Threshold host density

Threshold host density (NT), in the context of <a href="/facts/Wildlife_disease/qAhza8zJ">wildlife disease</a> <a href="/facts/Ecology/YE8upuJU">ecology</a>, refers to the <a href="/facts/Population_density/70zGTCFc">concentration of a population</a> of a particular organism as it relates to disease. Specifically, the threshold host density (NT) of a species refers to the minimum concentration of individuals necessary to sustain a given disease within a <a href="/facts/Population/ArxshNuB">population</a>.
Threshold host density (NT) only applies to density dependent diseases, where there is an "aggregation of risk" to the host in either high host density or low host density patches. When low host density causes an increase in incidence of parasitism or disease, this is known as inverse host density dependence, whereas when incidence of parasitism or disease is elevated in high host density conditions, it is known as direct host density dependence.
Host density independent diseases show no correlation between the concentration of a given host population and the incidence of a particular disease. Some examples of host density independent diseases are <a href="/facts/Sexually_transmitted_diseases/IYL1rxdI">sexually transmitted diseases</a> in both humans and other animals. This is due to the constant incidence of interaction observed in sexually transmitted diseases—even if there are only 20 individuals left of a given population, survival of the species requires sexual contact, and continued spread of the disease.
Density dependent diseases are significantly less likely to cause extinction of a population, as the natural course of disease will bring down the density, and thus the propinquity of individuals in the population. In other words, less individuals—as caused by disease—means lower infection rates and a population equilibrium.

Threshold host density open-in-new

Threshold host density