Collateral damage: epiphytic orchids at risk from myrtle rust

Abstract

Epiphytic orchids rely on the habitat provided by their plant hosts to survive. The naturalisation of Austropuccinia psidii (G. Winter) Beenken (the causal agent for myrtle rust) in Australia means that some of these plant hosts, from the family Myrtaceae, are at risk of serious decline. We aimed to identify orchid species that associate with myrtaceous host plants and determine which, if any, might be susceptible to loss of habitat as a result of myrtle rust. We reviewed species descriptions and herbarium records and identified 73 epiphytic orchid species that are commonly found growing on myrtaceous hosts. At least seven orchid species are predominantly reliant on myrtaceous hosts, are distributed predominantly in the myrtle rust zone, and have host species that are highly or extremely susceptible to myrtle rust. Four of these orchid species are already listed as threatened. The impact of myrtle rust is broader than causing decline of Myrtaceae species, with knock-on effects on other biota, including epiphytic orchids. Moreover, there is the potential for further impact on these orchids through fragmentation (e.g. affecting pollination) and interactive effects with fire. Increased effort is required to identify the relative frequency of myrtaceous and non-myrtaceous hosts for these epiphytic orchid species, especially in relation to the compound effects of myrtle rust and other perturbations, such as fire and climate change. Where this is not possible, ex situ conservation may be required.