SPIKY EVOLUTION

You won’t find any growing in the wild on our sites and none are native to the UK, although a few, from cooler climes such us upland Chile and Mexico, are hardy enough to grow outside if protected from the wet, although occasional species are found even in the humid tropics. I am talking about cacti, for which those who know me are aware that I have had a penchant since childhood. Like marmite, people seem to love or hate these plants, but I find them utterly fascinating, and many have stunning flowers. I was interested, therefore in a recent New Scientist article that caught my eye[1].       

The cactus family, Cactaceae, comes in a large and very varied range of shapes and sizes, approaching 2,000 species. The family is reckoned to be over 30 million years old but, because they mostly grow in such xeric habitats, fossils of them are rare and their evolutionary pathways, which occurred surprisingly quickly to result in such a large morphological range, have proved difficult to unravel.

Enter Thompson et alii[2] who used machine learning to unravel more about the evolution of this prickly group of plants. Their work produced preliminary results suggesting thirty-nine factors that might have led to the unusually large diversification of the Cactaceae, three of which appeared to be the main architects of cactus speciation. These three are the difference between diurnal and nocturnal ambient temperature, the sandiness of the soil and plant size.

When temperatures varied by only about 10^oC between night and day, the researchers found more speciation. Cacti conserve water in their thickened tissues but that makes them vulnerable to freezing if nights are too cold. And cacti employ “crassulacean acid metabolism” or CAM photosynthesis which allows their stomata to remain closed during the day and protect the plants from dehydration in hot, arid environments[3]. If it is too hot, though, even cacti can cook, despite their waxy coating and thick shade-producing spines.

Many cacti do best in soils that are somewhat sandy. This favours a shallow web of roots that allows rapid uptake of water from a wide area during rare rain events. Other plants are less competitive in such conditions, but for cacti it seems to favour speciation.

The third speciation factor suggested by the research, plant size, indicated that small cacti and large cactus types displayed the greatest tendency to form new species, rather than medium-sized ones. This seems to be related to genetic factors such as faster mutation rates and generation times, and reduced gene flow in smaller species, and pollination factors in larger species. You will have to read the researchers paper for the details!

Betts Ecology may not have any cacti on our Estates but there are many plants with prickles and spines and others which can withstand drought. See how many you can find.

[1] New Scientist 19 September 2024 by Penny Sarchet (Wild Wild Life Nature Newsletter.

[2] https://www.nature.com/articles/s41467-024-51666-2 Nature Communications, 15, Article 7282.

[3] See https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism for more.