The aim of this study was to identify trends in plant diversity, and examin
e their predictive relationships with environmental variables (water stress
, sail texture and grazing intensity) and Vegetation characteristics (total
vegetation and individual species cover), along a 340 km transect through
an area of hyper-arid desert in southern Egypt. The major part of the trans
ect is located within the boundaries of the Wadi Allaqi Man and Biosphere (
MAB) Reserve. In total 55 plant species were recorded from the transect. A
TWINSPAN classification of the data identified four of the seven main veget
ation types, previously identified as present in the area by Springuel et a
l. (1997). Sites dominated by Vegetation type I occurred exclusively in the
lowest stretch of the Wadi Allaqi system, most heavily influenced by flood
and seepage from Lake Nasser, and characterized by dense stands of Tamarix
nilotica (Ehrenb.) Bunge and Glinus lotoides L. Vegetation type IIIb was r
epresented at sites higher up the wadi system, characterized by sparser veg
etation dominated by Acacia ehrenbergiana Hayne, Acacia tortilis subsp. rad
diana (Savi) Brenan and Aerva javanica (Burm. f.) Juss.ex Schult. Vegetatio
n type IIIa occurred in the middle region of Wadi Allaqi and was characteri
zed by Salsola imbricata Forssk. and Cullen plicatum (Delile) C. H. Stirt.
Vegetation type IV, the most species rich, was limited to upstream sites of
Wadis Allaqi and Eiqat. Plant beta-diversity las species turnover between
individual sites along the transect, assessed using Whittaker's beta(w) ind
ex) showed a rising trend of plant species turnover from sites low in the w
adi system to sites located in the upper part of the wadi. Plant alpha-dive
rsity within these vegetation types (as species richness R: number of speci
es present per 500 m(2) unit area) was linearly correlated with plant diver
sity measured using Simpson's index (D). R was reasonably well predicted fr
om three variables: plant cover, soil moisture and grazing intensity (follo
wing a third order polynomial model). This model predicted R in the origina
l data set with 52.9% success ( r = 0.73, p < 0.001). The study suggested t
hat vegetation cover estimates, in association with simple estimates of int
ensity of habitat stress (water availability) and disturbance (grazing inte
nsity), can provide a rapid indication of the biodiversity support capacity
of desert habitats. (C) 2000 Academic Press.