Food Webs and Energy Flow

In a woodland ecosystem, each tree provides food for numerous aphids which feed on the sap of the tree. The aphids are eaten by carnivorous beetles, as shown in the food chain.

Oak Tree → Aphids → Beetles

Which pyramid of energy represents this relationship?

• A

• B

• C

• D

In a typical terrestrial ecosystem, which trophic level would have the highest biomass?

• Autotrophs

• Primary Consumers

• Secondary Consumers

• Detritivores

Plankton are major producers in marine ecosystems. Only a small proportion of the energy harvested by plankton is passed to the primary consumers. Which process in phytoplankton results in the largest loss of energy that could otherwise be utilized by consumers?

• Reproduction

• Homeostasis

• Excretion

• Respiration

The diagram shows the food web for an aquatic ecosystem in which letters R–Z represent individual species.

Which organism is a tertiary consumer?

• T

• U

• W

• Y

The diagram shows interactions between food chains in an ecosystem in the African savannah. Which arrows indicate the flow of chemical energy?

• I and II

• I and III

• II and III

• I, II, and III

Answer explanation

This is a tricky question! The key phrase here is CHEMICAL ENERGY. Arrow 1 is showing light energy from the Sun. Arrows 2 and 3 show the flow of chemical energy in organic compounds transferred between producers and primary consumers as well as primary consumers and secondary consumers.

Animals in the highest trophic level of a food chain will often be the largest in body size but will be few in numbers. What accounts for the small numbers?

• Food eaten by animals at the highest trophic level has a lower energy content per gram

• Energy losses through the food chain

• Conversion of heat energy into chemical energy

• Biomass of producers is small

Which processes occur in an ecosystem?

I. Biomass increases in each successive trophic level.

II. Inorganic nutrients are recycled.

III. Chemical energy is stored in carbon compounds.

• I and II

• I and III

• II and III

• I, II, and III

Answer explanation

Inorganic nutrients include things like water, nitrogen, and phosphorus. These are all recycled (think of the water cycle), meaning they stay within the ecosystem in various forms. While these are recycled within an ecosystem, energy is not.

What is lost between trophic levels in ecosystems and cannot be recycled?

• Heat

• Nitrogen

• Carbon Compounds

• Biomass

Answer explanation

Heat is a form of energy, and energy is not recycled within an ecosystem.

What restricts the length of a food chain?

• Energy losses between the trophic levels

• A greater biomass at the higher trophic levels

• The number of species in the food web

• The consumption of waste by detritivores

Which category of organisms is correctly described by its method of nutrition and site of digestion?

• A

• B

• C

• D