Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 102, Issue 42, Pages 15140-15143Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0505139102
Keywords
density dependence; frequency dependence; population dynamics; parasite; infection
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Funding
- Wellcome Trust Funding Source: Medline
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Sexually transmitted diseases (STDs) occur throughout the animal kingdom and are generally thought to affect host population dynamics and evolution very differently from other directly transmitted infectious diseases. In particular, STDs are not thought to have threshold densities for persistence or to be able to regulate host population density independently; they may also have the potential to cause host extinction. However, these expectations follow from a theory that assumes that the rate of STD spread depends on the proportion (rather than the density) of individuals infected in a population. We show here that this key assumption (frequency dependence), which has not previously been tested in an animal STD system, is invalid in a simple and general experimental model. Transmission of an STD in the two-spot ladybird depended more on the density of infected individuals in the study population than on their frequency. We argue that, in this system, and in many other animal STDs in which population density affects sexual contact rate, population dynamics may exhibit some characteristics that are normally reserved for diseases with density-dependent transmission.
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