Department of Biology, Indiana University, 1001 E. 3rd St., Bloomington, IN 47405-3700, USA.
Oecologia. 2011 Jul;166(3):833-42. doi: 10.1007/s00442-011-1905-4. Epub 2011 Feb 9.
Parasites frequently reduce the fecundity, growth, and survival of individual hosts. How often do these virulent effects reduce the density of host populations? Spectacular examples show that recently invaded parasites can severely impact host populations--but what about parasites persisting long-term in host populations? We have addressed this issue using a zooplankton host (Daphnia dentifera) that becomes infected with a fungal microparasite (Metschnikowia bicuspidata). We combined observations of epidemics in nine lakes over 6 years, fine-scale sampling of three epidemics, and a mesocosm experiment. Most epidemics remained small (<10% maximum prevalence) and exerted little influence on host densities. However, larger epidemics more severely depressed the populations of their hosts. These large/severe epidemics started and peaked earlier than smaller/benign ones. The larger epidemics also exerted particularly negative effects on host densities at certain lags, reflecting the delayed consequences of infection on fecundity reduction and host mortality. Notably, negative effects on the juvenile stage class manifested later than those on the adult stage class. The results of the experiment further emphasized depression of host density by the fungus, especially on the density of the juvenile stage class. Consequently, this common parasite reduces the density of host populations when conditions foster larger outbreaks characterized by an earlier start and earlier peak. Given these considerable effects on host density seen in a number of large epidemics, parasitism may sometimes rank highly among other factors (predation, resource availability) driving the population dynamics of these hosts.
寄生虫经常会降低宿主个体的繁殖力、生长和存活率。这些毒性效应会在多大程度上降低宿主种群的密度？一些引人注目的例子表明，最近入侵的寄生虫会严重影响宿主种群——但在宿主种群中长期存在的寄生虫呢？我们使用一种浮游动物宿主（Daphnia dentifera）和一种真菌微寄生虫（Metschnikowia bicuspidata）来解决这个问题。我们结合了 6 年来在 9 个湖泊中观察到的流行病、对 3 次流行病的精细采样以及一个中观实验。大多数流行病规模较小（<10%的最大流行率），对宿主密度的影响很小。然而，较大的流行病更严重地抑制了宿主的种群。这些大/严重的流行病比小/良性的流行病更早开始和达到高峰。较大的流行病也对特定滞后时间的宿主密度产生了特别负面的影响，反映了感染对繁殖力下降和宿主死亡率的延迟影响。值得注意的是，对幼体阶段类的负面影响比成虫阶段类的负面影响出现得更晚。实验的结果进一步强调了真菌对宿主密度的抑制作用，尤其是对幼体阶段类的密度的抑制作用。因此，当条件有利于更大规模的爆发时，这种常见的寄生虫会降低宿主种群的密度，这些爆发的特点是更早开始和更早达到高峰。鉴于在许多大流行中看到的对宿主密度的这些相当大的影响，寄生作用可能在某些情况下在驱动这些宿主的种群动态的其他因素（捕食、资源可用性）中排名很高。
