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

所有的多细胞生物都是较小生物的“栖息地”，这些较小生物共生地生活在较大宿主的身体上或体内，或者生活在宿主创造的巢穴中。对于强制性单寄主共生体，寄主的身体或巢是必要的栖息地；如果宿主不在一个给定的区域，共生体就不能在那里存在。但是宿主的存在并不能保证共生体也会存在。共生体的丧失对于正在扩大其活动范围的宿主物种最为明显。在这种范围扩张过程中，特定的共生体可能偶然不在定殖宿主个体中，或者共生体的传播可能落后于其宿主。前一种现象是传统生物防治的基本原理，在传统生物防治中，入侵害虫相对没有寄生虫，而寄生虫是在害虫的原始范围内寻找并引入新范围的。后一种模式的一个例子是澳大利亚甘蔗蟾蜍的寄生虫。在其范围扩张的前沿，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。对于具有外部生命阶段的外共生体或内共生体，另一个因素可能在其地理分布中起作用：它们的宿主所占据的栖息地。这些共生体暴露在与其宿主相似的环境变量中，如果它们对温度、湿度等的耐受性与宿主不同，则可能存在适合宿主的栖息地对共生体有害的情况。因此，从共生体的角度来看，不仅需要合适的栖息地（=宿主），而且需要该栖息地的合适栖息地（=元栖息地）。后生境对植物寄生虫的重要性已被充分认识，但作为动物共生体地理分布的解释因素却很少被研究。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、片脚类动物）及其无脊椎寄生虫和共栖动物（螨虫和虱子、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宿主一起殖民新地区方面的作用。

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.