level

The position an organism occupies in a food chain or food web, measured by the number of feeding steps separating it from the primary producers.

How are trophic levels numbered in a typical ecosystem?

Primary producers occupy level 1, herbivores or primary consumers level 2, secondary consumers level 3, and higher-level consumers occupy successively larger integers. Apex predators are generally at the highest level in their food web.

Primary producer (autotroph)

An organism that makes organic food from inorganic substances. Plants, algae, and phytoplankton commonly use photosynthesis, while some organisms in light-free environments use chemosynthesis.

Why are primary producers the foundation of most food chains?

They convert energy from sunlight into chemical energy stored in organic molecules through photosynthesis. This chemical energy supports consumers at higher trophic levels.

Chemosynthesis

The production of organic molecules using energy released by chemical reactions rather than light. It supports primary production in environments such as deep-sea hydrothermal vents.

Consumer (heterotroph)

An organism that cannot manufacture its own food from inorganic materials and must obtain organic molecules by consuming other organisms or organic matter.

Differentiate herbivores, carnivores, and omnivores by their food source and usual trophic role.

Herbivores eat producers and are primary consumers. Carnivores eat animals and may be secondary or higher-level consumers, whereas omnivores eat both plants and animals and can occupy more than one trophic level.

Decomposer

An organism, often a bacterium or fungus, that breaks down dead organisms and wastes. Decomposers convert organic matter into inorganic substances that can be reused as mineral nutrients by producers.

Do decomposers recycle energy or nutrients in an ecosystem?

Decomposers recycle nutrients, not energy. Energy ultimately flows through the ecosystem and is dissipated as heat, whereas inorganic nutrients can return to producers.

Food chain versus food web

A food chain is a single sequence of feeding relationships. A food web is a network of interconnected food chains and better represents ecosystems in which organisms have multiple food sources and predators.

A rabbit eats grass and a fox eats the rabbit. What trophic levels do the grass, rabbit, and fox occupy?

The grass is a primary producer at level 1, the rabbit is a primary consumer at level 2, and the fox is a secondary consumer at level 3.

Apex predator

A consumer at the highest trophic position in a particular food web that normally has no predators outside its own species. Its trophic level may differ among populations if their diets differ.

Why can one species occupy different trophic levels in the same ecosystem?

A species may consume organisms from different trophic levels. Its trophic position therefore depends on its diet and can vary among populations or individuals.

Ecological efficiency

The fraction of energy or biomass transferred from one trophic level to the next. On average, only about 10% of the chemical energy consumed becomes new organic tissue at the next level.

What is the approximate energy-transfer relationship between adjacent trophic levels?

$E_{n+1} \approx 0.10E_n$. The remaining energy is largely used in metabolism, released as heat, or remains in uneaten or undigested material.

Why do food chains usually contain no more than about five or six trophic levels?

Because only about 10% of energy is transferred at each step, the energy available decreases exponentially up the chain. Higher trophic levels therefore support progressively less biomass and fewer organisms.

If producers convert about 1% of incident sunlight into chemical energy, what fraction of the original sunlight is approximately embodied in a tertiary consumer?

Using three transfers of about 10% each, the fraction is $(0.01)(0.10)^3 = 0.00001$, or approximately $0.001\%$ of the incident sunlight.

Why can producer biomass be lower than consumer biomass at a particular moment, especially in an ocean ecosystem?

Biomass is a snapshot, while production is a rate. Phytoplankton may reproduce and be consumed so rapidly that their standing biomass remains low even while they support a larger standing biomass of zooplankton.

What does an energy pyramid represent?

It represents the amount of energy available or transferred at successive trophic levels. Because energy is lost at each transfer, energy pyramids generally narrow toward the top.

How does energy differ from nutrients in its movement through an ecosystem?

Energy generally enters through sunlight or, in some ecosystems, chemical reactions, then flows from producers to consumers and is eventually dissipated as heat. Nutrients are recycled when decomposers return inorganic materials for producers to reuse.

Why can increased biodiversity produce more complex trophic pathways?

More species create more potential feeding relationships, so food webs contain additional overlapping chains. This can make organisms' trophic positions less accurately described by a single integer.

Why is an organism's trophic level not necessarily the same as the number of species in a food chain?

Trophic level counts feeding steps from producers, not the total number of organisms or species present. A level-3 consumer is two feeding transfers above a producer, regardless of how many species occur elsewhere in the food web.