Variation in Population Size
- A period of slow growth as the initially small number of individuals reproduce to slowly build-up their numbers.
- A period of rapid growth where the ever-increasing number of individuals continue to reproduce.
- Population growth declines until its size remains more or less stable.
- Food supply or increased predation may limit numbers.
Abiotic Factors
(1) Temperature
- Each species has a different optimum temperature at which it is able to survive.
- If temperatures fall below the optimum, enzymes will work more slowly so organisms’ metabolic rate will be reduced, however if temperatures rise too far above the optimum, the enzymes will be denatured.
(2) Light
- Light is a basic necessity of life and is the ultimate source of energy for ecosystems.
- As light intensity increases, so does the rate of photosynthesis, and growth rate of plants is increased and in turn, their population size increases.
- The population of herbivores will therefore also increase.
(3) pH
- This affects the action of enzymes.
- Each enzyme has an optimum pH at which it operates most effectively.
- A population will be larger where the appropriate pH exists.
(4) Water and Humidity
- Where water is scarce, populations are smaller and consist only of species which are well adapted to living in such conditions.
- Humidity affects the transpiration rate of plants and the evaporation of water from the bodies of animals.
Competition
Intraspecific competition
- It is the driving force behind natural selection since individuals with the best characteristics are more likely to win the competition and go on to breed and pass on their genes.
Interspecific competition
- It is the competition for resources between two individuals of different species.
- Where populations of a species initially occupy the same niche, one will normally have a competitive advantage over the other.
- The population of this species will gradually increase in size while the other will diminish.
Predation
- Predation occurs when one organism is consumed by another.
- The predator-prey relationship shows cyclical changes.
- Predators eat their prey, reducing the population of prey.
- With fewer prey available, there is greater intraspecific competition between predators for the prey that are left.
- The predator population is reduced as some individuals are unable to obtain enough prey for their survival.
- Fewer prey are eaten so their population increases.
- With more prey now available, the predator population in turn increases.
Human Populations
- The human population, like that of other organisms, has for most of our history been kept in check by food availability, disease and climate.
- The development of agriculture and manufacturing has led to exponential growth.
- Wars, disease and famine have caused only temporary reversals in this upward trend.
Population growth = (births + immigration) – (deaths + emigration)
% population growth rate = (population change during the period) / (population at the start of the period) x 100
Factors Affecting Birth Rate
- Economic conditions - Countries with low GDP per capita tend to have higher birth rates.
- Culture and religion - Some cultures encourage larger families while many religions are opposed to the use of contraception.
- Social pressures and conditions - In some Countries, a large family improves social standing.
- Contraception - The use of birth control is banned in some countries while in others it is expensive/difficult to acquire.
- Political factors - Education, taxation, policies and incentives influence birth rates.
Factors Affecting Death Rate
Age profile - Ageing populations tend to have a higher death rate.
Life expectancy - MEDCs have longer life expectancies than LEDCs.
Food supply - An adequate, balanced diet reduces the death rate.
Clean water and sanitation - Water-borne diseases such as cholera are spread through contaminated water and poor sanitation.
Medical and educational provision - Access to sufficient health care and education reduces the death rate.
Natural disasters - Areas prone to flooding, earthquakes, volcanic eruptions etc will have a higher death rate.
War - Deaths during wars produce an immediate drop in the population and a longer term fall as a result of fewer fertile adults.
Population Structure
- The population of countries go through changes where the birth and death rates may rise and fall, altering the levels of natural increase/decrease.
- This is known as demographic transition and it leads to a levelling-off of the population and the re-establishment of the typical sigmoid population growth curve.
- It is illustrated in the Demographic Transition Model (DTM).
Further information about birth and death rates, as well as life expectancy and infant mortality, can be obtained through analysis of population pyramids.
They can show whether a population is stable and the birth and death rates are balanced, if it is increasing and a there is a high birth rate shown by a wide base, or if the population is decreasing, shown by a narrow base.
Survival Curves
- Survival curves show the percentage of all individuals born in a population that're still alive at any given age.
- They allow us to calculate life expectancy by simply reading off the graph the age at which 50% survive.
Type I
- Long life expectancy with low infant mortality and most of the cohort dying in old age.
- Type I curves are shown by large mammals and human societies where families are small and there is high investment in parental care.
Type II
- There is an intermediate life expectancy and a roughly constant death rate regardless of age.
- Type II curves are shown by animals that are equally susceptible to predation or disease at any age, such as small mammals and birds.
- They are also shown by human societies facing a serious epidemic such as the AIDS epidemic in many African countries.
Type III
- Short life expectancy, with most of the cohort dying in infancy and few surviving to old age.
- Type III curves are shown by animals that do little to no parenting and produce large numbers of offspring to compensate, such as insects and fish.
That's the end of the topic!
Drafted by Bonnie (Biology)