Why vampires would have a population problem

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clipped from www.newscientist.com Michael Marshall, reporter Maths blog Punk Rock Operations Research points out a major problem with the notion of vampires, which believers in our bloodsucking cousins would do well to note. Assuming vampires are effectively immortal, and that they can reproduce by turning normal humans into vampires, their population would explode. So in the imagined worlds in which they exist, what prevents them from over-running the planet? Colleagues in the New Scientist office suggest that there must be a high death rate caused by Slayers and other natural hazards, balancing out the high "birth" rate or mortals converted by bites. Such a mechanism could be modelled using the classic Lotka-Volterra equations for predator-prey populations. Although, of course, vampires are capable of making prey out of their supposed predators.