December 9, 2011
To be posted on

Phil Lester
School of Biological Sciences
Victoria University of Wellington in New Zealand
PO Box 600
Wellington 6140
New Zealand

Dear Dr. Lester:
I read with interest the review (Boom and Bust ECONOMIST vol. 401 no. 8762 December 3, 2011 page 98) of the work done by you and Meghan Cooling published in Biology Letters on Argentine ants, how they will invade an area in New Zealand, become very numerous, and then after 12.9 to 15.3 years frequently experience a population collapse or extinction. 

There is a reference I’m sure you know but which did not make it into the review and I would like to take occasion to recommend it to you again.  A team led by a man named Sibly (On the Regulation of Populations of Mammals, Birds, Fish, and Insects.  Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hone, Mark Pagel SCIENCE VOL 309 22 JULY 2005 page 609) amassed more than a thousand field studies of animals suitable for their analysis and found a negative relationship between population size and fertility.  This appeared to be independent of environmental limitations.  The same relationship has been seen in humans.  (An Association Between Kinship And Fertility of Human Couples,  Agnar Helgason, Snaebjoern Palsson, Daniel F. Guobjartsson, Pordur Kristjansson and Karl Stefanson, SCIENCE vol. 319 8 February 2008 page 813)  The fact that their curves are so similar suggests a single mechanism.  This cannot be environment, since the Icelanders were not starving at the time, and it cannot be due to economic factors, since animals are affected.

The phenomenon develops too fast to be genetic and therefore must be epigenetic. 

Obviously if fertility declines with population size, there must be some population size so large that extinction is a threat.  Darwinian evolution mandates that the fittest survive, but that does not appear to be the case all the time.  This is very odd.

I would like to suggest that what you have documented requires no special pleading invoking occult disease but is simply an extreme and well studied case of the general principle. 

I say a single mechanism, but my own work (unpublished) suggests that the mechanism may be post-zygotic at times and a combination of pre-zygotic and post-zygotic at others.  If you would like to see the results that lead me to such a brazen claim, I should be happy to oblige. 

I shall even be heedless enough to make a guess.  The fact that there are a characteristic number of years between the invasive colony being noticed and its crash is familiar.  That sounds like my mechanism.  With a limited number of cases it appears to me that ten generations is pretty typical, so I infer that the average generation time of Argentine ants is between one and two years. 

As for why some populations die out and others decline without either rebounding or going extinct, I am not so sure.  I would have to wave my hands around, and indeed the cause may vary from case to case. 

If you wish to see more, go to and scroll down to “Orlando meeting” and click on it.  There is more evidence and my idea as to why nature might have pulled such a dastardly trick on us. 

Let me know what you think.


M. Linton Herbert MD 

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