The demographic model

A stable population would be that one that is kept in balance, that is to say, that the number of incorporations is equal to that of losses of individuals of the population.

An example would be a population with 50 pairs (100 reproductive individuals), in which every year 10 adults died, 50 juveniles were produced (a chick successfully bred by pair), of whom only 10 survived to join later the population again (the 40 rest would have died throughout the 3 years of stay in the dispersal areas).

       10 adults dead

Adult mortality   = ------------------------------- x 100 = 10%

       100 population individuals


 50 chicks

Productivity =  ---------------------- = 1 chick / pair

50 pairs


     40 young dead

Pre-adult mortality = ------------------------ x 100 = 80%

     50 young produced


The populations are not closed and independent structures. Interchanges of individuals among them exist, so that its evolution is also determined by the incorporations or the exit of individuals to other neighbouring populations.

From data gathered of different populations from the Iberian Peninsula and the south of France, it has been possible to create a mathematical model that allows predicting how the demography of a population based on its productivity, adult mortality and pre-adult mortality will evolve.

The model indicates that adult mortality has sensitivity 4 times superior to pre-adult mortality and 10 times superior to the productivity. Therefore, if it is wanted to obtain a stable population, is 10 times more effective to take measures to try to reduce the adult mortality than to promote an increase of the productivity.

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