Chagas vector:
There is a disease called Chagas disease that is endemic in Latin America.. It is caused by a parasite named Trypanosoma cruzi.  The parasite can enter a wound.  The disease is carried in the wild by a number of kinds of mammals and can be transmitted to humans by more than 100 species of triatomine insects, sometimes called the “kissing bug” because it tends to bite on or near the lips. It deposits the parasite in its feces.  This disease is important but tends to be neglected.  The issue at this moment is the fact that although the disease has been recognized in mummies thousands of years old, in the past two or three centuries it has increased a lot.  (Cagas Disease: a New Worldwide Challenge, Pendro Albajar Viñas NATURE vol. 465 no. 7301 June 24, 2010 page S6 special section on Chagas disease)  The causes have been increased opportunity for the bug to spread and the fact that deforestation has reduced the availability of wild animals for the bug to feed on.

This assessment is made without fanfare.  That is because the logic is not new.  In medical school we were told that ships used to be pulled through the locks of the Panama canal by draft animals.  When these were replaced with little motorized tractors, the mosquitoes which had been biting the donkeys started biting people so that malaria increased.  A student, more alert than I, expressed astonishment.  The professor explained that this was to be expected.  In fact he told us that there were tunnels under the buildings of the school.  During the winter mosquitoes living there could be caught and found to contain human blood.  But in the summer, when the medical students were gone, they contained pigeon blood.  They just took what was available to them.

But from what we knew then, and from what most people think now, it really was astonishing.  It is axiomatic with people that population depends on the environment.  Population will increase until it reaches the carrying capacity of the environment; that is what people think.  And the food supply is usually the limiting factor. 

But his observation among insects, now we have seen three examples, flies directly in the face of that assumption.  Insect populations are not always, and I dare say not usually, limited by the food supply.  Think about it.  When was the last time you saw a wild animal?  It was enviably lean, no doubt, but it was not gaunt, not starving.  If the population had grown to the limits of the food supply, then it should be on the edge of starving to death. 

Better yet, go to a park and look at pigeons.  They are fat.  That’s because people feed them.  But there are only so many people and they bring only a finite amount of food.  If the pigeon population had grown to the limits of the food supply you would see starving pigeons even though the food was easy to get.  But no, they are fat.

The paper by Sibly I refer to so often (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) of course tells the story.  The population is regulated because with increasing size there is falling fertility, pure and simple. 

The evidence is everywhere.  One need but notice. 

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