CONCEPTS
 

 
Population - a group of individuals of the same species living in the same area.

Population density - the number of individuals in a population in a given area at a given time.

 
Distribution patterns of individuals within an animal population will change with conditions.

Population age structure can tell more about population dynamics than either density or distribution.

Because of the difference in lifespans, the ages of these functional age groups can be very different from one type of animal to another.

  • Prereproductive - too young to reproduce.
     
  • Reproductive - able to reproduce.
     
  • Postreproductive - too old to reproduce.
A population age profile is an estimation of the number of individuals in each age class.
  • When the number of prereproductive individuals forms a high percentage of the population, the population is likely to be increasing in size.
     
  • A stable population has a fairly even number of individuals in each age class.
     
  • When the number of postreproductive individuals is large, the population is declining. There are too few individuals maturing to provide population replacement.

A species with a short life span will show all three types of age profiles during one seasonal cycle.

 
Biotic potential - the rate at which a population will grow if all individuals survive and reproduce at their maximum capacity.

This is a theoretical number that is never realized in nature. While the term is not usually applied to human populations, health and census data are available for such calculations.
Realized intrinsic rate of growth - a measure of the difference between natality (birth rate) and mortality (death rate).
r = n - m
Since environmental conditions are rarely ideal, the realized intrinsic rate of growth more closely represents the actual growth of organisms in nature than does the biotic potential.
Zero population growth is reached when r = 0, natality equals mortality, and population size remains constant, even though individuals continue to be born and continue to die.

Carrying capacity - the maximum number of individuals that an ecosystem is capable of supporting.

The World's Human Population

As population numbers increase, certain limiting factors in the environment become limits to growth. All living things need a certain amount of space in which to live, food, water, and oxygen.







 

CONCEPTS (continued)
 

Two things can be said about populations:

  1. Population density varies from habitat to habitat.
     
  2. No population increases indefinitely.
 
Law Of Minimums - essential material available in amounts most closely approaching the minimum needed by an organism will tend to limit the organism's growth and development.

Limits Of Tolerance - organisms can only tolerate certain extremes in environmental factors. Populations cannot exist outside the tolerance limits of its individuals.

Competitive Exclusion Principle - competition between populations of two species for the same limiting resource eventually leads to the elimination of one of the species populations.

 
Crayfish are found in most freshwater streams, but not all. Why are they missing from some streams?

 
Two Models of Population Growth

The exponential growth model assumes that populations grow without being influenced by environmental resistance.

The logistic growth model may seem more realistic, but it makes three assumptions:

  • assume the carrying capacity is constant
     
  • assume each individual affects the growth of the entire population by increasing environmental resistance as soon as it is born
     
  • assume the number of offspring produced by an individual relates directly to the resources available at the time of birth
Logistic population growth can be expressed by the equation:

d N / d t = r N x (K - N) / K

  • d - means an instantaneous change in
  • N - the number of individuals already in a population
  • t - a unit of time
  • r - realized intrinsic rate of population growth
  • K - carrying capacity
Species Types:

  • r-selected species are capable of very rapid population growth, approximating an exponential growth pattern followed by a crash in the adult population .

  • K-selected species have more or less stable populations, existing at or near carrying capacity in relatively stable habitats.
     
  • euryecious species are widely distributed with a high tolerance for many environmental factors.
     
  • stenoecious species have a narrow distribution with a low tolerance for changes in environmental factors.
 
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