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If you are given an unlimited growth model in the form:

$\frac {dP(t)}{dt} = k P(t)$

Obviously the population growth would never be unlimited, but let's presume for the moment that we are introducing a species into an environment where there is the possibility for unlimited growth, at least on for a given time -- i.e. invasive species.

$k$ is some rate of growth of the population at time $t$, denoted by $P(t)$

What are some feasible values of $k$? In other words, if a number is way above or way below $k$, where would I know that the research I am reading is preposterously off-base?

I am sure it is different for different types of animals, including mammals, birds, bacteria, etc.

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The feasible value for K depends on the growth rate and is variable. The good thing with using mathematical models is that you can provide a range of values for a parameter and observe the effect on the system. You can get an idea of the feasible values based on literature evidence. You first decide what that invading species might be. –  WYSIWYG Apr 9 '13 at 9:05
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A solid limit: k must be greater than zero. Unless you're talking about some cannibalistic species or something that isn't suited to the model at all.

As long as the species is productive in the new environment: k is greater than 1. The population is probably growing or again you probably won't be using an exponential growth model.

As mentioned before you would need to know the species for more information. But if you look at generation times and litter sizes:

  • Some bacterial generation times (from here) range from 10 to 2000 minutes (33 hours). So that is $k=2$ per generation time. Per day you're looking at a lower bound of 2 per day and an upper bound of $k=2^{14}=10^{43}$ per day.
  • Mice are something like 12 week generation time and a litter of 10 giving something like $k=10^{10}$ per year.
  • Elephants are one young every 25 years. So $k=16$ per century or so.

Of course this is all based on gross assumptions. But you're looking for guidelines for a unrealistic model so hopefully they'll do.

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