• “Study of the growth, abundance, and distribution of populations”
  • Population abundance and distribution are described by…
  1. Size (N): total number of individuals in the population
  2. Density: total number of individuals per area occupied
  • Adding = thru birth or immigration; Removing = deaths or emigration
  • Resource availability affects population density
    • More food → population can become denser → higher reproductive rates & limited space
  1. Dispersion: how individuals in a population are distributed
  • May be clumped (like humans in cities), uniform (like trees in an orchard), or random (trees in a forest)
  1. Age structure: description of individuals of each age
  • Rapidly growing population is indicated when the large proportion is young
    • Pyramid-shaped = rapidly growing populations
    • Tiers of equal width = stable populations with little/no population growth (zero population growth)
  1. Survivorship curves: Describes mortality rates of individuals in a species
  • Type I: species in which most individuals survive to middle age and after that age mortality is high
    • Ex: humans
  • Type II: die randomly (any age)
    • Ex: many rodents
  • Type III: most individuals die young; only a few survive to reproductive age

      6. Biotic potential: maximum growth rate of a population under ideal conditions (unlimited resources and no growth restrictions) → rMax

  • Ex: some bacteria can divide every 20 minutes
  1. Factors that contribute to the biotic potential & reproductive success of a species
  • Age at reproductive maturity
  • Clutch size (number of offspring produced at each reproductive event)
  • Frequency of reproduction
  • Reproductive lifetime
  • Survivorship of offspring to reproductive maturity
  1. Carrying capacity: max number of individuals of a population that can be sustained by a particular habitat
  • Largely contributed by competition for resources
  1. Limiting factors: factors that prevent a population from attaining its biotic potential and determine carrying capacity → can be density-dependent or density-independent factors
  • Energy, shelter, nutrient & water availability can all be limiting factors
    • Limited quantities of these resources result in intraspecific competition
  • Density-dependent factors: abiotic and biotic factors whose limiting effects become more intense as the population density increases
    • Ex: competition for resources, territoriality, disease, predation, waste
    • Process that maintains a stable population
      • Factors are a negative feedback which stops population growth by reducing birth rates or increasing death rates
    • Density-independent factors: occur independently of the density of the population
      • Ex: Natural disasters, pollution, and extremes of climate

Population Growths

Equation for Growth of Population

Growth of Populations:

  • R is the reproductive rate (or growth rate), N is the population size, numerator is net increase in individuals
  • When deaths exceed births, r will be negative and population size will decrease
  • Intrinsic rate of growth: when reproductive rate (r) is at maximum (biotic potential)

Patterns of Population Growth

  1. Exponential growth: occurs whenever the reproductive rate is greater than one. Results from reproduction w/o constraints
  • A population will usually exhibit exponential growth when they have more resources and space that they need                       
  • Formula:
  • ΔN is the change in population size → add to original N to find total population
  • ΔT is change in time
  • A population’s growth rate stays the same regardless of population size

Logistic growth: when limiting factors restrict size of the population to the carrying capacity of the habitat 

  • Cause for logistic growth: density-dependent factor becomes limiting and then population stabilizes (population at carrying capacity = stable)
    • Population will usually switch to logistic growth bcuz of competition for resources (food, space, mates)
  • Formula: 
  • K = carrying capacity.
  • If question gives you max population, use logistic formula
  • When N = K, the reproductive rate is 0 & the population is at its carrying capacity (stable)
  • A population’s growth rate gets smaller as population reaches K
    • A population is stable when it is not growing/at carrying capacity
  • Population cycles: fluctuations in population size in response to varying effects of limiting factors                         
    • Population may grow exponentially beyond carrying capacity of the habitat before limiting factors inhibit growth
    • Population increases above carrying capacity → N decreases → factors ease → renew population grow
      • Other times new, lower carrying capacity may be established (perhaps because the habitat was damaged by the excessively large population)
    • When the population size is small, limiting factors exert little negative feedback → population growth
      • Ex: since many limiting factors are density-dependent, will have a greater effect when population size is large
  • Life History of an organism is its strategy for maximum fitness.
  1. R-selected species exhibit rapid (exponential) growth
  • Opportunistic species that quickly invade a habitat, reproduce and die
    • Produce many offspring that are small, mature quickly, and require little parental care
  1. K-selected species exhibit logistic growth as they remain at K
  • Species (ex: humans) produce a small number or large offering that require extensive parental care until they mature

Key Idea: What maintains a stable population ?

  • Density-dependent factors, stability of the environment and climate