GRE Subject Test: Biology : Understanding Food Webs and Food Chains
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Example Questions
Example Question #61 : Ecosystem Development And Relationships
Which of the following would be a secondary consumer?
A black-tailed deer that browses on grasses and shrubs
People who are keen to eat bear, which eats small mammals that live off berries and seeds
A western diamondback rattlesnake that preys on field mice that eat seeds and berries
Earthworms, bacteria and fungi that decompose plant matter on the forest floor to replenish the soil
A maple tree that stores energy harnessed from the sun in the form of sugars through a process called photosynthesis
A western diamondback rattlesnake that preys on field mice that eat seeds and berries
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 #1 : Understanding Food Webs And Food Chains
What is the ratio of energy generated by producers to the energy absorbed by the next trophic level up, that of primary consumers?
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 : Organismal Ecology
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?
The conversion of food into biomass results in heat loss
All the available food cannot be eaten
Not all ingested food can be absorbed into the body
Not everything absorbed into the body is used for growth
All of these
All of these
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 #3 : Organismal Ecology
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?
Herbivores are not diverse enough nor do they have enough adaptations to eat all plants
Herbivores are limited by their predators
Herbivore population sizes are controlled by the climate
Most herbivores are very small
Herbivores generally only eat one plant
Herbivores are limited by their predators
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 #1 : Organismal 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?
There is more detritus than live biomass
Dung is much easier to digest because its already been digested by another animal
There is less competition for detritus than other resources
There are no special adaptations required to be a detritivore
Nutrient content is much richer in detritus
Dung is much easier to digest because its already been digested by another animal
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?
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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