Remarkable and unexpected diversity of scurvy grasses discovered on the Chatham Islands

1. Lepidium oleraceum – Cook’s Scurvy Grass is of course not a grass but a large shrubby cress with a flavour not unlike watercress.1. Lepidium oleraceum – Cook’s Scurvy Grass is of course not a grass but a large shrubby cress with a flavour not unlike watercress.The New Zealand scurvy grasses (Lepidium species) include the famous Cook's scurvy grass (L. oleraceum) (Fig. 1), a species which has gained almost legendary status as the plant that saved Captain Cook and his crew from the depredations of scurvy. Whilst modern research has shown that this is gross exaggeration (de Lange & Norton 1996) it cannot be doubted that this plant and its allies were important green foods for not only scurvy ridden sailors but iwi (who in New Zealand knew the plants collectively as 'nau').

It has long been recognised that the Chatham Islands populations of Cook's scurvy grass were variable (de Lange et al. 2010) but until they were subjected to critical study by Department of Conservation and Landcare scientists it was not realized that this variability had a firm genetic and molecular basis. A study initiated in 2005 is now drawing to its final stages prior to publication. In that work, aside from Cook's scurvy grass (known on the islands from only one 2006 Mangere Island collection), a further five species are recognised for the islands. Of these, with the exception of L. flexicaule, the other four are new and are now in the process of formal taxonomic description.

The new species are currently known by informal tag names using an alphabetic system. Thus Lepidium aff. oleraceum (a) (Fig. 2) is the unnamed species best known to islanders and visitors from the salt flats and cobble beaches running to Kaingaroa Point. Lepidium aff. oleraceum (b) (Fig. 3) is now rarely seen on Chatham Island but it is the typical species on wave washed reefs like Western Reef and from Tarakoikoia (The Pyramid). It is this species too that has an unexpected occurrence on the remote Antipodes Islands. Lepidium aff. oleraceum (d) (Fig. 4) is a remarkable species known so far only from a small part of Mangere Island, and also Little Mangere. This plant is closely related to a now extinct species (L. obtusatum) that was only ever known from the rocks at the entrance to the Wellington Harbour. A final unnamed species, L. aff. oleraceum (i) (Fig. 5) was only recognised for what it was in late 2010. In 2006 it had been collected from several sites on Mangere Island, the specimens of which has been thought to be hybrids involving L. oleraceum and L. aff. oleraceum (d). Seedlings raised in New Zealand, and DNA data extracted from these and the 2006 collections told a very different story, that these 'hybrids' were in fact another new species.

2. Lepidium aff.  oleraceum (a) – photographed here at Kaingaroa is the largest of New Zealand’s 16 species of “Cook’s Scurvy Grass”2. Lepidium aff. oleraceum (a) – photographed here at Kaingaroa is the largest of New Zealand’s 16 species of “Cook’s Scurvy Grass”
Lepidium aff.  oleraceum (b) – this is the most widespread of the species recorded from the Chathams. It is always associated with sea bird and seal coloniesLepidium aff. oleraceum (b) – this is the most widespread of the species recorded from the Chathams. It is always associated with sea bird and seal colonies
4. Lepidium aff.  oleraceum (d) – this species is only known from a small area on Mangere and Little Mangere. Remarkably it is the closest living relative of a now extinct species that was once present around the entrance to Wellington Harbour4. Lepidium aff. oleraceum (d) – this species is only known from a small area on Mangere and Little Mangere. Remarkably it is the closest living relative of a now extinct species that was once present around the entrance to Wellington Harbour
5. Lepidium aff.  oleraceum (i) – this species was originally thought to be a hybrid. It is so far only known from Mangere and a few nearby islands5. Lepidium aff. oleraceum (i) – this species was originally thought to be a hybrid. It is so far only known from Mangere and a few nearby islands6. Mangere Island as viewed from the ‘Top Plateau’. For its size this island has the greatest diversity of scurvy grasses known on the Chatham Islands.6. Mangere Island as viewed from the ‘Top Plateau’. For its size this island has the greatest diversity of scurvy grasses known on the Chatham Islands.
Now in case people are wondering the missing letters c, f-h (and yes there is even a "j") relate to other unnamed species found on the Kermadecs, the southern South Island, Stewart and subantarctic Islands. However, at the onset of this study no one expected the Chatham Islands to be a key centre of diversity and endemicity for the genus. DNA data provides some rough clues as to why. It's now thought that these scurvy grasses occasionally hitched rides from sea birds flying between the Cook Strait and northern North Island to the Chathams. After which each new arrival was temporarily isolated from the New Zealand gene-pool and so underwent speciation; indeed the 2006 discovery of the real Cook's scurvy grass (L. oleraceum) on Mangere Island suggests this.

Considering the Chatham Islands as a whole Mangere Island (Fig. 6) is regarded as especially remarkable, considering that for its size and past gross habitat modification it provides a home to three species, and two of these are virtually endemic to it!

Lepidium flexicaule was only recognised from the Chathams in 2005. This species has a very unusual distribution, occurring on Chatham (but so far not on any of the other islands), then on the north-western coastline of South Island, and in scattered, mostly westerly sites in North Island from Wellington to Auckland. There are few records of it from the eastern side of Coromandel Peninsula, along the shores of the Firth of Thames and from an island in the Hauraki Gulf. Outside New Zealand it then turns up on the western side of Tasmania. Interestingly some DNA data suggests that the Chatham Island plants have a closer relationship to Tasmanian rather than New Zealand populations. However this needs further study.

Some other patterns are puzzling. For example no Lepidium species have ever been recorded from Pitt Island, and South East Island only has the one species (L. aff. oleraceum (b)). Whether these patterns are real we cannot say. Sadly historical collections of Lepidium made from the Chathams by Travers in the 1860s don't mention specific localities. Despite that it is hard to believe that Pitt Island never supported these species, and even harder to understand by Mangere would support three species, two seemingly near endemic.

The research is now nearing completion and formal submission of a paper is likely in the next few months.

References

  • de Lange, P.J.; Heenan, P.B.; Norton, D.A.; Rolfe, J.R.; Sawyer, J.W.D. 2010: Threatened Plants of New Zealand, Christchurch, Canterbury University Press.
  • de Lange, P.J.; Norton, D.A. 1996: To what New Zealand plant does the vernacular "scurvy grass" refer? New Zealand Journal of Botany 34: 417-420.