C4.1

• Populations


  Interacting groups of organisms of the same species living in an area
• Community


  All of the interacting organisms in an ecosystem
• Estimation of population size by random sampling

  1. Avoid choosing areas that don't represent the whole site

  2. Determine if distribution of species is random or there is an association with other species

  3. Use stratified sampling in an area with two or more habitat types

  4. Use systematic sampling along an environmental gradient

  5. Minimise sampling error by using different methods and sharing data for peer review
• Quadrats


  • Used to randomly sample a population by dividing the area into grids and randomly selecting coordinates
• Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms


  1. Capture and mark a sample of the population

  2. Release the marked individuals

  3. Recapture a sample and count the marked individuals

  4. Use the Lincoln index formula to estimate the total population size
• Carrying capacity


  The maximum number of species which can be sustainably supported by a given environment
• Exponential growth


  • When limiting factors are not restricting growth of population
  • Plentiful resources
  • Lack of competition
  • Favourable abiotic conditions
  • Growth starts slow then increases rapidly
• Logistic (Sigmoid) growth


  • When there are limiting factors restricting the growth of the population
• Negative feedback control


  Any response that counteracts an initial stimulus, in ecology this will be the return of a population to the carrying capacity
• Density-dependent factors


  • Competition for resources
  • Predation
  • Disease
  • Parasitism
• Density-independent factors


  • Natural disasters
  • Habitat destruction
  • Pollution
  • Climate change
• Population growth


  1. Increases from reproduction and immigration

  2. Decreases from mortality and emigration
• Population size = (Natality + Immigration) - (Mortality + Emigration)
• Population density


  Indication of how closely packed individuals are in a given space
• Density dependent factors


  • Competition for resources
  • Predation
  • Disease
  • Parasitism
• Density-independent factors


  External factors that can cause sudden death and alter the availability of resources
• Population growth curves


  1. Increases to population size through reproduction

  2. Increases to population size from external populations

  3. Decreases to population size as a result of death

  4. Decreases to population size as a result of loss to external populations
• Population Size


  Natality + Immigration - Mortality + Emigration
• Most important factor for population growth is birth, without which the population cannot grow
• If individuals die before reproducing, the population size of the next generation will be reduced / population growth is negatively affected
• Density independent limiting factors such as environmental change affect the very young, the old and the weak individuals the most, with minimal impact on population growth as individuals of reproductive age are affected the least
• The more genetically diverse a population is, the more likely individuals are to survive (e.g. disease) to reproduce
• Increasing population density and the associated competition promotes emigration from a population (and therefore immigration to other populations)
• Exponential population growth


  Requires an ideal environment with all individuals surviving to reproduce successfully and unlimited resources with no competition or limit to the population
• Exponential population growth


  • American Plains bison population growth from 1902 to 1915
  • Human population growth over the last 10,000 years
• Limiting factors


  Environmental factors that control the maximum rate at which a process, e.g. population growth, can occur
• Density dependent limiting factors

  • Build-up of toxic by-products of metabolism
  • Injury
  • Senescence (death from age-related illness)
• Density independent limiting factors

  • Climate/weather
  • Availability of light (for plants)
  • Natural disasters such as volcanic eruptions and fire
• Sigmoidal population growth curve

  1. Rapid increase in population size/growth as natality rate exceeds mortality rate

  2. Competition increases as resources are reduced, leading to falling natality and rising mortality

  3. Mortality rate equals natality rate, population reaches carrying capacity with no net growth
• Duckweed (Lemna sp.) is a good model organism for measuring sigmoidal population growth
• If the fish population is very small

  There will be plenty of resources and the population will grow quickly, showing exponential growth
• If fish are harvested only down to a certain level

  Fishing can be continued indefinitely and the population will quickly recover
• If greater numbers of fish are taken

  The population will decline
• Otolith (ear bone)


  Contains rings that can be used to determine the age of the fish
• If a population is in decline, the proportion of older fish will be higher, while if a population is growing, the relative number of younger fish will be higher
• Restrictions to ensure fish can breed before being caught

  • Regulations or international agreements to restrict net mesh size
  • Closed seasons to allow undisturbed breeding
  • Exclusion zones with all fishing banned
  • Quotas and moratoria for species with low stocks
• Fish are very mobile and often school, so are unevenly distributed, making it difficult to count/estimate their numbers
• Sampling methods for estimating fish population size

  • Random sampling (ineffective)
  • Capture-mark-release-recapture (used in lakes and rivers, but limited in open waters)
  • Echo sounders (only useful for schooling fish species)
  • Fish catches (biased due to unreported landings)
• Intraspecific competition


  Individuals of the same species competing for the same resources
• Intraspecific cooperation


  Complex behaviours that select the individuals working together to minimize the impact of competition