Rocky Areas as "refuge islands" in the Deserts of the Middle East

פורסם: August 29th, 2010 | עודכן: 13/01/15


A common principle in islands research is that in many systematic groups there is a correlation between the number of species present in the island and the size of the island. In the scientific literature three explanations of the correlation between species diversity and island size are commonly accepted:

  1. The ecological approach – suitability of islands as habitats for diverse species. This approach was also named habitat diversity. Hence, the number of species increases with the enlarged number of habitats.
  2. The theory of equilibrium between colonization and extinction MacArthur & Wilson (1976).
  3. The evolutional approach, which considers the equilibrium between colonization and speciation of species from parents already colonized.

“Island biogeography” does not have to deal with land islands in a water ocean; they can be lake islands in an “ocean of land”; mountain tips are islands of high elevation; forest clearing patches are light islands in the “forest shadow ocean.”A colleague was doing theoretical-mathematical research on island colonization using insect species penetrating tree leaves and forming tunnels in their mesophil. He considered each leaf as an island. In the present discussion I shall deal with islands of a relatively moist habitat within the “desert ocean” surrounding them. This habitat enables the existence of mesophytes (considered here as an antonym of xerophytes) which are commonly growing in neighboring or remote mesic areas (Figs. 3.1.1, 3.1.2).

Fig. 3.1.1: Water in a depression on a rock-outcrop in the desert. Soil pockets should be regarded as such depressions filled with soil.

Years ago I met a botanist in California and showed him my distribution map of [Origanum] species in the Middle East (Fig. 3.1.3). He said that the map highly resembled his distribution maps of the genus he studied in Hawaii. I started to consider my studies in the desert as a kind of “Island Biogeography.” Theoreticians often make equations and compile theories on situations in nature and then search for field-proof of their theories. I am not a theoretician and I deal mainly with the study of flora and vegetation of the Middle East. I shall try to present here test cases of a few genera in this area, and consider the smooth-faced rock-outcrops in desert as a parallel to land islands in oceans.

Fig. 3.1.2: A soil pocket in the desert. The marked plants are typical Mediterranean plants, surviving due to efficient run-off from the exposed smooth rock (P=80 mm). 1. Umbilicus intermedius 2. Urospermum picroides.

Fig. 3.1.3: World distribution of Origanum, section Campanulatocalyx.

The habitat

Smooth-faced rock-outcrops are the desert habitat with islands of water regime resembling that of moister areas with higher quantities of rainfall (Fig. 3.1.4 lower right). In highly fissured rocks (Fig. 3.1.4 lower left) the water is distributed in many fissures and each of them receives relatively small amounts of water. One of the most important factors in the formation of continuous rock outcrops is epilithic lichens (Figs. 3.1.5, 3.1.6). When epilithic lichens form a continuous film on the rock surface, they prevent the direct contact of raindrops with the rock surface. Each water drop passes first through the lichen body while losing the potential energy it had when falling on the rock. Since the lichens completely cover the rock surface, relatively homogeneous weathering processes take place which result in the formation of a smooth-faced outcrop, with a very few rock fissures and soil pockets. The distribution of epilithic lichens, as of any other organism, depends on moisture regime.

Fig. 3.1.4: The influence of the most common rock and soil types on the water regime under desert conditions.

Fig. 3.1.5: Urginea maritima in a soil pocket on a smooth-faced rock outcrop covered with epilithic lichens (P=100 mm).

Fig. 3.1.6: A cross-section in limestone covered with an epilithic lichen that protects it from the destructive energy of the rain drops. 1. Epilithic lichen, 2. Intact rock.

Fig. 3.1.7: Rakhama ridge, a hard dolomite layer covered by white epilithic lichens on the north-facing slope and pitted on the brown south-facing slope.

The epilithic lichens enjoy being wetted by rain water, dew and heavy mist. In addition to the distribution of these three sources, slope direction has important significance in our area. The effective solar radiation on the south-facing slopes makes them much drier than the other slopes. The radiation on the north-facing slope is less effective, as shown in Figs. 3.1.7 and 3.1.8. The organisms growing on the south-facing slope lead to the formation of pits at the rock surface (Figs. 3.1.8, 3.1.9). The amount of runoff water from the pitted slope is considerably less than the amount of runoff from the north-facing slope. The rain-water falling on the northern slope leads to the formation of a thin film of water on the smooth-faced rock surface (these are the luminous or glossy areas in Figs. 3.1.10 and 3.1.11). Such glittering takes much longer to develop on the surface of the fissured rock slope. The running water is absorbed and accumulates in the few soil pockets and below the smooth rock outcrops (Figs. 3.1.12, 3.1.13).

Fig. 3.1.8: Hard dolomite rock covered by white epilithic lichens on the north-facing slope (N) and pitted on the brown south-facing slope (S).

Fig. 3.1.9: Pitted rock surface formed by biogenic weathering in the desert.

Fig. 3.1.10: A hard rock outcrop near Mash’abbe Sade, Negev Highlands, during a light shower (ca. 1 mm/hr). The shiny faces are sites where a continuous water film developed on the rock and run-off started.

Fig. 3.1.11: Limestone outcrops and an asphalt road: places where run-off started.

Fig. 3.1.12: An outcrop of chert (SiO2) and a narrow belt of herbaceous vegetation flourishing on the run-off water from the exposed rock. 1. Umbilicus intermedius leaves.

Fig. 3.1.13: An aerial view of a typical hill slope in the Negev Highlands near Sde Boker. 1. Shivta Formation (Turonian) outcrops, 2. Below the layer, 3. A wadi.

Plant species diversity

Plant species diversity in rock outcrops is linked to the size of the area in each of the organization levels with which we are dealing. Let us start with the impact of outcrop size in one mountain – in other words, we shall deal with the plant association level. The larger the rock outcrop, the greater the likelihood of the repeated occurrence of certain micro-habitats. Thus, the number of species that can grow in these micro-habitats increases as well. I compared plant species diversity between the vegetation of the smooth-faced outcrops (lower right in Fig. 3.1.4) and the vegetation developing in the shrub-steppe on the fissured rocks (lower left in Fig. 3.1.4). These were studied in various parts of the Negev Highlands. Plant names and the number of individuals of each species were recorded in stands of 100 square meters. Plant species diversity was calculated according to three equations:

  • 1. d = (S-1)/logN
  • 2. H – Shannon-Wiener Index = -SUM Pi ln Pi
  • 3. eH – Hill 1973 index

N = number of individuals in the record, Ni = number of individuals of the ith species; Pi = Ni/N, S = number of species.

Pairs of habitats in the Negev Highlands, in the 70-100 mm annual rainfall range, were compared in four sites situated along a climatic gradient (Fig. 3.1.14; the sites on the right side are moister than those on the left side). The results are repeated in the four sites; species diversity, estimated in the three forms, is lower on the lithosol than on the smooth-faced rock outcrops.

Fig. 3.1.14: Values of species diversity calculated as d, H, and e^H.

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