Last year the Chatham Islands lost one of its unique genera. The endemic Chatham Islands sow thistle was originally described by New Zealand botanist Thomas Kirk (1828–1898) as a new species of Sonchus, S. grandifolius (Kirk 1894). Then in 1965 botanist Loufty Boulos transferred S. grandifolius and the Australian endemic S. megalocarpus to a new genus Embergeria (Eichler 1965). Later Nicholas Lander (Lander 1976) transferred the Australian Embergeria megalocarpus to another new endemic Australian genus Actites leaving the Chatham Islands E. grandifolius the sole representative of the genus Embergeria, which by default was now a Chatham Islands endemic.
Despite these moves acceptance of Embergeria has always been problematic (see comments in Webb et al. 1988) and last year Garnock-Jones (2014) relegated the genus (and another New Zealand endemic one, Kirkianella) to synonymy within Sonchus. While the decision taken could be challenged (see for example discussion on the matter by Heenan et al. 2010) for now at least it has been decided to follow this move (see http://nzflora.landcareresearch.co.nz/default.aspx?selected=NameDetails&TabNum=0&NameId=2CB14B4F-4AE0-447C-B06C-DEF50B6585A1 accessed 25 August 2015).
For the Chatham Islands this now means that there is only one endemic plant genus left for the islands, the Chatham Island forget-me-not (Myosotidium). The move also means that New Zealand now has seven species of Sonchus, three endemic (one to the Chatham Islands) and the rest naturalised (de Lange et al. 2011).
Those Sonchus naturalised to New Zealand (often called sow thistle or pūhā) are for the most part weedy species of European origin. Of the four naturalised species found in New Zealand, the Chatham Islands has inherited three, field sow thistle (S. arvensis), prickly sow thistle (S. asper) and pūhā (S. oleraceus). Field sow thistle is the least common; it is the large, rhizomatous species that grows mostly along the banks of the Nairn River near Waitangi. The other two species are widespread on all of the main islands of the Chatham group. Of the native species, aside from Sonchus grandifolius, the Chatham Island also has S. kirkii, which is not particularly common. It seems to have its stronghold along the south Chatham Island coastline, where it sometimes grows with S. grandifolius.
Although the loss of Embergeria from the Chatham, and indeed New Zealand indigenous plant flora is sad, this does not detract from the global significance of S. grandifolius, which is still a very distinctive and biologically important component of the Chatham Islands vascular flora.
This importance is perhaps best demonstrated by the least expected avenue. A pathogen endemic to Sonchus grandifolius, a minute rust, known as Puccinia embergeriae ‘embergeria rust’). This rust was first discovered at Kaiangaroa Point, Chatham Island by New Zealand rust expert Eric McKenzie, and it was described as recently as 2004 (McKenzie & Johnston 2004). At the time of its description Puccinia embergeriae was only known from a few Sonchus grandifolius plants growing at Kaiangaroa Beach, Kaiangaroa Point and along the coastal portion of Ocean Mail, and that is how the wild situation has remained despite diligent searching. As such it is regarded as a globally threatened species. That conservation agencies would be concerned about rust may seem odd from a human perspective, after all the rust undoubtedly damages its host (itself a threatened species (see de Lange et al. 2013)) but it’s important to realise that the rust is also part of the unique Chatham Islands biota, having evolved alongside its host plant and that it too has its place in the ecosystem that Sonchus grandifolius occupies.
While Chatham Island sow thistle has made a spectacular recovery from the brink of extinction, the same cannot be said for Puccinia embergeriae whose ongoing scarcity remains somewhat of an enigma. For a start its unlikely to have been overlooked as the rust is fairly conspicuous, being evident as yellow circular lesions (often with darker orange spots) on the upper leaf surface of infected Sonchus grandifolius. Leaves exhibiting these lesions, if turned over reveal numerous black ‘dusty’ spots (the uredinia) where the lesions erupt through the lower leaf surface.
Nevertheless despite this rust’s apparent scarcity in the wild it recently (2015) appeared on Sonchus grandifolius plants at the Department of Conservation Te One Area Office front car park. The same rust also turned up briefly on cultivated S. grandifolius growing at Oratia, West Auckland in 2010, suggesting that it is possible to establish this rust outside its natural haunts on Chatham Island, and also in cultivation back in New Zealand.
de Lange, P.J.; Heenan, P.B.; Rolfe, J.R. 2011: Checklist of vascular plants recorded from the Chatham Island Islands. Department of Conservation, Wellington. 57pp.
de Lange, P.J.; Rolfe, J.R.; Champion, P.D.; Courtney, S.P.; Heenan, P.B.; Barkla, J.W.; Cameron, E.K.; Norton, D.A.; Hitchmough, R.A. 2013: Conservation status of New Zealand indigenous vascular plants, 2012. New Zealand Threat Classification Series 3. Department of Conservation, Wellington.
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