There are a number of reasons why a plant may be considered rare, says Paul Rees, Nursery Manager at Kew Gardens.
It may be that a species is taking advantage of a specific habitat or ecological niche, meaning its range can be restricted but remain locally common. Plants may be rare because we have a poor understanding of distribution or it could be due to how we classify some species.
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Grouping taxa together will make species appear more common, whereas splitting taxa could increase their rarity. Understanding distribution and relations is vital.
Extinction is to some degree natural, as plants compete and evolve meaning some species will become more successful and out-compete others for resources. Plants may also be rare in some countries yet abundant in others, particularly when they are at the edge of their natural range. Some may even be rare in their natural range but considered invasive outside of their natural range.
Ultimately, however, many species are experiencing increasing pressures - whether it be a changing climate, poaching, habitat destruction or land use change - and it is sad to see species going extinct for these manmade reasons. It is all of our responsibility to do everything we can to prevent this.
Rarest plants in the world
Granite banksia or Albany banksia
- Banksia verticillata.
- Critically Endangered (last assessed 2018)
The Granite Banksia is endemic to Western Australia and is found growing in the wild in only two separated clusters around Walpole and Albany, says Jess Francis, Botanical Hortiuclturist at Kew Gardens. It grows on or beside granite outcrops and in deep acidic sand.
A small bird with a bright yellow wing flash, the New Holland honeyeater (Phylidonyris novaehollandiae) is the primary pollinator of the granite banksia, attracted over long distances by the abundant nectar. Other pollinators include bees and small mammals.
This species grows in areas prone to bushfires, which kill the plant but will induce the seeds to disperse and germinate, regenerating the population. It can take a decade for a plant to reach maturity and produce seeds of its own and at least 20 years is needed between fires to maintain a good seed bank and genetic diversity.
An increased frequency of fires in the region is the biggest threat, restricting the distribution of this species even further. Harvesting the flowers from the wild is also common and will reduce the availability of seed. Further, like with other members of the genus, B. verticillata is at risk from pathogens like honey fungus, canker and the soil mould Phytophora cinnamomi which cause dieback and eventual death of the plant affected.
Monkey tail cactus and golden rat tail cactus
- Cleistocactus winteri
- Endangered (last assessed 2010)
The Cleistocactus winteri is endemic to Bolivia, where it grows in the seasonally dry forests of the inter-Andean valleys on rock faces as a lithophyte, says Neil Alderson, Botanical Horticulturist at Kew Gardens. It has a naturally restricted distribution, occurring across a range of 250 km2 at an elevation of 1,300 – 1,400 m.
There are two subspecies: Cleistocactus winteri ssp. colademono, known locally as the monkey tail cactus, and C. winteri ssp. winteri, the golden rat tail cactus. The golden rat tail is recorded across a range of about 10 km whereas the monkey tail is recorded in only a single location.
Threats include habitat loss and poaching, and the species is listed as Endangered by IUCN. Unlike a lot of cacti in Bolivia this species is not severely threatened by agriculture and urban development due to the mountainous terrain of its native range, but both subspecies are growing fairly near roads and could potentially be accessed by poachers willing to traverse the sheer cliffs it grows on.
Both are popular as ornamental plants around the world but within Bolivia the monkey tail is particularly iconic. It is used in horticultural displays in urban centres and is very common in the gardens of Bolivians. Cuttings are easy to root and commercial nurseries in Bolivia reproduce the monkey tail in large numbers, but there is apparently still poaching pressure on the wild populations.
Maiden's quiver tree
- Aloe ramosissima
- Endangered
There are three closely related threatened Aloes, the Aloe ramosissima, A. dichotoma and A. pillansii, says Paul Rees, Nursery Manager at Kew Gardens.
They all have a very small distribution growing mostly in the |Ai-|Ais/Richtersveld Transfrontier Park. This region is arid, receiving very little rainfall per annum. Summers are getting hotter and droughts are lasting longer, which is impacting species in this area.
Longer droughts are reducing the palatable species that goats in the region would usually graze on, leading them to turn to Aloe ramosissima. There is generally very little seed recruitment seen, which is possibly due to the lack of good rainfall events to allow them to establish and the seed pod often being parasitised by insect grubs.
After genetic studies, Aloe as a genus was found not to have a common ancestor as a result the genus having been spilt into multiple genera, with Aloe ramosissima becoming Aloidendron, one of ‘7 tree Aloes’
Lady’s Slipper Orchid
- Cypripedium calceolus
- Least Concern - but Critically Endangered in Britain.
This charismatic orchid has always grown very locally in Britain, formerly occurring in around 30 locations in the limestone districts of Derbyshire, Yorkshire, Durham, Lancashire and Cumbria, says Bala Kompalli. It declined dramatically in the 19th century, primarily due to over-collection, and by the early 20th century it was thought to be extinct.
Since its re-discovery in 1930, just a single mature plant of Cypripedium calceolus surviving in the wild, it has been closely guarded. Here at Royal Botanic Gardens, Kew, Dr. Viswambharan Sarasan leads a project with Yorkshire Wildlife Trust, National Trust, Plantlife, Botanic Society of the British Isles and the Cypripedium Committee working to improve population numbers in the wild funded by the Natural England.
Seedlings are grown in vitro as test tube babies in defined growth media under laboratory conditions of temperature and light. My role as a Botanical Horticulturist at Kew involves acclimatising these delicate lady slipper orchid seedlings under glasshouse conditions.
The aim is to produce high quality seedlings of different genetic lineages for conservation translocation trials back in the wild. This process can take few years and various stages of horticultural techniques for this species hoping to become a successfully established populations in wild.
Ravenea moorei
- Critically Endangered (last assessed 1998)
This elegant, dioecious species was introduced to Kew as seed in 1963, and has been in its current location since the late 1980s where it flowered for the first time in 2017, says Will Spoelstra, Palm House Supervisor at Kew Gardens.
At the time, seeds were also distributed to other botanic gardens, but we now know this is the last specimen left of that collection and likely the only cultivated specimen in the world.
Ravenea moorei is one of four known endemic palm species to the Comoros islands and, so far, the only recorded specimens have been found on Grande Comore, the largest of four islands in the archipelago. This species grows in the rainforest at altitudes between 700 and 1000m and can grow up to 20m tall. Most recent sightings have been on Mount Karthala, an active volcano in the centre of Grande Comore.
The forests in the Comoros Islands are highly degraded and at risk due to agriculture, invasive weeds and cyclones. A Kew team of horticulturists and scientists are working with local partners on a conservation project to protect all four endemic palms from extinction.
Dwarf ebony
- Trochetiopsisebenus
- Critically Endangered
Clinging onto life, St Helena ebony, or Trochetiopsis ebenus was believed to be extinct for 100 years, says Malgorzata Czarnecka, Botanical Horticulturist at Kew Gardens.
They were overgrazed by goats, and the last two plants left were spotted growing on the side of a cliff face. A local guide called Charles Benjamin retrieved cuttings from one of these last wild specimens in 1980 by abseiling down a cliff.
He bravely carried a stem back up the mountain between his teeth(!), using a makeshift harness. From the two surviving wild St Helena ebony plants, Kew's horticulturists and conservationists have taken part in propagation programmes with colleagues in St Helena to produce thousands of new plants from cuttings and seeds
White-flowered Aloe
- Aloe albiflora
- Critically Endangered
Aloe albiflora is a small succulent with a spike of pretty, white, bell-shaped flowers, says Silke Strickrodt, Botanical Horticulturist at Kew Gardens.
It is classified as Critically Endangered (possibly extinct in the wild) by the IUCN. The species is endemic to southern Madagascar and has not been found in the wild for nearly 70 years. It is known from only two collections in a small area of grassland (comprising about 8 Km2 ), which is now utilised for agriculture. This makes the reintroduction of Aloe albiflora to its natural habitat impossible.
The species is widely cultivated, including in many botanic gardens, but the inability to reintroduce this plant to its natural habitat shows the fundamental importance of conserving not just individual species but also their habitats. To be able to achieve this conservation of natural habitats, it is important for botanic gardens to cooperate with each other and relevant agencies in the countries where the species live (or used to live).
Rhododendron kanehirai
- Extinct in the Wild
Rhododendron kanehirai, first introduced to the UK by famous plant collector Ernest Wilson, is extinct in the wild, says Charles Shi, Botanical Horticulturist at Kew Gardens. Once native to Taiwan, it is one of the rarest rhododendrons in the world.
This small-leaved evergreen shrub prefers a temperate climate, and grows between 1-3 metres tall, producing crimson funnel-shaped flowers in April.
It was once found along the Peishi River, in the mountains of northern Taiwan, but excessive flooding led to its disappearance.
In recent years, only two plants in the world remained of this species, grown at Logan Botanic Garden – the plant has now been distributed to gardens worldwide, including Royal Botanic Gardens, Kew.
The Red List of Rhododendron 2011 states that: 25% of all Rhododendron taxa are under threat of extinction in the wild – with the major threat being habitat clearance and, increasingly now, climate change.
Saving endangered Rhododendron species in mountainous regions of East Asia is critical for preserving these ecosystems. To achieve this, we urgently need horticultural expertise to conserve and distribute these plants.
St Helena redwood
- Trochetiopsis erythroxylon
- Extinct in the Wild
The St Helena redwood, Trochetiopsis erythroxylon, is extinct in the wild, says Tom Pickering, Senior Glasshouse Manager at Kew Gardens. All surviving garden specimens have descended from a single tree found on St Helena in the 1950s.
In 1659, the East India Company first established a permanent colony on St Helena. They cleared the fertile, temperate hills for fuel and timber to make way for farmland. T. erythroxylon grew up to 8m tall with straight trunks, and its wood quickly became the favoured building material in the area. Only trees unsuitable for construction purposes survived the habitat destruction. Cultivated survivors rarely grow beyond 3m and often have curved limbs.
Ruizia mauritiana
- Threatened
Ruizia mauritiana is native to Mauritius and nowhere else, says Carlos Magdalena, Botanical Horticulturist at Kew Gardens. Horticulturists have observed that at higher temperatures the plants only produce male flowers, whilst at lower temperatures, female flowers are produced.
It is thought that the plant is carrion-fly pollinated, and the temperature dependence is a mechanism to ensure cross pollination, thus maintaining maximum genetic diversity. Kew is the only place in the world that material from the highland tree is in cultivation, and in 2018 the tree was returned to the Temperate House following the largest restoration project in Kew’s history.
Several of these plants will be on display in the Temperate House at Kew Gardens this autumn for Rare and Extinct, which runs from Saturday 19 October to Sunday 17 November