Competition between organisms interfere in host and pathogen dynamics in ways that are difficult to predict. By one side, competitors can reduce the food supply and cause nutritional stress. Such stress could further modulate the susceptibility to infection by altering immune response or metabolic rate of the host. Alternatively, competitors may trap pathogens before they reach the focal host, and therefore reduce, enhance, or have no effect on infection according to the competitor's susceptibility to the infection. To better understand how competition influences host and pathogen interactions, we experimentally assessed the relative importance of competition for pathogens and resources on the severity of a viral disease infecting the Pacific oyster Crassostrea gigas. We designed an open‐flow system where food enriched seawater flowed to filter‐feeding competitors (or empty controls) before being delivered to recipient oysters. We tested a range of competing species that exhibit both low (ascidians, European oysters, mussels) and high (Pacific oysters) susceptibility to the virus. We assessed the physiological condition of the recipient oysters during acclimation, we added virus‐contaminated seawater upstream of the distribution system, and we monitored host and pathogen dynamics. We found that the presence of competitors, regardless of susceptibility to the virus, indirectly reduced the infection rate of hosts by decreasing their food ingestion and growth rates. Although competitors can reduce viral particles from the seawater, this had no effect on the host population. Our data suggest that the effect of competition for food overwhelmed that of competition for pathogens, thus emphasizing the importance of considering resource availability in host and pathogen dynamics. More particularly, resource availability can have positive effects at the individual level, fostering physiological condition and growth, but negative effects at the population level, increasing magnitude of epidemics.

中文翻译：

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食物竞争减少了海洋无脊椎动物的疾病易感性

生物之间的竞争以难以预测的方式干扰宿主和病原体的动态。一方面，竞争者可以减少食物供应并引起营养压力。这样的压力可以通过改变宿主的免疫反应或代谢率来进一步调节对感染的敏感性。替代地，竞争者可以在病原体到达病灶宿主之前将其捕获，因此根据竞争者对感染的易感性来减少，增强或对感染没有影响。为了更好地了解竞争如何影响宿主和病原体之间的相互作用，我们通过实验评估了病原体和资源竞争对感染太平洋牡蛎Crassostrea gigas的病毒性疾病严重程度的相对重要性。。我们设计了一个开放式水流系统，使富含食物的海水在流向接受过滤器的牡蛎之前先流向过滤器饲喂的竞争对手（或空控件）。我们测试了一系列竞争性物种，它们对病毒表现出低（海生，欧洲牡蛎，贻贝）和高（太平洋牡蛎）敏感性。我们评估了驯化过程中受体牡蛎的生理状况，在分布系统的上游添加了受病毒污染的海水，并监测了宿主和病原体的动态。我们发现，竞争者的存在，无论对病毒的易感性如何，都通过降低食物摄入量和生长速度而间接降低了宿主的感染率。尽管竞争对手可以减少海水中的病毒颗粒，但这对寄主种群没有影响。我们的数据表明，争夺食物的影响胜过争夺病原体的竞争，因此强调了在宿主和病原体动力学中考虑资源可用性的重要性。更具体地说，资源的可获得性在个体层面上可以产生积极影响，促进生理状况和生长，而在人口层面上则可以产生负面影响，从而增加流行的程度。