Predictive modelling of Namibian fairy circles and their co-occurrence with Euphorbia species using Geographical Information Systems (GIS)

Abstract

Fairy circles are an example of patterned vegetation and have only been documented to occur along the coastline of southern Africa, although a very similar vegetation pattern has recently been documented in north Western Australia. After several decades of investigation, the cause of fairy circles is still open to debate. The current Namibian-based project studied the “Euphorbia spp. allelopathy” theory which states that fairy circles are the product of a localized allelopathic affect in the soil caused by the previous presence and subsequent death of E. damarana and E. gummifera. In the first part of this study, a GIS-based site suitability analysis for the occurrence of fairy circles was done using rainfall, altitude and land cover as site selection criteria. Thereafter 100 random sample points, each within a 10 km radius, were generated within the predicted site and investigated using Google Earth. It was found that 65 of the 100 locations contained fairy circles including a large distribution of fairy circles located in the south east of Namibia and also in the Kalahari Desert, that had not previously been documented. It was observed from the satellite images that several of the random sites did not have grass cover, and therefore would not have ‘shown’ fairy circles if they were present. From the site suitability analysis several additional sites where fairy circles co-occur with Euphorbia spp. were documented. The second part of the study focused on the landscape scale characteristics of fairy circles, E. damarana and E. gummifera. Aerial and satellite imagery were used to make observations of Euphorbia plants over time, while a number of analytical techniques were used to compare the size and spatial patterning of fairy circles to Euphorbia plants. In terms of results, first, the aerial image comparison showed that E. gummifera is eventually replaced by fairy circles. Second, using a T-test it was found that there is no statistical difference in size between fairy circles and E. damarana. Third, the spatial patterning of fairy circles was examined and compared to the distribution of E. gummifera and E. damarana using a range of geospatial point pattern techniques. Sites where fairy circles co-occur with either E. damarana or E. gummifera were included in the point pattern analysis as mixed sites. The analysis using point pattern analysis showed that fairy circles have the most uniform distribution, compared to the Euphorbia and mixed plots. Interestingly, the regularity of the pattern associated with the mixed plots are in-between that of fairy circles and the Euphorbia plots. E. damarana have also been shown to have a regular spatial pattern, while the regularity of the pattern associated E. gummifera only slightly departs from complete spatial randomness.