Figure 3 code.R 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.

宿主密度通过两类相悖的生物学现象调控感染风险。其一，当寄生虫感染期被分配至多个宿主个体时，更高的宿主密度可通过**群体安全效应（safety in numbers）**降低感染风险；然而宿主作为寄生虫的寄生资源，更高的宿主可利用性也会促进寄生虫繁殖，因此宿主密度会通过**密度依赖传播（density-dependent transmission）**提升感染风险。现有理论指出，这两类现象并非独立的不同结局，而是在不同时间尺度下发生的：换言之，更高的宿主密度可能在短期降低感染风险，但由于其支持寄生虫繁殖，反而会提升长期感染风险。本研究依托**浮游动物-病害系统（zooplankton-disease system）**，通过室内实验与野外观测对该理论进行了验证。研究结果支持该理论：我们发现密度-风险负相关关系（即群体安全效应）有时会在短期尺度下出现，但这类关系会在两代宿主内反转至密度依赖传播模式。通过让寄生虫的数量响应充分显现，时间可以改变宿主密度的生态效应，从即刻降低感染风险转变为未来放大感染风险。