GRE Subject Test: Biology : Understanding Food Webs and Food Chains

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Food Webs And Pyramids

Which of the following would be a secondary consumer?

Possible Answers:

A western diamondback rattlesnake that preys on field mice that eat seeds and berries

A black-tailed deer that browses on grasses and shrubs

Earthworms, bacteria and fungi that decompose plant matter on the forest floor to replenish the soil

People who are keen to eat bear, which eats small mammals that live off berries and seeds

A maple tree that stores energy harnessed from the sun in the form of sugars through a process called photosynthesis

Correct answer:

A western diamondback rattlesnake that preys on field mice that eat seeds and berries

Explanation:

A secondary consumer is a step above the primary consumer (herbivore) on the food chain, consisting of omnivores and carnivores. A mouse that lives off plant matter and is thus a primary consumer. When a snake eats the mouse, it is the secondary consumer in the food web.

Example Question #62 : Ecosystems And Biology

What is the ratio of energy generated by producers to the energy absorbed by the next trophic level up, that of primary consumers?

Possible Answers:

Correct answer:

Explanation:

With every advancement in the trophic level, energy converts on a ten-to-one scale. For example, ten kilograms of grain fed to a steer produces roughly one kilogram of beef. This is true for every step up the tropic food pyramid.

Example Question #1 : Understanding Food Webs And Food Chains

Only 10% of the energy at each level of the trophic pyramid is available energy for the following trophic level. Why is so much energy lost between each level?

Possible Answers:

Not everything absorbed into the body is used for growth

The conversion of food into biomass results in heat loss

All the available food cannot be eaten

All of these

Not all ingested food can be absorbed into the body

Correct answer:

All of these

Explanation:

These are all sources of lost energy in between levels of the trophic pyramid. In the context of herbivores and carnivores: Not all food at each level can be eaten, because some prey escape their predators or they can't be found. When a predator eats its prey, not all of that tissue is digestible, such as cellulose and lignins. Lastly, everything that the predator digests is not used for new growth, and some is lost through excretion and respiration (heat).

Example Question #1 : Understanding Food Webs And Food Chains

There are a huge number of herbivores in the world, with insects being the largest and most diverse group. Given how successful these herbivores are and how abundant their plant resources are, why haven't all plants in the world been eaten by now?

Possible Answers:

Herbivore population sizes are controlled by the climate

Herbivores generally only eat one plant

Most herbivores are very small

Herbivores are limited by their predators

Herbivores are not diverse enough nor do they have enough adaptations to eat all plants

Correct answer:

Herbivores are limited by their predators

Explanation:

Herbivores are very likely limited by the predators in their own food webs, preventing them from completely overtaking the plants that they feed on. This is called the Earth is Green hypothesis, originally proposed by Hairston, Smith, and Slodobkin.

Example Question #2 : Ecology

Detritivores employ an evolutionarily successful feeding strategy of animals, in which they feed primarily on other animals' waste. Why is this an efficient approach?

Possible Answers:

Nutrient content is much richer in detritus

There is more detritus than live biomass

There are no special adaptations required to be a detritivore

There is less competition for detritus than other resources

Dung is much easier to digest because its already been digested by another animal

Correct answer:

Dung is much easier to digest because its already been digested by another animal

Explanation:

Detritivores are successful because it is much more efficient to digest dung because it doesn't require much extra digesting, as another animal has already done it. Detritivores can generally have a much less complicated digestive system and save themselves the energetically expensive process of digesting new plants or animals. Detritivores are also important to the ecosystem because they cycle the nutrients in dung back into the food chain. There is no evidence that detritivores experience less competition, nor that detritus is in excess to live organisms.

Example Question #3 : Ecology

Plants, which are capable of primary production via photosynthesis, are the base of many global food webs. However, this is an inefficient process relative to how much total solar energy is available. What percentage of incoming solar radiation is actually converted into plant tissue and is available to the next trophic level?

Possible Answers:

Correct answer:

Explanation:

Less than half of total solar energy is within the photosynthetically active wavelength range, and plants to not absorb all of this energy due to reflection and refraction. Thus, only about 1-5% of incoming solar radiation is actually converted to plant biomass, which serves as the base for all food chains. This is why herbivorous animals generally have to eat extremely high quantities of plants to achieve adequate nutrition.

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