IB ESS 2.3

Which of the following is an energy store in the nitrogen cycle?

• Deforestation

   

• Decomposition of feces

• Nitrogen fixation by bacteria

• Fossil fuels

Answer explanation

Fossil fuels are the only store of nitrogen in this list. The others are how nitrogen flows from one store to another.

Approximately how much of incoming solar radiation that reaches the ground is photosynthesized by producers?

•   1%

•   99%

•   5%

• 10%

Answer explanation

About 1 to 2 % of the sunlight made available to the plant is actually able to be photosynthesized into biomass. Most solar radiation is at wavelengths that make it unsuitable for photosynthesis.

A herbivore population is isolated for this experiment and the following data is collected. The organism is given 50 kg of feed per day, but only consumes 40 kg. 20 kg of feed per day is used for respiration, while 15 kg of faeces/waste per day is collected. Choose the correct statement?

• GP=35 kg/day and NP=15 kg/day

• GP=40 kg/day and NP=25 kg/day

• GP=25 kg/day and NP=5 kg/day

• GP=40 kg/day and NP=5 kg/day

Answer explanation

The total energy consumed and available to the organism is 40 kg/day. From this the undigested food that becomes feaces and is unavailable to the body for energy, needs to be subtracted, leaving GP = 25 kg/day. From this GP value, respiration of 20 kg/day needs to be subtracted, leaving NP = 5 kg/day.

To calculate the gross secondary productivity (GSP) over a period of time, what data would you need to collect?

• The biomass of food consumed and feces/waste produced

• The biomass of food consumed only.

• The biomass of food consumed, feces/waste produced and equivalent amount of biomass respired.

• The biomass of food consumed and equivalent amount of biomass respired.

Answer explanation

Gross productivity of consumers (GSP) is determined from the amount of food available for energy to its body. Only digested food can be turned into energy, so the biomass of feaces/waste needs to be subtracted from the food consumed. Respiration is not required to calculate gross productivity.

In energy flow diagrams, the symbol used to indicate energy storage is a 

• triangle

• arrow

•   box / rectangle

• circle

Answer explanation

Boxes or rectangles are used to indicate energy stores. Arrows are used to indicate the direction of energy flow from one store to the other.

Net Primary Productivity is the amount of energy

• fixed by the heterotrophs  in an ecosystem

• fixed in an ecosystem by photosynthesis, minus the losses due to respiration by producers

• produced from energy in developing countries

• fixed in an ecosystem by  the process of photosynthesis

In energy flow diagrams, how would an organism be indicated that consumes plants and is eaten by consumers?

• An arrow pointing towards the box representing the organism

• A box with both arrows pointing to and away from it.

• An arrow pointing away from the box representing the organism

• A box with no arrows pointing to or away from it.

Answer explanation

An arrow pointing towards the box represents the energy entering the organism and the arrow leaving the box represents the energy moving to the consumer.

Why do most marine food chains rarely have more than four trophic levels? 

• Energy is transferred inefficiently as it moves along the food chain. 

• Many species have become extinct. 

• Fishing activities have increased significantly

• There is great competition for food. 

Which of the following is/are needed to calculate gross primary productivity:

I. Energy fixed by photosynthesis

II. Energy lost by respiration

III. The biomass gained by heterotrophic organisms

• I and II

•   I only

•   III only

•   II only

Answer explanation

GPP is equal to the amount of energy fixed by photosynthesis. When you subtract losses due to respiration then you calculate NPP.

Net Primary Production represents the energy

• captured by photosynthesis in producers in an ecosystem, minus losses due to respiration.

•   captured by photosynthesis in producers in an ecosystem.

• captured by herbivores in an ecosystem, minus losses due to respiration.

•   captured by herbivores in an ecosystem.

Answer explanation

Net primary productivity is the amount of energy stored in the biomass of a producer. This is calculated by subtracting respiration from the energy gained in this case through photosynthesis.