|MadSci Network: Environment/Ecology|
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: Number 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 . . . etc. 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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