Biogeography and Conservation

There is hardly a region in the world where mycoheterotrophy does not occur. As far as we know, all orchid species are dependent on fungi during germination and early development and are therefore classi fi ed as initial mycoheterotrophs (Chap. 1 ). Orchidaceae have a worldwide distribution and occur in almost every terrestrial ecosystem apart from deserts and permafrosts. The distribution of fully mycoheterotrophic species, however, is much more restricted and often shows intriguing patterns. Fully mycoheterotrophic fl owering plants almost exclusively inhabit closed-canopy forests, and the majority of species occur in the tropical regions of the world. Many families and genera of mycoheterotrophic plants have remarkably widespread distributions that cross major dispersal barriers, and several botanists have suggested that these widespread lineages must be ancient (Engler 1888 ; Malme 1896 ; Jonker 1938 ; Rübsamen 1986 ; Leake 1994 ) . In addition, the distribution ranges of mycoheterotrophic plants often show wide gaps (“disjunctions”). The mycoheterotrophic genus Thismia (Thismiaceae) even holds the status of what Robert Thorne considered one of the strangest distribution patterns in fl owering plants (Thorne 1972 ) , with two allegedly related species: one in northern USA and the sister species in Australia and New Zealand. At local scale, the distribution of mycoheterotrophs is often highly patterned. Mycoheterotrophs can be extremely scarce, and some genera and species are known from only a few localities. It is not always clear whether this re fl ects true rarity or the plant’s ability to remain unnoticed by collectors. Even in well-collected areas, the ephemeral nature of mycoheterotrophs makes their discovery challenging. In addition, some tropical species seem to have intriguingly fragmented distribution ranges, which, in some cases, appears to be linked with postulated glacial rainforest refugia (Cheek and Williams 1999 ; Franke 2004 ) . Despite this rarity, certain localities and habitats seem to be more suited for mycoheterotroph survival: both in tropical and temperate zones, there is a tendency of unrelated species to grow together (e.g., Wallace 1975 ; Maas and Rü bsamen 1986 ) . In general terms, the study of biogeography includes two distinct components. First, the distribution of species is determined by historical V. S. F. T. Merckx (*) Naturalis Biodiversity Center , Leiden University , P.O. Box 9514, 2300 RA Leiden , The Netherlands e-mail: [email protected]