Department of Biology, University of Evansville, 1800 Lincoln Avenue, Evansville, Indiana 47722.
J Parasitol. 2021 Nov 1;107(6):923-932. doi: 10.1645/20-6.
There has been a growing interest in characterizing the parasite faunas of species populations as they expand their geographical ranges as a result of climate change. Expanded-range populations often exhibit lower parasite diversity than historical-range populations, and reduced parasitism may, in part, be attributable to expanded-range populations escaping their native range parasites. The present study compares the helminth faunas of green treefrogs (Hyla cinerea) from 4 historical and 4 expanded-range populations to determine whether these latter populations have undergone parasite escape. Results of this study found relatively high degrees of similarity in species composition among helminth assemblages within historical or within range-expansion locations, with marked differences in the composition of helminth faunas between historical and expanded-range populations of these frogs. Because green treefrogs from expanded-range locations exhibited significant decreases in helminth species diversity compared with those from historical sites, they appear to be escaping levels of parasitism typically experienced by these frogs in their native range. Most notably, there was a decrease in the abundance of helminths with direct life cycles and the absence of trematode assemblages with indirect life cycles among expanded-range populations of H. cinerea. The low prevalence of trematode assemblages among historical populations of green treefrogs could limit these parasites' ability to be introduced and propagated in expanded-range locations. However, the lack of trematode assemblages among populations of H. cinerea in its expanded range may also be due to the absence or limited availability of other aquatic hosts that are required to complete the life cycles of these parasites. The reduction in helminth diversity among expanded-range populations of green treefrogs lends some credence to the notion that individuals at the front of a range expansion often invest less energy in reproduction and in doing so allocate more energy to dispersal and other life-history traits, including resistance to parasites. There may, however, be other explanations for differences in parasite species diversity between historical and expanded-range populations of H. cinerea. Because many of the helminths reported from this study are host generalists of amphibians whose recruitment and transmission among intermediate and paratenic hosts are known to be constrained by water and/or soil moisture conditions, we cannot ignore the role of both local amphibian diversity and local abiotic factors in influencing helminth diversity between the 2 population types of green treefrogs. These latter factors would decrease the role of parasite escape or energy trade-offs in driving helminth diversity among populations of H. cinerea and instead would suggest that local conditions play a more prominent role in structuring their helminth communities.
人们越来越感兴趣的是，随着物种种群的地理范围因气候变化而扩大，描述寄生虫区系。扩展范围的种群的寄生虫多样性通常低于历史范围的种群，而寄生虫减少的部分原因可能是扩展范围的种群逃避了其原生范围的寄生虫。本研究比较了来自 4 个历史种群和 4 个扩展范围种群的绿树蛙（Hyla cinerea）的寄生虫区系，以确定这些后者种群是否经历了寄生虫逃逸。本研究的结果发现，在历史或范围扩张地点内的寄生虫组合中，物种组成具有相当高的相似性，而这些青蛙的历史和扩展范围种群的寄生虫区系组成存在显著差异。由于来自扩展范围地点的绿树蛙的寄生虫物种多样性明显低于来自历史地点的寄生虫，因此它们似乎正在逃避这些青蛙在其原生范围内通常经历的寄生虫感染水平。值得注意的是，在 H. cinerea 的扩展范围种群中，直接生活史的寄生虫数量减少，而间接生活史的吸虫组合不存在。历史种群中吸虫组合的低流行率可能限制了这些寄生虫在扩展范围地点的引入和传播能力。然而，H. cinerea 扩展范围种群中吸虫组合的缺乏也可能是由于完成这些寄生虫生命周期所需的其他水生宿主的缺失或有限可用性。绿树蛙扩展范围种群中寄生虫多样性的减少为这样一种观点提供了一些可信度，即范围扩展前沿的个体通常在繁殖过程中投入较少的能量，并且通过这种方式将更多的能量分配给扩散和其他生活史特征，包括对寄生虫的抵抗力。然而，H. cinerea 的历史和扩展范围种群之间寄生虫物种多样性差异可能还有其他解释。由于本研究报告的许多寄生虫都是两栖动物的宿主泛化者，其在中间宿主和副宿主之间的招募和传播受到水和/或土壤湿度条件的限制，因此我们不能忽视局部两栖动物多样性和局部非生物因素在影响这两种绿树蛙种群之间寄生虫多样性方面的作用。后一类因素将减少寄生虫逃逸或能量权衡在驱动 H. cinerea 种群寄生虫多样性方面的作用，而是表明局部条件在构建它们的寄生虫群落方面发挥更突出的作用。
