population and ecosystem

• Population


  A group of organisms of the same species
• Factors affecting population numbers


  1. Births increase population size

  2. Deaths decrease population size

  3. Immigration increases population size

  4. Emigration decreases population size
• Stable population


  Births + immigration = deaths + emigration
• When writing about bacteria or yeast, use cell reproduction
• Factors affecting death rate


  • Density independent (abiotic)
  • Density dependent (biotic)
• Density independent factors


  • Abiotic (non-living), e.g. wildfire kills small and large populations equally
• Density dependent factors


  • Biotic (living), affect a greater proportion of the population if it is larger and more dense, e.g. more competition for food, more predation, diseases and parasites spread more easily
• Phases of bacterial/yeast population growth curve


  1. Lag: enzymes synthesised, DNA replicated

  2. Exponential: population doubles every unit of time, nutrients abundant

  3. Stationary: death and cell reproduction happen at same rate, competition for nutrients

  4. Death/decline: nutrient depletion and toxin accumulation, death rate higher than cell reproduction
• Carrying capacity


  Maximum number of individuals of a species that the environment can support indefinitely
• Population changes in a more complex organism colonising a new area


  1. Lag: population limited by low numbers to reproduce

  2. Exponential: doubling of numbers with abundant resources and low predation

  3. Stationary: fluctuation as population increases, competition for resources and predation increases, death rate higher than birth rate, population decreases, then increases again

  4. Carrying capacity: set point around which the fluctuation takes place
• Ecosystem


  An area that has a particular community of plants and animals interacting with their environment
• Community


  All of the organisms of all species in an ecosystem
• Habitat


  The place in an ecosystem where an organism lives
• Niche


  An organism's role in the ecosystem, particularly its feeding role
• Energy flow through ecosystems


  Sunlight → Producers (autotrophs) → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary and quaternary consumers

  
• Food web


  Shows the complex feeding relationships within an ecosystem as some organisms feed on more than one trophic level or consume different types of food
• Photoautotrophs


  Mainly green plants, use light energy from the sun to fix carbon dioxide into organic molecules by photosynthesis
• Gross Primary Production (GPP)


  The rate at which photoautotrophs fix carbon dioxide into organic molecules, units are kJ m-2 yr-1
• Net Primary Production (NPP)


  The plant biomass that is available to be consumed by the next trophic level (herbivores), calculated as GPP - Respiration
• Not all sunlight energy is used in photosynthesis; some is the wrong wavelength, transmitted through the leaf or reflected

  
• All organisms respire so energy is lost as heat at every trophic level
• Dead organisms, faeces and urine are fed on by decomposers (fungi and bacteria), which also respire
• The number of trophic levels is limited because of energy losses; eventually, there is not enough left to sustain another level
• Efficiency of transfer


  A percentage calculated as (little number ÷ big number) x 100
• Efficiency of transfer from sun to plant is about 0.2%, from plant to herbivore is about 10%

  
• Herbivores consume mainly cellulose, relatively indigestible, and lose a lot of energy as faeces
• Transfer from herbivores to carnivores is often more efficient as less energy is lost as faeces
• Endotherms have higher respiratory rates to produce heat and are less efficient than ectotherms
• Aquatic food chains can be longer as they are more efficient
• In agriculture, farmers keep animals warm, reduce movement and give high protein feed to reduce energy losses
• Succession


  A change in community and species composition over time
• Primary succession


  1. Starts from bare rock, where no organism lives

  2. Pioneer species colonise the rock

  3. Mosses colonise

  4. Grasses become established

  5. Herbaceous plants

  6. Shrubs

  7. Woodland (climax community)
• Climax community


  The stable end point of succession; no further changes take place
• Climax communities have high species diversity and many habitats and niches
• The speed of succession depends on temperature and proximity to a source of spores and seeds to be blown in
• Sere


  Each stage of a succession, characterised by particular communities
• Succession can be deflected by grazing, mowing and burning, maintaining particular subclimax communities
• Random sampling


  Grid the area, generate random numbers to select coordinates, place quadrat and assess % cover/count organisms/ACFOR scale
• Transect


  1. Line: place quadrats at regular intervals along the line and use to assess % cover/count organisms/ACFOR

  2. Belt: assess vegetation by continuous quadrats along the line
• Measurements


  Abiotic factors that may be affecting distribution like pH/light intensity/soil water