3.7.4 Populations in ecosystems

• Ecosystem- self-sustaining unit in a defining area with it's own community of organisms and physical factors interacting with each other
• Species- group of individuals with similar characteristics that can interbreed to produce fertile offspring
• Habitat-where particular organisms live within an ecosystem
• Population- all of the interbreeding organisms of one species living in a particular habitat at the same time
• Community- all of the populations of all of the species that live together in a particular ecosystem
• Niche- the role an organism has in it's community
• Abiotic factors- non-living, physical conditions in an ecosystem (temperature, light intensity, humidity, pH)
• Biotic factors- effect of the activities of living organisms on other organisms (food availability, competition, disease, predator-prey relationships)
• Fundamental niche- full range of environmental conditions that a population of a species can use without the effect of limiting factors
• Realised niche- limited amount of environmental conditions that a population of a species can use due to limiting factors
• Stronger competitors utilise recourses such as water availability or space better, therefore outcompeting other species.
• Population growth curve of yeast cells:

  • Lag phase- small number of cells to start with, needs time to switch on enzymes, trigger the correct biochemistry.
  • Exponential phase- optimum conditions, more available recourses.
  • Stationary phase- less resources such as nutrients/ respiratory substrate, so more competition. Less oxygen, more toxins/ waste produced. Maximum population size.

  
• Carrying capacity- maximum population size that can be supported by available resources
• Abiotic factors affecting population sizes:

  • Temperature
  • Food/ water/ nutrient availability
  • Light intensity
  • Soil pH
  • Wind movement
• Biotic factors affecting population sizes:

  • Competition
  • Predator-prey relationships
  • Food/ nutrients
  • Pathogens
• Interspecific competition- competition for recourses between members of different species
• Intraspecific competition- competition for recourses between members of the same species
• PoE:

  • Abiotic and biotic factors act together as a combination
  • Competitive advantage means better utilisation of recourses
  • Species exclusion principle- two organisms occupy the same niche and one species outcompetes the other
• Predator-prey relationships:

  • Population size of both predator and prey is determined by the population density of each other.
  • P-p relationships are best seen when predator exclusively predates on one prey species, but not as clear when a variety of prey species are eaten.
  • P-p is a cyclic relationship.
  • There is a delay in the peaks of predators/ prey after one decreases.
  • P-p is not an isolated relationship- abiotic and biotic factors influence the population size of predators and prey.
• 1 . Predator eats prey, prey number drops. This increases intraspecific competition between predators for prey.

  2 . Predator numbers drop due to intraspecific competition for prey, which means less prey are eaten.

  3 . Prey numbers increase due to less prey being eaten, allowing predators to eat the prey, due to higher numbers of prey, which decreases interspecific competition.

  
• Estimating population sizes- must be without bias (using quadrats) and representative (large sample size)
• Mark, Release, Recapture:

  • Used for mobile organisms.
  • Capture a sample from the population (N1).
  • Mark the sample.
  • Release the marked sample back into the population and allow enough time for marked sample to redistribute into population.
  • Capture a second sample (N2).
  • Estimate population size (N1 x N2)/N3
• Limitations of Mark, Release, Recapture:

  • Samples sizes need to be as large as possible
  • Method of marking must be recognisable yet durable
  • Marking cannot interfere with survival chances
  • Must be ethical
  • Time between N1 and N2 may affect population size
  • Organism cannot become trap happy
  • Migration behaviours could influence population size
• Random sampling:

  • Used for sessile organisms (e.g. plants)
  • Use of a grid or quadrats
  • Quadrats can be used to estimate % cover, count no of individuals in quadrat or record frequency in quadrat divisions
  • Large sample of at least 30
  • Running mean
  • Representative sample is when population size from readings becomes stable
• Systematic sampling:

  • Used for sessile organisms
  • Used to measure an environmental gradient (using a transect)
• Ecological succession- the natural changes that take place to a community over time, notably the change in species
• Primary succession starts on a bare piece of ground with pioneer species.