Biology • Year 11 • Module 4 • Lesson 11
Comparing Ecosystems: Abiotic and Biotic Differences
Lock in the vocabulary and structural knowledge for terrestrial and aquatic ecosystem comparisons — abiotic conditions, competition types, mutualism, thermocline, and the diversity-productivity paradox.
1. Term–definition match
The twelve definitions below are shuffled. In the right-hand column write the matching term from this list: abiotic factor, biotic factor, interspecific competition, intraspecific competition, mutualism, commensalism, parasitism, photic zone, thermocline, zooxanthellae, phytoplankton, biodiversity. 12 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The number and variety of species living in a given area or ecosystem. | |
| 1.2 | A non-living environmental variable such as temperature, rainfall, or light intensity. | |
| 1.3 | An influence on an organism caused by other living things, such as predation or competition. | |
| 1.4 | Competition for resources between individuals of different species. | |
| 1.5 | Competition for resources between individuals of the same species. | |
| 1.6 | A symbiotic relationship in which both organisms benefit (+/+). | |
| 1.7 | A symbiotic relationship in which one organism benefits and the other is unaffected (+/0). | |
| 1.8 | A symbiotic relationship in which one organism benefits at the expense of the other (+/−). | |
| 1.9 | The layer of water where light is sufficient for photosynthesis; extends to approximately 200 m depth. | |
| 1.10 | A steep temperature gradient between warm surface water and cold deep water that blocks vertical mixing. | |
| 1.11 | Photosynthetic dinoflagellates that live inside coral polyp tissue and provide up to 90% of the coral’s energy. | |
| 1.12 | Microscopic photosynthetic organisms (algae and cyanobacteria) that form the base of planktonic food webs. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the correct version on the line below it. 10 marks (1 for T/F, 1 for each correction where needed)
2.1 In a tropical rainforest, the dominant form of competition among plants is intraspecific competition for water. T / F
2.2 The thermocline in a tropical ocean prevents nutrients from sinking below the surface. T / F
2.3 Coral reefs are the most biodiverse marine ecosystem despite having nutrient-poor surface waters. T / F
2.4 The Southern Ocean has higher biodiversity than the Great Barrier Reef because it is more productive. T / F
2.5 Semi-arid scrublands tend to have lower species richness than tropical rainforests due to unpredictable rainfall and temperature extremes. T / F
3. Cloze — fill the blanks
Complete the paragraph using the word bank below. Each word is used once only. 10 marks
In a tropical rainforest, plants compete primarily through 1_____________ competition for 2_____________, which drives the development of vertical layers. The high rainfall and temperature 3_____________ also support elaborate mutualistic relationships such as pollination and mycorrhizae. In contrast, a semi-arid scrubland experiences intense 4_____________ competition for 5_____________, and mutualistic relationships are more facultative.
In the open ocean, a steep 6_____________ prevents nutrients from deep water reaching the sunlit surface layer, limiting primary 7_____________. Coral reefs bypass this limitation through the coral–8_____________ 9_____________, which recycles scarce nutrients within the coral tissue. The complex three-dimensional structure of the reef also creates many micro-10_____________, supporting extraordinary species diversity.
4. Function recall
Answer each question in 1–2 sentences using precise lesson terms. 10 marks (2 each)
4.1 What is the function of the thermocline in structuring primary productivity in tropical oceans?
4.2 What is the function of mutualism with zooxanthellae for coral polyps living in nutrient-poor water?
4.3 What is the function of vertical stratification in a tropical rainforest?
4.4 What is the role of decomposers in a tropical rainforest, and why are the soils still nutrient-poor?
4.5 What does the term diversity-productivity paradox describe?
5. Build a concept map
Draw labelled arrows between the six terms below to show how abiotic conditions shape biotic outcomes in an ecosystem. Each arrow must carry a short linking phrase (e.g. “limits”, “supports”, “drives”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: rainfall • biodiversity • competition type • mutualism • habitat stability • niche specialisation.
Q1 — Term–definition matches (12 marks)
1.1 biodiversity • 1.2 abiotic factor • 1.3 biotic factor • 1.4 interspecific competition • 1.5 intraspecific competition • 1.6 mutualism • 1.7 commensalism • 1.8 parasitism • 1.9 photic zone • 1.10 thermocline • 1.11 zooxanthellae • 1.12 phytoplankton.
Q2 — True / false with correction (10 marks)
2.1 False. In a tropical rainforest, the dominant competition among plants is interspecific competition for light — not water. The dense canopy creates intense competition among species at different vertical layers. (Semi-arid scrublands show intraspecific competition for water.)
2.2 False. The thermocline prevents nutrients from deep water rising to the surface (i.e., it blocks upwelling / vertical mixing), not from sinking. Nutrients produced in surface waters can sink below the thermocline where they become trapped.
2.3 True.
2.4 False. The Southern Ocean has lower biodiversity than the Great Barrier Reef despite being more productive. High productivity does not guarantee high biodiversity — the diversity-productivity paradox shows that stability, habitat complexity, and evolutionary time are equally important.
2.5 True.
Q3 — Cloze paragraph (10 marks)
1 — interspecific • 2 — light • 3 — stability • 4 — intraspecific • 5 — water • 6 — thermocline • 7 — productivity • 8 — zooxanthellae • 9 — mutualism • 10 — niches.
Q4.1 — Function of the thermocline (2 marks)
The thermocline is a steep temperature gradient between warm, less-dense surface water and cold, denser deep water. Because the layers do not mix, nutrients that sink below the thermocline cannot return to the photic zone, limiting the nutrient supply available to surface phytoplankton and therefore capping primary productivity in tropical ocean surface waters.
Marking criteria: 1 mark for identifying that the thermocline blocks vertical mixing / upwelling; 1 mark for linking this to nutrient starvation of surface waters / reduced primary productivity.
Q4.2 — Function of coral–zooxanthellae mutualism (2 marks)
Zooxanthellae (photosynthetic dinoflagellates) live inside coral tissue and supply up to 90% of the coral’s energy through photosynthesis. In return the coral provides shelter and CO₂ for photosynthesis. This tight internal nutrient cycle allows corals to thrive in nutrient-poor water by recycling scarce nitrogen and phosphorus within the symbiosis rather than relying on external supply.
Marking criteria: 1 mark for identifying the energy/photosynthate supply from zooxanthellae to coral; 1 mark for linking the mutualism to survival in nutrient-poor conditions / nutrient recycling.
Q4.3 — Function of vertical stratification in a rainforest (2 marks)
Vertical stratification partitions the light resource across different height layers (canopy, understory, shrub layer, forest floor), allowing different species to occupy different light niches. This reduces direct interspecific competition and supports the extremely high biodiversity found in tropical rainforests.
Marking criteria: 1 mark for explaining stratification as partitioning of the light resource into height layers; 1 mark for linking this to reduced competition and/or increased biodiversity.
Q4.4 — Decomposers and nutrient-poor soils (2 marks)
Decomposers (bacteria and fungi) are highly active in warm, moist rainforest conditions and rapidly break down leaf litter and dead organic matter, releasing nutrients. However, living plants absorb these nutrients so quickly that very little accumulates in the soil — the vast majority of nutrients are locked in living biomass (trunks, leaves, roots) at any given time, making the soils paradoxically nutrient-poor.
Marking criteria: 1 mark for identifying that decomposers break down organic matter rapidly; 1 mark for explaining that nutrients are quickly absorbed into living biomass, leaving soils nutrient-poor.
Q4.5 — Diversity-productivity paradox (2 marks)
The diversity-productivity paradox refers to the observation that ecosystems with very high productivity (e.g. the Southern Ocean) do not necessarily have high biodiversity, while some low-productivity ecosystems (e.g. the Great Barrier Reef) can have extraordinary species richness. This shows that biodiversity depends on environmental stability, habitat complexity, evolutionary time, and resource partitioning — not just on energy availability alone.
Marking criteria: 1 mark for correctly stating that high productivity does not equal high biodiversity (and vice versa); 1 mark for identifying at least one factor other than energy that drives biodiversity (stability / habitat complexity / evolutionary time / resource partitioning).
Q5 — Sample concept map (6 marks)
A correct map should include arrows such as:
- rainfall — determines availability of → competition type (high rainfall → interspecific for light; low rainfall → intraspecific for water)
- rainfall — supports high → biodiversity
- habitat stability — allows → niche specialisation
- niche specialisation — increases → biodiversity
- habitat stability — enables elaborate → mutualism
- mutualism — reduces competition and supports → biodiversity
Any biologically valid linking phrase is accepted. Award 1 mark per correctly labelled arrow that accurately reflects causal direction, maximum 6 marks.