All orchids are at least partially mixtrophic. It’s being able to parasitise fungi that allows them to germinate. In the longer-term, they can build specialised relationships that might still be parasitic and remain heterotrophic, getting their food from elsewhere. Or they might same some of their own food through photosynthesis and have a mix of nutrients from the air and their fungal partners, a mixotrophic lifeway.
Not all fungi are the same, and orchids might require their own partners. Could this place a limit on orchid distribution, leaving them unable to germinate where their fungal partners don’t live? Oktalira and colleagues examined two closely related Caladenia orchid species pairs to find out. (try this if the DOI is broken)
The first pair, C. procera (Carbunup king spider orchid), and C. pectinata (king spider orchid) are found in Western Australia. In the case of it’s a tiny part of Western Australia near Busselton. It’s critically endangered.
The second pair are in eastern Australia. C. atrovespa (thin-clubbed mantis orchid) is found in a few sites in New South Wales and the Australian Capital Territory. C. tentaculata, in contrast, is found throughout south-east Australia.
They looked to see what the orchids needed to germinate and what they needed as adults and got mixed results.
For the western orchids, both orchids seemed to associate with similar fungi. In the east, in contrast, the orchid with the wider distribution also seems to have a wider range of fungal partners. One reason for this might be sampling. If you examine an orchid that lives in just a few sites, and another that lives in many, you could expect to see more fungal diversity anyway, as one orchid has a more geographically diverse range of samples. Against this, the authors note they saw C. tentaculata associating with multiple fungi at a single site, which might also hint that it’s more tolerant of variety.
For germination, it was the narrow-range C. procera that was more tolerant of variety in its fungi than the wider-ranging C. pectinata. C. atrovespa, the rare orchid in the east remained fussy, and the authors think this is a case of mycorrhizal specificity. While the need was specific for one fungal group, that group itself was fairly widespread.
The authors conclude that mycorrhizal specificity is unlikely to be the limiting factor for rare Caladenia orchids. Instead, they look at the pollination system, which is often through sexual deception as the critical issue. Plants need pollinators to continue in ranges.
While this raises other conservation problems for orchids, such as how do you conserve the pollinators? it also provides some opportunities. The authors conclude: “For ex situ conservation, our result indicates a benefit of using a number of isolates from other Caladenia species to germinate seeds of species such as C. pectinata, or even the endangered species C. procera.” These fungi can be found with the widespread orchids, so you don’t need to disturb a fragile C. procera colony to get the fungi you need for germination. Orchids will remain fussy, so it’s not just a case of lifting a trowel of earth and hoping for the best. But tests seeds of endangered orchids could provide a huge boost for ex situ conservation.
Oktalira, F. T., Whitehead, M. R., & Linde, C. C. (2019). Mycorrhizal specificity in widespread and narrow-range distributed Caladenia orchid species. Fungal Ecology, 42, 100869. https://doi.org/10.1016/j.funeco.2019.100869