Populations (notes)


  1. Populations and ecosystems
  2. Investigating populations
  3. Variation in population size
  4. Competition
  5. Predation
  6. Human Populations
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  • Created by: Emilie
  • Created on: 15-02-15 16:20
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Variation in Populations
Ecology:the study of the inter-relationships between organisms and their environment
Biosphere:the life supporting layer of land, air and water that surrounds the Earth
Ecosystem: more or less self-contained functional unit comprising of all the biotic and abiotic
features in a specific area
Population:a group of interbreeding organisms of one species in a habitat
Community: all the populations of different organisms living and interacting in a particular place
at the same time
Habitat:where a community of organisms lives
Ecological niche:the role an organism has in its environment. No two species occupy exactly the
same niche
Investigating populations
To study a habitat, it is often necessary to measure the abundance of a species in a given space.
Random and systematic sampling techniques are used to obtain a representative sample,
therefore conclusions will be valid.
There are three factors to consider when using quadrats:
Size of the quadrat . This will depend upon the size of the plants or animals being counted
and how they are distributed within the area.
The number of quadrats to record . The larger the number of sample quadrats, the more
reliable the results will be. However, as the recording of a species within a quadrat is time
consuming, a balance must be struck between the validity of the results and the time
The position of each quadrat . To produce statistically significant results, random sampling
must be used in order to avoid bias.
To carry out random sampling:
1. Lay out two long tape measures at right angles along two
sides of the study area
2. Obtain a series of coordinates by using random numbers
taken from a table or generated by a computer
3. Place a quadrat in the intersection of each pair of
coordinates and record the species within it

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Issues may arise where organisms are only partly within the quadrat. It
must be decided beforehand how these will be counted and the
decision reached must be carried out consistently throughout the
It is sometimes more logical to measure abundance and distribution
using a systematic method, particularly where some form of transition in the community takes
place. A line transect comprises a string or tape stretched across the ground in a straight line.…read more

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Where possible, the organisms should be studies in situ. If it is necessary to remove them,
the numbers taken should be kept to a minimum.
Any organisms removed should be returned to their original habitat, even if they're dead
A sufficient period of time should elapse before the site is used for fieldwork again
Disturbance or damage should be avoided
Variation in population size
1. A period of slow growth as the initially small number of individuals reproduce to slowly
build-up their numbers
2.…read more

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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.
Intraspecific competition: competition for resources between two individuals of the
same species.…read more

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Social pressures and conditions . In some Countries, a large family improves social
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 .…read more

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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.…read more


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