What Causes Namibian Fairy Circles? Some experimental tests.

in 2008 we spent three weeks at the nami brunt nature reserve in namibia setting up experiments for testing hypotheses of the causes of fairy circles fairy circles are gaps in otherwise continuous grassland this is a satellite image with a wheel track for scale the causes of these circles have stimulated a great deal of scientific and public interest with the profusion of suggestions from many quarters the namibran nature reserve is a six hour drive from windhoek the capital of namibia most of the route is on graded dirt roads such as this we rented a wonderful farmhouse our field site was seven miles away on sand tracks leading to a prominent peak the jacob the track passes by one fairy circle after another the deep red wind-blown sands of the namib desert formed this plain and inundate inundate some of the lower hills the grassland is composed of two dominant species of bushman grass a short one forming the matrix and a tall one around the perimeter of the fairy circles the desert is green only for short periods after rare summer rains that drift in from the indian ocean and drop their last moisture over this region the region is home to several large grazers zebra oryx antelope ostrich springbok and hart the beast seen in the early morning light the perimeter of taller grass is clearly visible as is the bareness of the circles and the grassiness of the matrix the desert quickly turns green after substantial rains but the circles remain bare note the wheel tracks for scale whoever what we've done here is to set up a series of experiments uh there are four of them cited at the same place and they test several different hypotheses about what forms these fairy circles that we're standing in the first question we tested is is the soil and fairy circles inhibitory to grass germination and growth the soil transfer experiment tests whether the soil of fairy circles inhibits the growth of grasses by moving soil between natural and artificial fairy circles that's 10 okay it's 9 10. so in this experiment we are moving soil from natural circles to these artificial ones these artificial ones those are six meters diameter the bigger and we're moving soil from the artificial circles to natural circles and from natural to natural as a control and from artificial to artificial as the other control so two controls two treatments if the soil in the fairy circles is inhibitory to germination of plants and growth of plants then moving matrix soil through the circle should result in reduced growth that we're standing in right now is one in which we've transferred soil from fairy circles to artificial circles that we've made by pulling up the grass and so that if there if there's something inhibitory in the very circle soil then it should reduce growth in the artificial circles and if we transfer artificial circle soil to fairy circles and the fairy circles are inhibitory then that should also reduce growth and then we also do controls where we move it from like to light so now elton is going to explain the same thing more or less in normal language we monitored this experiment for over five years all replicates were the same so i'm only presenting one of them here this experiment failed to support an inhibitory effect of fairy circle soil circles remained bare and matrix circles filled in as you see in this image here former matrix circles are shown as ellipses both fairy circles remain bare the second question was is the soil and fairy circles depleted in micronutrients thereby failing to support plant growth we're testing the hypothesis that these very circles are depleted in trace elements so it's a very simple experiment all we've done is to make a mixture of the major trace elements and we've watered one set of plots with the trace elements and the other one we just watered with plain water so if micronutrients are depleted then the water micronutrient treated clots should grow grass now there's a little wrinkle here because the um the berry circles are very depleted in grass scenes the wind blows them away they don't stay so what we did was to collect grass seeds from the matrix around here and sprinkle the grass seed on the plot before we water it with either the control or the treatment simplest outcome would be that these things burst into grass and everything grows when you add micronutrients and then everything else is obvious we monitored the micronutrient treatments for over five years all replicates were alike so i'm showing only four of the twelve replicates even after abundant rains the circles receiving micronutrients did not fill in we conclude that fairy circles do not form as a result of micronutrient depletion the third question we tested was does a toxic vapor emanating from below prevent plant growth in fairy circles so in this particular experiment there is a theory that the fairy circles are caused by some kind of a vapor or toxic material coming up from below ground and killing the grass so what we've done here is we've dug a hole marked by the four stones and we've lined it with an impervious tarp so that the vapors can't get through of course that also restricts the movement of water down but we'll sort that out later so if there's a vapors are coming up from below and killing the plants then the in this exp in this particular experiment the plants should grow because we've blocked the vapors we also did a control in which we just dug up the soil [Music] as in other experiments we monitored the two replicas of the barrier experiment for over five years barriers are on the right controls on the left five years later no plants had grown over the vapor barrier whose outline is shown by the rectangle the toxic vapor hypothesis was thus not supported question four was given the regular spacing of fairy circles is there any kind of interaction among them that might be detectable in plant growth and that would be related to circle distance and size if a fairy circle influences plant growth in its neighborhood we should see differences among artificial circles of various sizes and distances that's two meters from this one and that is a two meter circle that's six meters from this one and over there here that is a two meter circle that's two meters from this one and finally this is a uh four meter circle that's six meters from the center so now if if there is an interaction among neighbors then the vegetation that grows in these different sizes and distances should be different it's kind of a long shot experiment i'm not sure i expect much from it but it's something that had to be done so the treatments are are simple we just simply pulled up all the grasses on these artificial surfaces and then the monitoring will be well there are two dependent measures that we're looking for germination in general and the species that germinate and secondly whether the grass around the perimeter is taller than the grass in this surface so and that could be because different species do well there you know the tall bushman grass or because there's more water or maybe the two are the same thing yeah so that's the one experiment we monitored these experiments for over five years as well all replicates showed the same outcome so only one is presented here size and distance of the artificial circles from the real circles had no effect all artificial circles filled in after the abundant rains of 2011. here the original artificial circles are shown as ellipses none of them became real circles or show differences in vegetation altogether these experiments have failed to support several popular hypotheses circle soil is not inhibitory no toxic vapors emanate from below and micronutrient depletion does not play a role in formation in addition natural fairy circles have little local influence on grass growth in artificial circles nearby you

Channel: Walter Tschinkel--- ants and much more

Views: 692

Keywords: desert ecology, Tschinkel, fairy circles, Namib Desert, self-organization, Turing patterns, Namib Rand Nature Reserve, Nama language, ecology experiments

Id: HZBnHITB4Og

Length: 14min 40sec (880 seconds)

Published: Fri Oct 15 2021