Doubling Time
When a deer population is on the increase, it is often useful to consider the time that it will take for it to reproduce itself, to double in number. In some situations, a person will claim that "the population has doubled!" and then the rate at which it may have been reproducing will be of interest to the deer herd manager.
Deer herds have a rate of increase (or decrease) like money in the bank. A herd of 100 bucks and 100 does producing 100 healthy yearlings has a rate of increase of 0.5.
To get the doubling time, diving 0.7 by the rate (and round up). For example:
T = 0.7/r = 0.7/0.5
T= 1.4
or 2 years . You must assume that the rate will be constant.
At a more realistic rate of increase (including mortality and other factors), perhaps 0.10, it will take 7 years for a population to double. A stable population will have a zero rate of increase (maybe 0.01 one year, 0.01 the next).
If a population is claimed to have doubled in 12 years, then we rearrange the equation and
12 = 0.7/r
r = 0.7/12
r = 0.96
Computer models of the populations provide much better insight into these population changes over the years than do simple rules of thumb like 0.7 over the rate.
The numbers do not tell us what to do to reach a specific point but they can help clarify the direction for the herds, where we have been, and help us know when we have arrived.
Halfing Time
Like population "doubling time", it is possible to use the same concept for finding out the time it will take for a population to be reduced to about half of its present number. In the region, where deer numbers have expended rapidly and pest conditions exist, this may be a number of greater interest than that for doubling time. To get halfing time, use:
T = 0.7/r
where T is the number of years and r is the rate of change and round up to the next higher year.
For example, where the population can be reduced at an average constant annual rate of about 0.04, then it will take 17.5 or 18 years.
To protect a deer range, forests, or motorists, the rate may have to be increased-18 years may be too long.
Halfing time is an approximation, but using a simple tool like "0.7 over the rate" can be useful in planning and in conversations. Then the real work can begin on the computer and in the field.
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This Web site is maintained by R. H.
Giles, Jr.
Last revision January 17, 2000.
