Populations and ecosystems

• What is ecology?

  The study of the way in which living organisms interact with their environment
• Definition of environment:

  An organisms surroundings
• Definition of population:

  All of the organisms of the same species living in the same environment
• Definition of community:

  All the organisms of different species living in a common environment
• Definition of habitat:

  the place where an organism lives
• Definition of niche:

  The role of an organism within its ecosystem
• What is an ecosystem?

  All the organisms living in a certain area (biotic), and all the non-living conditions found there (abiotic)
• Population growth curves:

  Slow growth - as there's only a few individuals present

  Rapid growth - as more individuals continue to reproduce

  Stable state - birth and death rates are balanced. The population fluctuates around the carrying capacity of the environment

  A) Slow growth

  B) Rapid growth

  C) Stable state

  
• What is the carrying capacity of an environment?

  The maximum population size that it can support
• Abiotic factors affecting population of photosynthetic organisms:

  1. Light - less light decreases rate of photosynthesis, which is used to obtain food for growth
  2. Oxygen - less oxygen reduces respiration, which affects the amount of ATP produced
  3. Carbon dioxide - Affects photosynthesis
  4. Temperature - As temp increases, enzyme activity increases up to a point at which they denature
  5. Minerals - Needed for growth
• Interspecific competition:

  When organisms of different species compete with each other for the same resources - means that well adapted organisms are likely to out-compete the other species

  e.g grey squirrels are a better competitor than red squirrels, as they reproduce more quickly. The red squirrel population is declining because of this.

  
• Intraspecific competition:

  When organisms of the same species compete with each other for resources - causes a cyclical change in population size

  1. Species population increases when resources are plentiful
  2. Resources become limiting, which causes the population to start declining
  3. Smaller population means less competition for resources - population starts to grow again

  
• Predation:

  Where an organism (predator) kills and eats another organism (prey)

  1. As the prey population increases, there's more food for the predator, so the predator population also increases
  2. As the predator population grows, more prey is eaten, therefore the prey population begins to fall
  3. This means there's less food for the predator, so its population also decreases

  
• what is succession?

  The change in communities over time due to a change in the environment, caused by the species present.
• Primary succession

  1. Species colonise a new land surface

  2. Seeds/spores blow in from the wind and begin to grow

  3. First organisms to invade and colonise a new site = pioneer species

  4. Pioneer species adapted to harsh abiotic conditions

  5. Pioneer species die, bacteria decompose dead organic matter (humus) forming basic soil

  6. Conditions become less hostile, basic soil helps retain water and provides minerals

  7. More competitive species establish in more favourable conditions and out-compete present species

  8. Each stage of succession is a sere

  9. More organisms die and add to organic material in soil, making it richer in minerals

  10. Large plants can grow in deeper soil which retains more water and contains more nutrients

  11. Changes result in a climax community, ecosystem supporting largest and most complex community of plants and animals it can, in a steady state
• what are the hostile abiotic conditions faced by pioneer species?

  Limited water available - no soil to retain water

  Few minerals/nutrients - as there is no soil

  May be high light intensity/exposure to wind or rain/fluctuating temperatures - as the area is exposed to the sun and elements
• What are the changes during succession?

  • Soil depth and availability of minerals increases
  • Humus content and moisture in the soil increases
  • Species diversity increases
  • height and biomass of plants increases
  • Structural diversity of the community increases
  • plants create more habitats for animals
  • abiotic conditions become less hostile
  • Biomass increases
• What is deflected succession?

  When human activity prevents succession continuing - this is important in the conservation of endangered species that can only survive in a certain stage of succession. Examples:

  • Sheep grazing prevents grassland from developing into woodland
  • Trampling preventing succession to woodland on sand dunes
  • Mowing/using herbicides on grassland
• How do you measure the dry biomass of a plant in a sample area?

  1. Dig up the plants in a sample area
  2. weigh them
  3. heat in an oven at a low temperature for 24 hours
  4. reweigh
  5. repeat heat and reweigh until a constant mass is reached
• How do you measure population size?

  1. Abundance - the number of individuals of one species in a particular area
  2. Frequency - the number of samples of a species is recorded
  3. Percentage cover (for plants only) - how much of the area you're investigating is covered by a species
  4. Distribution - where a particular species is within the area you're investigating
• Random sampling:

  Used to estimate the population size of a stationary species or plants (uniformed ecosystems with uniformed distributions of organisms)

  Procedure:

  1. Divide area to be sampled into a grid
  2. Generate a large number of random coordinates for the grid - this avoids bias/ minimises errors in obtaining samples
  3. Count plant species in using a quadrat at each coordinate - repeat this many times to ensure there's a more reliable mean and a true representation of the population
  4. Calculate a mean
  5. Estimated population size= Mean x size of area / area of 1 quadrat
• Advantages of random sampling:

  Advantages:

  • Random
  • Removes bias
  • Representative of the whole population
• Disadvantages of random sampling:

  Can lead to a poor representation of the overall population or biodiversity if large areas aren't hit by the random numbers generated.
• Systematic sampling:

  Investigate change in distribution of species across an area - sample ecosystems with very different areas within it

  Used with a transect e.g. use a quadrat to take samples at every 10m interval along a transect
• Advantages and disadvantages of systematic sampling:

  Advantages:

  A good coverage of the study area can be more easily achieved than random sampling along a transect

  Disadvantages:

  Species with small populations may not be sampled, and this may lead to over/under representation of biodiversity