These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Data for every question: percentage change = (new − original) ÷ original × 100. A calculator is assumed. All species named by letter and all data sets are fictional.
Question 1 (multiple choice · foundation)
A new fungal disease of trees arrives in a region. Which area is most likely to keep a functioning forest ecosystem over the next twenty years?
- (A) A plantation of one fast-growing tree species, all grown from cuttings of a single parent tree
- (B) A plantation of one tree species grown from seed collected from many parent trees
- (C) A natural woodland with many tree species of different ages, including several species that fill a similar role
- (D) A large lawn of a single grass variety next to a small group of trees
Answer and explanation
Answer: (C). Many tree species, with several filling a similar role, give back-up: if the disease kills one species, others can keep producing, sheltering animals and holding the soil. This woodland has the most components and the most variety among them.
- (A) has one species with almost no genetic variation, since all the trees are clones. If one tree is susceptible, nearly all are.
- (B) has some genetic variation, so some trees may resist, but there is still only one species. It is more resilient than (A) but less than (C).
- (D) is a low-diversity system that is not even a forest; the few trees have no back-up species.
Question 2 (multiple choice · core)
Which observation would be the strongest evidence that species S is a keystone species in a lake?
- (A) Species S makes up 60% of the total animal biomass in the lake.
- (B) Species S makes up 0.5% of the animal biomass, and in lakes where S has died out, the number of other animal species is less than half that of lakes with S.
- (C) Species S is found in every lake in the region.
- (D) Species S is rare, and the number of other animal species is the same in lakes with and without S.
Answer and explanation
Answer: (B). A keystone species has an effect far larger than its abundance. A species with 0.5% of the biomass whose loss more than halves the number of other species fits exactly.
- (A) describes an abundant, dominant species. A large effect from a large species is not disproportionate.
- (C) says S is widespread, which tells you nothing about its effect on the community.
- (D) shows a rare species with no effect: rarity alone does not make a keystone species.
Question 3 (multiple choice · core)
A disease kills most of the seagrass, a producer, in a shallow bay. Seagrass is eaten by grazing fish and gives shelter to young fish and shrimp. Which is the most likely short-term effect?
- (A) The number of grazing fish and young fish in the bay falls.
- (B) Predatory birds that eat fish in the bay increase, because fish are easier to see.
- (C) The bay quickly develops a new community with more species than before.
- (D) Nothing changes, because the animals can get their energy from sunlight instead.
Answer and explanation
Answer: (A). The first effects fall on the species that interact directly with the lost producer: grazers lose food and young fish lose shelter, so their numbers fall.
- (B) might happen very briefly, but with fewer fish the birds soon have less food; it is not the most likely main effect.
- (C) is a possible long-term outcome, not a short-term one, and losing a producer usually lowers diversity.
- (D) is wrong: animals cannot photosynthesise. Producers capture the energy that everything else uses.
Question 4 (constructed response · core)
Two sets of outdoor water tanks hold communities of algae: one set with 2 algal species, the other with 8. A heat wave occurs in week 2. The mean oxygen production of each set (mg O₂ L⁻¹ day⁻¹) is:
| Tanks | Before heat wave | During heat wave | Three weeks later |
|---|---|---|---|
| 2 species | 8.0 | 2.4 | 4.4 |
| 8 species | 8.4 | 6.3 | 8.2 |
(a) Calculate the percentage fall in oxygen production during the heat wave for each set. (b) Calculate the production three weeks later as a percentage of the production before. (c) Explain the difference between the two sets. (d) A fish farm grows fish in ponds fed by one cultured algal species. Use your answer to give one piece of advice.
Worked solution and suggested Marlbridge rubric
(a) 2 species: (8.0 − 2.4) ÷ 8.0 × 100 = 70%. 8 species: (8.4 − 6.3) ÷ 8.4 × 100 = 25%.
(b) 2 species: 4.4 ÷ 8.0 × 100 = 55%. 8 species: 8.2 ÷ 8.4 × 100 = 97.6% (about 98%).
(c) The 8-species tanks fell less (more resistant) and recovered almost fully (more resilient). With eight species, some are probably heat-tolerant and kept photosynthesising while sensitive species struggled; afterwards they could also regrow quickly. With only two species, if both are heat-sensitive there is no back-up, so production fell steeply and recovered slowly.
(d) Grow a mixture of algal species (or varieties) with different temperature tolerances, so that food production for the fish continues during a heat wave. Any advice that adds functional back-up earns the point.
| Point | What earns it |
|---|---|
| 1 | Both falls correct (70% and 25%) |
| 1 | Both recoveries correct (55% and about 98%) |
| 1 | Explanation uses back-up species / variety of responses |
| 1 | Distinguishes the smaller fall from the fuller recovery (resistance and resilience) |
| 1 | Advice in (d) increases diversity, with a reason |
Question 5 (constructed response · stretch)
In a tropical forest, tree species F makes up about 3% of the trees. It is the only tree that produces fruit in the three-month dry season. During those months, about 40 species of fruit-eating birds and mammals feed mainly on its fruit. For the rest of the year, these animals eat the fruit of many other tree species and carry their seeds away in their droppings. Logging removes nearly all F trees from one area.
(a) Explain why F can be called a keystone species. (b) Predict the short-term effect of removing F on the fruit-eating animals. (c) Predict the long-term effect on the tree community, and explain the chain of events.
Model answer and suggested Marlbridge rubric
(a) F is only about 3% of the trees, yet 40 animal species depend on it for food during the dry season, and through them many other trees depend on it for seed dispersal. Its effect on the ecosystem is far larger than its abundance.
(b) In the first dry season after logging, fruit-eating animals have little food. Many will starve, fail to breed or leave the area, so their populations fall.
(c) With fewer fruit-eaters all year round, fewer seeds of the other fruiting trees are carried away from the parent tree. Seedlings then grow mostly near their parents, where competition and disease are heavier, so fewer survive. Over decades, tree species that rely on animal dispersal decline, and trees dispersed by wind increase. The forest becomes less diverse in both trees and animals, and the community has a different structure.
| Point | What earns it |
|---|---|
| 1 | Keystone justified by small abundance and large effect |
| 1 | Short-term: fruit-eater populations fall in the dry season, with the reason |
| 1 | Long-term: loss of seed dispersal for other trees |
| 1 | Long-term: change in tree composition and overall loss of diversity |
Accept other reasonable long-term consequences if the chain is complete, for example predators of fruit-eaters also declining.
Question 6 (constructed response · core)
A dam is built upstream of a river floodplain. Before the dam, the river flooded the plain every wet season. After the dam, the regular floods stop. Ten years later, the area of seasonal wetland has fallen from 120 km² to 45 km², and the number of fish species that breed on the floodplain has fallen from 34 to 21.
(a) Calculate the percentage change in wetland area and in fish species. (b) Explain why the floods count as an essential abiotic factor for this ecosystem. (c) Predict one further long-term change in the floodplain community, and explain it.
Worked solution and suggested Marlbridge rubric
(a) Wetland: (45 − 120) ÷ 120 × 100 = −62.5%. Fish species: (21 − 34) ÷ 34 × 100 = −38.2% (about −38%).
(b) The floods are a non-living factor that many species need. They create the shallow, food-rich water where fish breed, spread nutrient-rich silt that fertilises plants, and keep the wetland habitat in existence. When the floods stop, the habitat shrinks and species that depend on it are lost, so diversity falls.
(c) Any one with reasoning, for example: wetland plants are replaced by dry-land shrubs and grasses, because the soil no longer floods; birds that feed on the fish or nest in wetland decline as their food and habitat shrink; plant growth on the plain falls because floods no longer bring nutrient-rich silt.
| Point | What earns it |
|---|---|
| 1 | Both percentage changes correct with signs |
| 1 | Floods linked to habitat, breeding or nutrients for many species |
| 1 | A valid long-term change explained through a chain of effects |
Question 7 (constructed response · stretch)
On a large island with no large predators, a deer-like herbivore had grazed so heavily that few young trees survived. A predator of the herbivore returns naturally to the island. Over the next eight years:
| Year after predator returned | 0 | 3 | 8 |
|---|---|---|---|
| Herbivores per km² | 40 | 31 | 22 |
| Young trees (saplings) per hectare | 150 | 480 | 1900 |
| Plant species in survey plots | 12 | 15 | 23 |
(a) Calculate the percentage change from year 0 to year 8 in herbivores, saplings and plant species. (b) Explain how the predator could cause the changes in plants, even though it does not eat plants. (c) Describe how the short-term and long-term effects differ. (d) A student claims: “These data prove the predator caused the increase in plant diversity.” Evaluate the claim.
Model answer and suggested Marlbridge rubric
(a) Herbivores: (22 − 40) ÷ 40 × 100 = −45%. Saplings: (1900 − 150) ÷ 150 × 100 = +1167% (about 12.7 times as many). Plant species: (23 − 12) ÷ 12 × 100 = +91.7% (about +92%).
(b) The predator eats herbivores, so herbivore numbers fall. Fewer herbivores eat fewer saplings and small plants, so more survive and grow. More kinds of plant can establish, so plant diversity rises. The predator affects the plants indirectly, through the herbivores.
(c) In the first few years, the main effects are on the predator’s direct prey: fewer herbivores, and more saplings surviving (150 to 480). Over eight years, the effects spread through the community: saplings rise more than twelvefold and plant richness almost doubles, which in turn will provide new food and habitat for other species.
(d) The data support the claim but do not prove it. The pattern is a correlation over time on one island, with no control island. Other changes could have contributed, such as wetter years, disease in the herbivores or people hunting them. A stronger test would compare islands with and without the predator over the same years, or fence plots to exclude herbivores.
| Point | What earns it |
|---|---|
| 1 | All three percentage changes correct |
| 1 | Indirect chain: predator → fewer herbivores → less grazing → more plants |
| 1 | Short-term (direct prey) contrasted with long-term (community-wide) using the data |
| 1 | Claim evaluated: supports, not proves, with a named alternative explanation |
| 1 | A better test proposed (control island or exclusion plots) |
How did you do?
- Q1 or Q4 wrong: re-read “Diversity and resilience” and Worked example 1 in the study guide.
- Q2 or Q5 wrong: re-read “Keystone species” and Worked example 2; the key is effect relative to abundance.
- Q3, Q6 or Q7 incomplete: re-read “What keeps an ecosystem diverse” and “Adding or removing a component”, and write your prediction as a chain of steps.
Then tick off the topic checklist.