Population Size and Ecosystems

• Birth Rate - Number of new individuals per unit time

  Death Rate - Number of individuals that die per unit time

  Immigration - The movement of individuals into a population of the same species

  Emigration - The movement of individuals out of a population of the same species
• Phases of population growth:

  • Lag phase
  • Log phase
  • Stationary phase
  • death phase

  
• Lag Phase:

  • In single celled: cells have to make enzyme, have to synthesise ATP, dried cells have to rehydrate
  • In animals: time taken for individuals to reach sexual maturity
• Log phase:

  • In single celled: bacterial cells double per unit time as no limiting factors such as lack of glucose or build up of toxic substances
  • In animals: rapid increase as more individuals available for reproduction as no limiting factors such as limited space for habitats and food sources
• Stationary phase:

  Environmental resistance is introduced

  Population has reached carrying capacity

  • In single celled: limiting factors such as space in nutrient broth/ agar plate is low, intrapecific competition for glucose
  • In animals: limiting factors such as food source or space for habitats/nests leading to intraspecific competition
• Death phase:

  Environmental factors now result in death rate exceeding birth rate
• The numbers of most species lie near an equilibrium - the carry capacity
• Density dependent factors - factors that have a greater effect the larger the population size (more likely to slow down population growth rate)

  Density independent factors - factors that have an effect regardless of the size of the population (more likely to cause a population crash)
• Density dependent factors:

  • disease and parasitism
  • food availability
  • toxic waste build up

  Density independent factors:

  • floods
  • fires
  • sudden temperature changes
  • pH changes in soil
• Predator-prey relationships - as the population of the prey increases, so does the population of the predator as they have more food available. This causes a drop in prey populations as they get killed more and so also a drop in the predator population as they have less food available
• Intra-specific competition - competition between members of the same species

  inter-specific competition - competition between members of different species
• No two species can have the same niche and habitat as one species will be more adapted as out compete the other, causing it to die off
• Random quadrat sampling:

  1. set up a 10m x 10m grid and use a random number generate to generate integers between 1 and 10, use these as coordinates to determine where to place 1m^2 quadrats
  2. count the number of species or percentage cover in each quadrat
  3. repeat at 10 random coordinates
  4. calculate a mean for each species
• Density - with some species it is easy to count individuals

  Percentage Cover - if it is difficult to count individual plants (such as grass), estimating percentage cover is useful, where we see how many quadrats the species covers
• Line Transect - record the species touching the line transect at each point. Would be used to describe the difference in plants as you enter a wood
• Belt Transect - a quadrat is placed at regular intervals along a line transect and estimate the density or percentage cover of plant species in each quadrat. This is used to show the effect of abiotic factors on species distribution
• $population =$$(N_1 *N_2)/N_m$
• Assumptions for estimating population using mark, release, recapture:

  • population number stays the same between captures
  • animals integrate back into population (usually takes 2-3 days)
  • no migration in or out
• Photosynthetic efficiency = (energy incorporated into photosynthetic products / total light falling on plant) x 100
• Gross Primary Productivity (GPP) - rate of production of chemical energy in organic chemicals by photosynthesis (kJ m^-2 year^-1)
• Net Primary Productivity (NPP) - GPP minus the energy used up by the producers in respiration in a year. It is therefore the energy in the plant’s biomass
• Secondary Productivity - the rate at which consumers accumulate energy from assimilated food in biomass in their cells or tissue
• As energy is passed from one tropic level to the next, there is loss from the food chain at each level, this is due too:

  • energy lost in egested molecules (cellulose)
  • energy lost as heat generation in respiration
  • energy remains in inedible parts of animals
• Gross Ecological Efficiency = energy in tropic level / energy in previous tropic level x 100

  
  

  often around 10%, up to 40% in oceanic food chains
• Pyramid of Numbered - raw number of organisms at each trophic level per area of volume

  Pyramid of Biomass - amount of dry carbon matter at each trophic level per unit area of volume

  Pyramid of Energy - shows the quantity of energy transferred from one tropic level to another per unit area/volume per unit time
• Primary Succesion - The introduction of plants/animals into areas that have not previously supported a community
• Secondary Succession - The reintroduction of organisms into a bare habitat previously occupied by plants and animals that had been damaged
• Primary Succesion:

  1. Pioneer species are the first organism to colonize the rock (lichen)
  2. These organisms slowly erode the rock and the accumulation of dead and decomposing organic material leads to the formation of primitive soil
  3. As the soil develops, grasses become established
  4. As the soil builds up and nutrient levels increase, deep rooted shrubs appear
  5. Over a very long period of time trees and woodlands become established
  6. This results in a stable, long lived community known as a climax community