Population size and factors affecting it

What you'll learn

This revision guide covers population dynamics and the factors that control population size, a fundamental concept in ecology for CIE IGCSE Biology. You'll understand how birth rates, death rates, immigration and emigration affect population numbers, and how limiting factors prevent unlimited population growth. These concepts are essential for questions on ecosystems, food chains and human impact on the environment.

Key terms and definitions

Population — all the organisms of one species living in the same area at the same time

Birth rate — the number of live births per 1000 individuals per year in a population

Death rate — the number of deaths per 1000 individuals per year in a population

Immigration — the movement of individuals into a population from another area

Emigration — the movement of individuals out of a population to another area

Limiting factor — an environmental factor that restricts population growth and prevents it from reaching its full biotic potential

Carrying capacity — the maximum population size that an environment can support sustainably with the available resources

Predator-prey relationship — an interaction where one organism (predator) hunts and feeds on another organism (prey), affecting both populations

Core concepts

Factors that increase population size

Population size increases when more individuals are added than removed. Two main processes contribute to population growth:

Births

• Reproduction adds new individuals to the population
• Higher birth rates lead to faster population growth
• Birth rate is influenced by:
  • Availability of food and water
  • Suitable breeding sites and nesting areas
  • Absence of disease
  • Favourable climate conditions

Immigration

• Organisms moving into an area from elsewhere increase the population
• Common when resources are plentiful or conditions are favourable
• Examples include:
  • Birds migrating to the UK from Africa for the breeding season
  • Fish moving into coral reefs with abundant food
  • Wildebeest migrating to areas with fresh grass in East Africa

Factors that decrease population size

Population size decreases when more individuals are removed than added. Two main processes cause population decline:

Deaths

• Mortality removes individuals from the population
• Higher death rates lead to population decline
• Death rate increases due to:
  • Food shortages and starvation
  • Disease outbreaks
  • Extreme weather conditions (drought, floods, cold)
  • Predation
  • Competition for resources
  • Lack of shelter or suitable habitat

Emigration

• Organisms leaving an area reduce the population
• Common when resources become scarce or conditions deteriorate
• Examples include:
  • Lemmings migrating away from overcrowded areas
  • Caribbean birds leaving during drought periods
  • Salmon leaving freshwater streams to migrate to the ocean

The population size equation

The change in population size can be calculated using this relationship:

Population change = (Births + Immigration) − (Deaths + Emigration)

When births plus immigration exceed deaths plus emigration, the population grows.

When deaths plus emigration exceed births plus immigration, the population declines.

When these factors are balanced, the population remains stable.

Limiting factors and population control

Populations cannot grow indefinitely because environmental factors restrict growth. These are limiting factors.

Food supply

• Insufficient food reduces birth rates and increases death rates
• Organisms must compete for limited food resources
• In the Caribbean, limited fruit availability during dry seasons restricts parrot populations
• Herbivore populations are controlled by plant availability

Water availability

• Essential for all life processes
• Particularly limiting in arid environments
• Drought reduces population sizes dramatically
• Water holes in African savannahs support only limited animal populations

Space and territory

• Organisms need space for shelter, breeding and feeding
• Territorial behaviour limits population density
• Overcrowding increases stress and disease transmission
• Seabird colonies on UK cliffs are limited by nesting space

Predation

• Predators kill and consume prey, reducing prey populations
• High predator numbers lead to decreased prey populations
• When prey becomes scarce, predator populations decline due to starvation
• This creates cyclical population changes in both species

Disease

• Spreads more rapidly in dense populations
• Can cause dramatic population crashes
• Parasites weaken hosts, increasing death rates
• Coral diseases in Caribbean reefs reduce fish populations dependent on coral habitat

Competition

• Organisms compete for the same resources (food, water, space, mates)
• Intraspecific competition occurs between members of the same species
• Interspecific competition occurs between different species
• Competition is more intense when populations are large and resources are limited

Predator-prey cycles

Predator and prey populations show characteristic cyclical changes linked to each other:

The cycle pattern:

1. Prey population increases when predator numbers are low

   • Abundant food allows high birth rates
   • Low predation pressure reduces deaths

2. Predator population increases due to abundant prey

   • More food available increases predator birth rates
   • Predator death rates decrease

3. Prey population decreases due to increased predation

   • High predator numbers kill more prey
   • Prey birth rates may also fall due to stress

4. Predator population decreases due to food shortage

   • Prey scarcity causes predator starvation
   • Predator birth rates fall and death rates rise

5. The cycle repeats as prey populations recover when predator numbers fall

Key features of predator-prey graphs:

• Prey population peaks before predator population
• Predator population changes lag behind prey population changes
• Both populations fluctuate but rarely reach zero
• The amplitude (size of fluctuations) depends on reproductive rates and environmental factors

Carrying capacity and population growth curves

When organisms colonise a new environment, the population follows a characteristic growth pattern:

Phase 1: Lag phase

• Initial slow growth as organisms adapt to the new environment
• Small number of breeding individuals
• Time needed for reproduction to begin

Phase 2: Exponential (log) growth phase

• Rapid population increase
• Abundant resources available
• Little competition or disease
• Birth rate far exceeds death rate
• Forms a J-shaped curve if unlimited

Phase 3: Stationary phase

• Population growth slows and stabilises
• Reaches carrying capacity of the environment
• Birth rate approximately equals death rate
• Limiting factors restrict further growth
• Forms an S-shaped (sigmoid) curve

Phase 4: Fluctuation around carrying capacity

• Population oscillates slightly above and below carrying capacity
• Responds to variations in limiting factors
• May decline if resources are depleted or conditions worsen

Human population growth

The human population shows distinctive patterns different from other species:

Historical growth:

• Slow growth for thousands of years due to high death rates
• Limited by disease, food shortages and natural disasters
• Population remained well below 1 billion until 1800

Modern growth:

• Exponential growth from the 1800s onwards
• Global population exceeded 8 billion in 2022
• Growth due to:
  • Improved food production through agriculture and technology
  • Better sanitation reducing disease transmission
  • Medical advances (vaccines, antibiotics) reducing death rates
  • Reduced infant mortality
  • Increased life expectancy

Regional variations:

• Rapid growth in many developing countries
• Slower growth or decline in developed countries
• Birth rates falling globally but still exceed death rates overall

Sustainability concerns:

• Finite resources (fossil fuels, minerals, fresh water)
• Food production must increase to feed growing populations
• Habitat destruction and biodiversity loss
• Climate change from increased carbon dioxide emissions
• Waste production and pollution

Worked examples

Example 1: Calculating population change

Question: A population of rabbits on an island had 450 individuals at the start of the year. During the year, 120 rabbits were born, 35 died, 18 immigrated from a nearby island, and 25 emigrated. Calculate the population size at the end of the year. [3 marks]

Solution:

Step 1: Identify the factors that increase population size:

• Births = 120
• Immigration = 18
• Total increase = 120 + 18 = 138 ✓

Step 2: Identify the factors that decrease population size:

• Deaths = 35
• Emigration = 25
• Total decrease = 35 + 25 = 60 ✓

Step 3: Calculate final population:

• Final population = 450 + 138 − 60 = 528 rabbits ✓

Mark scheme:

• 1 mark for correct addition of births and immigration
• 1 mark for correct addition of deaths and emigration
• 1 mark for correct final answer

Example 2: Interpreting predator-prey graphs

Question: The graph below shows population changes of lynx (predator) and snowshoe hares (prey) over 20 years.

(a) Describe the relationship between the two populations. [2 marks]

(b) Explain why the lynx population peaks after the hare population. [2 marks]

Solution:

(a)

• The populations show cyclical changes / fluctuate ✓
• When the hare population increases, the lynx population also increases (after a delay) ✓
• When the hare population decreases, the lynx population also decreases ✓ (Any 2 points for 2 marks)

(b)

• When hare numbers are high, lynx have abundant food ✓
• This leads to increased lynx birth rate / survival, so lynx population increases ✓
• There is a time delay because lynx must reproduce and offspring must grow ✓ (Any 2 points for 2 marks)

Mark scheme:

• (a) 1 mark for identifying cyclical pattern; 1 mark for describing the relationship
• (b) 1 mark for explaining increased food availability; 1 mark for explaining time delay

Example 3: Limiting factors

Question: A farmer introduces 20 pheasants to a woodland area of 5 hectares. After 3 years, the population reaches 200 birds and then remains stable.

(a) State the term used to describe the maximum population the woodland can support. [1 mark]

(b) Suggest two limiting factors that prevent the pheasant population from continuing to grow. [2 marks]

Solution:

(a) Carrying capacity ✓

(b) Any two from:

• Limited food supply (seeds, insects, berries) ✓
• Limited nesting sites / space ✓
• Predation by foxes or birds of prey ✓
• Disease transmission increases in dense populations ✓
• Limited water sources ✓

Mark scheme:

• (a) 1 mark for correct term
• (b) 1 mark per valid limiting factor, maximum 2 marks

Common mistakes and how to avoid them

• Confusing immigration and emigration — Remember: immigration is organisms moving IN (both start with 'i'); emigration is organisms moving out (EXIT). Use these memory aids in exam answers.

• Stating predator and prey populations change at the same time — The predator population peak always lags behind the prey population peak. Clearly state "time delay" or "after a period of time" in explanations.

• Forgetting that limiting factors vary — Don't assume food is always the limiting factor. Consider the specific environment: water in deserts, nesting sites on cliffs, shelter in exposed areas. Match your answer to the context.

• Claiming populations reach zero — In predator-prey cycles, populations fluctuate but rarely become extinct. State that populations "decrease significantly" or "fall to low levels" rather than "reach zero".

• Mixing up intraspecific and interspecific competition — Intraspecific is within one species (intra = inside); interspecific is between different species (inter = between). Be precise with terminology for marks.

• Writing vague answers about population change — Always reference specific processes: birth rate, death rate, immigration, emigration. Avoid phrases like "the population just grows" without explaining the biological mechanisms.

Exam technique for "Population size and factors affecting it"

• Command word recognition: "State" requires the term only (e.g., carrying capacity = 1 mark). "Explain" requires a reason or mechanism (minimum 2 marks). "Describe" needs observable patterns without explanation. "Suggest" accepts any reasonable answer based on biological principles.

• Use the context provided: Questions often specify environments (woodland, ocean, desert) or organisms (specific animals/plants). Tailor limiting factors to match: mention water for desert species, light for rainforest floor plants, salinity for marine organisms.

• Graph interpretation skills: For population graphs, always describe trends over time (increases/decreases/fluctuates), quote data values when shown, compare multiple populations if present, and explain patterns using limiting factors or predator-prey interactions.

• Calculation questions: Show all working clearly. Use the population change formula: (births + immigration) − (deaths + emigration). Units matter — state whether answers are in individuals, thousands, or per 1000 population for rates.

Quick revision summary

Population size increases through births and immigration, and decreases through deaths and emigration. Limiting factors such as food, water, space, disease, predation and competition prevent unlimited growth and establish a carrying capacity. Predator and prey populations show cyclical changes with predator peaks lagging behind prey peaks. Human populations have grown exponentially due to improved food production, medicine and sanitation, raising sustainability concerns about resource depletion and environmental impact.