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

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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.