Plant Records: The world’s most common group of plants is… 'rare' plants. A recent article in Science Advances labelled over a third of species as 'exceedingly rare'.

Name: Rare plants
Scientific names: Far too many to mention
Known for: Appearing the newspaper headline “Botanists fight to protect rare plant in local area.”
Record broken: Biggest group of plants on the planet.

How do scientists group plants? In reality, whatever way they like, so long as it’s a useful classification. But if you come up with a group like ‘rare plants’ then you’d be hard-pressed to find a taxonomist who takes it seriously. Scientists tend to like their categories monophyletic. This means coming from a common ancestor. It helps when classifying plants and looking for common features. 

Sometimes paraphyletic groups are useful. Succulents share no close common ancestor, but they share environmental challenges and some adaptations to those problems. In contrast, ‘rare plants’ seems like a chaotic group. Nothing is unifying them other than they’re rare. They seem to be everywhere – and maybe that’s why they’re worth noticing.

Skippy the Bush, Wollemi Pine and occasional Botany One mascot.

An article published this year in Science Advances by Brian Enquist and colleagues says that over a third of the Earth’s plant species are ‘exceedingly rare’. Rare plants tend to be found in hotspots. The authors propose two reasons for this. “First, current hotspots of rare species likely reflect areas with lowered risk of historical extinction. Rare species are often found in geographic localities that have had more stable climates that have likely lowered the probability of extinction… Second, rare species are spatially clumped in ways that support mechanisms for generating and maintaining rare species articulated by early theorists, who proposed roles for mountains and climate stability in influencing both rates of speciation and dispersal.”

The paper has some useful discussion of rarity. “[M]ost species tend to be simultaneously common in a few parts of their ranges and rare in most of their ranges,” they note. An example would be the Radnor lily, county flower of Radnorshire that is only found in one small location in the whole of the UK. It was seen in flower earlier this year. While that might be news in Wales, the species Gagea bohemica is found across southern Europe and North Africa. It’s rarity is local.

Rarity is a problem, as rare species are more likely to become extinct, and there were a lot of plant extinctions reported this year. Aelys Humphreys and colleagues published their findings in Nature Ecology and Evolution. The team found that while extinctions were a problem, so too was finding out if a plant was extinct or only astonishingly challenging to find. “Almost 600 species have become extinct, at a higher rate than background extinction, but almost as many have been erroneously declared extinct and then been rediscovered,” they write.

This difficulty in knowing that a plant is genuinely extinct means that a plant has to have been gone for many years before botanists accept it’s not merely very very very rare. “How are you going to check the entirety of the Amazon for your lost plant?” Dr Maria Vorontsova of Kew told the Guardian.

Another difficulty is that some extinct plants survive in seed banks. An article in Nature Plants by Sarah Dalrymple and Thomas Abeli argues that these could be used to revive extinct species. “There is no technical reason why a species should go extinct,” they write. “In addition to in situ management options, a variety of facilities can deliver ex situ plant conservation according to the needs of the species — living collections are cultivated in botanic gardens (including nurseries and arboreta), while viable genetic material can be stored in gene and seed banks, and occasionally found in herbaria.”

When this material survives, an extinct label can be a significant handicap for a species. “For many species, ex situ seed banks might be the last resort, but their classification as EX presents a bureaucratic barrier to any meaningful attempts at species restoration,” write Dalrymple and Abeli.

So it is quite possible to see extinct plants and not too difficult to find a rare plant in trouble. And if you can’t find a rare plant locally, then you might just need to wait a few years.

Florian Jansen and colleagues have published a study on how common plants are becoming rarer. They saw a significant decrease in occupancy for over half of the plants they analysed. This massive change has ecological consequences, the scientists warn.

“The observed decline of moderately common plant species might have large effects on ecosystem services as well as the abundance of animals,” Jansen and colleagues write in their paper. “The subsequent consequences for food webs cannot be underestimated. The observed decline in insect biomass is most probably related to the decline in occupancy and abundance of formerly common plant species. If this is true, conservation focus mainly on rare plant species threatened by extinction will not be sufficient to counteract the losses of dependent taxa from other trophic levels.”

“Knowledge of the world’s plant and fungal diversity and its uses is incomplete and fragmented. There are many opportunities to accelerate the process of filling knowledge gaps in these areas,” write Alexandre Antonelli and colleagues in Nature Plants. Their contribution to filling the gaps will be the State of the World’s Plants & Fungi report, due September next year from Kew.

Sadly, if extinctions continue at the current rate, there’ll be over four hundred plant species that won’t be in any state to report. You’ll be able to find out more about the report at Plants, People, Planet next year.