Introduction to Ecology

Population Ecology

  • Population ecology: the study of populations in relation to their environment 
    • Environmental influences on density  and distribution 
    • Age structure
    • Population size

Population

  • Population: a group of individuals of a single species living in the same general area
  • Populations are described by their boundaries and size
    • Density: the number of individuals per unit area or volume
    • Population density is determined by immigration and emigration
    • Dispersion: the pattern of spacing among individuals within the boundaries of the population

Dispersion Patterns

  • Clumped: members of a population are clustered in groups
    • Most common
    • Resources tend to be clustered in nature
    • Social behavior may promote this pattern
  • Uniform: members of a population are dispersed relatively evenly
    • Competition may cause this pattern
    • May also result from social interactions such as territorality
  • Random: members of a population are distributed without a predictable pattern
    • Rarest
    • Resources are rarely randomly spaced
    • May occur where resources are common and abundant
    • Absence of strong attractions or repellents. 

Life History

  • An organism’s life history comprises the traits that affect its schedule of reproduction and survival
    • The age at which reproduction begins
    • How often the organism reproduces
    • How many offspring are produced during each reproductive cycle
    • Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

Reproductive Strategies

  • Semelparity: produce all offspring in single reproductive event
    • Individuals reproduce once and then die
  • Iteroparity: reproduce in successive years or breeding seasons
    • Seasonal iteroparity: distinct breeding seasons
    • Continuous iteroparity: reproduce repeatedly at any time of the year

Age Classes

  • Reproductive strategy has a strong effect on subsequent age classes of a population
  • Semelparous organisms have batches of young the same age, called cohorts
  • Iteroparous organisms have young of different ages
  • Expect a population increasing in size to have many young and a decreasing population to have few young

Per Capita Rate of Increase

  • Change in population size = births + immigrants entering population - deaths - emigrants leaving population
  • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
  • Exponential population growth: population increase under idealized conditions
  • Under these conditions, the rate of increase is at its maximum, denoted as rmax

Carrying Capacity

  • A population grows more slowly as it nears its carrying capacity
  • A more realistic population model limits growth by incorporating carrying capacity
    • Carrying capacity(K): the maximum population size the environment can support
    • Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small
  • Carrying capacity varies with the abundance of limiting resources
  • Density-dependent factors: mortality factor that varies with population density
    • Parasitism, predation, and competition
    • Predators kill few prey when the prey population is low, more prey when the population is higher
    • Detected by plotting mortality against population density and finding positive slope
  • Density-independent factors: mortality factor whose influence is not affected by changes in population size or density
    • Physical factors – weather, drought, flood, fire
  • r-selection: density-independent selection, selects for life history traits that maximize reproduction
    • In density-independent populations, birth rate and death rate do not change with population density
  • K-selection: density-dependent selection, selects for life history traits that are sensitive to population density
    • In density-dependent populations, birth rates fall and death rates rise with population density

Mechanisms of Density-Dependent Population Regulation

  • Density-dependent birth and death rates are an example of negative feedback that regulates population growth
    • Affected by factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors
    • Increasing population density intensifies competition for resources and results in a lower birth rate
    • Accumulation of toxic wastes
    • As a prey population builds up, predators may feed preferentially on that species
    • Some populations, intrinsic (physiological) factors regulate population size
  • Population density can influence the health and survival of organisms
    • In dense populations, pathogens can spread more rapidly