2.2 Energy and biomass in ecosystemsIB Environmental Systems and Societies SL: Subtopic test

1. 1
   Neptune grass (Posidonia oceanica) forms underwater meadows in the shallow Mediterranean Sea. Its leaves capture sunlight and the plant makes glucose, some of which is stored as biomass in leaves, roots and rhizomes. Fish such as the salema (Sarpa salpa) graze on the leaves. The meadows receive a steady supply of sunlight but only a small fraction of the light energy reaching the leaves ends up in the grazers.

   (a)

   Which energy transformation is carried out by the seagrass when it makes glucose?

   [1 mark]

   • AChemical energy to light energy
   • BHeat energy to chemical energy
   • CLight energy to chemical energy
   • DChemical energy to kinetic energy only

   (b)

   Which statement is a consequence of the second law of thermodynamics in the meadow?

   [1 mark]

   • AEnergy is created by the grazers as they feed
   • BSome energy is transformed to heat at each transfer, so energy conversions are inefficient
   • CAll the light energy reaching the leaves is stored in biomass
   • DEnergy is destroyed when glucose is respired

   (c)

   Explain why the meadow needs a continuous input of light energy to keep functioning.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A research group measured energy flow through a temperate grassland food chain, in kJ m⁻² year⁻¹. The grass (producers) had a gross productivity of 12,000 and lost 4,800 as heat through cellular respiration. Grasshoppers (primary consumers) gained 900 in their biomass from eating grass. Shrews (secondary consumers) gained 90 from eating grasshoppers. Kestrels (tertiary consumers) hunt the shrews and have only ever been seen to feed on shrews and other small mammals in this grassland.

   (a)

   What is the net productivity of the grass, in kJ m⁻² year⁻¹?

   [1 mark]

   • A16,800
   • B4,800
   • C12,000
   • D7,200

   (b)

   Which is the main reason why food chains rarely have more than four or five trophic levels?

   [1 mark]

   • AEnergy is lost at each trophic level, so too little remains to support further levels
   • BPredators prefer smaller prey
   • CProducers cannot make enough glucose
   • DMatter is destroyed when it is eaten

   (c)

   Explain why only 900 kJ m⁻² year⁻¹ of the grass's net productivity ends up in the grasshoppers.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Clear Lake in California was treated with the insecticide DDD (a compound closely related to DDT) in the 1940s and 1950s to control gnats. Measurements afterwards found about 0.02 parts per million (ppm) in the lake water and about 5 ppm in the plankton. Small fish that fed on plankton had higher concentrations, and fish-eating western grebes (Aechmophorus occidentalis) had concentrations in their fat of well over 1,000 ppm. Grebe numbers on the lake fell sharply and the birds failed to rear young.

   (a)

   Calculate how many times more concentrated the pesticide was in the plankton than in the lake water, and distinguish between bioaccumulation and biomagnification.

   [3 marks]

   (b)

   Explain why the concentration of the pesticide was highest in the grebes and why this endangered the grebe population.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers in Sabah, Malaysia, compared an area of lowland rainforest with an oil palm plantation that replaced similar forest. The plantation is a monoculture of one species, is fertilised with nitrogen and phosphorus, and its palm fruit is harvested and transported away by lorry. Some of the forest was cleared by burning, including peat soils. The researchers want to compare the feeding structure of the two ecosystems using ecological pyramids.

   (a)

   Explain how the conversion of rainforest to oil palm plantation changes the flows of energy and the transfers of matter in the ecosystem.

   [6 marks]

   (b)

   Evaluate the use of pyramids of numbers, biomass and energy to compare the trophic structure of the forest and the plantation.

   [6 marks]

   Total for question 4: 12 marks

End of questions