IGCSE · Practice questions · Populations

Name the phase, then the limit.

Six original Cambridge-style questions on population ecology: definitions, factors affecting growth rate, reading sigmoid growth curves and explaining each phase with limiting factors.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Lag, log, level, fall.

Population: one species, same area, same time. Community: all the populations. Ecosystem: community plus environment, interacting.
Growth rate depends on food supply, competition, predation and disease.
Phases: lag, exponential (log), stationary (births = deaths), death (deaths > births).
Extended: name the limiting factor and link it to the birth and death rates.

Recall

[3 marks]

Define the terms population, community and ecosystem.

Population: a group of organisms of one species, living in the same area, at the same time. ✓
Community: all of the populations of different species in an ecosystem. ✓
Ecosystem: a unit containing the community of organisms and their environment, interacting together. ✓

Recall

[4 marks]

State four factors that affect the rate of growth of a population and, for each, say whether an increase in the factor speeds up or slows down growth.

Food supply: more food speeds up growth. ✓
Competition: more competition slows growth. ✓
Predation: more predators slow growth. ✓
Disease: more disease slows growth. ✓

Data

[4 marks]

The number of yeast cells in a culture was counted every 4 hours.

Time / h	0	4	8	12	16	20	24	28
Cells / million per cm³	2	3	10	40	110	150	152	150

(a) Identify the times of the lag phase and of the stationary phase. (b) Calculate the mean rate of increase between 8 and 16 hours.

(a) Lag phase: 0 to about 4 hours (little increase). ✓
Stationary phase: about 20 to 28 hours (numbers level at about 150 million per cm³). ✓
(b) Increase = 110 − 10 = 100 million per cm³ in 8 hours. ✓
Mean rate = 100 ÷ 8 = 12.5 million cells per cm³ per hour. ✓

Explain

[4 marks]

Extended. Explain the shape of the curve for the yeast culture in the previous question between 0 and 28 hours, referring to limiting factors.

Lag (0-4 h): few cells, adjusting to new conditions and making enzymes, so few divisions. ✓
Exponential (4-16 h): plenty of sugar and no limiting factors, so the birth rate is far greater than the death rate and numbers double regularly. ✓
Slowing then stationary (16-28 h): sugar (food) becomes a limiting factor and competition increases; ethanol starts to build up. ✓
So the birth rate falls until it equals the death rate and the population stays constant. ✓

Analysis

[3 marks]

Extended. Predict how the yeast curve would change if the culture were continued for another 48 hours without adding any more sugar. Explain your prediction.

The population would enter the death phase and fall. ✓
The sugar would run out, so cells could not respire and reproduce. ✓
Toxic waste (ethanol) would build up, so the death rate would be greater than the birth rate. ✓

Explain

[3 marks]

A small number of rabbits is released onto an island with plenty of grass and no predators. Describe and explain how the rabbit population is likely to change over the following years.

At first the population grows slowly (lag), because there are few rabbits and they must mature and find mates. ✓
It then grows rapidly (exponential), because food is plentiful and there is no predation. ✓
Eventually food runs short and competition (and disease) increase, so growth slows and the population levels off at the size the island can support. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.