3.7.4 POPULATIONS IN ECOSYSTEMS

• Population - a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.

  Community - all of the populations of different species living and interacting in a place at the same time.

  Ecosystem - the dynamic interaction between the (biotic) community and non-living (abiotic) factors in a given area. Can be very small to very large. Dynamic systems.
• Within a habitat/ecosystem, every organism occupies a specific ecological niche:

  • A niche includes all the abiotic and biotic conditions of the environment which organisms are adapted to.
• Carrying capacity - the maximum population size that can be supported by an ecosystem. 
• Abiotic factors limiting carrying capacity:

  • Temperature & pH - each species has its optimum levels, and deviations from this optimum reduces population growth
  • Light - low light levels reduce the carrying capacity of producers, reducing the population size of consumers
  • Water - low water availability reduces the population size
• Biotic factors limiting carrying capacity:

  • Interspecific competition (between different species - ensures only 1 species occupies a particular niche at a particular time)
  • Intraspecific competition (within the same species - helps control pop. size)
  • Predation (Predator-prey cycles. Lag is time for reproduction)
• Estimating Population Size - Slow-moving/non-motile organisms: 

  Randomly placing quadrats, or quadrats along a belt transect. 

  • Lay out tapes to make grid of sample area
  • Generate random coordinates using random number generator
  • Place quadrats at coordinates
  • Can count the number of individuals of each species in the quadrat or percentage cover.
  • Scale up (with calculation) 

  Random to avoid bias.
• Estimating Population Size - Motile organisms:

  Mark-Release-Recapture.

  Assumptions:

  • Population size stays constant - no immigration/emigration, births or deaths
  • Mark is not toxic/impacts organism's chance of survival
  • Mark is not removed
  • Time for even redistribution (random mixing)
• marked/x = marked (recapture)/total recapture
• Succession - communities changing over time.

  • Primary succession - the progressive colonisation of bare rock or other barren terrain by living organisms.

  Colonisation by pioneer species --> stable climax community.

  • Secondary succession - the recolonization of an area after an earlier community has been removed or destroyed.
• Succession

  • Area colonised by pioneer species - change abiotic factors to be less hostile for other species to survive.
  • At each stage, species change the env so it becomes more suitable for other species with different adaptations. New species may change the env so it becomes less suitable for the previous species.
  • Changes organisms produce in their abiotic environment can result in a less hostile environment and change biodiversity.
  • Climax community - stable state reached, where there is high biodiversity and a number of new species. Population sizes + abiotic factors remain constant.
• Conservation - the maintenance of biodiversity, including diversity between species, genetic diversity within species and maintenance of a variety of habitats and ecosystems.
• Conservation frequently involves management of succession in a community. Eg. halting succession, to preserve species that would be extinct by the climax community being established.

  • Rate of growth of the human population creates an increasing demand for raw material and food. A balance between conversation and human needs is necessary in order to maintain the sustainability of natural resources