Capturing the Diversity – Invasion of the Mexican Thorn Trees

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The Thorn trees dominatiing

Of all the species that have spread themselves over the island, the most invasive is the Mexican Thorn Tree.  Much has been done to try and stop this plant spreading , especially by the Royal Botanic Gardens at Kew in London.  But watch out!  There is suggestion that Kew Gardens gave some advice to the BBC in the 1950’s to plant Mexican Thorn as an ornamental bush in the BBC’s own village at Two Boats, to stabilise slopes and pretty up the harsh environment.  It spread, and spread and spread.  Some estimates originally gave it as covering over 2/3 of the island, and people who travelled round would say it was “everywhere”.  My own exploration of Ascension showed it to be by no means everywhere, it was not up Green Mountain, it was not round the south eastern side of the island.  For Edsel’s and my project I had obtained a couple of satellite images of Ascension, and given the size that these spiky bushes could grow to, I could clearly see many of them on the images.  I made a decision at home one night to put the argument to rest.  Scouring over the images square by square, I started to digitize the crowns of the trees.  Instead of drawing every little nook and cranny I used circles to represent the largest extent of the canopy; after all, unimpeded they tended to grow evenly out from a central trunk.  Of course I could not capture every single plant, some were too small to be identified on a satellite image, even one of high resolution.  Other places plants were growing too close together to be identified individually, but eventually I made over 31,500 circles on my map.  What a job!  But I love digitising when it is nice and easy  – like some people can do knitting for hours.  I tried to work out where the densest patches were, and then started looking at the factors that might be affecting the presence.  It appeared they favoured the looser lava flows, gravels of scoria cones and the like, over the more consolidated rocks of the south east of the island, where anyway there was much more competition from existing vegetation.  There also seemed to be a trend that they were gradually spreading down water catchments.  Now that is a very difficult thing to discern on Ascension Island.  Being essentially a big load of volcanoes, the geology is so loose and open that water usually get soaked up inside the rock very quickly.  Only in a few places to watercourses form, and they only rarely get filled.  But I suppose there have been several serious rainfall events, causing temporary flash flooding.  This could have flushed seeds and even small plants down the water courses.  Certainly those valleys which come out from Two Boats where the first trees were planted appear to have the highest concentrations of these trees.  Some people have blamed the donkeys for spreading them, but I think water is a much more likely solution.

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The thorn trees take over

The trees could be said to have prettified much of the landscape of the western part of the island, and indeed when the fresh green growth comes out they stand aesthetically pleasing against the red rocks.  But they are so prevalent now and continue to grow.  In the few years of trips I have been on, they have crept up the side of the Two Sisters Hills, which looked beautiful when they were pure red screes of rock.

They have grown over old relics of the past like the storage tanks between Two Boats and Georgetown.  They are reaching out on to the beaches where the turtles lay their eggs, their extensive roots frustrating females from digging deep down.  And no one knows where they will stop. They also have provided food and cover for some of the more invasive animal species, especially the rat.

Nobody has the resources to eradicate this tree; it is an alien that has to be lived with, but some control in key sites to stop it harming other species is being carried out.  There is a moth whose caterpillars can devour thorn trees, but consideration needs to be given to work out if its introduction  could damage other species more than the control of the thorn.

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