Population Dynamics

• Population


  A group of individuals of the same species that occupy a specific area over a certain period of time
• Population dynamics


  The ecological interactions (biotic and abiotic interactions) that lead to fluctuations in population size. It refers to how populations of a species change in size and structure over time. Rates of reproduction, death and migration are important factors in population dynamics.
• Carrying capacity


  The maximum population size of the species that the environment can sustain
• Population abundance


  • The relative representation of a species in a particular ecosystem. It can be measured as either population size or density, and knowing the abundance in a sample of areas of an ecosystem can help determine population distribution. Total abundance refers to the total number of organisms of each species present, or, the number of organisms per unit area which live in a particular part of their distribution.
• Population composition


  • Includes the characteristics of a population, such as age, sex ratios, age structure (the number of organisms for each age group), fertility rate and average number of offspring per female.
• Abundance is not solely a measure of the number of individuals, it includes specifically where in the habitat they live, breed or hunt.
• Stable populations


  • Relatively predictable ecosystems, larger, longer-lived species out-compete the smaller, faster-growing species. Such species have a steadier population growth pattern, and can exist in numbers close to the carrying capacity of their environment.
• Unstable populations


  • Unpredictable ecosystems, such as those that occur after a fire or land clearing, opportunistic species move in and colonise quickly. Colonising populations show a pattern of rapid increase, where the maximum reproductive potential of the species is reached, then decline (or crash) in numbers.
• k-selected species


  • Kangaroos, elephants, whales, gorillas, oak trees and eucalypts
• r-selected species


  • Frogs, rats, weeds, swarming insects such as locusts
• k-selected species


  • Longer life cycle, relatively large size, parental care
• r-selected species


  • Short life cycle, relatively small size, lack of parental care
• Population growth rate


  Rate refers to the number of individuals being added or subtracted per hundred or per thousand present in the population per unit time (usually per year). Birth rate is usually given as the number of births per thousand of a population over a given time. Death rate is usually given as the number of deaths per thousand of a population over a given time.
• A useful method of representing collected data is to construct population pyramids. The figure below shows population pyramids for two different human populations. You can see that the pyramid for population A is wider at the bottom, indicating a healthy future reproduction rate because the number of children is quite high. Population A has a greater proportion of children than does population B.
• Distribution patterns


  • Exactly where in the physical space members of the different species are found.
• Quadrat


  A quadrat (usually a square) frame is positioned in random locations. The average number of individuals are counted and averaged per quadrat then multiplied to gain total population estimation.
• Transect


  A line drawn through a community. Information is gathered from along the line and is used to estimate the distribution of species within that community.
• Similarities between quadrats and transects: Useful method with species that are immobile, data can be used to calculate density in specific locations, can be used together to gain data, using a significantly large sample size and a random number generator can increase reliability.
• Differences between quadrats and transects: A quadrat is a square frame positioned in random locations, a transect is a line drawn through a community.
• Density


  Measuring the number of individuals per unit area or volume.
• Capture-mark-recapture


  The timing and method of the captures need to be appropriate to ensure each individual has an equal chance of being captured. Enough time is required for adequate mixing of animals after the release, but not so long that individuals have died. The proportion of marked organisms in the total population is equal to the proportion of marked organisms in the second sample.
• Capture-mark-recapture assumptions


  • There is no death, immigration or emigration (i.e., the population is closed)

  2. The sampling methods used are identical for the capture and recapture

  3. The marking (tag) does not affect the survival rate of the animals.

  4. All members of the population mix randomly. Enough time was given between captures for the organisms to mix. In other words, each member of the population has an equal chance of capture each time.
• The capture-mark-recapture technique gives an estimate of population size
• Capture-mark-recapture practice question


  A pest controller wanted to find out an estimation of the population of rats needing to be exterminated. He caught a first sample of rats – 26 of them, and attached ear tags to them, before releasing them. A week later, the pest controller caught 27 rats and 12 of them had tags. Estimate the total population.
• Contemporary technologies, including satellite sensing and remote monitoring enable improved monitoring of habitat and species population change over time.
• Remote tracking using telemetry is used to monitor the foraging habits (spatial and temporal) of little penguins.
• Carrying capacity


  A habitat has limited resources and therefore can only support a certain number of individuals in a population. Limited resources can consist of availability of resources such as water, food, sunlight, shelter, predation, and disease.
• Population growth curves


  • When a few members of a species colonise a new and favourable habitat, their population increases rapidly. However, this population growth cannot be sustained; resources are used and the population begins to level off. Despite minor fluctuations in the size of populations, there tend to be upper and lower limits. For a given species in a particular habitat, there is a certain equilibrium population that the ecosystem can support.
• A population growth curve (also known as an S-curve)
• Density dependent factors


  Factors whose effects on the population vary depending on the density of the population. The greater the density, the more individuals die or fail to reproduce.
• Density independent factors


  Factors that affect all individuals in a population regardless of how dense the population is. All individuals have an equal chance of survival.
• Density-dependent factors


  • Predation, interspecific and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites
• Density-independent factors


  • Severe weather conditions, volcanic activity or habitat destruction by clearing or fire
• Limiting factors


  • Density-dependent and density-independent factors that regulate populations
• Populations are regulated by density-dependent and density-independent factors.
• Population changes that can be measured

  Population size and distribution

  Population density

  Population abundance

  Population composition
• Immigration


  refers to an increase in population due to migration of individuals into the habitat
• Emigration 


  refers to an decrease in population due to migration of individuals out of the habitat
• Random distribution


  organisms are spaced irregularly (specific tree species in a forest)
• Uniform distribution 


  organisms are evenly spaced (penguin colony)