Published in enviro.philica.com
Large-scale patterns can arise in the vegetation of homogeneous areas due to biotic interactions of animals and plants. If the satellite imagery has a high enough resolution, such patterns can be easily observed. Here we show some large polka-dot arrangements, probably created by the red harvester ants, in Arizona, near the Grand Canyon.
Patterned Vegetation Created by Ants and Observed in Satellite Images of Arizona
Amelia Carolina Sparavigna
Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy
Abstract: Large-scale patterns can arise in the vegetation of homogeneous areas due to biotic interactions of animals and plants. If the satellite imagery has a high enough resolution, such patterns can be easily observed. Here we show some large “polka-dot” arrangements, probably created by the red harvester ants, in Arizona, near the Grand Canyon.
Keywords: Patterned Vegetation, Ecosystems, Satellite images, Google Earth.
It seems difficult to imagine that, from satellites, it is possible to investigate some aspects of the life of insects. However, it is so. Of course, we are not considering a single small organism, but the colonies that insects can create, such as those of ants and their nests. For instance, the red harvester ants create nests that, with their surrounding area can reach a dimension which is possible to be observed in satellite images having a high enough resolution. Moreover, these nests are not isolated; each one can be surrounded by several other nests, giving rise to a patterned local vegetation, which is visible from above, much better than from the ground. In this article, we will show some patterns, visible in the Google Earth images of an area 10 km long in a valley of Arizona, near the Vulcan’s Throne of Grand Canyon.
In fact, the use of satellites for monitoring ant colonies is not a new idea. In , it was proposed for supporting natural resource managers that were interested in locating red harvester ant mounds. These ants can have a negative impact due to heavy infestations on pastures. However, they are also important for the survival of other animals, such as lizards. In , the researchers evaluated the QuickBird imagery as a tool for detecting the ant mounds. The research gave the result that natural resource managers can use this imagery to determine the severity of infestations. Besides QuickBird imagery, the satellite images of Google Earth have also been used for the "fairy circles" of Namibia [2,3].
As discussed in , large-scale regular spatial patterns can arise within homogeneous landscapes from local biotic interactions alone [5-7], that is, the organisms in nature can scale up to produce quasi-regular patterns across large landscapes. Therefore, due to the recent satellite imagery of high resolution, such patterns have been increasingly reported in the ecological literature . For instance, it is possible to observe, in semi-arid East Africa, the mounds built by Odontotermes termites frequently occurring in uniform “polka-dot” arrangements .
Polka-dots arrangements can be observed in satellite images also for the nests of red harvester ants. The nests are visible for the following reasons. They are characterized by a lack of plant growth and small pebbles surrounding the entrance of them. Hulls of seeds may be found scattered around the nest. As explained in , "in grassland areas, such as ranches, the lack of plant life makes red harvester ant colonies very easy to spot, and where they are very plentiful they may make serious inroads into the grazing available to livestock". The mound corresponding to the nest are typically flat and broad, 300 to 1,200 mm in diameter . Even larger bare areas, on the order of 10 m2, are created. Trails typically lead away from the mound, like "arms". These trails are used by ants to collect and bring food back to the mound.
In , the nest and the related bare areas are defined as nest discs. Alexander Wild, in his web article , is showing some examples of how they appear in images. One image from Google Earth is concerning an area South of Tucson, Arizona (31°38.097’ N, 111°03.797’ W). The area is showing a pattern of evenly-spaced “polka-dots” : the dots are the "nest discs of one of our most conspicuous insects in the Sonoran desert, the red harvester ant Pogonomyrmex barbatus. Down on the ground it is harder to get a sense of the even spacing of the nests, but the discs are plenty obvious. The ants keep the large area around their nest entrance free of vegetation and other unwanted debris" .
Reference 9 is fundamental to understand the patterned vegetation that we can find in another area of Arizona. It is near the Vulcan's Throne, a cinder cone volcano and a prominent landmark on the North Rim of the Grand Canyon. This area is not reported in , probably because in 2008 the satellite images had a low resolution. Today, using Google Earth for the areas marked in the map shown in the Figure 1, we can see a large landscape covered by discs, that look like those shown in Ref.9. For this reason, we are here proposing that these discs are nest discs of ant colonies. The number of these discs, disseminated in a valley 10 km long is impressing.
Figure 1: The markers show a large area, 10 km long, near the Grand Canyon, where it is possible to observe a patterned vegetation, like that given in the following Figure 2.
Figure 2: Patterned vegetation having a “polka-dot” arrangement (Courtesy Google Earth, coordinates: 36°17’16.79’’ N, 113° 05’ 57.66’’ W). A part of the image has contrast and brightness enhanced.
In the Figure 1, the markers are showing a large area, near the Grand Canyon rim, where, using Google Earth, we can see a patterned vegetation like that given in the Figure 2. The patterned vegetation has a “polka-dot” arrangement of discs. In the following Figure 3 and 4, the pattern is given in images with smaller scales.
Figure 3: Patterned vegetation having a “polka-dot” arrangement (Courtesy Google Earth, coordinates: 36°15’19.18’’ N, 113° 04’ 55.38’’ W).
Figure 4: Discs in patterned vegetation (Courtesy Google Earth, coordinates: 36°14’55.07’’ N, 113°05’05.22’’ W).
Of course, question could arise about the cause of such discs. If we compare the discs in the Figure 4, with a red harvester ant nest sketched in Ref.10, it seems that they are in fact the nests of these ants. However, since the dryland ecosystems can exhibit patterns of vegetation created by the competition between individual plants , a local investigation is required. However, in any case, due to the irregularity of the polka-dot pattern, I would like to exclude an unnatural origin.
 Fletcher, R.S., Everitt, J.H., & Drawe, L. (2007). Detecting red harvester ant mounds with panchromatic QuickBird imagery. Journal of Applied Remote Sensing, Vol. 1, 013556.
 Tschinkel, W.R. (2012). The Life Cycle and Life Span of Namibian Fairy Circles, PLOS ONE, DOI: 10.1371/journal.pone.0038056
 In Namibia, some grasslands that develop on deep sandy soils are punctuated by thousands of quasi-circular bare spots, usually surrounded by a ring of taller grass. As told in , the causes of these so-called “fairy circles” are unknown, although a number of hypotheses have been proposed.
 Pringle, R.M., Doak, D.F., Brody, A.K., Jocqué, R., & Palmer, T.M. (2010). Spatial pattern enhances ecosystem functioning in an African Savanna. PLOS Biology, DOI: 10.1371/journal.pbio.1000377
 Hassell M.P., Comins H.N., & May R.M. (1991). Spatial structure and chaos in insect population dynamics. Nature 353: 255–258.
 Gueron S., & Levin S.A. (1993). Self-organization of front patterns in large wildebeest herds. J. Theor. Biol. 165: 541–552.
 Comins H.N., Hassell M.P., & May R.M. (1992) The spatial dynamics of host parasitoid systems. J. Anim. Ecol. 61: 735–748.
 Vv. Aa, (2016). Red harvester ant, Wikipedia. Retrieved on 6 January 2016 at https://en.wikipedia.org/wiki/Red_harvester_ant
 Wild, A.L. (2008). Ants from a Kilometer Up. Web page created on 20 January 2008, retrieved on 6 January 2016, at http://www.myrmecos.net/2008/01/20/ants-from-a-kilometer-up/
 Davis, J.M. (unknown year). Management of the Red Harvester Ant Pogonomyrmex barbatus. Retrieved on 6 January 2016, at https://tpwd.texas.gov/huntwild/wild/wildlife_diversity/texas_nature_trackers/horned_lizard/documents/harvester_ant_management.pdf
 Penny, G.G., Daniels, K.E., & Thompson, S.E. (2013). Local properties of patterned vegetation: quantifying endogenous and exogenous effects. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 371(2004), 20120359.
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Sparavigna, A. (2016). Patterned Vegetation Created by Ants and Observed in Satellite Images of Arizona. PHILICA.COM Article number 555.
1 added 6th January, 2016 at 15:28:08
On Fairy Circles, Science had published the paper by Norbert Juergens, entitled “
The Biological Underpinnings of Namib Desert Fairy Circles”
The sand termite Psammotermes allocerus generates local ecosystems, so-called fairy circles, through removal of short-lived vegetation that appears after rain, leaving circular barren patches. Because of rapid percolation and lack of evapotranspiration, water is retained within the circles. This process results in the formation of rings of perennial vegetation that facilitate termite survival and locally increase biodiversity. This termite-generated ecosystem persists through prolonged droughts lasting many decades.
2 added 28th January, 2016 at 10:04:45
On January 26, 2016, I have received a very important e-mail from Dr. Robert Nagell, DVM, San Diego. He was so kind to tell me that the past weekend he visited the area shown in the article; this is the Toroweap Valley of Arizona. He can confirm that the spots are ant mounds. In the pictures he sent me, it is visible the cone of the nest and the surrounding area cleared of vegetation. As he observed, due to the cold, the ants were not active.
3 added 20th March, 2016 at 07:39:24
Recently it had been reported that the “fairy circles pattern” exists in the remote outback of Australia. Using fieldwork, remote sensing, and spatial pattern and mathematical modeling, the researchers proposed a pattern-formation theory for the Australian environment.
Getzin, S., Yizhaq, H., Bell, B., Erickson, T.E., Postle, A.C., Katra, I., Tzuk, O., Zelnik, Y.R., Wiegand, K., Wiegand, T., and Meron, E. (2016). Discovery of fairy circles in Australia supports self-organization theory, PNAS 2016; published ahead of print March 14, 2016, doi:10.1073/pnas.1522130113