Population:  all individuals of the same specie in the same life zone/ecosystem/habitat
     ex:  human population on Planet Earth; gypsy moth population in Ballard, etc.
          over 6 billion H.sapiens on Earth and about 1 moth in Ballard
     population growth can reflect favorable conditions for reproduction and survival
          substantial habitat, adequate nutrition, few predators, little disease, etc.
     population decay/decline can reflect poor reproduction and/or low survival rates
     populations CAN vary without much change in external/"forcing" variables
       some organisms have "boom-bust" cycles despite uniform conditions

Population Structure/Composition:  related to reproduction and survival
     age structure:  distribution of ages within the population
          each age group has its own death rate, highest in elderly, low in youth
          age important for reproduction too:  females have reproductive window
               in humans, from about 15-45 years.
     sex structure:  distribution of males and females for each age bracket
          in humans, males>females in infancy, females>>males in old age
     combine these two variables to make population pyramid
          barrel-shaped pyramid indicates relatively stable or declining population
          cone-shaped pyramid typical of high population growth

Population Variables:  birth, death and movement into and out of population
     crude birth rate: number of live births per 1000 individuals in population
          humans, continent to continent, 10--40/1000 (mean = 23/1000)
           2.3% is a very high birth rate, but has declined during 20th century
     refined birth rate: live births per 1000 females between 15-45 years of age
     death rate:  deaths per 1000 individuals.  humans, 7-15/1000, 9/1000 = mean
     growth/decay rate: birth - death rates.  mean human growth rate = .014
          1.4% growth is very high, but down from 1963 value of 2.2%
     immigration rate:  number of individuals immigrating INTO the population
          humans:  quite variable, few countries have substantial immigration (US, CAN...
     emigration rate:  number of individuals emigrating OUT of the population
          wars, famine, disease, politics, religion, povertyóall drive emigration in humans
     net migration rate:  immigration rate ? emigration rate
          immigrants and emigrants change the birth and death rates, depending

Population Modelling:  % change indicates exponential growth: J shaped curve
     BUT linear component as well with fixed amounts of immigrants and emigrants
          leads to combined linear and exponential difference equation model
     BUT rates of birth and death may change with time as well
          ex:  growth rate often decreases as population reaches limit of resources
          produces S-shaped curve of X = time versus Y = population
          population may reach a stable value = carrying capacity of ecosystem
 reliable modelling of future populations depends upon accurate predictions of rates
 

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