Dear Dr. Gibbs

 

I am currently studying your book on Problem-solving in Conservation Biology for possible inclusion in my course. I have read many of your chapters with interest. In chapter 10, on habitat loss and fragmentation, I would like to report a possible problem with the way the weighted average population growth is calculated.

In your example on page 77, you present the following calculation:

r(average) = ((0.02 x 30) + (-0.02 x 60))/100 = -0.0060

 

I would argue that the right way to proceed would be to weigh r as a function of the total (for instance, here, the total is 90).

So the alternate calculation gives:

R(average) = (0.02 x 30/90) + (-0.02 x 60/90) = -0.0067

 

For this example, the difference seems subtle. However, consider the extreme example where no interior forest remains. For instance, if we read the first table of Chapter 10 in your instructor's manual, one can read that when only 10% of the forest remains, all the remaining forest is made of edge.  Since all remaining forest is forest edge, one would expect a growth rate equal to the growth rate in forest edge, i.e. -0.02.

If we calculate using my proposed approach, we get this result:

R(average) = (0.02 x 0/10) + (-0.02 x 10/10) = -0.02

 

Instead, in the table, we get a different result:

R(average) = ((0.02 x 0) + (-0.02 x 10))/100 = -0.002

 

So the final r is 10 times too high.

 

Could you tell me if I am missing something, or if a correction is in order?

 

Thanks a lot for your time, and congratulations for this great book.

 

All the best,

Marc

 

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Marc Amyot

Professeur agrégé / Associate professor

GRIL

Département de sciences biologiques

Université de Montréal