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

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