Timescale reverses the relationship between host density and infection risk

Host density shapes infection risk through two opposing phenomena. First, when infective stages are subdivided among multiple hosts, greater host densities decrease infection risk through 'safety in numbers'. Hosts, however, represent resources for parasites, and greater host availability...

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Bibliographic Details
Published in:	Proceedings of the Royal Society. B, Biological sciences Vol. 289; no. 1980; p. 20221106
Main Authors:	Stewart Merrill, Tara E, Cáceres, Carla E, Gray, Samantha, Laird, Veronika R, Schnitzler, Zoe T, Buck, Julia C
Format:	Journal Article
Language:	English
Published:	England The Royal Society 10-08-2022
Subjects:	Animals Daphnia - physiology Ecology Host-Parasite Interactions - physiology Parasites Reproduction Zooplankton - physiology Daphnia safety in numbers density-dependent transmission encounter dilution timescale Metschnikowia
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Summary:	Host density shapes infection risk through two opposing phenomena. First, when infective stages are subdivided among multiple hosts, greater host densities decrease infection risk through 'safety in numbers'. Hosts, however, represent resources for parasites, and greater host availability also fuels parasite reproduction. Hence, host density increases infection risk through 'density-dependent transmission'. Theory proposes that these phenomena are not disparate outcomes but occur over different timescales. That is, higher host densities may reduce short-term infection risk, but because they support parasite reproduction, may increase long-term risk. We tested this theory in a zooplankton-disease system with laboratory experiments and field observations. Supporting theory, we found that negative density-risk relationships (safety in numbers) sometimes emerged over short timescales, but these relationships reversed to 'density-dependent transmission' within two generations. By allowing parasite numerical responses to play out, time can shift the consequences of host density, from reduced immediate risk to amplified future risk.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare.c.6115282.
ISSN:	0962-8452 1471-2954 1471-2954
DOI:	10.1098/rspb.2022.1106