extensive and understandable population ecology notes summarized from textbook with additional notes from IEB life science examiner. Graphs and diagrams included. Also aesthetically pleasing :)
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ENVIRONMENTAL STUDIES
1. Population Ecology
Ecology = study of interactions of organisms with their physical and biological environments
Population ecology = concerned with fluctuations in population size and factors that regulate populations
POPULATION SIZE
- Population size = total no. of individuals in a population.
- Population density = no. of individuals of a population per unit area
POPULATION PARAMETERS
→ 4 main factors, known as parameters, effect population size:
o Natality
Birth rate
o Mortality
Death rate
o Immigration
One-way movement into an area
o Emigration
One-way movement out of an area
→ A population where immigration/emigration don’t occur = closed population
DETERMINING POPULATION SIZE
- Can be determined by direct or indirect method.
- Method is determined by:
• The size
• The mobility
DIRECT TECHNIQUES
- Involve the total count of all individuals in the populations
- Direct counting (census)
- Census methods include:
• Direct contact counting of humans.
• Direct counting from vehicle/helicopter.
• Counting from aerial photographs.
INDIRECT TECHNIQUES
- Includes counting only part of the population
- This count is used as a sample to estimate population by statistical calculation.
- It includes:
• Mark-recapture technique
• Quadrat technique
Mark-recapture technique
- A specific area is demarcated
- A no. of individuals are caught, counted and marked , and they are known as the 1st sample.
- The marked individuals are released into the environment.
- Sufficient time must be allowed for the marked individuals to mix with the rest of the population.
- Another group of individuals is caught and counted and they are known as the 2nd sample.
- The number of marked individuals in the 2nd sample are counted.
- This data is used to determine the estimated size of the population:
, 𝑚×𝑠
P=
𝑡
p = estimated population size
m = total number of animals captured and marked in 1st sample
s = total number of animals captured in 2nd sample
t = number of marked individuals in 2nd sample
▪ Population must be closed
▪ Time must be short enough that no births/deaths occur
Quadrats
- Quadrat = a square frame
- An area where organisms must be counted is demarcated and the surface area determined.
- The quadrat is placed on the ground in the demarcated area and the organisms concerned are
counted inside the frame i
- This is the no. of individuals per unit area
- This process is repeated a few times in diff. areas of the demarcated area by placing the quadrat
randomly of the ground.
- The average no. of individuals per quadrat is calculated.
- The total number of individuals in the population is determined using the following formula.
POPULATION GROWTH FORMS
- Populations have distinctive growth patterns (population growth forms)
- Two growth forms are distinguished:
▪ Geometric J-shaped growth form
▪ Logistic or S-shaped (sigmoid) growth form
Geometric (J-shaped) growth form
- A population can result its full reproductive potential when it has unlimited resources such as food, water
and space and no predators.
- These ideal environmental conditions result in the maximum possible growth rate and population figures
increase rapidly.
- This type of growth is called geometric or exponential growth.
Example
→ If a population of bacteria in an environment has sufficient food and oxygen as well as the ideal
temperature, the bacteria will increase rapidly.
→ If the no. of bacteria in the population is recorded over a period of 14 days and the data represented
as a line graph, it will form a curve.
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