Salvadora and the Strange Oasis
Written and photographed by Oliver Halsey
We drove northwards, surrounded by a vastness of water and sand, the Atlantic Ocean to the west, the Namib Desert to the east. After a couple of hours, rocky outcrops, previously hazy in the distance, came into focus; the Brandberg and surrounding mountains rose sharply from the plains. The sky was filled with low-lying cloud and a cold wind blew fog inland from the ocean. The sun had begun its descent several hours prior and the stark, blackened mountains stood ominously in the dimming light. Eventually we turned off the main coastal road and trundled down a dusty, corrugated track heading towards the mountains. The track veered around the dry ephemeral Messum River channel, bordered by precipitous rock shelves, sculpted by millennia of erosion. The track, enclosed by the walls of crumbling rock, eventually opened out into a desolate expanse. We had entered the caldera of the Messum Crater, an extinct supervolcano some 18 kilometres in diameter; last active some 130 million years ago. Dotted around the surrounding desolation stood thousands of Welwitschia plants, barely visible in the dim evening light. The ringing sound of silence filled my ears. This foreboding landscape exuded a primeval air that instilled pure wonderment.
In early 2017 I had the privilege of travelling to and camping in the Messum Crater with several knowledgeable and inspirational scientists. The Messum Crater, a harsh environment, unforgiving and remote, is one of the less visited, yet accessible parts, of the Namib Desert. Its eponym is Captain William Messum whom travelled across this land from the ocean at Cape Cross circa 1850. The Messum Crater has numerous archaeological sites, rock paintings and many Welwitschia plants, living fossils that date back some 200 million years. Being here was the closest I have come to travelling through time to some distant prehistoric era. Despite my veneration for this bygone landscape, the result of this excursion shifted my focus from the past, to a novel observation.
At our campsite, on the second morning of our stay in the Messum Crater, J Scott Turner, a professor of biology from State University of New York College of Environmental Science and Forestry (USA), pointed out to us several small mounds of soil, several centimetres high. “These are soil dumps created by termites”, explained Professor Turner. Harvester termites excavate vast subterranean nests with tunnels that lead to the surface; excess soil gets dumped above ground resulting in these miniature mounds. However, these soil dumps were damp. The termites had dug their nest deep enough to reach underground water. This may seem surprising in a place as arid as the Namib, however we were in the vicinity of the Messum River, which despite being dry at the time, indicated the presence of underground water from past flow. As we continued on into the Messum Crater and beyond, I put Professor Turner’s observation to the back of my mind, however it would soon surface again.
At the time of our Messum Crater visit I was based at the Gobabeb Research and Training Centre, a remote scientific station a few hundred kilometres south of Messum in the Namib-Naukluft Park. Gobabeb is located in the central Namib Desert alongside the Kuiseb River. The Kuiseb, much like the Messum River and most other Namibian rivers, is ephemeral and dry the majority of the time. The Kuiseb rises in the Khomas Hochland region, near the capital of Windhoek, and runs in a southwesterly direction into the heart of the Namib Desert. Gobabeb rarely sees water flood this dusty channel, only when enough rain falls hundreds of kilometres away in the interior catchment area does the river flow this far. In the sun-soaked central Namib, the Kuiseb can go from flooded to dry in a matter of days, with much of the water draining under the sandy surface and resting in an aquifer. This is why we see mass vegetation along the dry Kuiseb River channel in the middle of the desert; roots extend deep under the surface to extract the water.
There is a plant found along parts of the Kuiseb that intrigued me from first encounter. I remember asking if it was an invasive species (a plant not native to the area, but introduced, which can have a detrimental effect on native wildlife and/or cause economic damage). It wound around the native trees, seemingly choking them with its grasp of intertwining branches and bushy leaves. Its green leaves were relatively large, unusual for desert-adapted plants, which usually have small leaves or none at all; however, for plants with roots capable of reaching the underground water, this was an oasis.
The plant was indeed native to this area but is also found in many other African countries, parts of Saudi Arabia and India. The plant in question was Salvadora persica, which goes by many common names such as, miswak and arak in Arabia, or the mustard bush or toothbrush tree; its branches can be used as a natural toothbrush. Much research has been conducted on the chemical properties of the plant, with results showing that it really is beneficial for oral health and dental hygiene. The dental benefits of the plant have been known for generations and are acclaimed in a poem written by Suwayd ibn Abi Kahil al-Yashkuri in the 17th century A.D.
"Free born is she: she shows when she smiles a row of white teeth regularly spaced
Like the rays of the sun breaking forth from midst of a cloud
She has polished it with a green sappy twig of arak
Sweet of savour, so that it is perfectly lustrous"
Walking down the sandy Kuiseb riverbed, I would often hear rustling coming from within the dense, tangled Salvadora bushes. I wondered what creatures could be lurking within. African wildcats and brown hyenas have been spotted along the Kuiseb River in the central Namib and may shelter in the larger bushes; jackals and foxes probably use Salvadora for shelter during the day too. Snakes, geckos, spiders and scorpions almost certainly reside within too, not to mention countless insects. I even accidently frightened off a small antelope one day who was in a large Salvadora bush seeking respite from the sun. The unseen water was supporting plant life in one of the driest places on earth, which in turn was aiding countless creatures. I began to think of Salvadora from the perspective of a small animal, a sprawling biological metropolis literally buzzing with life.
So how does all of this relate to the damp termite soil dumps from the Messum Crater?
I decided to develop this idea of a ‘biological metropolis’, mixing art with science into a short film. As I spent more time in and around Salvadora, I began to notice that the harvester termite, Hodotermes mossambicus, was more likely to be seen foraging detritus from Salvadora than anywhere else in the vicinity. Harvester termites, as their name suggests, harvest organic matter, in this case small pieces of Salvadora twigs and leaves. The workers drag the twigs, which are often far larger than their own bodies, back to their nests using their powerful jaws and then store the plant matter underground for use as food. Colonies can be gigantic. I once observed thousands foraging under a Salvadora bush; the sheer quantity of the tiny insects produced an audible rustling.
One day in the Kuiseb I noticed the same damp termite soil dumps that Professor Turner had noticed in the Messum Crater. This time hundreds of tiny flies, barely visible, were crowded at the top of the damp mounds. They were drinking. I began to notice this frequently as I spent more time around Salvadora, not just with flies but other insects too. Dune ants, Camponotus detritus, and the beetle, Zophosis moralesi, would become transfixed on the damp soil, thirstily guzzling all possible moisture. The termites were providing a way for other insects to access the underground water, otherwise unobtainable. As long as the termites continued to expand their underground nests, fresh damp soil would often appear above ground and other insects would exploit this. These were termite-constructed oases.
Insects in the central Namib quench their thirst from rain that fell hundreds of kilometres away, owed to the digging termites. Combinations of seemingly unrelated events and organisms have come together to allow access to moisture in a hyper-arid desert environment. My observation of this interaction, not previously documented in the Namib, stemmed from the seemingly trivial patches of damp soil in the Messum Crater. Considering how numerous harvester termites along the Kuiseb River are, this poses the question of how valuable these termite-constructed oases are for insects, and confirms that desert organisms are willing to go to any convenient source of moisture to exploit it. No matter how seemingly inconsequential, the scientific value of observation is crucial to advancing our knowledge and understanding of the world around us.
If we are to put ourselves into the figurative shoes of the wandering beetle, these mounds of damp soil are no longer unremarkable. They are instead a welcome, refreshing respite. What initially may appear insignificant is often revealed to be astonishing upon closer inspection. Wonders are often found within the subtleties of life; all we have to do is look.
Oases for Insects Frontiers EcoPics published note
All images unless stated otherwise are copyright © Oliver Halsey.