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Are fog-driven ecosystems being driven out?

By Joh Henschel, SAEON Arid Lands Node
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How will climate change affect fog along the west coast? How will it affect the fog-dependent ecosystems of Namaqualand and the rest of the Namib Desert?

The extraordinary biodiversity of this area, both in terms of numbers of endemics and in terms of unexpectedly high species numbers for such an arid region, could disappear along with the fog, leaving the land almost barren. Can this be?

West coast fog comes off the cold Benguela Current, which generates low-lying stratocumulus clouds that intercept objects at ground level on the adjacent land, where it deposits water droplets. Rain falls so rarely that the regularly occurring fog provides most of the moisture, most of the time for most of the creatures.


Fog rolls into the Namib Sand Sea, keeping this UNESCO world heritage alive (photo: Joh Henschel)

Fog and fauna

The connection between fog and fauna has fascinated scientists for over a century, giving rise to numerous studies in the Namib Desert that are reviewed in a recent publication*. This publication is the culmination of many years of cooperation between the University of the Witwatersrand, Gobabeb Namib Research Institute, South African Environmental Observation Network and the Namibian University of Science and Technology.

The reader gets a grounding concerning the Namib fog-belt, on how organisms acquire water from fog and other sources, such as food, vapour or metabolism, and on the physiological challenges of harvesting fog water, especially osmoregulation – sudden rehydration when quickly consuming as much water as possible while available requires suitable checks and balances.

The paper ends with an outlook to the future, whether and how fog may change with global change, but also what we can learn from the Namib’s organisms with such huge scope of coping with extremes of dehydration and rehydration, including concern if even these extremophiles were to be pressed beyond their limits due to global change.

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A fog-basking beetle drinks at an awkward time when it is too cold for it to be active (photo: Joh Henschel)

This discoid beetle bulldozes through the sand to enhance fog harvesting (photo: Joh Henschel)

At least 48 animal species consume fog water, and quite a number of plants, too, although relatively few studies have examined this with plants, or, for that matter, with animals. Iconic are two species of fog-basking beetles and several other beetles that construct fog-collecting trenches from which they harvest water. Achieving this entails specialised fog-collecting behaviours, enabling these beetles to drink up to a third of their body weight of water during one fog incident, which was first highlighted 44 years ago in the journals Nature and Science.

Many other species get fog from droplets off their own bodies or vegetation, and spiders collect it from soaking wet silk on trapdoors — even mammals as large as elephant and giraffe benefit by ingesting fog-wetted vegetation. Fog-consuming species manage to sustain active adult populations during long intervals between rainfall events that boost their populations.

The general pattern is one of brief periods of boom, followed by long periods of not quite going bust, and it is fog that is largely responsible for keeping things “not quite” bust and thus keeping the desert alive.

Is there any indication that the fog is changing?

The long-term patterns of fog are complex, as is the underlying climatology in detail. Changes in fog could be due to changes in wind patterns and ocean temperatures or increasing elevation of warmer stratocumulus clouds reducing their interception of land.

At the Gobabeb Namib Research Institute, rainfall patterns have changed during the past 30 years. Astonishingly, the mean annual precipitation doubled across this period due to the occurrence of more extreme events, but the number of rainfall events decreased by almost a third. During long intervals between rainfalls, the occurrence of fog provides critical relief, but thirsty creatures will be left wanting if intervals between fog events were also to lengthen.

Fog is only one of several moisture sources, also including dew that some animals can tap. Some small desert creatures even manage to get water from atmospheric vapour.

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Deposited fog droplets coalesce into drops that water the desert (photo: Joh Henschel)

Dew occurs less frequently than fog but also represents a useful source of water (photo: Joh Henschel)

With fog, dew and vapour, arid lands are living deserts despite sparse rainfall. Global warming may affect all these moisture sources and could cause the most reliable water sources along the west coast to disappear.

Detecting this requires long-term research on land and ocean. SAEON can play a significant role in the case study on the west coast of southern Africa. This would benefit from international collaboration between South Africa, Namibia and Angola, as well as participation of the global scientific community. It entails observations of fog in relation to climate as well as fog-driven ecosystems and their constituents. These are special not only because of their uniqueness along this 2 000-km strip of land, but even more so because this area constitutes the very essence of a living desert sustaining nature and people.

* Mitchell, D., J. R. Henschel, R. S. Hetem, T. D. Wassenaar, W. M. Strauss, S. A. Hanrahan, and M. K. Seely. 2020. Fog and fauna of the Namib Desert: past and future. Ecosphere 11(1): e02996. 10.1002/ecs2.2996 

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