Ecology & Ecosystems

Question 1 (12 marks)

(a) GPP $= 1\% \times 1{,}000{,}000 = 0.01 \times 1{,}000{,}000 = 10{,}000\ \text{kJ/m}^2/\text{year}$.

• Correct method (×0.01) (1); correct value with units (1).

(b) Apply ×0.1 each step:

• Primary consumers: $10{,}000 \times 0.1 = 1{,}000\ \text{kJ/m}^2/\text{yr}$ (2)
• Secondary consumers: $1{,}000 \times 0.1 = 100\ \text{kJ/m}^2/\text{yr}$ (2)
• Tertiary consumers: $100 \times 0.1 = 10\ \text{kJ/m}^2/\text{yr}$ (2)

Why: energy transfer efficiency between successive trophic levels is ~10%, so each level multiplies by 0.1.

(c) Any three (1 mark each, max 4 with a well-explained point):

• Energy lost as heat during respiration at each level.
• Not all of an organism is eaten/digestible (bones, cellulose, roots).
• Energy lost in egestion (faeces) and excretion (urine/nitrogenous waste).
• Some organisms die and pass energy to decomposers rather than the next consumer.
• Energy used for movement/metabolism is not stored as biomass.

Question 2 (10 marks)

(a) Pyramid of biomass (1). Diagram: three bars stacked, width proportional to value — phytoplankton (4, narrowest) at base, zooplankton (11) middle, small fish (37, widest) top (2); correct labels of trophic levels (producer → primary consumer → secondary consumer) (1).

(b) Explanation (4):

• Biomass is a standing crop measured at one instant, not a rate.
• Phytoplankton have a very high turnover / reproduction rate — they reproduce and are eaten so fast that the mass present at any moment is small.
• Over a year, the total biomass produced by phytoplankton still exceeds that of consumers; the snapshot merely misses this.
• Energy (a rate over time) still decreases up the chain, so a pyramid of energy would be upright — no contradiction.

(c) Advantage of energy pyramid (2): it is always upright/never inverted because it accounts for the rate of energy flow over time, giving a true picture of productivity independent of organism size or turnover.

Question 3 (12 marks)

(a) Diagram (6): cycle showing $N_2 \xrightarrow{\text{fixation}} NH_3/NH_4^+ \xrightarrow{\text{nitrification}} NO_2^- \rightarrow NO_3^- \xrightarrow{\text{assimilation}} \text{plant/animal protein} \xrightarrow{\text{ammonification}} NH_4^+$, and $NO_3^- \xrightarrow{\text{denitrification}} N_2$.

• All five processes labelled (3); correct chemical species in correct positions (3).

(b) Organisms (5), 1 each:

• Nitrogen fixation: Rhizobium (root nodules) / Azotobacter (free-living) / cyanobacteria.
• Nitrification: Nitrosomonas ($NH_4^+ \to NO_2^-$) and Nitrobacter ($NO_2^- \to NO_3^-$).
• Assimilation: plants (roots absorbing nitrate).
• Ammonification: decomposers — saprotrophic bacteria and fungi.
• Denitrification: denitrifying bacteria e.g. Pseudomonas.

(c) (1) Atmospheric $N_2$ has a strong triple bond and is chemically inert; most organisms cannot use it directly, so it must be fixed into ammonia/nitrate before it can be assimilated.

Question 4 (10 marks)

(a) Food web (4) — arrows point from food to feeder (direction of energy flow). Correct arrows: grass→rabbit, grass→grasshopper, grasshopper→frog, grasshopper→shrew, rabbit→fox, rabbit→hawk, frog→hawk, shrew→hawk, shrew→fox.

• Award ~0.5 per correct arrow, all directions correct.

(b) (3):

• Producer: grass (1).
• Organisms at more than one trophic level: hawk (eats rabbit = 2nd trophic level as primary carnivore, but eats frog/shrew which are themselves carnivores/insectivores → 3rd/4th level) and fox (eats rabbit AND shrew) (1). Justification that they feed on prey at different levels (1).

(c) (3): Removing foxes → predation on rabbits and shrews decreases → shrew population increases → shrews eat more grasshoppers → grasshopper population decreases (1 for prediction). Must trace chain through shrews (1) and note reasoning (1). (Accept nuance: foxes don't eat grasshoppers directly, so effect is indirect via shrews.)

Question 5 (8 marks)

(a) (3):

• Primary succession: development of a community on newly exposed land with no pre-existing soil (e.g. bare rock, lava, sand dune) (1).
• Secondary succession: development on land where a community was removed but soil remains (e.g. after fire, flood, clearance) (1).
• Distinguishing feature: presence/absence of soil (or starting point) — secondary is faster because soil already present (1).

(b) (5):

• Pioneer community: lichens/mosses colonise bare rock (1).
• They secrete acids and, on death, add organic matter → begin soil formation (1).
• Thin soil allows small herbaceous plants/grasses to establish; they retain water and add more humus (1).
• Shrubs then larger plants can root as soil deepens (1).
• Trees establish → climax community (e.g. forest); each stage modifies conditions making them suitable for the next, less tolerant species (1).

Question 6 (8 marks)

(a) (3): Niche = the role/function of an organism in its ecosystem — how it obtains food, its interactions, the conditions it tolerates (1). Habitat = the physical place where an organism lives (1). Distinction: habitat is the "address", niche is the "profession/way of life" (1).

(b) (3): Factor = rainfall/water availability (1). Rainforest has high, year-round rainfall → high productivity → supports many species/high biodiversity (1). Desert has very low rainfall → water is limiting → few species with special adaptations, so low biodiversity (1).

(c) (2): e.g. Camel (desert) — primary consumer/herbivore (1); adaptation: stores fat in hump / concentrates urine to conserve water (1). (Accept any valid organism + trophic level + adaptation.)

[
  {"claim":"GPP = 1% of 1,000,000 = 10,000 kJ",
   "code":"gpp=0.01*1000000; result=(gpp==10000)"},
  {"claim":"Primary consumer energy = 1000 kJ via 10% rule",
   "code":"pc=0.01*1000000*0.1; result=(pc==1000)"},
  {"claim":"Tertiary consumer energy = 10 kJ (three 10% steps from GPP)",
   "code":"gpp=0.01*1000000; tert=gpp*0.1*0.1*0.1; result=(tert==10)"},
  {"claim":"Biomass pyramid is inverted: fish(37)>zooplankton(11)>phytoplankton(4)",
   "code":"phyto,zoo,fish=4,11,37; result=(fish>zoo>phyto)"}
]