Coastal Peppercress in the Abel Tasman National Park
Coastal Peppercress or Lepidium banksii is one of New Zealand’s rarest plants. It is a tough little coastal herb, with a pleasant peppery taste. It faces a wide range of threats and is virtually extinct in the wild. It only still survives due to the tenacity of a small group of botanists and volunteers that have been keeping it going for the last 20 years or so. It only occurs naturally in Tasman Bay and is another reason that the Abel Tasman Coastline is so special as it is the last stronghold of this unique species.
Lepidium banksii is usually found below the forest above the rocky margins of the NW Nelson coastline. It thrives in nutrient rich soils, fertilised by seals and seabirds, who deposit nitrogen from the sea back onto the land through feeding their young, defecating, breeding and occasionally dying. The plants live a tough life, preferring exposed sites with bare soils and lots of light. They need lots of nutrients, and so rely on marine mammals and birds to survive as they disturb the soil, creating new habitat. When thriving, the peppercress produce masses of sticky seeds, which stick to the birds and seals, who aid in it’s distribution, by preening, using it for nest building or brushing up against rocks. A large plant can be up to 1m high, however they are usually much smaller in areas where they are squashed down by animals. It’s small white flowers are pollinated by insects, to create a small capsule which splits and releases the seed.
There are 21 native cress species in New Zealand, with all but three occupying a similar niche to Coastal Peppercress, and only one is not endangered. The early sailors called it ‘Scurvy Grass’ because it is high in Vitamin C and was eaten to prevent them from getting that scourge of mariners in times gone by. It was first possibly discovered by Cook’s botanist Joseph Banks in 1770, which is where it gets the species name from. Recently is was rediscovered by Alan Esler in 1961, just north of Totaranui in the Abel Tasman National park, this sparked a search for more populations, of which five were found, with the largest being up to 300 individuals at Mutton Cove. Tragically these were lost one year later, as they were eaten by a wild pig. It has now been recorded at 10 known sites, although most of these have been affected by storms and there are thought to be only two fully wild plants in existence.
It is thought that one of the reasons for the plant’s decline is associated to the decline in seal and bird numbers in the last 100 years from predation by humans and introduced species causing a reduction in their available habitat. Salt water inundation is also a threat, as is erosion from wave action, although this can also be seen as an opportunity as it creates areas in which to scatter more seed in the work to ensure the plant’s survival. The list of threats goes on with infestations of Diamond Back Moth, Pigs uprooting and eating the plants, weeds competing for space and light, White rust (Albugo candida), White cabbage butterfly caterpillars, rabbits, hares, rats, mice possums, deer, snails, whitefly and turnip mosaic virus all having an impact, and these are just the threats that we know about. It seems that climate change, exacerbating sea level rise and extreme weather events is having catastrophic effects on the species.
Coastal peppercress was classified ‘Nationally Critical’ in 2004 and a plan for conserving New Zealand’s coastal cress species was written in 1999. The objectives of this plan included promoting the plight of coastal cress, researching the taxonomy, habitat requirements and threats to coastal cresses, searching for coastal cress sites, managing coastal cress populations so that they persist and preferably flourish, establishing cultivated populations and establishing coastal cresses at new locations. This plan has largely been successful for many of the coastal cresses, and many people and organisations have devoted considerable time and money to the conservation of these charismatic plants. This effort has resulted in slowing their decline, but unfortunately most (if not all) species are still declining in numbers and range, and conservation of coastal cresses has proved to be difficult because of the range of threats they face.
The population of coastal peppercress has decreased from over 500 individuals in 1994 to possibly just 2 wild plants in 2015. A dedicated team from DoC have been spearheading the conservation through nursery propagation at DOC in Motueka and Auckland and Dunedin botanical gardens and the spreading of seed in situ at 21 sites along the Tasman Bay coastline. Plants have even been hydroponically grown at Tasman Bay Herbs, however they attracted too many diseases and threatened the operation. It is thought that as well as the threats listed above, this limited seed source is creating a genetic bottleneck in the species.
The project is being funded by Project Janszoon, a 30 year project aimed at reversing the biodiversity decline in the park. Much of the pest control, biosecurity and restoration on these islands is undertaken by volunteers from the Abel Tasman Birdsong Trust. This sort of thing simply wouldn’t be possible without their contribution.
It is considered that to save the species, more sites need to be located for cultivating plants and selective breeding of a disease resistant strain developed. Also concentrating the re-introduction effort to a selection of varied sites including:
1) A large seal haulout site on a predator-free offshore island.
2) A dense burrowing seabird nesting site on a predator-free offshore island.
3) A sandy strip beach which backs onto native forest with little weed presence.
4) A coastal area of steep, unstable soils which regularly slip and create fresh bare-ground.
5) A cobble beach.
6) Several artificial coastal seawalls.
7) The inner margin of a coastal estuary mouth where debris accumulates.
8) The inner margin of a coastal estuary mouth where regular flooding keeps an area clear of weeds.
9) In the estuary saltmarsh meadow transition zone to taller grasses.
Reintroductions should consist of annually broadcasting as much seed as possible into these sites. Post-broadcast care of the plants (including weed control, fungal or insecticide treatments) should occur at all sites, except where this is not possible (see 4.). Seed used in the reintroduction must be from a known, disease-free source.
Why save Coastal PepperCress?
When I asked Roger Gaskell, the lead on this project this question, this was his answer:
“Why bother ? Yep there are some pretty large $ figures in Mike Thorsen’s document. Of course these are theoretical and aspirational (No one has that sort of money to save peppercress). But maybe our peppercress work is a bit like using 1080, not ideal but the most effective option at present to stop loads of critters going down the gurgler while more effective tools are developed. If Project Janszoon can get ground-roosting birds back onto mainland ecosystems we may find peppercress can fend for it’s self. Ground roosting birds, the main drivers of the entire coastal ecosystem have vanished. No fertility and not the right kind of disturbance. Peppercress is right up there for threat of extinction, one of the most threatened plants nationally. Without doubt it would be extinct by now but for saving seed and growing new plants every year. Threatened plants don’t get much press compared to feathered creatures.”
Abel Tasman EcoTours is actively involved in the conservation of this species, we provide subsidized boat charters twice a year, we fund-raise for conservation in the Abel Tasman region through Abel Tasman Tree Collective and we communicate for those that can’t speak to the wider world, like these little plants.
Simon Walls, Roger Gaskell and Shannel Courtney at DOC for leading this project. Project Janszoon for funding and technical support. The Abel Tasman Birdsong Trust volunteers for giving their time scrambling off boats and around seals to save this cool wee plant.
What can you do?
Volunteer at the Department of Conservation nursery in Motueka, working bees are held every Monday at 9 am – contact Helen Lyndsay firstname.lastname@example.org
Volunteer for Abel Tasman Birdsong Trust to help maintain low predator numbers in the park – Contact Abby Butler email@example.com
Donate to the Abel Tasman Tree Collective, who are actively fundraising to improve the DoC community nursery facility.
Book an Abel Tasman EcoTour, we are involved in conservation and we can show you exactly where this little gem grows.
References and Further Reading
The information for this article was taken from Mike Thorsen’s Coastal Peppercress Factsheet, Roger Gaskell’s Update for Seed Grower’s and Personal Communications with Roger Gaskell.
Website: NZ Plant Conservation Network. Link
Scientific paper: New Lepidium (Brassicaceae) from New Zealand. By P. J. de Lange, P. B. Heenan, G. J. Houliston, J. R. Rolfe, A. D. Mitchell. PhytoKeys Vol. 24, pages 1-147, 2013. Link
Book: Threatened Plants of New Zealand. By Peter de Lange, Peter Heenan, David Norton, Jeremy Rolfe, John Sawyer. Canterbury University Press, Christchurch, 2010.
News article: Defying a ‘death wish’. By Vanessa Phillips. Nelson Mail, 20/3/2009. Link
Scientific paper: First record of Turnip mosaic virus in Cook’s scurvy grass (Lepdium oleraceum agg.) – an endangered native plant in New Zealand. By J.D. Fletcher, S. Bulman, P. J. Fletcher and G. J. Houliston. Australian Plant Disease Notes Vol. 4, pages 9-11, 2009.
Report: Molecular detection and pathology of the oomycete Albugo candida (white rust) in threatened coastal cresses. By T. Armstrong. DOC Science and Development Series No. 274. Department of Conservation, Wellington, 2007. Link
Scientific paper: The role of seabirds and seals in the survival of coastal plants: lessons from New Zealand Lepidium (Brassicaceae). By D. A. Norton, P. J. de Lange, P. J. Garnock-Jones, D. R. Given. Biodiversity and Conservation Vol. 6, pages 765-785, 1997.
Plan: Coastal cresses (nau) Recovery Plan. By David A. Norton & Peter J. de Lange. Threatened Species Recovery Plan No. 26. Department of Conservation, Wellington, 1999.