Secondary 3 Biology Ecology Quiz

Secondary 3 Biology Quiz - Ecology: Answer Key

Total Marks: 40

Section A: Multiple Choice Questions (10 marks)

1. B [1]
An ecosystem is defined as a community of organisms interacting with each other and their physical (abiotic) environment. Option A describes a population. Option C describes population size. Option D describes a niche.

2. B [1]
Dissolved oxygen is a non-living (abiotic) factor. Algae, water lily, and mosquito larvae are all living (biotic) components of the ecosystem.

3. C [1]
Producers (autotrophs such as plants and algae) form the first trophic level in a food chain because they convert light energy into chemical energy through photosynthesis.

4. C [1]
If snakes decrease, the predation pressure on mice will be reduced, so the mouse population is likely to increase. Frogs would also increase (not decrease), and grass would likely increase due to reduced herbivory. Foxes are not directly affected by snake numbers in this web.

5. B [1]
Energy flows in one direction through an ecosystem (from producers to consumers) and is lost as heat at each trophic level through respiration. Energy is not recycled — matter is recycled, but energy is not.

6. B [1]
An inverted pyramid of numbers occurs when a single large producer (e.g., one tree) supports many smaller consumers (e.g., insects and birds). The base of the pyramid (one tree) is smaller than the level above it (many insects).

7. D [1]
Carbon dioxide is returned to the atmosphere through respiration (by all living organisms), decomposition (by decomposers breaking down dead organic matter), and combustion (burning of fossil fuels and biomass). Photosynthesis removes CO₂ from the atmosphere.

8. B [1]
Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia or nitrates by nitrogen-fixing bacteria, making it available for plant uptake.

9. C [1]
Deforestation removes trees that store carbon. When trees are cut down and burned or left to decompose, the stored carbon is released into the atmosphere as CO₂. This increases atmospheric CO₂ levels.

10. C [1]
When prey population increases, there is more food available for predators. After a time lag (time needed for predators to reproduce and increase in number), the predator population will increase. The prey population will not increase indefinitely due to limiting factors such as food availability and predation.

Section B: Structured Questions (20 marks)

11.

(a) [3 marks]
Correct graph plotting:

• Axes correctly labelled: x-axis = "Year", y-axis = "Population" [1]
• Both lines plotted accurately with data points joined [1]
• Lines clearly labelled ("Grass" and "Snakes") with a key or direct labelling [1]

Expected pattern: Grass population decreases steadily from 500 to 300 (×1000). Snake population increases steadily from 10 to 20.

(b) [2 marks]
As the population of grass decreases over the five years, the population of snakes increases. [1] This is an inverse/negative relationship between the two populations. [1]

(c) [1 mark]
The snake population increases because there is less competition for prey (frogs and mice), or because the ecosystem has changed in a way that favours snakes (e.g., fewer predators of snakes, or snakes are generalist feeders that can find alternative food sources). [1]

Accept any reasonable suggestion consistent with the data.

12.

(a) [1 mark]
Mangrove leaves and algae. [1] (Both must be named for the mark.)

(b) [2 marks]
Any two of the following: crabs, mudskippers, small fish. [1 mark each]
Secondary consumers feed on primary consumers (herbivores). Crabs eat mangrove leaves (producer → primary consumer → crab is secondary). Mudskippers eat insects (primary consumer). Small fish eat algae (producer → primary consumer → small fish is secondary).

(c) [1 mark]
Mangrove leaves → Insects → Mudskippers → Herons [1]
OR: Algae → Snails → Mudskippers → Herons [1]

(d) [2 marks]
If the heron population decreases, there will be less predation pressure on mudskippers. [1] Therefore, the mudskipper population would increase. [1]

13.

(a) [1 mark]
Photosynthesis [1]

(b) [2 marks]
Any two of the following: respiration, decomposition, combustion (burning of fossil fuels). [1 mark each]

(c) [2 marks]
Human activities such as burning fossil fuels and deforestation have increased the amount of carbon dioxide released into the atmosphere. [1] This disrupts the natural balance of the carbon cycle by adding more CO₂ than can be absorbed by photosynthesis, contributing to global warming. [1]

(d) [1 mark]
The carbon cycle regulates Earth's climate by controlling the amount of CO₂ in the atmosphere. CO₂ is a greenhouse gas that traps heat. When the carbon cycle is balanced, CO₂ levels remain stable, helping to maintain a stable climate. [1]

14.

(a) [1 mark]
Nitrogen fixation [1]

(b) [1 mark]
Nitrification [1]

(c) [1 mark]
Ammonification [1]

(d) [2 marks]
Nitrogen is an essential element required for the synthesis of proteins and nucleic acids (DNA and RNA). [1] Proteins are needed for growth, repair, and enzyme function, while nucleic acids are needed for genetic information and cell division. [1]

(e) [2 marks]
Nitrogen-fixing bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium compounds. [1] This makes nitrogen available in a form that plants can absorb and use to make proteins, thus playing a crucial role in the nitrogen cycle. [1]

15.

(a) [2 marks]
As light intensity increases, the number of plants per m² also increases. [1] The number of plants rises from 2 per m² at 10 units of light to a maximum of 25 per m² at 95 units of light. [1]

(b) [1 mark]
Other abiotic factors may become limiting, such as water availability, soil nutrients, or temperature. Alternatively, competition between plants for space may prevent further increase. [1]

Accept any reasonable suggestion.

(c) [1 mark]
Transect sampling / line transect [1]

(d) [2 marks]
Recording light intensity at each sampling point allows the student to correlate the distribution of the plant species with light intensity. [1] This helps to determine whether light intensity is a factor influencing the distribution of the plant species along the transect. [1]

Section C: Extended Response (10 marks)

16.

(a) [3 marks]

• The student should place the quadrat randomly at several locations within the field to avoid bias. [1]
• The student should count the number of dandelions within each quadrat. [1]
• The student should calculate the mean number of dandelions per quadrat and multiply by the total area of the field divided by the area of one quadrat to estimate the total population. [1]

(b) [2 marks]
Mean = (3 + 5 + 2 + 4 + 6 + 3 + 5 + 4 + 2 + 6) / 10 [1]
Mean = 40 / 10 = 4 dandelions per quadrat [1]

(c) [2 marks]
Taking multiple samples reduces the effect of random variation and ensures the data is more representative of the entire field. [1] A single quadrat may be placed in an unusually dense or sparse area, giving an unreliable estimate. [1]

(d) [2 marks]
Any two of the following: light intensity, soil pH, soil moisture, temperature, nutrient availability, water availability. [1 mark each]

(e) [1 mark]
Example: Light intensity affects the rate of photosynthesis in dandelions. Higher light intensity increases the rate of photosynthesis, leading to faster growth and reproduction. [1]
Accept any valid explanation linked to a named abiotic factor.

17.

(a) [1 mark]
Mutualism is a type of symbiotic relationship in which both species benefit from the interaction. [1]

(b) [2 marks]
The fig wasp depends on the fig tree for reproduction (it lays eggs inside the fig) and for food (the larvae feed on fig tissue). [1] When fig tree populations decline, the wasps lose their breeding site and food source, causing their population to decline. [1]

(c) [2 marks]
Deforestation destroys the habitat of many species, leading to loss of shelter, food sources, and breeding sites. [1] This reduces species diversity and can lead to extinction of species that are unable to adapt or relocate. [1]

(d) [2 marks]
Any two of the following: establish protected areas/nature reserves, enforce laws against illegal logging, promote reforestation/afforestation, educate local communities about conservation, create wildlife corridors. [1 mark each]

(e) [1 mark]
It is obligate mutualism because both species are entirely dependent on each other for survival and reproduction — the fig tree can only be pollinated by this specific wasp, and the wasp can only reproduce in this specific fig tree. [1]

(f) [2 marks]
If a new pollinator were introduced, the fig tree would no longer be exclusively dependent on the fig wasp for pollination. [1] The fig wasp population might decline because the fig tree could survive without it, reducing the selective pressure to maintain the mutualistic relationship. Alternatively, the wasp might face competition from the new pollinator for access to figs. [1]

Accept any reasonable prediction with valid reasoning.

18.

(a) [1 mark]
A pyramid of biomass is a graphical representation showing the total mass of living organisms (biomass) at each successive trophic level in a food chain. [1]

(b) [2 marks]
Biomass decreases at each trophic level because energy is lost as heat through respiration at each level. [1] Only a small proportion (approximately 10%) of the energy/biomass at one trophic level is transferred to the next, so the total biomass decreases progressively. [1]

(c) [2 marks]
Percentage transfer = (Biomass at trophic level n+1 / Biomass at trophic level n) × 100 [1]
= (5 / 25) × 100 = 20% [1]

(d) [2 marks]
At each trophic level, a large proportion of energy is lost as heat through respiration, through uneaten parts of organisms, and through excretion. [1] By the fourth or fifth trophic level, there is insufficient energy remaining to support another level of organisms. [1]

19.

(a) [1 mark]
The pesticide concentration increases at each successive trophic level, from 0.02 ppm in algae to 50.0 ppm in the osprey. [1]

(b) [2 marks]
The pesticide is absorbed by organisms at the lowest trophic level and is not easily broken down or excreted (it is persistent). [1] As organisms at each trophic level consume large quantities of organisms from the level below, the pesticide accumulates in their tissues, leading to higher concentrations at each successive level. [1]

(c) [1 mark]
Bioaccumulation / Biomagnification [1]

(d) [2 marks]
Top predators accumulate very high concentrations of the pesticide in their bodies, which can reach toxic levels. [1] This can cause reproductive failure, weakened immune systems, behavioural changes, or death, potentially leading to population decline or extinction of the top predator species. [1]

20.

(a) [3 marks]
Pioneer species such as grasses and herbs would colonise the bare soil first. [1] Over time, shrubs and small trees would grow, followed by larger trees as the soil becomes richer in nutrients from decomposing organic matter. [1] Eventually, after many years, the land would develop into a mature forest community similar to the original forest that was cleared. [1]

(b) [2 marks]
In primary succession, the area starts with bare rock or no soil, so pioneer species such as lichens and mosses that can grow on bare rock and help form soil are needed. [1] In secondary succession, soil is already present, so pioneer species such as grasses and fast-growing herbs that can colonise existing soil are the first to appear. [1]

(c) [1 mark]
A climax community is the final, stable community that develops at the end of ecological succession, which remains relatively unchanged as long as environmental conditions remain constant. [1]

(d) [2 marks]
Biodiversity increases during the early and middle stages of succession as more species colonise the area and habitats become more complex. [1] In the climax community, biodiversity may stabilise or slightly decrease as competitive exclusion leads to dominance by a few well-adapted species, but overall species richness is higher than in the early stages. [1]

End of Answer Key