Populations Definitions

Abiotic: an ecological factor that makes up part of the non-biological environment on an organism

Biotic: an ecological factor that makes ip part of the living environment on an organism

Ecosystem: a self contained functional unit in ecology made up of all the interacting abiotic and biotic factors in a specific area

Population: a group of individuals of the same species that occupy the same habitat at the same time

Community: all the organisms of all the species that live in the same area

Habitat: the place where an organism normally lives, which is characterised by physical conditions and the species of other organisms present

Ecological Niche: all conditions and resources required for an organism to survive, reproduce and maintain a viable population

Abundance: the number of individuals of a species in a given space

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Factors to consider when using quadrats:

  • Size of the Quadrat: this will depend on the size of the organisms being counted and how they are distributed within the area. 
  • The number of sample quadrats to record within the study area: the larger the number of sample quadrats, the more reliable the results will be. A balence needs to be made between the validity of the results and the time available.
  • The position of each quadrat within the study area: to produce statistically significant results, random sampling must be used

                            (http://learnthings.co.za/content/secondary/Home/Lessons/Science/Biology/Grade12/energyand/ecolog_00/ecolog_00/random_00/diagram5.jpg)       Frame Quadrat (http://www.nuffieldfoundation.org/sites/default/files/PB_fieldwork-using-your-school-playing-field-2.jpg)

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Measuring Abundance

Random sampling with quadrats and counting along transects are used to obtain measures of abundance. Abundance is the number of individuals of a species within a given space. It can be measured in several different ways:

  • Frequency. This is the likelihood if a particular species occuring in a quadrat. This method is usefull where a species is hard to count, such as grass. It gives a quick idea of the species present and their general distriution within an area. But it lacks information about the density and detailed distribution of the species.
  • Percentage Cover. This is an estimate of the area within a quadrat that a particular plant species covers. It is useful where a species is particularly abundant or is difficult to count. It means data can be collected rapidly and individual plants do not need to be counted. It is less useful where organisms occur in several overlapping layers.
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Most animals are mobile and move away when approached, others are hidden and difficult to identify. Therefore, measuring abundance for animals requires a different technique to plants:

A known number of animals are caught, marked in some way, and then released back into the community. Later, a given number of individuals are collected randomly. The number of marked individuals is recorded.

Population Size =

        total no. of individuals in first sample x total no. of individuals in second sample 


                                  no. of marked individuals in second sample

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The mark-release-recapture technique relies on a number of assumptions:

  • the proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole
  • the marked individuals released from the first sample distribute themselves evenly amongst the remainder of the population and have enough time to do so
  • the population has a definite boundary so there is no immigration or emmigration of the population
  • the method of marking is not toxic to the individual nor does it make the individual more conspicuous and therefore more liable to predation
  • the mark or label is not lost or rubbed off during the investigation
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Population Sizes


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Population Sizes

The usual pattern of growth for a natural population has three phases:

  • a period of slow growth as the small number of individuals reproduce to slowly build up their numbers
  • a period of rapid growth where the increasing number of individuals continue to reproduce. The population size doubles during each interval of time. The curve is very steep
  • a period when the population growth declines until its size remains more or less stable. The decline may be due to the food supply limiting numbers or to increased predation. The curve levels out with only cyclic fluctuations due to variations in factors, e.g food supply or the population size of predators

Factors that influence population size

  • Abiotic Factors, temperature, light, pH, water and humidity
  • Biotic Factors, predation, available prey, competition
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Effect of predator-prey relationship on population size:

  • predators eat their prey, reducing the population of the prey
  • with fewer prey available, the predators are in greater competition with eachother for the remaining prey
  • the predator population is reduced as some individuals are unable to obtain enough prey to survive
  • with fewer predators, fewer prey are eaten so the prey population increases
  • with more prey now available, the predator population increases too


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Human Population Growth

Factors affecting the size of human population:

  • Birth Rate and Death Rate
  • Immigration and Emigration

Birth Rates are affected by:

  • economic conditions
  • cultural and religious backgrounds
  • social pressures and conditions
  • poitical factors
  • birth control

Death Rates are affected by:

  • age profile
  • life expectancy at birth
  • food and safe water supply
  • medical care
  • natural disasters
  • war
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Population Equations


Population Growth = (births + immigration) - (deaths + emigration)

% Population Growth Rate = population change/population at start x 100

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Age Population Pyramids

  • Stable Population: birth and death rate are in balenced so there is no change in population
  • Increasing Population: higher birth rate, giving a wider base to the population pyramid and fewer old people, giving a narrow apex to it. Typical of economically less developed countries
  • Decreasing Population: lower birth rate and a lower mortality rate leading to more elderly people. In more developed countries


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