Population Ecology

• What is the definition of a species?

  A group of organisms, consisting of similar individuals capable of interbreeding.
• What is a population?

  A group of the same species that occupies the same habitat at the same time and are able to interbreed.
• How does a community differ from a population?

  A community consists of different species populations that interact with each other within a specific habitat.
• What is an ecosystem?

  A biological community of interacting organisms and their physical environment.
• What does the term environment refer to in ecology?

  The sum of all biotic and abiotic factors in an ecosystem.
• What is a habitat?

  Where an organism competes to live successfully.
• What is natality in ecology?

  The percentage increase in a population because of the number of births in a given year.
• What does mortality refer to in a population?

  The death rate within a population, usually as a percentage.
• What is immigration in ecological terms?

  Act of entering an environment and becoming a part of its population.
• What is emigration?

  Act of individuals leaving an environment/population.
• What is migration in the context of ecology?

  The general movement from one place to another during a particular period of an organism’s life cycle.
• What is carrying capacity?

  The maximum number of individuals that an ecosystem can support at a given time.
• What characterizes a stable population?

  A population with a 0 growth rate, neither growing or shrinking.
• What is an unstable population?

  A population that exceeds the carrying capacity of an area.
• What are the key levels of ecological organization from smallest to largest?

  • Organism
  • Population
  • Community
  • Ecosystem
  • Environment
• What are the main factors affecting population dynamics?

  • Natality
  • Mortality
  • Immigration
  • Emigration
  • Migration
  • Carrying capacity
• Population Size Factors

  Increasing population

  • Natality - increased birth rates introduce new organism to a population. 
  • Immigration - introduces individuals into a population. These members could enter the population temporarily or permanently (species dependent)

  Decreasing population 

  • Mortality - increased death rates remove individuals from a population. 
  • Emigration - individuals leaving a population possibly due to the population splitting into sub-populations across a large geographical area. 
  • Population change = (B+I) - (M+E)
• Types of Environmental Resistance

  Carrying Capacity

  Limiting Factors
• What is environmental resistance?

  Environmental resistance refers to factors that limit population growth.
• What characterizes a stable population size?

  A stable population size fluctuates around the carrying capacity.
• What are the implications of exceeding carrying capacity on population dynamics?

  • Leads to resource depletion
  • Causes rapid population decline
  • Results in increased environmental resistance
• Stable Population 


  Population size may fluctuate around the carrying capacity.
• What defines a closed population?

  A closed population is influenced only by natality and mortality.
• What is the purpose of direct counting techniques?

  Direct counting techniques measure exact population sizes without estimations.
• What is a census?

  A census is an official count or survey of a population.
• How do indirect counting techniques work?

  Indirect counting techniques estimate population sizes, ideal for smaller organisms.
• What is the mark-recapture technique used for?

  The mark-recapture technique is used to indirectly calculate the size of a population.
• Describe the process of mark-recapture.

  Capture and mark a sample, release it, then capture a second sample and count marked organisms.
• What are quadrats used for?

  Quadrats are square frames used to study small sedentary animals and small plants.
• How are quadrats placed in a study area?

  Quadrats are placed randomly on an area containing the organism.
• What are the differences between direct and indirect counting techniques?

  • Direct counting: measures exact population sizes, no estimations.
  • Indirect counting: estimates population sizes, ideal for smaller organisms.
• What are the key features of the mark-recapture technique?

  • Capture a sample and mark it.
  • Release the marked sample.
  • Capture a second sample.
  • Count the number of marked organisms in the second sample.
• Limiting Factors

  Density Dependent Factors

  • Have a direct correlation to the size of the population. 
  • Exert a greater influence when a population is large. 
  • e.g. territorial behaviour, predation and competition, accumulation of waste, food, shelter, and parasitism and disease. 

  Density Independent Factors 

  • Exert an influence on a population regardless of its size. 
  • Abnormal and unforeseen events in nature. 
  • e.g. Tsunamis, floods, veld fires, landslides, and volcanic eruptions. 
• Direct Counting Methods

  Suitable for larger, slow moving organisms or organisms that are sessile. 

  1. Aerial photography: actual number of organisms are counted from an aerial photo. 
  2. Census: Determines the actual number of humans in an area 
• Indirect Counting Methods

  Ideal for smaller, fast moving organisms. 

  1. Mark-recapture: individuals are captured, marked, released, captured again, and then counted.
  2. Quadrats 
• Counting Precautions

  Mark-Recapture

  • Sample must be large enough to represent a population.
  • Markings must not interfere with the organism's behaviour.
  • Adequate time must be allowed for the population to mix after the initial marking.
  • This period must not be long enough to allow natality/ mortality and immigration/emigration.

  Quadrates

  • Several quadrant samples should be made in order to average the number of individuals per quadrat. 
  • Quadrat samples must be done randomly in the area. 
  • Process should be repeated several times to calculate the average number of organisms. 
• Population Calculations

  Mark-recapture

  • N = number of individuals estimated in the population 
  • M = number of marked animals in first sample. 
  • C = number of animals captured in the 2nd sample. 
  • R = number of individuals marked in the 2nd sample

  Quadrats

  • Population size = (avg no. of individuals per quadrat x surface area of land)/quadrat surface area
• What is the shape of the growth curve that represents exponential growth?

  1. S -shape
• What does logistic growth start with?

  A few individuals and many resources
• What are the two main factors that affect growth rate levels in logistic growth?

  Growth rate and resource availability