Data from: Linking host traits, interactions with competitors, and disease: mechanistic foundations for disease dilution

1.The size of disease epidemics remains difficult to predict, especially when parasites interact with multiple species. Traits of focal hosts like susceptibility could directly predict epidemic size, while other traits including competitive ability might shape it indirectly in communities with a ‘dilution effect’.

2.In a dilution effect, diluter taxa can reduce disease by regulating (lowering) the density of focal hosts (i.e., through competition), or by reducing encounters between focal hosts and parasites. However, these dilution mechanisms are rarely grounded in focal host traits, and the relative importance of host regulation vs. encounter reduction remains understudied.

3.Here, we map focal host traits to disease—via these dilution mechanisms—in communities with diluters. We measured two traits (competitive ability and susceptibility) for eight genotypes of a focal host (Daphnia), tracked the densities of each genotype in experimental mesocosms (+/- Ceriodaphnia competitor/diluters), and monitored their infections with a virulent fungal parasite (Metschnikowia) over 6-8 host generations. We disentangled the impacts of both traits on the density of infected hosts and partitioned dilution mechanisms using path models.

4.Higher susceptibility directly fueled larger epidemics. Simultaneously, weaker competitive ability indirectly suppressed epidemics by enabling higher densities of diluters. These higher densities of diluters reduced the density of infected hosts indirectly via host regulation. In contrast, encounter reduction was much weaker.

5.Our experiment strengthens the dilution effect paradigm with a predictable, traits-oriented framework. Similar traits—susceptibility, competitive ability, and their covariance—could help predict epidemic severity in a variety of other systems. Partitioning the direct and indirect effects of diluters could also delineate how they impact disease. Such trait-based insights could help broadly predict the size of epidemics in diverse communities.

1. 疾病流行规模的预测始终颇具挑战，尤其是当寄生虫与多种宿主物种发生互作时。焦点宿主（focal host）的相关性状（如易感性）可直接用于预测流行规模；而在存在“稀释效应（dilution effect）”的群落中，诸如竞争能力一类的其他性状则可能间接影响流行规模。
2. 在稀释效应中，稀释类群可通过两种途径降低疾病传播：一是通过竞争调控（即降低）焦点宿主的种群密度，二是减少焦点宿主与寄生虫之间的接触机会。然而，现有研究极少将稀释机制与焦点宿主的性状挂钩，且宿主密度调控与接触减少这两种机制的相对重要性仍未得到充分研究。
3. 本研究针对存在稀释类群的群落，通过上述稀释机制，建立了焦点宿主性状与疾病流行之间的关联。我们针对一种焦点宿主——水蚤（Daphnia）的8个基因型，测定了其竞争能力与易感性两项性状；在添加/不添加竞争者/稀释类群角突溞（Ceriodaphnia）的中型实验生态系统（mesocosm）中，追踪了各基因型的种群密度，并在6-8个宿主世代周期内，监测了其感染强毒力真菌寄生虫（Metschnikowia）的情况。我们厘清了两项性状对受感染宿主密度的影响，并通过路径模型（path model）对稀释机制进行了拆分。
4. 更高的易感性会直接推动疫情规模扩大。与此同时，较弱的竞争能力可通过提升稀释类群的种群密度，间接抑制疫情发展。这类更高密度的稀释类群可通过宿主密度调控途径，间接降低受感染宿主的种群密度；相较而言，接触减少机制的作用则微弱得多。
5. 本实验通过一套可预测的性状导向框架，进一步完善了稀释效应范式。类似的性状——易感性、竞争能力及其协方差——可助力预测多种其他生态系统中的疫情严重程度。拆分稀释类群的直接与间接效应，也可明确其对疾病传播的作用机制。这类基于宿主性状的研究结论，有望广泛用于预测不同群落中的疾病流行规模。