Disease as a stabilizing factor in the protection of landscape: The leishmaniases as models

It is suggested that disease limits populations only under exceptional circumstances, when other population-limiting factors are inoperative. This is precisely the situation with human populations, which have grown well (perhaps as much as three logs) beyond sustainability. It is inconceivable that any self-regulatory mechanism will bring human populations down to a level compatible with the survival of other large mammals, so the only hope for environmental survival is disease. In this essay, I discuss the inexorable rise of human populations and our seeming escape from all normal boundaries to population growth, apart, perhaps, from diseases. I then provide examples of how the various leishmaniases have affected and constrained human populations, or have failed to do so, providing an example of how diseases may ultimately protect landscapes from anthropogenic change.