Selection Pressures & Population Change
In 1926, prickly pear cactus had smothered 24 million hectares of Queensland and NSW farmland — an area the size of Britain made completely unusable. The Queensland Department of Agriculture, working with CSIRO, released the South American moth Cactoblastis cactorum as a biological control agent in 1926–1933. Within 7 years, 90% of the prickly pear had been destroyed by the moth's larvae. This remains the most spectacular biological control success in history, and it is a perfect demonstration of selection pressures at work.
A drought hits a population of birds. The next year, the surviving birds tend to have larger, stronger beaks than before.
Before reading: what do you think happened during the drought to change the population, and was the environment "biotic" (living) or "abiotic" (non-living) in this case?
Know
- What a selection pressure is
- The difference between biotic and abiotic selection pressures
- The cane toad and prickly pear as case studies of population change
Understand
- How selection pressures change a population's abundance and traits over time
- How an introduced species acts as a selection pressure
- How biological control reduces a pest population
Can Do
- Classify selection pressures as biotic or abiotic
- Predict the effect of a selection pressure on a population
- Interpret population-change data over time
Core Content
The environmental forces that decide who survives
Picture the Queensland farmland in 1926: prickly pear cactus had blanketed 24 million hectares, and every plant in the path of the infestation had to either tolerate the dense cactus shade or die. When CSIRO released the Cactoblastis moth larvae, plants with tougher, more fibrous tissue survived longer before being consumed. This scenario illustrates exactly what a selection pressure is: any environmental factor — living or non-living — that affects which organisms survive and reproduce, and therefore which traits become more common over time.
A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing. Selection pressures drive changes in species diversity and abundance over time.
Biotic pressures come from other living things (predators, competitors, disease); abiotic pressures come from the physical environment (temperature, water, light, salinity).
Pause — copy the highlighted definitions and the biotic/abiotic split into your book.
A non-living environmental factor such as temperature or drought is an _____ selection pressure.
From a pressure to a change in the population
We just saw the types of selection pressure. That raises a question: how does a pressure actually change a population? This card answers it → through differential survival and reproduction over generations.
A selection pressure acts on the variation already present in a population — favouring some individuals over others.
Within any population there is natural variation. When a selection pressure acts: individuals with advantageous variations are more likely to survive and reproduce (differential survival), so those traits become more common over generations, while abundance and species diversity may rise or fall.
Effects can include: a change in the proportion of a trait, an increase or decrease in population abundance, or even local extinction if no individuals can cope with the pressure.
Add the "predicting an effect" steps to your notes before the check below.
A selection pressure changes a population mainly by...
Classify & Predict
Pattern — Classify & Predict an Effect
For each scenario, (a) name the selection pressure and classify it biotic or abiotic, and (b) predict its effect on the population over time. Answer in your book:
- A long drought reduces water in a grassland.
- A new fast predator arrives on an island of slow lizards.
- A fungal disease spreads through a frog population, killing thin-skinned individuals.
- Increasing salinity affects a wetland plant population.
Two Australian case studies of dramatic change
We just saw how to predict effects in principle. That raises a question: what does dramatic population change look like in the real world? This card answers it → two famous Australian invaders.
Introduced species can act as powerful selection pressures, causing rapid change in both the invader's population and the native species around it.
Cane toad (Rhinella marina): introduced to Queensland in 1935 to control cane beetles. It failed to control the beetles, but bred and spread rapidly across northern Australia. Because it is toxic, native predators (e.g. quolls, goannas, snakes) that eat it die — so the toad acts as a biotic selection pressure that has reduced many native predator populations.
Prickly pear (Opuntia): an introduced cactus that spread across millions of hectares of Queensland and NSW. The Cactoblastis moth was introduced as biological control — its larvae feed on the cactus, and within a few years the prickly pear population collapsed and stabilised at a low level.
A biotic selection pressure (the Cactoblastis moth) caused a rapid collapse, then a low, stable prickly pear population
Add both case studies (species, pressure, effect) to your notes before the check below.
The toxic cane toad acts as a biotic selection pressure that has reduced some native predator populations.
The Cactoblastis moth is an example of biological control of the prickly pear.
A selection pressure changes an individual organism's own genes during its lifetime to suit the environment.
Interpreting Population-Change Data
Pattern — Analyse a Case Study
Use the prickly pear graph in Card 3 and your knowledge of selection pressures. Answer in your book:
- Describe what happens to prickly pear cover before and after the Cactoblastis moth is released.
- Explain why the moth acts as a selection pressure on the prickly pear population.
- Explain why the prickly pear stabilises at a low level rather than disappearing completely (think about what happens to the moth population as food runs low).
- The cane toad spread rapidly while prickly pear collapsed. Explain how the same idea — a selection pressure — produces an increase in one case and a decrease in the other.
Selection pressures
- = environmental factor affecting survival & reproduction.
- Biotic = living (predation, competition, disease); Abiotic = non-living (temperature, water, salinity).
Predicting effects
- Acts on existing variation → differential survival & reproduction.
- Over generations: trait proportion shifts; abundance ↑ or ↓; possible local extinction.
Case studies
- Cane toad (1935): toxic invader → biotic pressure reducing native predators.
- Prickly pear: invasive cactus controlled by Cactoblastis moth (biological control) → rapid collapse, low stable level.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Define a selection pressure and give one biotic and one abiotic example, explaining how each could affect a population.
1 mark: definition · 1 mark: biotic example + effect · 1 mark: abiotic example + effect
ApplyBand 4(4 marks) 2. Using the cane toad OR the prickly pear, explain how an introduced species acts as a selection pressure and changes populations over time.
1 mark: introduction/spread · 1 mark: how it acts as a selection pressure · 1 mark: effect on the relevant population(s) · 1 mark: change over time
EvaluateBand 5(4 marks) 3. Evaluate the use of biological control (e.g. the Cactoblastis moth) as a way of managing an invasive species, referring to its benefits and risks.
up to 2 marks: benefits (effective, targeted, self-sustaining) · up to 2 marks: risks (control agent itself becoming a pest / affecting non-targets) + judgement
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
Q1 (3 marks): A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing. A biotic (living) example is predation: a new predator favours individuals that are faster or better camouflaged, so over generations those traits become more common and prey abundance may fall. An abiotic (non-living) example is drought: reduced water favours individuals that tolerate dry conditions (e.g. deeper roots, water storage), so drought-tolerant traits increase and less tolerant individuals die, reducing the population.
Q2 (4 marks): The cane toad was introduced to Queensland in 1935 to control cane beetles and has since bred and spread rapidly across northern Australia. It acts as a biotic selection pressure because it is toxic: native predators such as quolls, goannas and snakes that attempt to eat it are poisoned and die. This means individuals (and species) unable to avoid eating the toad suffer high mortality, so the populations of several native predators have declined where the toad has spread. Over time, the cane toad population has greatly increased and expanded its range, while affected native predator populations have decreased — a clear change in both populations driven by the introduced selection pressure. (In some predator populations, individuals that avoid eating toads are now favoured, an emerging selective response.)
Q3 (4 marks): Biological control uses a living organism to reduce a pest population, and it has clear benefits: it can be highly effective and self-sustaining (the Cactoblastis moth caused the prickly pear to collapse across millions of hectares and kept it at a low, stable level without ongoing chemical spraying), and it can be specific to the target pest. However, it carries significant risks: the control agent is itself an introduced species and may attack non-target native species or become a pest itself (as the cane toad — introduced as a control agent — did). Outcomes can be hard to predict and irreversible. Overall, biological control can be a powerful and sustainable management tool, but only when the control agent is carefully tested for host-specificity beforehand; used carelessly it can cause more harm than the original pest.
Timed questions on biotic/abiotic selection pressures and population change. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Enter the arenaYou were asked what happened during the drought to change the bird population, and whether the pressure was biotic or abiotic.
The drought is an abiotic selection pressure. It didn't change individual birds — instead, the birds that already had larger, stronger beaks could crack the tougher seeds left during the drought, so they survived and reproduced more (differential survival). Over the next generation, the proportion of large-beaked birds increased. That's exactly how a selection pressure reshapes a population: by acting on existing variation, not by changing individuals during their lives.
The 1926–1933 Cactoblastis biological control programme by CSIRO and Queensland's Department of Agriculture demonstrated the same principle at a spectacular scale. The Cactoblastis moth larvae acted as a biotic selection pressure on the prickly pear population: plants that could not withstand larval feeding were killed, and within 7 years 90% of 24 million hectares of infestation had collapsed. The remaining cactus population stabilised at a low level — differential survival had reshaped the abundance of an entire plant across a continent-sized area.