Timescale reverses the relationship between host density and infection risk

Tara E. Stewart Merrill, Carla E. Cáceres, Samantha Gray, Veronika R. Laird, Zoe T. Schnitzler, Julia C. Buck

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Article number20221106
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1980
StatePublished - Aug 10 2022


  • Daphnia
  • Metschnikowia
  • density-dependent transmission
  • encounter dilution
  • safety in numbers
  • timescale

ASJC Scopus subject areas

  • General Agricultural and Biological Sciences
  • General Environmental Science
  • General Biochemistry, Genetics and Molecular Biology
  • General Immunology and Microbiology


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