II. Study Site
i. Location
The study site, Visnara (Visitor’s nara) is situated in the Namib Desert, Namibia, approx. 1km south of the Gobabeb Training and Research Centre (GTRC), formerly called DERU (Desert Ecological Research Unit of Namibia at Gobabeb), within the Namib-Naukluft National Park. Gobabeb (15º03’E, 23º33’S) is located on the banks of an ephemeral river, the Kuiseb River, approx. 60 km inland from the Atlantic Coast at 407 m above sea level (Map II-1). The linear oasis of the Kuiseb marks the border between the sand dunes (“sand sea”) of the Southern Namib, and the gravel plains of the Central Namib.
Map II-1: Map of the Namib-Naukluft Park and location of the Gobabeb Training & Research Centre (GTRC, marked by arrow). Map from Seely (1987)
Adaptations of Rhabdomys pumilio II. Study Site
The Namib forms a narrow strip (~ 200 km wide) between the coast and the inland plateau (“Hochland”), with a more or less distinct escarpment. It extends for more than 2000 km, from the Olifants River (South Africa) in the south, to the Carumjamba River (Angola) in the north. The eastern boundary (“Pronamib”) is about 1000 m above sea level, at the 100 mm rain isocline (Seely 1987). At the latitude of Gobabeb, the escarpment has a very gentle slope (1000 metres over a distance of 100 kilometres).
ii. Climate
The Namib Desert is a cool coastal desert, like the Atacama Desert in Chile and the Californian Desert in the USA (Louw & Seely 1982). A cool sea current, the Benguela current, and resulting advective fog influence its climate. Fog precipitation is highest at a distance 35 – 60 km inland from the coast at an altitude of 300 – 600 m above sea level. This zone along the coast where the most fog events occur is also the zone with the most substantial climate extremes, i.e. with high differences in humidity and temperature throughout the day (Seely 1987).
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
relative humidity %
0 10 20 30 40 50 60 70 80
fog days
0 5 10 15
relative humidity fog days
Figure II-1: Average number of fog days per month and average monthly relative humidity at Gobabeb between 01/02/1996 and 31/01/2000. Data collected at the first order weather station Gobabeb, data not available for the period between 20/06/1997 and 07/08/1997. Means ± SD
Over a period of four years (between 1996 and 2000), fog events were recorded every month, amounting to an average of 45.3 fog days per year (Figure II-1). This is 25% more than the 37.23 days per year reported earlier by Lancaster et al. (1984), based on data over a period of 15 years. The mean annual fog precipitation of this
Adaptations of Rhabdomys pumilio II. Study Site
period is 30.79 mm. Fog events are recorded more often in the second half of the year (August – December), but fog precipitation per day is highest during the winter months, in June and July (Lancaster et al. 1984). Fog events are rare in April, May and June, which is the hottest and driest season in the Namib Desert. During that time, the wind direction changes to northeast, and hot dry air flows down the escarpment, leading to the so-called “Eastwind” conditions. Corresponding with the occurrence of fog, relative humidity is highest during the second half of the year, and lowest during the “East Wind Season” from April to June (Figure II-1).
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
temperature (ºC)
0 5 10 15 20 25 30 35 40
rain (mm)
0 5 10 15 20 25 30
temperature rain
Figure II-2: Average monthly rainfall and mean monthly temperature at Gobabeb between 01/02/1996 and 31/01/2000. Data collected at the first order weather station Gobabeb, data not available for the period between 20/06/1997 and 07/08/1997. Means ± SD.
Figure II-2
As in most desert areas, rainfall in the Namib Desert is episodic and highly localised (Besler 1972, in litt.), and amount of precipitation is highly variable between years. As the Central Namib is situated at the southwestern edge of the summer rainfall zone, most rain falls between January and April (Schulze, in litt.). Rain may occur at any month, however, and mostly as short and heavy showers, but also as light rain of longer duration (Lancaster et al. (1984); C. Krug pers. obs.). Between February 1996 and January 2000, rain precipitation is only recorded in nine of 48 months observed, and within those months, on 22 days in total ( ). Total rainfall between February 1996 and January 2000 amounts to 100.2 mm, with an average of 25 mm per year, ranging from 2.6 mm (in 1998) to 78.2 mm (in 1997) per year. The average rainfall per month in this period ranges from 0mm to 8.1mm. In comparison,
Adaptations of Rhabdomys pumilio II. Study Site
mean annual rainfall at Gobabeb over a period of 18 years is 27.20 mm, with a range from 0-50 mm (Lancaster et al. 1984). On the whole, the Namib Desert is a hyper-arid region, receiving most of its moisture from fog events, and many of the plant and animal species living in the Namib Desert show unique adaptations to this phenomenon (Louw & Seely 1982; Costa 1995; Degen 1997).
Due to the location at the coast of Namibia, and the proximity to the cold Benguela current, temperatures in the Namib are not as extreme as in inland deserts at approximately the same latitude (e.g. Kalahari). In summer, mean daily temperatures rarely rise above 25°C at Gobabeb ( ), with the mean daily maximal temperatures seldom reaching more than 35°C (Lancaster et al. 1984). The highest maximum daily temperature ever recorded at Gobabeb is 42.6°C, in March 1998 (C.
Krug, pers. obs.). In winter, the mean daily temperature is approx. 17°C, ranging between 10°C and 25°C. Minimum temperatures rarely fall below freezing point (Lancaster et al. 1984). Mean daily soil temperatures recorded at various depths (5 cm, 10 cm, 20 cm, 30 cm, 60 cm and 120 cm) also show seasonal variation. Unlike the pattern shown by ambient temperature, highest temperatures in the soil at various depths occur during the summer months (November - January), when solar radiation is most intensive.
Figure II-2
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
soil temperature (°C)
15 20 25 30 35 40
5 cm 10 cm 20 cm 30 cm 60 cm 120 cm
Figure II-3: Average monthly soil temperatures at 5 cm, 10 cm, 20 cm, 30 cm, 60 cm and 120 cm soil depth recorded at Gobabeb between 01/02/1996 and 31/01/2000. Data collected at the first order weather station Gobabeb, data not available for the period between 20/06/1997 and 07/08/1997. Means ± SD.
Adaptations of Rhabdomys pumilio II. Study Site
Soil temperatures in the upper strata of the soil (5 cm, 10 cm and 20 cm) are lowest during June and July, while temperatures in the lower strata (60 cm and 120 cm) are lowest in July, August and September ( ). Mean daily soil temperature just below the surface ranges from 20ºC in the winter months to 36ºC in summer. At a soil depth of 120 cm, mean daily soil temperature ranges between 27ºC in winter and 31ºC in the summer months. At 5 cm depth, sand temperature can vary as much as 20ºC daily in summer and 10ºC in winter but at 30 cm and below, there is little seasonal or diurnal fluctuation (Figure II-4). Burrowing or fossorial animals can therefore escape the temperature extremes at the soil surface rather easily (Louw & Seely 1982; Du Plessis et al. 1992; Costa 1995).
Figure II-3
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
soil temperature difference (ºC)
-5 0 5 10 15 20 25 30
5 cm 10 cm 20 cm 30 cm 60 cm 120 cm
Figure II-4: Average diurnal fluctuations of soil temperatures per month at 5 cm, 10 cm, 20 cm, 30 cm, 60 cm and 120 cm soil depth recorded at Gobabeb between 01/02/1996 and 31/01/2000.
Data collected at the first order weather station, data not available for the period between 20/06/1997 and 07/08/1997. Means ± SD.
iii. Vegetation
The vegetation at the study site is typical for the dune valleys of the Southern Namib, but is also influenced by its proximity to the dry riverbed and the vegetation therein. The characteristic plant at the study site is Acanthosicyos horridus, the nara, a member of the Cucurbitaceae, and an endemic of the Namib sand dunes. The species grows in the dune valleys were there is access to underground water (Sandelowsky 1990). The nara is well adapted to the arid climate of the Namib Desert. Its leaves are
Adaptations of Rhabdomys pumilio II. Study Site
reduced to small stiff scales (Sandelowsky 1990), and the tendrils are reduced to thorns or spikes (Meeuse 1962). Photosynthesis takes place in the branches and thorns (Sandelowsky 1990). The species grows two types of roots: long, woody taproots that reach the ground water table (Meeuse 1962; Kutschera et al. 1997), and subsurface roots that take up the moisture from both fog and rain precipitation (Kutschera et al.
1997). The nara also has an endomycorrhizal system that enhances nitrogen fixation in nutrient limited ecosystems, thus leading to a C/N ratio in the plant tissue that is higher than usual in desert environments (Klopatek & Stock 1994).
The plant is dioecious, both sexes bearing fleshy, pale green flowers. Flowering usually starts in August and September, and ending in late autumn, around May. Male plants produce a large number of flowers that appear about 4 weeks earlier than the female flowers (Herre 1975). Female plants produce few flowers with an inferior ovary that are pollinated by two species of solitary bees (C. Meier, pers. com.). The large, spiny fruits set from December onwards and mature between February and April (Sarafis 1999). The indigenous Topnaar people have harvested this so-called
“nara melon” for many centuries (Herre 1975; Arnold et al. 1985; Sandelowsky 1990;
Moritz 1992; Dentlinger 1997), and utilize both the pulp and the seeds. It is also believed that the root has medicinal properties (Moritz 1992).
Nara plants form dense, tangled bushes and as sand is trapped underneath the branches, a hummock reaching considerable size is built up. These hummocks can reach sizes up to 4m in height and cover an area of up to 1000 m2, forming micro-ecosystems within the dunes of the Namib Desert. The species is regarded as a keystone species (Klopatek & Stock 1994). Nara fruit, seeds, growing tips (shoots) and flowers are highly nutritious, and provide food for a number of species, ranging from beetles to gemsbok and ostrich. In addition, the dense spiky branches offer shelter for many smaller animals, among them R. pumilio. Other small species encountered in the Nara hummocks are short-tailed gerbils (Desmodillus auricularis) as well as black-tailed tree rats (Thallomys nigricauda), which use nara plants located close to camelthorn trees. Another small dweller of the Namib, the pygmy hairy-footed gerbil, Gerbillurus paeba, was inhabiting the open sandy areas between the nara clumps, but was never found in the nara hummocks.
Other plants found at the study site are clumps of the grasses Eragrostis spinosa and Stipagrostis sabulicola, which grow in the sandy areas between the nara hummocks, and, as the study site was located in close proximity to the Kuiseb river,
Adaptations of Rhabdomys pumilio II. Study Site
Acacia erioloba (Camelthorn) and Salvadora persica (mustard bush or tooth brush tree). Map II-2 gives an overview over the study site. Ground cover underneath the camelthorn trees is rather low, and as domestic livestock use the seedpods of the acacias regularly, disturbances underneath and around the trees are rather high. The vegetation in the riverbed consists mainly of Acacia erioloba, Falherba albicans (Anaboom), Salvadora persica and few annual and perennial shrubs and grasses.
Ground cover in the riverbed is rather low, and food sources are limited. Floods can change the course of the river considerably (C. Krug, pers. obs.).
V-2
V-8
V-1
V-3 V-4
V-7
V-9
V-10
V-5 V-6
Kuiseb River
Riverbed Vegetation Camelthorn Mustardbush
riverbed
riverbed vegetation camelthorn
nara hummocks excluded nara hummocks included mustard bush
Track
20 0 20 40 60 80 100 Meters
S N
E W
Gobabeb
Map II-2: Overview over the study site with the location of nara hummocks, mustard bushes and camelthorn trees. Map created in ArcView GIS version 3.0a. Nara hummocks included: nara hummocks included in the study, nara hummocks excluded: nara hummocks that were not included in the study.