Do mycorrhizal symbioses cause rarity in orchids?

Abstract

While rarely tested, the rarity of a species may be linked to the rarity of symbiotic partners. The requirement of many terrestrial plants to form a symbiosis with mycorrhizal fungi may limit the distribution and abundance of plant species. Here, in just the second test of the role of mycorrhiza in host species rarity, we investigate the influence of mycorrhizal specificity, distribution and ecological requirements in the genus Drakaea (Orchidaceae). In situ seed baiting was used to resolve the distribution of mycorrhizal fungi for three common and two co-occurring rare species of Drakaea. Mycorrhizal fungi were isolated from wild adult orchids and protocorms and the ITS nrDNA regions sequenced. An in vitro study tested the range of Drakaea species of which each fungus can support germination. All Drakaea species studied used a narrow monophyletic clade of Tulasnella endophyte for protocorm, seedling and adult plant stages. In situ seed baiting revealed that germination of Drakaea is largely restricted to the same microhabitat as that of the orchid. However, within this habitat Drakaea exhibit comparable germination to other Western Australian orchid genera. Rare and common Drakaea exhibited no difference in germination rates and both germinated in suitable habitat not currently occupied. Synthesis. In contrast to the previous study of the role of mycorrhiza in plant rarity, there was no evidence that mycorrhizal specificity contributed to rarity in Drakaea. However, the formation of mycorrhiza being mostly restricted to a specific microhabitat may limit the abundance of Drakaea in some landscapes. In Drakaea, the highly specific pollination system of sexual deception may contribute to rarity. A trend towards high specificity in orchid mycorrhizal associations in southern Australia is hypothesized to result from the predominance of old landscapes affording the opportunity for specialization on a single or few mycorrhiza(s) best adapted to the landscape conditions.