By Dr Jennifer Lalley, Oxford University, University of Witwatersrand
Protecting Namibia’s deserts has been an ongoing conservation drive for several decades, but average visitors, the mining community and even residents often lack understanding of the boldly lettered warnings telling us not to drive off the road. While many take this seriously, others shrug it off and barrel off the road to find ‘idyllic’ campsites or even unload quad-bikes to venture further across pristine landscapes, leaving webs of tyre marks behind them. What few people understand is that a track across any desert surface will take time to recover, while a track across some of the Namib Desert’s gravel plains can take over 500 years to recover, if at all.
The fragility of the Namib’s gravel plains is due largely to the widespread growth of ground-dwelling lichens that discreetly hold many of this desert’s gravel plains together and form a critical base to the desert ecosystem. The colourful display of lichens that we admire on rock faces can also grow on soil surfaces, but often go unnoticed in this niche. In fact, lichens grow in most terrestrial habitats in the world and on many different substrates. In the harsher climates of the world, such as the Namib Desert, lichens thrive where higher plants struggle, making the survival of lichens even more critical to these ecosystems.
Little was known of the Namib Desert’s lichen communities until the late 1970s, yet the roles they play in the ecosystem continue to be poorly understood and therefore undervalued. Looking at the vast fields of lichens growing near Cape Cross, where the species Teloschistes capensis grows in thick shrub-like mats, it is easy to imagine that a world might lie beneath. But the more inconspicuous communities have been carelessly walked and driven over even by the most careful visitors. When fog filters inland, these little-noticed lichens come to life in bright hues of orange and green and begin to photosynthesise before drying out again and lying dormant until their next exposure to moisture.
Many of the roles played by ground-dwelling lichens are linked to the soil crusts they form. Lichens contribute to soil-crust formation (also referred to as cryptogamic crusts, microphytic crusts and biological soil crusts), entangling the gravelled surface along with cyanobacteria and green algae, and forming a protective layer that resists most wind and water erosion. By stabilising the surface, habitat is created for burrowing insects, beetles, spiders, reptiles and small mammals.
A recent study investigated arthropod populations in several different lichen-dominated areas and found that distinct assemblages of insects, beetles and spiders occur in each unique lichen area. Knowing the high rate of endemism in the Namib Desert’s arthropod populations, these lichen-rich habitats may be crucial to the existence of many species and the species feeding on them, and so on. In the late 1970s, Dr Dirk Wessels investigated this possibility and tested the feeding preference of different species of Tenebrionid beetle. He found that one species fed selectively on one species of lichen and nothing else.
In addition to providing habitat and a food source for many of the Namib’s unique species, lichen-dominated soil crusts also contribute to carbon and nitrogen cycling. Throughout the world, lichens contribute substantially to the storage of global carbon emissions – a role that has become increasingly important as evidence of global warming accrues. In deserts, the cycling of carbon and soil nutrients are slow pro-cesses and every little bit counts. It is no surprise that grass readily sprouts across lichen-rich areas of the Namib in years of exceptional rainfall. Lichens are known to facilitate the growth of higher plants in many ecosystems of the world through the cycled nutrients they provide and through the soil stabilisation required for the germination of many plant species.
Damage to Namib Desert lichens and their recovery
In the ecologically important ground niche, lichens are prone to long-term structural damage from human activities such as off-road recreational driving and mining. A study of seven damaged sites in the central and northern Namib demonstrated the extent of damage caused by these activities and estimated recovery times. An important finding was that a lichen community’s vulnerability to disturbance and ability to recover varies considerably according to soil structure, gravel cover, lichen community type and, of course, the magnitude of the impact.
For example, a well-established contiguous covering of lichen on silt and gypsum soil is highly sensitive to disturbance. An assessment of an old mining road in such an area was estimated to recover in only 500 plus years, based on a comparison of lichen cover in the disturbed area with adjacent undisturbed areas. Conversely, a single track across loose sand sediments where lichens grow in patches can recover within a decade. The lichen species occurring in this habitat are easily wind dispersed, and sand sediments do not compact as much as silt. Unfortunately most of the Namib’s gravel plains have some component of silt. Subsequently, soil compaction from vehicles is common and leads to the destruction of lichen-soil crusts and unsightly long-lasting scars across desert landscapes.
The recovery of lichens in track ruts is difficult for several reasons:
• lichens are incredibly slow growing and propagation is even slower for some species;
• compacted soil in vehicle tracks can make the reattachment of lichens difficult;
• track ruts can channel water during rainfall, thus dislodging lichens that have managed to establish; and
• if any lichen cover remains in a track or, alternatively, if wind-dispersed species of lichen have collected in the track, they will experience a significantly lower rate of production (photosynthesis, nutrient cycling, growth, et cetera) due to variations in fog deposition and temperature inside tracks.
What are the implications of losing lichen cover for such long periods? Soil erosion, lower inputs of nitrogen and less cycling of carbon, loss of habitat to animals, loss of a food source to animals, are all proven consequences of damage to soil-crust lichen communities. The destruction of any basal organism is a complicated story, given the dependence of the ecosystem on the processes occurring at this level. Hence a great deal more work is needed before we can fully understand the knock-on effects of lichen removal within the gravel plains of the Namib Desert.
So drivers, kindly take note of Namibia’s pleas to stay on the road. Will anyone want to visit a desert that resembles a motocross rally? Will tourism continue to grow and support the economy if Namibia’s pristine landscapes are shrinking? More importantly, the natural heritage of Namibia lies in its unique species and diverse habitats. Let’s not forget those we cannot always see…
The information reported here emanates from a study carried out by the Namibian Ministry of Environment and Tourism and Biodiversity Taskforce of Namibia and the University of Oxford. Contributions and support came from: Wilderness Safaris’ Wilderness Trust, the Royal Geographical Society, Explorer’s Club of New York, Africa Conservation Science Centres, Sindisa Foundation, Desert Research Foundation of Namibia and Gobabeb Training and Research Centre.
This article appeared in the 2007/8 edition of Conservation and the Environment in Namibia.