BIO330: Population vs Community and Population Dynamics

Population and Community

• A population consists of members of one species that live in a particular geographic area.
• A community includes all the different species that live in a particular geographic area.

Population Dynamics

• Population dynamics is the study of the factors that affect the increase, stability, or decrease of a population over time.
• It is influenced by the number of additions to the population (births and immigration) and the number of reductions in the population (deaths and emigration).
• Understanding population dynamics is key to understanding the relative importance of competition for resources and predation in structuring ecological communities.

Population Size

• Population size is the number of individuals present in a subjectively designated geographic range.
• Ecologists estimate population size by counting individuals within a small sample area and extrapolating that sample to the larger population.
• Small populations face a greater risk of inbreeding, are susceptible to random deaths, and are at a higher risk of extinction.
• Large populations experience greater competition for resources as they approach carrying capacity, resulting in lowered fecundity and may degrade available habitat.

Population Density

• Population density is the number of individuals per unit area or volume.
• It can be measured using various sampling techniques, such as:
  • Counting individuals in a few representative plots of an appropriate size.
  • Counting indirect indicators, such as the numbers of nests, amounts of droppings, or signs of tracks.
  • Capture-mark-recapture method.

Population Dispersion

• Population dispersion is the pattern of spacing among individuals within the geographical boundaries of the population.
• Types of population dispersion:
  • Random: Spacing varies in an unpredictable way.
  • Uniform: Spacing is even, a common result of behavioral or territorial interactions.
  • Clumped: Individuals are aggregated in patches, a common response to uneven distribution of resources.

Density-Dependent Factors

• Density-dependent factors are factors that influence the size and growth of a population depending on the density of the population.
• Examples of density-dependent factors:
  • Competition: interference competition and exploitative competition.
  • Disease and parasitism: high population density makes the spread of disease easier to transmit.

Survivorship Curves

• Survivorship is the percentage of an original population that survives to a given age.
• Survivorship curves are plots of the numbers in a cohort still alive at each age.
• Types of survivorship curves:
  • Type I: Curve is relatively flat at the start, reflecting low death rates during early and middle life.
  • Type II: Curve is intermediate, with mortality more constant over the lifespan.
  • Type III: Curve drops sharply at the start, reflecting very high death rates for the young.

Life History Adaptations

• K-selected species or populations: adaptations that enable organisms to survive and reproduce with few resources.
• r-selected species or populations: adaptations that promote rapid reproduction, such as increased fecundity and earlier maturity.
• Traits or characteristics of K-selected species/populations and r-selected species/populations:
  • K-selected: high parental investment, low reproductive rate, and large body size.
  • r-selected: low parental investment, high reproductive rate, and small body size.

Factors That Regulate Population Growth

• Factors that regulate population growth:
  • Density-dependent factors: influence the size and growth of a population depending on the density of the population.
  • Density-independent factors: influence the size and growth of a population regardless of the density of the population.

Learn the difference between population and community, and understand population dynamics, including the factors that affect population growth or decline over time.