master_SIN_microscopic evaluations.xlsx from 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 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.

宿主密度通过两种对立的作用机制调控感染风险。首先，当寄生虫感染期虫体（infective stages）在多个宿主间分配时，更高的宿主密度可通过“群体安全”效应降低感染风险；但宿主本身是寄生虫的生存资源，更高的宿主可获得性也会促进寄生虫繁殖，因此宿主密度又会通过“密度依赖传播”提升感染风险。现有理论指出，这两种现象并非独立的结局，而是在不同时间尺度下发生的。换言之，更高的宿主密度可降低短期感染风险，但由于其能够支持寄生虫繁殖，反而会提升长期感染风险。我们依托浮游动物（zooplankton）-寄生虫病害系统，结合室内控制实验与野外观测对该理论开展了验证。研究结果佐证了该理论：我们发现负密度-风险关联（即“群体安全”效应）在短期时间尺度下偶有出现，但在两代宿主的周期内，这种关联会转向“密度依赖传播”模式。通过让寄生虫的数量响应充分显现，时间可改变宿主密度的生态效应：从降低即时感染风险转向放大未来感染风险。