Ecosystems

• Ecology


  The study of inter-relationships between organisms and their environment. Its aim is to explain why organisms live where they do.
• Ecosystem


  A reasonably self-contained area together with all its living organisms.
• Habitat


  The physical or abiotic part of an ecosystem, i.e. a defined area with specific characteristics where the organisms live.
• Community


  The living or biotic part of an ecosystem, i.e. all the organisms of all the different species living in one habitat.
• Biotic


  Any living or biological factor.
• Abiotic


  Any non-living or physical factor.
• Population


  The members of the same species living in one habitat.
• Species


  A group of organisms that can successfully interbreed.
• Estimating Populations and Distribution

  1. Take samples

  2. Count the number of individuals

  3. Use the data to estimate population size
• Measuring population sizes and distributions is important for comparing differences between communities and species, impact assessments, restoration ecology, and setting harvest limits.
• It is usually difficult or not possible to census all individuals in the target area, so population size is estimated using sampling techniques.
• Sampling techniques


  • Designed for specific types of organisms
  • Numerous ways of arriving at estimates from each sampling technique
  • Accuracy depends on number of samples, method of collecting samples, and proportion of total population sampled
• Sampling is viewed by statistical ecologists as a science in its own right.
• Population size


  The number of individuals present in the population
• Density


  The number of individuals in a given area
• Spatial distribution


  The pattern of how individuals are distributed in an area
• Types of spatial distributions

  • Random
  • Uniform
  • Clumped
• Ecologists are interested in spatial distribution because it provides information about social behaviour and ecological requirements of the species.
• Positive association


  Two species are found together more often than expected by chance
• Negative association


  Two species are found together less frequently than expected by chance
• Random sampling with quadrats

  1. Divide census area into a grid

  2. Choose quadrats at random

  3. Count number of individuals in each quadrat
• Frequency


  An indication of the presence of an organism in a quadrat area
• Density

  The numbers of organisms per unit area
• Percentage cover


  An indication of how much the quadrat area is occupied
• Counting along Transects


  1. Stretch a rope or tape across the habitat

  2. Record all species that touch the line (line transect)

  3. Record all species between two lines (belt transect)

  4. Record all species in quadrats placed along the line (interrupted belt transect)
• Mark-release-recapture techniques


  1. Capture organisms, mark them, release them back into the environment

  2. Recapture the population and count marked and unmarked individuals

  3. Use the data to estimate the total population size
• Diversity


  Depends on the number of species (species richness) and the abundance of each species
• Factors limiting population growth

  • Abiotic factors (e.g. pH, light, temperature)
  • Biotic factors (e.g. intraspecific competition, interspecific competition, predation)
• Index of diversity


  A measure of the range and numbers of species in an area, taking into account the number of species and the number of individuals of each species
• In extreme environments, diversity of organisms is usually low, resulting in an unstable ecosystem dominated by abiotic factors.
• Index of diversity (d)

  N(N-1) / n(n-1), where N = total number of organisms of all species in the area, n = total number of organisms of each species in the area
• A higher index number indicates greater diversity
• Extreme environments


  Diversity of organisms is usually low, with few species adapted to the extreme abiotic factors. This results in an unstable ecosystem with simple food webs
• Less hostile environments


  Diversity of organisms is usually high, with many species adapted to the abiotic factors. This results in a stable ecosystem with complex food webs
• Population Ecology


  Concerned with understanding the factors that affect population size
• Logistic/sigmoid growth curve


  • 3 phases: lag phase, rapid growth phase, stable phase
  • Factors responsible for each phase depend on the ecosystem
• Carrying capacity (K)


  The maximum population supported by a particular ecosystem
• Factors affecting population size

  • Abiotic factors
  • Seasons
  • Food supply
  • Interspecific competition
  • Intraspecific competition
  • Predation
  • Parasitism and disease
• Ecological niche


  A population's role in its ecosystem, usually its feeding role in the food chain
• Specialist species


  • Have narrow niches, can coexist in the same habitat as they are not competing, found in abundant stable habitats