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Nature Notes – Ascension’s Mosses

26 February 2021


Nature notes is a monthly instalment bringing you information and updates about the natural world on Ascension Island. In this month’s notes – the humble moss!

Mosses were some of the very first species to colonise Ascension and begin greening Green Mountain hundreds of thousands of years ago. At first glance, you may think they all look the same and are little more than a green crust on the rocks. In fact, mosses are vital to the survival of our endemic plants, are one of the original food sources for our land crabs, and provide an irreplaceable habitat to countless microscopic creatures. There are sixty different species here, including thirteen found nowhere else on earth.

For plants that survive on such a dry island, mosses are strangely dependent on rain and fog to grow. They do not have complex root systems to get water from the soil like other plants. Instead, the water content of their leaves always balances with the water content of the surrounding air (this is called poikilohydry). So, they generally do best in moist microclimates such as shady crevices or foggy mountaintops. Windy Ridge off Elliot’s Path has just such a habitat and supports one of the island’s most diverse and colourful communities of mosses and related plants. Yet mosses are easily capable of surviving prolonged droughts as well. Look at the bottom of Grazing Valley and you’ll see that the ground in many places is actually a carpet of greyish mosses. These mosses switch off their metabolism when dry, surviving desiccated in a sort of suspended animation called cryobiosis until rainfall awakens them to grow.

The creation of the cloud forest radically changed Ascension’s habitats. However, our mosses have proven remarkably adaptable. Many have colonised the bark and leaves of introduced plants; species which do this are called epiphytes. A few liverworts and mosses naturally grew on the leaves of the endemic ferns, but many species which once lived only on rocks are now abundant in the forest. Bamboos, figs, white olives and ginger support the most diverse epiphyte communities, with eleven species growing on a single leaf in some places. We are now working to keep healthy populations of these beneficial cloud forest plants while controlling the harmful species.

Clumps of mosses growing on bamboos, figs and rock faces are now vital to the critically endangered Moss Fern, which has lost much of its original habitat to invasive species. Further down the mountain, the critically endangered Hedgehog Grass likewise needs healthy carpets of mosses to grow in. Not only do the mosses keep these two unique plants from extinction, but they also seem to be a delicacy for our land crabs. So next time you’re out walking, look a little bit closer at the mosses and see if you can spot some of their easily-missed diversity and colour.