Habitat and meta-habitat: influence of environment and colonization history on co-distribution of hosts and symbionts

所有多细胞生物体都充当较小生物的“栖息地”，这些生物生活在较大宿主的身体上或体内，或生活在宿主创建的巢中。对于必须的单宿主共生体，宿主的身体或巢是必要的栖息地；如果宿主不在给定的区域，共生体就不能在那里存在。但宿主的存在并不能保证共生体也会存在。对于正在扩大范围的寄主物种来说，共生体的丧失最为明显。在这种范围扩大的过程中，某些共生体可能偶然地从寄主个体中消失，或者共生体的扩散可能滞后于它们的寄主。前一种现象是经典生物防治的基本原理，即入侵害虫相对没有寄生虫，这些寄生虫是在害虫的原始范围内寻找的，并被引入新的范围。后一种模式的一个例子是澳大利亚的甘蔗蟾蜍寄生虫。处于活动范围扩展前沿的蟾蜍没有肺虫，肺虫的传播特性导致它们自己的分布落后于宿主数年。对于外生共生体，或具有外部生活期的内生共生体，另一个因素可能对它们的地理分布起作用：宿主占据的栖息地。这种共生体与它们的宿主暴露在类似的环境变量中，如果它们对温度、湿度等的耐受性与它们的宿主不同，那么在某些情况下，适合宿主的栖息地可能不利于共生体。因此，从共生体的角度来看，不仅需要适当的栖息地，而且需要该栖息地的适当栖息地。对于植物寄生虫来说，后生生境的重要性是众所周知的，但作为动物共生体地理分布的解释因素的研究却很少。在这里，我描述了几个研究地理上广泛分布的宿主(鸟类、小龙虾、双足类)及其无脊椎动物寄生虫和共生体(蠕虫和虱子、介形虫、蠕虫)的共同分布模式的项目。我和我的学生将测试殖民历史和当地环境条件在确定共生体是否能够与它们的宿主一起殖民新地区方面所起的作用。

All multicellular organisms act as 'habitats' for smaller organisms that live symbiotically on or inside the bodies of the larger hosts, or in nests created by the hosts. For obligatory single-host symbionts, the body or nest of the host is the necessary habitat; if the host is absent from a given area, the symbiont cannot exist there. But presence of a host is no guarantee that the symbiont will also be present. Symbiont loss is most obvious for host species that are expanding their ranges. During such range-expansion, particular symbionts may be absent from the colonizing host individuals by chance, or spread of the symbionts may lag behind that of their host. The former phenomenon is the rationale for classical biological control, in which an invasive pest is relatively free of parasites, which are sought for in the original range of the pest and introduced to the new range. An example of the latter pattern is parasites of cane toads in Australia. Toads at the leading edge of their range expansion are free from lungworms, whose transmission properties result in their own distribution lagging years behind that of their hosts. For ectosymbionts, or endosymbionts with an external life stage, another factor may play a role in their geographical distribution: the habitat occupied by their host. Such symbionts are exposed to similar environmental variables as their hosts, and if they have a different tolerance to temperature, humidity, etc. than their hosts, there may be circumstances in which a habitat suitable for the host is inimical to the symbiont. Thus from the symbiont's point of view, not only the appropriate habitat (= host) is necessary, but also the appropriate habitat of that habitat (= metahabitat). The importance of metahabitat is well understood for parasites of plants, but has been little investigated as an explanatory factor for the geographical distribution of symbionts of animals. Here I describe several projects examining co-distribution patterns of geographically widespread hosts (birds, crayfish, amphipods) and their invertebrate parasites and commensals (mites & lice, ostracods, helminths). My students and I will test the roles of colonization history and local environmental conditions in determining whether symbionts are able to colonize new areas along with their hosts.