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Biology OCR A level Populations and sustainability Summary Notes
Biology OCR A level Populations and sustainability Summary Notes
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1 of 5B24- Populations and sustainability
(a) The factors that determine size of a population
Key words:
Limiting factors- factors (environmental resources or constraints) which limits the rate of a
process.
Carrying capacity- the maximum population size that an environment can support.
Immigration- movement of individual organisms into a particular area, increasing population size.
Emigration- movement of individual organisms away from a particular area, decreasing
population size.
Density-dependent factors- the impacts of these factors vary with population density.
Density-independent factors- factors that affect a population regardless of its size.
• Human population can not be sustained indefinitely at the current rate of growth due to limiting
factors such as the availability of food, which will prevent the population rising above a certain
level.
Sigmoidal population growth curve
• Growth trends tend to be sigmoidal in shape and have
three main phases.
• Phase 1- slow growth. Small numbers of individuals
reproduce increasing total population. Birth rate higher
than death rate.
• Phase 2- rapid growth. Number of reproducing individuals
increases and there are no constraints to limit the
population growing.
• Phase 3- stable state. Growth prevented by external
constraints. Size flutes but remains relatively stable. Birth
rate and death rate equal.
Limiting factors
• Factors that prevent or limit the rate of a process.
• Examples: competition between the organisms for
resources, the build-up of the toxic by-products of
metabolism, or disease.
• Can be divided into abiotic and biotic factors.
• Maximum population size that an environment can support is known as its carrying capacity.
Migration
• Variable which may affect population size.
• Immigration- movement of individual organisms into a particular area, increase population size.
• Example: Christmas Island red crabs migrate from forest to coast to reproduce each year.
• Emigration- movement of individual organisms away from a particular area decreases
population size.
• Example: Norway lemming emigrates away from areas of high population density.
Density in/dependent factors
• Density dependent factors- impacts vary on population size.
• Examples: abiotic and biotic factors
• Density independent factors- affect population regardless of its size.
• Examples: natural disasters, which can remove whole populations of a species from a region.
(b) Interactions between populations
Key words:
Interspecific competition- competition between organisms of different species.
Intraspecific competition- competition between organisms of the same species.
Predation- the capturing of prey in order to sustain life.
Predator-prey relationships
, 2 of 5
• Size of predator and prey populations are interlinked.
• Peaks and troughs in the size of the prey population are
mirrored by peaks and troughs in the size of the predator
population after a time delay.
• In the wild other factors influence population size such as
the availability of the food plants of the prey or the
presence of other predators.
• Stage 1- increase in prey population provides more food
for the predators, allowing more to survive and reproduce,
in turn resulting in an increase in the predator population.
• Stage 2- increased predator population eats more prey,
causing a decline in the prey population. Death rate of the
prey population is greater than the birth rate.
• Stage 3- reduced prey population can no longer support
the large predator population. Intraspecific competition for
food increases, resulting in a decrease in the size of the
predator population.
• Stage 4- reduced predator numbers result in less of the prey population being killed. More prey
survive and reproduce increasing their population.
Interspecific competition
• Occurs when two or more different species of organism compete for the same resource,
resulting in a reduction of the resource available to both populations.
• Example: red and grey squirrels compete for the same food sources and habitats in the UK.
• Means that resources available to both are reduced, so they will both have a limited amount of
food, so less energy for growth and reproduction, so population size will be smaller.
• Can affect distribution as the better adapted species to its surroundings is likely to out-compete
the other species. Competitive exclusion principle.
Intraspecific competition
• Occurs when members of the same species
compete for the same resources.
• The availability of the resources determines
the population size, resulting in fluctuations
in the number of organisms present in a
population.
• Stage 1- when all organisms have enough of
the resource to survive and reproduce
resulting in an increase in population size.
• Stage 2- more individuals that share food or
space. Resources are now limited, not
enough is available for all organisms to
survive. Population will decrease.
• Stage 3- less competition due to smaller
population so less competitive. So more
organisms survive and reproduce so
population grows.
(c) The reasons for, and differences between, conservation and preservation
Key words:
Conservation- the maintenance of biodiversity.
Reclamation- the process of restoring an ecosystem that has been damaged or destroyed.
Preservation- protection of an area by restricting or banning human use- so that the ecosystem
is kept exactly as it is.
Conservation
• The sustainable management of ecosystems to maintain biodiversity, includes maintaining
diversity between species, genetic diversity within a species and of habitats.
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