Threshold host densitythreshold density
Threshold host density NT, in the context of wildlife disease ecology, refers to the concentration of a population 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 population
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 sexually transmitted diseases 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
- 1 Host density-dependent diseases
- 2 Host density-independent diseases
- 3 Contact between individuals within a population as it relates to density in host density-dependent disease
- 4 Contact between individuals within a population as it relates to density in sexually transmitted infections
- 5 Contact between individuals within a population as it relates to density in vector-borne disease
- 6 References
- 7 Further reading
- 8 External links
Host density-dependent diseases
- Avian flu
- Echinococcus multilocularis
Host density-independent diseases
- Chlamydia in koalas
- Simian immunodeficiency virus SIV
- Human immunodeficiency virus HIV
Contact between individuals within a population as it relates to density in host density-dependent disease
This graph shows the direct relationship between disease spread through contact and population density As the population density increases, so do transmission events between individuals
Contact between individuals within a population as it relates to density in sexually transmitted infections
There is a rapid initial increase in disease transmission as the population increases from zero, and then the plateau of transmission throughout most of the graph As sexual contact is required in nearly all sexually reproducing species, transmission is not very host density dependent It is only in cases of near-extinction where sexually transmitted diseases show any dependence on host density It is for this reason that sexually transmitted diseases are more likely than density dependent diseases to cause extinction
Contact between individuals within a population as it relates to density in vector-borne disease
This graph shows the relationship between population density and the transmission of vector-borne disease Initially, the number of contacts between individuals and vectors increases as population density increases Eventually, however, the advantage of host density diminishes as the density becomes too great for the vector to maintain its natural ecological relationship with the host, and transmission decreases
- ^ Begon, M; Harper, JL; Townsend, CR 2009 "Glossary: H" In Striano, T; Reid, V Essentials Of Ecology 3rd ed Wiley ISBN 1444305344
- ^ Holmes, E, Kareiva, P, Sabo, J 2004 "Efficacy of Simple Viability Models in Ecological Risk Assessment: Does Density Dependence Matter" Ecology 85 2: 328–341 doi:101890/03-0035 JSTOR 3450199 CS1 maint: Multiple names: authors list link
- ^ Wobeser, Gary A 2005 Essentials of Disease in Wild Animals Wiley ISBN 978-0-8138-0589-4
- ^ Cabaret, J; Hugonnet, L 1987 "Infection of roe-deer in France by the lung nematode, Dictyocaulus eckerti Skrjabin, 1931 Trichostrongyloidea: influence of environmental factors and host density" J Wildl Dis 23 1: 109–12 doi:107589/0090-3558-231109 PMID 2950246
- scientific journal articles pertaining to host density and disease
- Ebert, Dieter 2005 "Ch 8: Epidemiology" Ecology, Epidemiology, and Evolution of Parasitism in Daphnia Bethesda MD: National Center for Biotechnology Information US ISBN 1-932811-06-0 NBK2036
- Jaffe, B; Phillips, R; Muldoon, A; Mangel, M 1992 "Density-Dependent Host-Pathogen Dynamics in Soil Microcosms" Ecology 73 2: 495–506 doi:102307/1940755 JSTOR 1940755
- Lentz, Amanda Jean 2007 The Effect of Aphids in Parasitoid-Caterpillar-Plant Interactions PhD thesis Blacksburg, VA: University Libraries, Virginia Polytechnic Institute and State University etd-07262007-141303
- Wildlife Disease Association, WDA
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