Parasites are more diverse and numerous than their hosts and commonly control population dynamics. Whether parasites also regulate key ecosystem processes, such as resistance to climate stress, is unclear. In southern U.S. salt marshes, drought interacts synergistically with keystone grazing to generate extensive ecosystem die-off. Field manipulations of parasite prevalence and salt stress in sediments in healthy marshes demonstrated that trematode parasites, by suppressing feeding activity of grazers that overgraze on drought-stressed plants, have the potential to slow the rate of ecosystem loss. Surveys along 1000-km of coastline during an intense drought event revealed parasitism is common in grazers on die off borders and that increasing infection prevalence along marsh die-off borders is negatively correlated with per capita grazing. Combined, results from this field experiment and survey suggested, but did not show, that parasites could affect rates drought-driven salt marsh die-off. To test whether parasites can indeed protect marshes under real drought conditions, we experimentally manipulated parasite prevalence in grazers over a month-long period on active die-off borders in three North Carolina marshes. Experimentally reducing parasite prevalence markedly increased the rate of plant ecosystem decline, an effect that scaled positively with prevalence. Thus parasites, by generating a trophic cascade, indirectly enhanced ecosystem resistance to overgrazing under intense drought in these North Carolina marshes. The generality of these results across the entire range of this keystone grazer in the southeastern US needs to be tested, employing both experiments and extensive surveys that examine how the rate of ecosystem decline is mediated by parasitism. Given the ubiquity of parasites in ecosystems, our results suggest that more research effort should be invested in examining the possible roles for parasitism in regulating ecosystem function and stability.

中文翻译：

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寄生虫增强沿海生态系统对干旱的抵抗力

寄生虫比它们的宿主更加多样化和数量众多，并且通常控制着种群动态。寄生虫是否也调节关键的生态系统过程，例如对气候压力的抵抗力，尚不清楚。在美国南部的盐沼，干旱与基石放牧协同作用，导致生态系统大面积死亡。对健康沼泽沉积物中寄生虫流行和盐胁迫的实地操作表明，吸虫寄生虫通过抑制过度放牧在干旱胁迫植物上的食草动物的摄食活动，有可能减缓生态系统的丧失速度。在严重干旱事件期间沿 1000 公里海岸线进行的调查显示，寄生在濒临死亡边界的食草动物中很常见，而且沼泽死亡边界上感染率的增加与人均放牧呈负相关。结合，该田间试验和调查的结果表明，但并未表明，寄生虫会影响干旱导致的盐沼死亡率。为了测试寄生虫是否确实可以在真正的干旱条件下保护沼泽，我们在北卡罗来纳州三个沼泽的活跃死亡边界上，在长达一个月的时间内通过实验操纵了食草动物的寄生虫流行率。实验性地降低寄生虫流行率显着增加了植物生态系统衰退的速度，这种影响随着流行率呈正比例增长。因此，寄生虫通过产生营养级联，间接增强了北卡罗来纳州这些沼泽地严重干旱下生态系统对过度放牧的抵抗力。这些结果在美国东南部这个基石食草动物的整个范围内的普遍性需要进行测试，使用实验和广泛的调查来检查生态系统衰退的速度是如何通过寄生来调节的。鉴于寄生虫在生态系统中无处不在，我们的研究结果表明，应该投入更多的研究工作来研究寄生虫在调节生态系统功能和稳定性方面的可能作用。