Population size and ecosystem

• Ecology is the study of living things and their interactions with each other and their environment
• Ecosystems are a characteristic community of interdependent species interacting with the abiotic components of their habitat
• Population refers to all the members of one species in an area that can breed with each other
• Community includes all the members of all species in an area
• Habitat is the place in an ecosystem where an organism lives
• Niche is the role of an organism in an ecosystem, generally a feeding role
• Biotic components of an ecosystem include all the living and organic parts
• Abiotic components of an ecosystem include all the non-living parts
• An ecosystem is not static; it undergoes constant changes
• The sun is the primary source of energy for ecosystems
• In temperate climates, the duration and intensity of sunlight change over the year, impacting the energy flow through the ecosystem
• Nutrient cycles in ecosystems depend on fungi and bacteria, with their population growth influenced by temperature and other abiotic and biotic factors
• Nutrient cycling varies throughout the year due to the dependence of fungi and bacteria on temperature
• Community composition can change over time, such as ponds silting up and land plants becoming established, known as succession
• Population sizes change over time and are dependent on 4 factors: reproduction rate, death rate, immigration, and emigration
• In a stable population, the equation Reproduction + Immigration = Death + Emigration holds true
• This equation signifies that reproduction and immigration increase the population size, while death and emigration decrease it
• Populations colonizing new environments undergo a characteristic series of stages that can be presented graphically as a growth curve
• Bacteria in a nutrient broth growth stages:

  • Lag phase: synthesising enzymes and replicating DNA, slow rise in numbers
  • Log phase: exponential growth, rapid cell division, population doubles for each unit of time
  • Stationary phase: reproduction and deaths stabilize as nutrients deplete, death and reproduction are equal
  • Death phase: toxins build up or nutrients run out, deaths exceed reproduction, population decreases
• When describing animal growth curves:

  • Lag phase: slow population increase, time needed for individuals to reach sexual maturity
  • Log phase: exponential growth, low competition for food, territory, and habitats
  • Carrying capacity: maximum population level with high competition for food, environmental resistance slows growth
  • Fluctuation: population falls below carrying capacity, then rises as food becomes sufficient, leading to fluctuation in numbers
• Some populations, like locusts, follow a boom and bust pattern with rapid increases and decreases in population levels
• Even in "equilibrium" species, a new disease or predator can increase the death rate, leading to a new fluctuation point at a lower population level
• The range of numbers in a population can be very large, especially with bacteria, so the graph is often plotted as a log graph
• Population growth curve - Bacteria

  
• population growth curve - mammals

  
• Density dependent factors affecting population size:

  • Include competition, predation, and disease
  • Have a bigger effect on the population when the population is higher
  • For example, as a population increases and becomes more densely populated, a disease would spread more easily and affect more individuals adversely
  • Density dependent factors tend to be biotic and limit the size of the population, determining the carrying capacity
  • Competition can be interspecific (between different species) or intraspecific (between members of the same species)
• Density independent factors affecting populations:

  • Affect populations regardless of their size
  • Tend to be abiotic factors
  • Include sudden changes to the environment like tsunamis, volcanic eruptions, and wildfires
  • An oil spill on the coast can kill common species with large populations and rare species with few individuals
• To assess the biodiversity of a habitat or geographical area, you need to determine the number of different species present and the number of individuals of each species present
• Simpson’s biodiversity index can be calculated to assess biodiversity (Unit 2.1 of your AS year)
• In random sampling:

  • Every individual organism/species must have an equal chance of being included in the sample
  • The sampling method ensures that the sample is representative of the whole population of each species
  • This reduces subjectivity and removes bias from the data collected
• Quadrats are frames of a known area used to collect data on population sizes for organisms that do not move or move slowly
• The choice of the size of quadrat depends on the area being sampled and the size of the organisms:

  • Small quadrats are suitable for narrow areas or areas with high diversity
  • Large quadrats are used for scenarios like counting trees in a wood (may need 10 x 10m2 quadrats)
• Sampling method:

  • Area divided into a grid pattern with indexed coordinates
  • Coordinates selected at random
  • A quadrat placed at each coordinate on the grid
  • Numbers / frequency / percentage cover of each species in each quadrat recorded
• Species density is the number of individuals of a given species in a given area, calculated as mean number per quadrat converted to a mean number per m2 multiplied by the area of the habitat
• Species frequency is a measure of the probability of finding a given species with any one placement of a quadrat, found by recording the presence or absence of a species in a quadrat
• Species cover is a measure of the proportion of ground occupied by a given species, useful for plants that spread over the ground and are difficult to identify individually
• ACFOR scale: A = abundant, C = common, F = frequent, O = occasional, R = rare; a semi-quantitative and subjective estimate of abundance
• Preliminary work is needed to determine the minimum number of quadrats required to sample all species living in a habitat, by randomly placing quadrats and recording the cumulative number of species until no further increase is observed
• To study how an environmental factor that changes across a habitat affects the abundance and distribution of organisms, a transect can be used
• A transect is a line that runs across the habitat being studied with different values of an environmental factor along the line