B19 populations in ecosystems

• ecosystems are dynamic systems made up of a community and all the abiotic factors of its environment, they can be very large or very small
• the distribution and abundance of organisms in a habitat is controlled by both biotic and abiotic factors
• biotic factors are living
• abiotic factors are non-living
• a population is a group of individuals of one species that occupy the same habitat at the same time and are potentially able to interbreed
• an ecosystem supports a certain size of population, this is the carrying capacity
• a community is all of the populations of different species living and interacting in a specific place at the same time
• a habitat is the place where an organism normally lives and is characterised by physical conditions, and the other organisms present
• within each habitat there are microhabitats, each with their own microclimate
• a niche is where an organism fits into an environment, ie where it lives and what it does, it includes all the biotic and abiotic conditions to which an organism is adapted to be able to survive in
• the abiotic factors which influence population size are:

  • temperature
  • light
  • pH
  • water
  • humidity
• temperature affects population size as each species has an optimum temperature, the further away from this, the less likely individuals of that species are to survive
• light affects population size as light is the ultimate energy source, rate of photosynthesis increases as light intensity increases
• pH affects population size as enzymes have an optimum pH, the further away from this, the more likely they are to denature and no longer function
• water affects population size as if there is no water, only small populations of species well-adapted to dry conditions will survive
• humidity affects population size as it affects transportation in plants
• a predator is an organism that feeds on another organism, the prey
• the populations of predators and prey affect each other:

  • when prey are eaten, the prey population falls and the predator population rises
  • once the levels of prey fall to a certain point, the predator population falls as there is not enough food to support them
  • there are fewer predators so the prey population rises
  • this cycle repeats itself over and over
• there will always be a gap between the rises and falls of the populations of prey and predators because the differences are not immediate, they happen over time
• to estimate the size of a population of non-motile or slow organisms, use randomly placed quadrats, or quadrats along a belt transect
• the main ways to measure abundance with quadrats are percentage cover, and frequency
• to estimate the size of a population of motile organisms, use the mark-release-recapture method
• the mark-release-recapture method is:

  • known number of organisms are captured and marked in a way that does not decrease their chance of survival
  • marked organisms are released into the same area they were caught in
  • after a length of time long enough for organisms to redistribute randomly, capture another known number of organisms, record the percentage of these that were marked
  • use population size = (no. in first sample)*(no. in second sample)/(no. of marked recaptured)
• succession is the change over time of one community of organisms into another
• primary succession is when an area previously devoid of life is colonised by communities of organisms called pioneer species
• pioneer species are adapted to survive in inhospitable conditions
• the process of succesion is:

  • pioneer species specially adapted to harsh conditions colonise an environment that is inhospitable to others
  • as pioneer species such as lichen die, they are decomposed by microorganisms
  • this creates soil and eventually releases enough nutrients, changing the environment in a way that makes it more suitable for another species
  • the new species outcompetes the old species
  • this process repeats until a balanced equilibrium is reached, called a climax community
• a climax community has the following features:

  • stays stable over long periods of time
  • abiotic factors are constant
  • population stays stable around carrying capacity
• secondary succession is when an area which previously supported life is recolonised
• as soil is already present in secondary succession, the process of succession occurs from a later point in the cycle
• conservation is the human management of the Earth’s resources, typically involving the management of succession