Populations — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 16Population definitions

Population definitions

Population

Population a group of organisms of one species, living in the same area at the same time.

Community

Community all of the populations of different species in an ecosystem.

Ecosystem

Ecosystem a unit containing the community of organisms and their environment, interacting together, for example a decomposing log or a lake.

Section 2 of 16Population growth

Competition

• Competition all organisms compete for food, water and living space.

Best adapted

• Best adapted they generally increase their populations at the expense of less well adapted organisms.

Controlling factors

• Controlling factors food supply, predation and disease.

Section 3 of 16The sigmoid growth curve

1Sigmoid curve

• Sigmoid curve the S-shaped growth of a population in an enclosed environment, such as microorganisms in a fermenter.

2Four phases

• Four phases lag, log (exponential), stationary and death.

Section 4 of 16Phases of the curve

Phase	What happens	Reason
Lag	Slow growth	Organisms adapt and there are very few of them
Log (exponential)	Rapid growth	Abundant food, high birth rate, low death rate
Stationary	Population levels out	A factor such as a nutrient becomes limiting; birth rate equals death rate
Death	Population falls	Death rate exceeds birth rate as food runs short or wastes become toxic

Section 5 of 16Limiting factors

1Carrying capacity

• Carrying capacity the maximum population the environment can support.

2Limiting factor

• Limiting factor a factor, such as a nutrient that is not replenished, that restricts growth.

3Stationary phase

• Stationary phase lasts until the nutrient is replenished or severely limited.

Section 6 of 16Natural environments

Real populations

• Real populations rarely show a perfect sigmoid curve.

Other factors

• Other factors changing temperature or light, predators, disease, immigration and emigration.

Section 7 of 16More from the specification

1

Sigmoid Population Growth Curve Phases

• Microbial and animal populations in enclosed systems exhibit four distinct phases: lag phase, log/exponential phase (rapid uninhibited division), stationary phase (birth rate equals death rate),
  • and death phase.
    • (exhaustion of nutrients and toxic accumulation)
  • (cellular adaptation)

2

Limiting Factors and Environmental Carrying Capacity

• Abiotic factors and biotic pressures (predation, interspecific competition, infectious diseases) establish an upper environmental carrying capacity that stabilizes population size.
  • (light, temperature, water, mineral availability)

3

Human Population Growth and Resource Consumption

• Exponential expansion of the global human population exerts escalating demands on arable land, freshwater reservoirs,
  • and fossil energy reserves, intensifying environmental degradation.

4

Source

• A part of a plant that releases sucrose or amino acids.

5

Photosynthesis

• The process by which plants make carbohydrates from raw materials using energy from light.

6

Bacteria

• Single-celled microorganisms, much smaller than plant or animal cells.

7

Carbon cycle

• The movement of carbon between air, living things, soil and fossil fuels.

8

Fossil fuel

• Fuel formed from long-dead organisms.

9

Energy transfer

• The movement of energy between trophic levels through feeding.

10

Primary consumer

• Organism that eats producers.

11

Secondary consumer

• An organism that eats primary consumers.

12

Nitrogen

• Element in amino acids and nitrogenous bases.

13

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

14

Biomass

• The dry mass of living matter in an organism or trophic level.

15

Consumer

• An organism that gets energy by feeding on other organisms.

Section 8 of 16Population size and growth

Population

Population a group of organisms of one species living in the same area at the same time.

Increase

• Increase births and immigration increase population size.

Decrease

• Decrease deaths and emigration decrease it.

Unlimited resources

• Unlimited resources growth is rapid, giving a steep exponential curve.

Section 9 of 16Sigmoid growth curve

1Lag phase

• Lag phase slow growth while a few individuals adjust and start to reproduce.

2Exponential (log) phase

• Exponential (log) phase rapid growth with plenty of food and little competition.

3Stationary phase

• Stationary phase birth rate equals death rate and the population levels off at the carrying capacity.

4Death phase

• Death phase the population falls if resources run out or waste builds up.

Section 10 of 16Factors that limit population size

Factor	How it limits the population
Food supply	Shortage raises competition and death rate and lowers birth rate
Predation	Predators kill prey, and numbers rise and fall in linked cycles
Disease and parasites	Spread faster in crowded populations
Competition	For food, water, light, space or mates
Migration	Immigration adds and emigration removes individuals
Climate, water, shelter	Affect survival and reproduction

Section 11 of 16Carrying capacity and predator-prey cycles

1Carrying capacity

• Carrying capacity the maximum population size an environment can support over time.

2Prey increases

• Prey increases predators have more food and increase a little later.

3Predators eat many prey

• Predators eat many prey prey numbers fall.

4Prey falls

• Prey falls predators then fall.

Section 12 of 16Human population growth

1Growth

• Growth the human population has risen sharply over the last 200 years.

2Lower death rate

• Lower death rate improved medicine, hygiene, vaccination and clean water.

3Birth rates

• Birth rates remained high in many regions.

4Consequences

• Consequences greater demand for food, water, housing and energy, more waste and pollution, and habitat loss.

Section 13 of 16Estimating population size

Estimating population size

Place quadrats at random in the area, using random numbers for coordinates.

Count the organisms in each quadrat and calculate the mean per quadrat.

Estimated population = mean per quadrat × (total area ÷ area of one quadrat).

• Mobile animals
  • use mark–release–recapture.
  • Capture and mark a sample (M), release it, then capture a second sample (C) and count the marked ones (R).
  • Estimated population = (M × C) ÷ R.

Section 14 of 16More from the specification

Carbon emissions from burning fossil fuels

• Carbon emissions from burning fossil fuels industrial combustion of coal and petroleum fossil fuels elevates atmospheric carbon dioxide, disrupting global population ecosystems.

Population growth phases

• Microbial and animal populations exhibit an initial lag phase, an exponential log phase, a decelerating transitional phase,
• and a plateau at environmental carrying capacity.

Limiting factors and carrying capacity

• Density-dependent factors and density-independent factors (climate extremes) cap maximum sustainable population sizes.
  • (food scarcity, predation, infectious disease)

Energy flow and ecological pyramids

• Pyramids of biomass and energy narrow across successive trophic levels due to respiratory heat loss and unconsumed or undigested biomass.

Trophic energy transfer efficiency

• Trophic energy transfer efficiency producers such as green plants transfer only ten percent of stored chemical biomass energy to subsequent trophic levels.

Pyramids of biomass and numbers

• Pyramids of biomass and numbers ecological pyramids illustrate feeding relationships, representing total dry biomass or organism numbers across progressive food chains.

Nutrient cycling in food chains

• Nutrient cycling in food chains carbon and nitrogen atoms cycle continuously between living plants, consumer organisms,
  • and decomposers within balanced ecosystems.

Ecosystem

• Community plus environment.

Photosynthesis

• The process by which plants make carbohydrates from raw materials using energy from light.

Microorganism

• A living organism so small that it can only be seen clearly with a microscope.

Bacteria

• Single-celled microorganisms, much smaller than plant or animal cells.

Fossil fuel

• Fuel formed from long-dead organisms.

Energy transfer

• The movement of energy between trophic levels through feeding.

Primary consumer

• Organism that eats producers.

Nitrogen

• Element in amino acids and nitrogenous bases.

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

Biomass

• The dry mass of living matter in an organism or trophic level.

Consumer

• An organism that gets energy by feeding on other organisms.

Section 15 of 16Key terms

Term	Meaning
Population	Organisms of one species in an area at the same time.
Community	All populations in an ecosystem.
Ecosystem	Community plus environment.
Sigmoid growth curve	An S-shaped curve of population growth.
Limiting factor	A factor that restricts population growth.
Carrying capacity	The maximum population an environment can support.

Term	Meaning
Carrying capacity	The maximum population an environment can support.
Immigration	Individuals moving into an area.
Emigration	Individuals moving out of an area.
Predation	One animal killing and eating another.
Quadrat	A square frame used for sampling.
Mark-release-recapture	A method of estimating the population of mobile animals.

Section 16 of 16Exam technique

1

Saying the log phase is slow — it is the fastest growth phase.

2

Saying the stationary phase has no births — births equal deaths.

3

Confusing population with community — a population is one species.

4

Saying natural populations always follow a sigmoid curve — many factors disturb them.

1

Saying a population rises only through births — immigration also adds individuals.

2

Saying predators and prey change at the same time — predator numbers follow prey numbers.

3

Placing quadrats where plants look dense — positions should be random.

4

Saying carrying capacity is the largest ever population — it is what the environment can support over time.