The "Kill the Winner" hypothesis (KtW) is an ecological model of population growth involving prokaryotes, viruses and protozoans that links trophic interactions to biogeochemistry.[1] The model is related to the Lotka–Volterra equations. It assumes that prokaryotes adopt one of two strategies when competing for limited resources: priority is either given to population growth ("winners") or survival ("defenders").[1] As "winners" become more abundant and active in their environment, their contact with host-specific viruses (also known as phages) increases, making them more susceptible to viral infection and lysis.[2] Thus, viruses moderate the population size of "winners" and allow multiple species (both "winners" and "defenders") to coexist.[3] Current understanding of KtW primarily stems from studies of lytic viruses and their host populations.[4][5][6]
KtW provides a possible solution to the paradox of the plankton.[7] It provides a mechanism for species coexistence despite resource limitations. Some investigations into virus-bacteria interactions in laboratory settings have suggested viruses play a major role in maintaining microbial diversity and provided more evidence in support of KtW.[8][9][10]
Competition specialists, or “winners”, are often the fastest growing populations.[1] Their abundance and activity increases when they outcompete other species for a shared limiting resource (e.g. phosphate). The resource can exist as a free form or as something that needs to be sequestered from biomass. Competition specialists (predators, grazers, parasites) are expected to dominate in oligotrophic environments where competition is a large ecological constraint.[1] When competition specialists are found at uncharacteristically low abundances in oligotrophic environments, viruses may be responsible for moderating their population size.
Defence specialists invest resources in strategies to avoid viral infection, but these strategies may result in reduced growth. Hence, the “defender” does not increase viral predation.[1] Defence specialists are expected to dominate in eutrophic environments where competition pressure is reduced.[1]
While the KtW model is widely applicable to different trophic levels and complex microbial systems, it has many limitations.[1][2] The KtW model represents an idealized microbial food web with mathematical parameters that only account for viral predation studied in vitro.[1][3] Because it assumes environmental conditions are stable, it can only predict population dynamics over a small time frame relative to a microbial community's history. It also fails to account for the fact that a prokaryotic species can be attacked by multiple viruses at once. The KtW model may be modified as other models that assess its limitations (e.g. CKTW) are developed.[11][12]
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