Biology • Year 11 • Module 3 • Lesson 1
Selection Pressures & Population Change
Apply the biotic/abiotic split, predict the effect of pressures on populations, and interpret the prickly pear collapse data.
1. Classify and predict an effect
For each scenario, complete the empty cells: classify the pressure as biotic or abiotic, and predict its likely effect on the population over time. 8 marks, 1 per correct cell
| Scenario | Selection pressure | Biotic or abiotic | Predicted effect over time |
|---|---|---|---|
| A long drought reduces water in a grassland | Drought / water scarcity | ||
| A new fast predator arrives on an island of slow lizards | Predation | ||
| Increasing salinity affects a wetland plant population | Salinity | ||
| A fungal disease kills thin-skinned frogs | Disease |
2. Data response, prickly pear collapse
Use the information below to answer the questions. 8 marks
Scenario. By 1926, introduced prickly pear cactus (Opuntia) had smothered about 24 million hectares of Queensland and NSW farmland. In 1926, the South American moth Cactoblastis cactorum was released as a biological control agent. Its larvae feed on the cactus. Within about 7 years, around 90% of the prickly pear had been destroyed, and the cactus population then stabilised at a low, persistent level rather than disappearing completely.
2.1 Identify the selection pressure acting on the prickly pear population, and classify it as biotic or abiotic. 2 marks
2.2 Describe what happens to prickly pear cover before and after the moth is released. 2 marks
2.3 Explain why the prickly pear stabilises at a low level rather than disappearing completely. Consider what happens to the moth population as its food runs low. 2 marks
2.4 This is described as biological control. Define biological control and state one reason it was used instead of repeated chemical spraying. 2 marks
3. Cause-and-effect chain, how a population changes
Complete the cause-and-effect chain by filling in each empty box. The first box is done for you. 5 marks
Step 1: A long drought sets in. This is an _________________________ selection pressure, because it is a non-living condition.
Step 2: During the drought only large, tough seeds remain. Birds with larger, stronger beaks can crack them, so they are more likely to _________________________ and reproduce.
Step 3: Because beak size is _________________________ (passed to offspring), the surviving birds pass on their large-beak trait.
Outcome: Over the next _________________________, the proportion of large-beaked birds in the population _________________________.
4. Applied scenario, increase or decrease?
The same idea, a selection pressure, can make one population grow while another shrinks. Read each scenario and explain the population change. 4 marks
4.1 After 1935, the cane toad (Rhinella marina) population grew and spread rapidly across northern Australia. Explain why the cane toad population increased. 2 marks
4.2 At the same time, populations of some native predators (e.g. quolls, goannas) declined where the toad spread. Explain why those predator populations decreased. 2 marks
Q1, Classify and predict table (marking criteria)
- Drought: Abiotic. Effect: favours drought-tolerant individuals (deeper roots, water storage); less tolerant individuals die, so the population may shrink and tolerant traits become more common.
- Predation: Biotic. Effect: favours faster or better-camouflaged lizards; slow individuals are caught and die, so prey abundance may fall and faster/hidden traits increase.
- Salinity: Abiotic. Effect: favours salt-tolerant plants; salt-sensitive plants die, so salt tolerance increases and overall abundance may drop.
- Disease: Biotic. Effect: favours individuals resistant to the disease; thin-skinned/susceptible frogs die, so resistance increases and the population may decline.
Award 1 mark per correct classification and 1 mark per valid predicted effect. Accept other biologically reasonable effects.
Q2.1, Selection pressure
The selection pressure is the Cactoblastis moth (specifically its larvae feeding on the cactus) [1]. It is a biotic (living) pressure [1].
Q2.2, Before and after
Before release, prickly pear cover is very high (about 24 million hectares of dense infestation) [1]. After release, cover falls rapidly, around 90% is destroyed within about 7 years, then levels off at a low, stable amount [1].
Q2.3, Why it stabilises at a low level
As the cactus is destroyed, the moth's food supply runs low [1]. With less food, the moth population also crashes, so the pressure on the remaining cactus eases and a small prickly pear population can survive and persist at a low level rather than being wiped out entirely [1].
Q2.4, Biological control
Biological control is the use of a living organism (such as a predator, parasite or disease) to reduce a pest population [1]. It was used because it can be self-sustaining and effective over a huge area without the cost and repeated effort of chemical spraying [1]. (Accept: targeted to the pest; long-lasting; no chemical residue.)
Q3, Cause-and-effect chain (marking criteria)
- Step 1: abiotic [1]
- Step 2: survive [1]
- Step 3: heritable / inherited [1]
- Outcome: generations [1]; increases [1]
Q4.1, Why the cane toad population increased
The cane toad faced few selection pressures in Australia: it is toxic, so native predators that tried to eat it were poisoned, meaning it had almost no effective predators [1]. With abundant food and no control, it bred and spread rapidly, so its population and range increased greatly [1].
Q4.2, Why native predator populations decreased
The toxic cane toad acts as a biotic selection pressure on native predators: individuals that attempt to eat the toad are poisoned and die [1]. This high mortality reduces the predator populations where the toad has spread (although individuals that avoid eating toads may now be favoured) [1].