Understanding Food Webs

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AP Biology › Understanding Food Webs

Questions 1 - 10
1

Only about 10% of the energy stored in a trophic level can be converted to matter in the next trophic level. Which of the following is not a consequence of this fact?

A species can occupy different trophic levels, depending on what it is eating; for example, an omnivore is a primary consumer when it eats plant leaves, but could be a secondary or tertiary consumer when it eats other animal species.

Food chains almost never have more than four or five trophic levels

Fluctuating population sizes at different trophic levels cause longer food chains to be less stable than shorter food chains

Producers always have the greatest biomass of any trophic level

Explanation

Though all of these statements are true, the fact that species can occupy different trophic levels depending on what they're eating is not a consequence of the fact that only 10% of all stored energy can ascend from one trophic level to the next. These facts are related, but one does not cause the other.

Food chains only have four or five trophic levels at maximum because the food chain is rapidly depleted of stored energy after each trophic level increase. Logically, producers always have the most biomass of any trophic level because they must produce all of the energy that will sustain the trophic levels above them. Finally, it makes sense that fluctuating population sizes threaten the stability of longer food chains because if even one trophic level suffers a population decrease, then all of the trophic levels above it are potentially jeopardized.

2

Which of the following is true regarding herbivores?

Herbivores eat plant material and are primary consumers

Herbivores eat plant material and are secondary consumers

Herbivores are secondary consumers

Herbivores are primary consumers

Explanation

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.

3

Which of the following best represents a component of biodiversity?

All of these

Variety of organisms in an ecosystem

Genetic variation across the planet

Ecosystem variation

Explanation

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.

4

Which of the following contributes to the phenomenon of biological magnification?

The biomass at higher trophic levels needs a larger amount of biomass at lower levels to support it

The biomass at higher trophic levels needs a smaller amount of biomass at lower levels to support it

Decomposers consume at every trophic level

Plants cannot absorb toxins from the environment

The biomass at higher trophic levels needs to be equal to the amount of biomass at lower levels to support it

Explanation

Because energy transfer between trophic levels is only about 10% efficient, it takes several organisms at a lower trophic level to support one organism at a higher trophic level. If each fish (lower level organism) has 10 toxin molecules in its body, and each eagle (higher level organism) eats 10 fish, the eagle now has 100 toxin molecules in its body. Molecules stored in an organism accumulate at higher trophic levels; this accumulation is known as biological magnification.

5

Of the following types of organism, which can directly obtain energy from any of the other types of organisms in an ecosystem?

Saprotroph

Producer

Carnivore

Herbivore

Omnivore

Explanation

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.

6

Which of the following would be considered as a secondary producer?

Antelope

Flowering plant

Cacti

Kelp

Explanation

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.

7

Which of the following processes is an example of primary production?

Photosynthesis

Carnivorous consumption

Decomposition

All of these

Explanation

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.

8

Animals are an example of which of the following categories of organisms?

Heterotrophs

Autotrophs

Photoautotrophs

Biosphere producers

Explanation

Animals are a type of heterotrophs. Heterotrophs are unable to make their own organic materials and so they must consume other organic materials in the form of autotrophs and other heterotrophs.

9

The energetic hypothesis states that __________.

The length of a food chain is limited by insufficient energy transfer through the food chain

The lower trophic levels provide sufficient energy for higher trophic levels

The abundance of organisms at higher trophic levels are supported by lower trophic energy

Several food chains linked together create food webs

Biomass increases with energy and trophic level

Explanation

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.

10

In terms of trophic structure, the cascade model follows which of the following viewpoints?

There is a unidirectional influence from higher to lower trophic levels

There is a unidirectional influence from lower to higher trophic levels

Lower trophic levels affect higher trophic levels unpredictably

Higher trophic levels affect higher trophic levels unpredictably

Trophic relationships are unpredictable and are dependent on environmental factors

Explanation

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.

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