The Peppered moth of the plant world?

A short article in the Guardian last month caught my eye – maybe partly because one of the scientists involved is based at Kunming Botanical Gardens, which we visited on a family trip to China to see our son, last year. Yang Nui and colleagues found that a tiny fritillary growing on the eastern edge of the Himalayas seems to be rapidly evolving to produce muddy-brown leaves and flowers instead of its original lime green ones.  Fritillaria delavayi bulbs take five years to produce their first flower; until then only a few small leaves are apparent.  The brightly coloured flowers may be a good way of attracting insect pollinators but, sadly, that’s not all they attract.

‘Original’ Fritillaria delavayi, from Nui et al., 2021

F. delevayi bulbs, known as Lu Bei, have been collected for use in Traditional Chinese Medicine (TCM) for more than 2000 years.  Tinctures and powders made from the bulbs are used to treat phlegmy coughs including Tuberculosis.  The bulbs are tiny (weighing about a quarter of a gram) but now fetch up to $480 per kilo, so the plants are at increasing risk of being over-harvested.

We’ve long known that animals camouflage themselves to avoid being eaten and the same is true of plants which want to protect themselves from herbivores – plants, after all, can’t just relocate when the going gets tough.  When we think of plants protecting themselves, we may think of spines and toxins but camouflage, in terms of matching their background, can also be important.  What is interesting about this work is that Nui et al. found little evidence of herbivore damage to F. delavayi plants – these plants seem to be changing their leaf and flower colour to protect themselves from human predators rather than animals. 

New form of F. delavayi

Not only that, but the degree to which populations of the plants were changing colour seemed to depend on how intensely they were being harvested.   Plants subject to intense harvest pressure, because of either collection intensity or ease of collection, were significantly more likely to be closely colour matched with their background than plants under less pressure. 

Nui and colleagues came up with an intriguing way of testing whether they were right, that a better match with the background meant plants took longer to detect and also whether herbivores or humans were driving the colour changes they observed. They created a Citizen science computer game called Spot the Plant (which you can still play)where participants had to mark the location of a selection of plants of the two colour forms against their scree-like natural habitat, as quickly as they could. Players can elect to have the dichromatic vision (blue and yellow colour channels) of a Yak or the trichromatic (red, blue, green) vision of a human.  Nui et al. found that ‘human’ players located all colour forms of the plants faster than ‘yaks’, however well these matched their background. They also showed that it took ‘humans’ longer to detect both flowers and leaves which matched their background well, whether that was pale limestone scree or darker shales. 

In any case, there seem to be few large herbivores such as yak in this part of Yunnan and little evidence that rodents eat the bulbs.  Bulbs of eight different Fritillaria species are important in TCM, ironically, largely because of the alkaloids they produce as chemical defences to deter herbivores! It seems that visibility/invisibility of the plant to human collectors is highly likely to influence its biological fitness, i.e. its chances of surviving long enough to reproduce successfully.

It does make me wonder how many other plants, over millennia, have evolved to protect  themselves from humans or, conversely, to make use of human activities to bolster their success. Maybe more of this in a future post…

Niu Y., Stevens M. & Sun H. (2021).  Commercial Harvesting Has Driven the Evolution of Camouflage in an Alpine Plant. Current Biology 31, 1–4. https://doi.org/10.1016/j.cub.2020.10.078

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