MadSci Network: Environment/Ecology

Re: I would like to know how to use 'Life Tables' for Ecology.

Date: Mon Mar 23 09:12:17 1998
Posted By: Tim Susman, Staff Zoology, University of Minnesota, St. Paul, MN
Area of science: Environment/Ecology
ID: 890004975.En

Life tables are a pretty simple, concise way to summarize the basic 
variables affecting population growth for either plants or animals, though 
I'll stick to animals in this example.  You say you already know what the 
basic variables of a life table are, but I'll summarize them quickly for 
other readers:

x    = age or age class
l(x) = survivorship of that age class, i.e. percent of animals born that   
     live to enter that age class
m(x) = mortality rate, i.e. percentage of animals that reach age or age
       class x and then die
s(x) = survival rate, i.e. percentage of animals that reach age or age
       class x that then survive to reach age x + 1 (note that this is
       equal to 1 - m(x) )
b(x) = fecundity at age x, i.e. number of offspring produced by a female
       at age x

Some tables also include e(x), a measure of how many more years each 
individual that reaches that age class will live, on average.

There are a few different ways to present these data in the form of a life 
table.  The way I've seen most frequently is to list the variables for all 
ages for which you have data:

   x     alive     l(x)     m(x)     s(x)     b(x)
 -----   -----    -----    -----    -----    -----
   0      1000    1.000    0.400    0.600       0
   1       600     .600    0.500    0.500      20
   2       300     .300    0.200    0.800     100


In this way, given the population and age structure in any year, you can 
calculate the population and age structure the following year, and 
eventually extract information such as the growth rate.

A few things to note:

* x need not be discrete years -- it could be an age class such as 0-1, 
  or 1-2.  Years are used most often because breeding seasons generally
  follow a yearly schedule.
* "Number alive" usually starts with either a hypothetical population of
  1,000 or an actual population number taken from real data.

I hope this helps you out.  For more information on life tables, check any 
population ecology or basic ecology textbook.  The one I used is "Ecology," 
by Dr. Robert Ricklefs, which is an excellent reference.  

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