Populations (Unit 4 Section 1)

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1.1 Populations and Ecosystems

  • Ecosystem-The community of organisms living in an area and their interrelationships with their abiotic environment. Withing an ecosystem there are two major processes to consider: the flow of energy and the cycling of elements. Physical processes can connect different ecosystems allowing organisms to migrate.
  • Populations- All the individuals of one species living in a particular area. Where exactly a boundary lies between two populations can be unclear and is dependant on the obstacles beween two potential popualtions as well as the physiology/capiability of the organism.
  • Community-All the populations of different organisms living and interacting in a particular place at the same time.
  • Habitat- The place where a community of organisms live. Withing an ecosystem there are many habitats and within each habitat there are smaller units each with their own micro climate known as microhabitats.
  • Ecological Niche-Describes how an organism fits into the environment. It includes all the biotic and abiotic conditions required for an organism to survive and reproduce. To species cannot occupy the same niche, the greater the degree of niche overlap, the more likely that one species will die out (natural selection).
    Key Words: Species, Biome, Biosphere, Species, Taxonomy

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1.2 Investigating Populations

  • Quadrats- Three factors to consider: 1.The size of quadrat to use (Depends on size of organism and distribution) 2.The number of sample quadrats to record within the study area (Balance needs to be found between validity and time available, the greater the number of species, the more quadrats needed) 3.The position of each quadrat  within the study area (Random sampling must be used)
  • Random Sampling- Used to emliminate bias, best method is to use a grid and co-ordinates.
  • Transects- A systematic form of sampling, allows you to see the distribution of organisms over a changing environment.
  • Measuring Abundance- The number of individuals of a species in a given space, methods inc.: 1.Frequency (The likelihood of a particualr species occuing in a quadrat does not provide info on desitity or detailed distribution) 2.Percentage cover (Estimation of the area within a quadrat, a plant species covers, not useful when organisms overlap)
  • Mark-release-recapture- Estimate pop.size= 1st sample x 2nd sample/Number of marked individuals recaptured Assumptions- 1.1st sample evenly redistributed, mark didn't rub off/increase predation 2.Closed population
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1.3 Variation and Population Size pt. 1

Population Growth Curves

A (LAG phase)- Slow phase as organisms reproduce to build populations.
B (LOG phase)- Rapid growth as an incresing number reach a reproductive maturity.
C (Stable phase)- Births+Immigration=Deaths+Emmigration
D (Death phase)- Deaths and emmigratin is greater than births and immigration

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1.3 Variation and Popualtion Size pt. 2

Population Size
No population grows indefinintely because certain factors limit growth, Each population has a maximum size that determined by its limiting factors. There are two basic types: abiotic and biotic factors.

Abiotic Factors:

  • Light- Greater light intensity results in faster rate of photosynthesis which causes an increase in plant growth increasing their population as well as the population of organisms that feed on them.
  • pH- Each enzyme has an optimum pH, the closer the pH is to its optimum, the greater the population.
  • Water and humidity- In dry conditions, species adapted to this will be larger than those that are not. Humidity affects the transpiration of plants and the evaporation of warer from the bodies of animals.
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1.4 Competition

Intraspecific Competition-
Competition between members of the same species. Members compete for resources such as food, water, breeding sites ect.

Interspecific Competition-
Occurs between individuals of different species. This is when two species occupy the same niche, one species will  have a competitive advantage over the other causing the population of the weaker species to decrease. This is known as the competitive exclusion principle (where two species are competing for limited resources, the one that uses these resources most effectively will ultimately eliminate the other).

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1.5 Predation

Effect of predator-prey relationships of population size-

  • Predators eat their prey, thereby reducing the population of the prey
  • Wither fewer prey available the predators are in greater competition with each other for the prey that are left
  • The predator population is reduced as some individuals are unable to obtain enough prey for their survival
  • Few predators left, prey popualation increases
  • With more prey now available as food, the predator popualtion in turn increases
    These population crashes create a selection pressure whereby only those individuals who can escape predators, withstand diseases ect. will survive resulting in population evolving to be better adapted.

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1.6 Human Populations pt. 1

Human popualtion size and growth rate-
Two major and relatively recent events have led to an explosion in the human population: 1.The development of agriculture 2.The industrial revolution
Wars, disease and famine have caused only temporary reversals in this upward trend, therefore the usual sigmoid population curve is not followed by the human population. The exponential phase, inwhich the population grows rapidly, continues.
Factors affecting the growth and size of human populations-

  • Immigration- where individuals join a population
  • Emmigration- where individuals leave a population
  • Birth rates- affected by: economic conditions, cultural and religious backgrounds, social pressures and conditions, birth control and political factors
    Birth rate= (number of births per year/total popualtion in same year) x1000
  • Death rates- affected by: age profile, life expectancy at birth, food supply, safe drinking water and effective sanitation, medical care, natural disasters and war
    Death rate= (number of deaths per year/total population in the same year) x1000


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1.6 Human popualtions pt. 2

Population structure (age population pyramids)-

  • Stable population: no increase/decrease in popualtion size
  • Increasing/expansive population: high birth rate, fewer older people (3rd world countries)
  • Decreasing/constrictive population: low birth rate, low mortality rate (Japan, Italy)

Survival rates and life expectancy-
A survival curve plots the number of people alive as a function time. The average life expectancy is the age at which 50% of the individuals in a particular population are still alive. It follows that life expectancy can be calculated from a survival curve.

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