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cause plant circles in African deserts
These fairy rings of perennial grass species dot arid, sandy sweeps from Angola to South Africa and have inspired ecological and mythological speculation about their origins. After 40 trips to study the water distribution and life around the fairy rings, Norbert Jrgens of the University of Hamburg in Germany concludes that the sand termite Psammotermes allocerus is the hidden force behind them.
Among the hundreds of species that thrive in these rings, the sand termite is the only one found throughout the range, he reports in the March 29 Science.
Termites unintentionally engineer these marvels by eating the roots of grasses, creating a bald patch that becomes the rings center. The subsurface depths of that patch stay moister than neighboring areas, where plants draw the water out of the soil. The circles bulls eye favors not only the moistureloving termites, but also a belt around its edge of perennial grasses and many other species that couldnt survive baked sand.
As ecosystem engineers, Jrgens says, the sand termite rivals the beaver.
Am I correct in my interpretation that the rings of perrenial grasses are there because they can draw on the moisture not only of the soils directly below themselves, but also on the adjacent soils within the rings. This would indicate that the grasses can be present in otherwise inhospitable conditions because of the lack of competition for the water within the circles. They can draw from a larger area than would be possible were the vegetation more densely packed. In the latter case, a different species would crowd them out.
The termites should be eating their roots too. The ability of the grasses to survive where other plants dont suggests either the grasses have some mode of discouraging termite foraging some biodefense, better ability to repair termite damage, or alternately that the moisture in rootfree soils is being drawn laterally through the soils to the roots by the capillary suctions so the plants in the ring dont have to have roots extend into the ring.
One other thought concerns the termite food supply. Perhaps the continuing production of roots by the surrounding grass ring provides a sustainable condition. The termites dont overgraze during most moisture conditions so the grasses can survive the grazing losses while still getting enough water to produce enough biomass to keep the termites from expanding the rings and destroying the grasses. Probably during droughts the rings would expand, but during wetter periods perhaps the grasses would actually encroach on the bare areas. This would be an interesting study. I also wonder how far the plants could draw moisture laterally from the soils beyond the extent of their roots. An interesting question for soil physics. I dont know the answer, but am curious.
I also wonder what limits the termite population so they dont overeat their food supply? As the ring diameter increases, geometry will have certain effects. The amount of biomass in the ring will increase linearly with the radius by the simple formula relating perimeter to radius. The travel distance from the center of the circle also increases, increasing the energy cost to the termites to travel to their food supply. Also, as the ring diameter increases, the curvature of the grassy ring decreases, becoming progressively more like a straight line. If the distance of effective capillary draw from within the ring is significant and is less than the radius of the circle a guess, I dont know, then the area of the wedge of soil from which adjacent grasses draw will increase as the curvature decreases, adding a bit more water to the adjacent grass plants as the ring grows. The significance of this geometric factor depends on the distance of capillary draw and radius of the circle and will vary as the ring grows.
All of this speculation is building fairy castles in the air as this is outside my field of expertise. I know little about soil physics and capillary movements in soils and plant root action and next to nothing about termite ecology, its energy balances, and other factors that control termite population. This is not how I would proceed were I actually studying this as a scientist, but is fun as a dilletante exercise.
In case anyone is wondering, I am only making dilletante comments on this because I dont subscribe to Science so I cant read the article itself to learn more about the underlying study and also because I cant afford the time to pursue all the things I find interesting or I really would become just a dilletante. Unfortunately, there is only one of me and I can readily generate more work than ten of me could do in the first iteration and then exponentially more than that as the multiple mes pursues the ongoing expansion of the things that interest me. As I have leisure to broaden my interests, I will still have to continually truncate what I wish to pursue to keep things manageable. The world has multiple wonders that keep a curious mind interested, but one cant pursue everything and still achieve any real understanding of anything.