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The effects of vegetationcover on the structure of birdcommunities in a hyperariddesertFares Khoury a , Mohammad Al-Shamlih a , HatimSultan b & Younis Abu-Ghalyun aa Department of Biological Sciences , HashemiteUniversity , P.O. Box 150459 , Zarqa 13115 , Jordanb Hatim Sultan, Jordanian Society for SustainableDevelopment , Amman , 11181 , JordanPublished online: 28 Feb 2013.

To cite this article: Fares Khoury , Mohammad Al-Shamlih , Hatim Sultan & YounisAbu-Ghalyun (2007) The effects of vegetation cover on the structure of birdcommunities in a hyperarid desert, Zoology in the Middle East, 40:1, 11-20, DOI:10.1080/09397140.2007.10638199

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The effects of vegetation cover on the structure of bird communities in a hyperarid desert

by Fares Khoury, Mohammad Al-Shamlih, Hatim Sultan and

Younis Abu-Ghalyun

Abstract. The hyperarid deserts of Wadi Araba, SW Jordan, include a variety of habitats ranging from barren alluvial fans and sand dunes to arid acacia savannahs and salt marshes. The diversity of resident birds was highest in acacia stands growing on alluvial fans and stone plains. Although open sand dune habitats have a distinct bird community contributing to the overall diversity, the local diversity within these habitats is relatively small. In habitats with both rocky and sandy sub-strates, species richness, avian abundance and the diversity of feeding guilds increased locally with the density of shrubs and/or trees. Additionally, avian diversity was related to vertical struc-ture and linked to the mean height of shrubs and trees. Ephemeral annual cover had no general ef-fect on the diversity of the native bird community. Shrub and tree cover appear to be reliable habi-tat cues used by many bird species when choosing their breeding habitats, as the presence of shrubs and trees increases the diversity of food resources and of secure nesting sites.

Kurzfassung. Die hyperaride Wüste des Wadi Araba, SW-Jordanien, beinhaltet eine Vielfalt an natürlichen Habitaten. Akazienbestände auf felsigen und steinigen Böden wiesen die höchste Vo-geldiversität auf. Sanddünen beinhalteten ihre eigenen Vogelgesellschaften, obwohl die lokale Diversität relative niedrig war. Sowohl in felsigen als auch in sandigen Gebieten stieg die Anzahl der Arten, die Abundanzen aller Arten und der Gilden mit zunehmender Strauch- und Baumdich-te. Eine Korrelation wurde auch zwischen mittlerer Baum- bzw. Strauchhöhe, die auf vertikale Strukturierung hindeutet, und Vogeldiversität nachgewiesen. Die ephemere, annuelle Vegetation hatte keinen Einfluss auf die Diversität der Vogelgemeinschaft. Die Dichte and Struktur der pe-rennierenden Vegetation werden als wichtige Faktoren bei der Habitatwahl vieler Vogelarten vermutet.

Key words. Bird communities, diversity, hyperarid desert, Wadi Araba, Jordan.

Introduction Bird community structure can be influenced by a variety of factors at different spatial scales (WIENS & ROTTENBERRY 1981). Recent studies in the arid deserts of Saudi Arabia have reported spatial and temporal variations in the abundance and diversity of resident and mi-gratory birds (NEWTON & NEWTON 1997, VAN HEEZIK & SEDDON 1999). These reports have indicated that, at a local scale, structural complexity of vegetation might be the principal factor affecting bird diversity in arid areas. Further studies in North American deserts have shown that avian diversity usually rises along a scale of increasing habitat complexity, due to an increase of the variety of available nest sites and food niches (TOMOFF 1974).

The structure of resident/breeding bird communities in the natural desert landscapes of Jordan, as well as other parts of the world, has generally received little attention. Most stud-ies in the deserts of Asia and North Africa have focused on the distribution of bird species Zoology in the Middle East 40, 2007: 11–20. ISSN 0939-7140 © Kasparek Verlag, Heidelberg

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rather than on quantitative analysis yielding information on community structure and eco-logical relationships (NEWTON & NEWTON 1997). The south-western desert landscapes of Jordan, including the hyperarid Wadi Araba, demonstrate a variety of habitat types ranging from sand dunes and alluvial fans to salt marshes and acacia pseudo-savannahs, each with its distinct composition of bird species (AL-SHAMLIH et al. 2005). This spatial variation in de-sert habitats, and local variations in vegetation cover, allowed a comparative study of the community structure of breeding and resident birds in a hyperarid desert ecosystem with limited human impact.

The aim of this study was to assess bird diversity in Wadi Araba, and determine the main environmental factors, other than human impact, that affect community structure in deserts. We focus in this paper on the impact of naturally occurring perennial vegetation on the di-versity and total abundance of resident and breeding bird species in desert habitats. Previous studies included the study of the distribution and habitat associations of a few common spe-cies (AL-SHAMLIH et al. 2005) and the impact of intensive agriculture on bird communities in sand dune habitats in the same general area of Wadi Araba (KHOURY & Al-SHAMLIH 2006).

Material and methods Study area Wadi Araba, the northern extension of the Great Rift Valley, is a low-lying, flat strip of land between the mountain range of southwest Jordan and the Negev plateau of southern Israel. It stretches between the Gulf of Aqaba in the south and the Dead Sea in the north, with a maximum width of 30 km (Fig. 1). The study included the southern and central parts of the Jordanian side of Wadi Araba (approx. 30°00’N, 35°10’E), 50% of which consists of sand dune desert, while the remaining areas consist of stone plains, rocky foothills and alluvial fans. According to the domi-nant flora, Wadi Araba can be biogeographically classified as Saharo-Arabian (BAIERLE 1993). However, the geographical location of Wadi Araba and its favourable environment for several floral and faunal elements of Afro-tropical origin (e.g. Acacia tortilis) has produced a Sudanian penetration zone, which is evident in the study area by the presence of Acacia stands, mainly on alluvial fans (BAIERLE 1993). The climate is generally hot and hyper-arid; the mean annual pre-cipitation is approximately 50 mm, while the annual mean evaporation rate lies around 2000 mm (NATIONAL ATLAS OF JORDAN 1984). Local disparities in soil and hydrological conditions cause variations in vegetation type and cover (BAIERLE 1993), which allowed us to measure the impact of vegetation cover on bird numbers and diversity by studying different sites varying in vegeta-tion cover, all other habitat variables being constant or insignificantly different. The sand dune habitats considered in this study vary from highly mobile sand dunes almost devoid of vegetation, to more stable and flat sand dunes, where the surface is occasionally covered with a thin layer of fine gravel. The vegetation usually consists of a sparse shrub cover dominated by Haloxylon persicum and patches of annual cover following the rainy season. A few shallow waterbeds trav-erse the sites studied, which are lined with relatively dense vegetation dominated by H. persicum, and occasionally include the shrubs Calligonum comosum and Retama raetam. The boundaries between sand dunes and salt marshes were also included in this study of habitat type, as the ground was also covered by sand, the vegetation being relatively dense and dominated by H. persicum and Nitraria retusa and including scattered Tamarix sp. shrubs. Rocky habitats, mainly alluvial fans, are generally characterised by rocky and stony ground surface varying in vegetation cover from sites nearly devoid of shrubs to others containing a variable cover of shrubs and aca-cia trees.

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Fig. 1. Map of the study area, comprising the southern and central parts of the Jordanian side of Wadi Araba. As well as the nomadic Bedouins there are a few villages, but most of the area is unpopulated and designated a closed military zone due to its proximity to the political borders. Nevertheless, grazing by goats and camels of local Bedouins is permitted everywhere and is widespread. Natu-ral herbivores such as Gazella dorcas occur in small numbers near the borders to Israel. The establishment of large projects of intensive, irrigated farming is in progress in several parts of the study area. These exploit ground water for irrigation and include fenced areas lined with intro-duced trees as windbreaks. These sites and their surroundings were omitted from this study to preclude the impact of agricultural developments on diversity. This impact has been studied recently in sand dune deserts (KHOURY & AL-SHAMLIH 2006). Field methods Quantitative data on birds was collected using the line transect method (BIBBY et al. 2000) in the period from March to May 2004, which covered the major part of the breeding season. Thirty-seven line transects were randomly selected in sand dunes, 24 in rocky acacia habitat, 22 in open rocky habitat and 9 along the edges of the salt marsh. The number of transects in each sample corresponded approximately to the area covered by each habitat type in the southern part of Wadi

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Araba. All transects were at least one km distant from farming projects to preclude the impact of agriculture. Each line transect was 1 km in length, with a belt of 100 m on either side (see KHOURY & AL-SHAMLIH 2006 for further details).

We excluded from the analysis all bird species that do not breed in the study area such as mi-grants, and birds resident and breeding in different habitat types in neighbouring areas that occa-sionally used the study area for feeding, as well as birds detected beyond 100 m and those just crossing the transects in flight. Habitat variables were quantified at three points (at both ends and in the middle) along each line transect to a range of 50 m before being averaged. This allowed a general description and comparison of the habitat types. Data analysis Analysis conducted at the community level included the construction of a non-metric multi-dimensional scaling (MDS) ordination using the software PRIMER v5 (CLARKE & GORLEY 2001), to demonstrate the association of the different bird species according to their log-transformed abundances and Bray-Curtis similarities (KHOURY & AL-SHAMLIH 2006). MDS is very good for accurately representing complex relations in low-dimensional space, without the need to delete any rare species. The MDS selects a configuration of points, which minimises the degree of stress (distortion between the similarity ranking and the corresponding distance rank-ings in the ordination plot). A stress value of around 0.1 gives a good ordination with little chance of a misleading interpretation (CLARKE & WARWICK 2001). Diversity magnitudes were calculated for all samples (transects) in each habitat type and included species richness (S = number of species), Shannon-Wiener index (H), in addition to the number of feeding guilds

s H = �Pi ln Pi i=1

Birds were classified into feeding guilds (insectivore-foliage, insectivore-ground, mainly granivore, frugivore, etc...) according to diet data provided in SNOW & PERRINS (1998).

The different magnitudes of avian diversity and abundance in four different habitat types, which included barren rocky habitats, acacia stands in rocky habitats, the edges of salt marshes and sand dune habitats, were tested for significant variations using the non-parametric Kurskal-Wallis test, to measure their departure from the norm. We then pooled these habitats into two groups: rocky habitats (open rocky + acacia) and sandy habitat (sand dunes + salt marsh edges), as the individual habitats pooled in each group differed signifi-cantly only in vegetation cover and type. A Spearman correlation analysis was then carried out in each group to link bird community measures to variables related to vegetation.

Results Bird assemblages and diversity measures

The 92 line transects recorded 22 species of breeding birds, some of which were present in more than one sample (habitat type). Two main clusters emerged in the MDS ordination. The first (left in Fig. 2) included species which occurred only or mainly in sand dune habitats, while the second comprised of species inhabiting habitats with a rocky or stony surface including open Acacia tortillis stands.

All species recorded in open rocky habitats, such as Desert Lark Ammomanes deserti and Trumpeter Finch Rhodopechys githaginea, were also included in the acacia bird community. This produced a loose cluster in the two-dimensional MDS ordination (Fig. 2) with no clear

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Tab. 1. Diversity magnitudes, number of individuals of all species (avian abundance), and num-ber of guilds in each habitat type. The mean � standard deviations are given. * p < 0.05, ** p < 0.01.

Salt marsh Acacia Rocky Sand dunes N 9 24 22 37 Shannon diversity 0.48�0.17 0.53�0.25 0.43�0.13 0.40�0.20 Species richness 4.33�1.5 5.30�2.6 ** 3.50�1.1 3.22�1.7 No. guilds 2.89�0.78** 3.13�1.45** 1.73�0.98 1.65�0.75 No. individuals 10.33�4.2 19.88�5.4 ** 8.73�5.4 6.16�4.5

Tab. 2. Dominant (first row) and other frequent perennial plant species occurring in the four habitat types.

Salt marsh borders Acacia Open rocky Sand dunes

Nitraria ritusa Acacia tortilis Anabasis articulata Haloxylum persicum Tamarix sp. Haloxylum persicum

Acacia raddiana Anabasis articulata Lycium shawii Nitraria retusa Zygophyllum dumosum

Zygophyllum dumosum Retama raetam Calligonum cumosum

Tab. 3: Spearman Rank correlation coefficients of bird community measures versus vegetation variables in Wadi Araba. Only significant coefficients (p < 0.01) are shown. Total = all samples pooled. H’(B) = Shannon diversity of birds. S(B) = Species richness of birds. I(B) = Total num-ber of individuals of birds; G(B) = number of feeding guilds; CS = Total perennial cover (dwarf shrubs + shrubs + trees); S(V) = plant species richness (dwarf shrubs + shrubs + trees); Mh = mean height of shrubs and trees; An = Annual/ground cover.

Total (n = 92) Rocky habitat (n = 46) Sandy habitat (n = 46) H'(B) S(B) I(B) G(B) H'(B) S(B) I(B) G(B) H'(B) S(B) I(B) G(B)

CS 0.48 0.67 0.79 0.59 - 0.47 0.61 0.51 0.77 0.93 0.90 0.72 S(V) - - 0.37 0.46 - - - - - - - 0.48 Mh - 0.40 0.55 0.49 - 0.43 0.52 0.60 - - - - An - - - - - - - - - - - -

differentiation between open rocky habitats and acacia stands. The Arabian Warbler Sylvia leucomelaena was considered the most characteristic species of acacia stands, due to its association with relatively dense stands of acacia trees and shrubs (AL-SHAMLIH et al. 2005). It occurred in acacia stands together with Little Green Bee-eater Merops orientalis, Yellow-vented Bulbul Pycnonotus xanthopygos, Palestine Sunbird Nectarinia osea, Blackstart Cer-comela melanura and locally with the White-crowned Black Wheatear Oenanthe leucopyga, in addition to a few other species such as Desert Lark, which comprised the bird community in rocky habitats without acacia trees or stands.

A few species were found to be typically characteristic of the sand dune habitats, as they were virtually absent in other habitat types These included Hoopoe Lark Alaemon alaudipes

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Fig. 2. Non-metric MDS configuration of the birds found in the habitats of the natural landscape. Spotted Sandgrouse, Pterocles senegallus: SSG; Lichtenstein’s Sandgrouse, P. lichtensteinii: LSG; Collared Dove, Streptopelia decaocto: CD; Laughing Dove, S. senegalensis: LD; Crested Lark, Galerida cristata: CL; Desert Lark, Ammomanes deserti: DL; Bar-tailed Lark, A. cincturus: BTL; Thick-billed Lark, Ramphocoris clotbey: TBL; Temminck’s Lark, Eremophila bilopha: THL; Hoopoe Lark, Alaemon alaudipes: HL; Yellow-vented Bulbul, Pycnonotus xanthopygus: YVB; Little Green Bee-eater, Merops orientalis: LGB; Desert Wheatear, Oenanthe deserti: DW; White Crowned Black Wheatear, Oenanthe leucopyga: WCW; Blackstart, Cercomela melanura: BS; Arabian Warbler, Sylvia leucomelaena: AW; Scrub Warbler, Scotocerca inquieta: SW; Great Grey Shrike, Lanius excubitor: GGS; Palestine Sunbird, Nectarinia osea: PS; Arabian Babbler, Turdoides squamiceps: AB; House Sparrow, Passer domesticus: HS; Trumpeter Finch, Bucanetes githagineus: TF; Desert Finch, Rhodospiza obsolete: DF. and Desert Wheatear, Oenanthe deserti which co-occurred locally with the Bar-tailed Lark Ammomanes cincturus and Spotted Sandgrouse Pterocles senegallus; and other rare species such as Temminck’s Lark Eremophila bilopha. A number of species occurred in several habitat types, and included the Collared Dove Streptopelia decaocto and Little Green-Bee-eater, both of which, however, were most frequent and abundant in Acacia habitat.

The most frequent species along the edges of the salt marsh were Crested Lark Galerida cristata, which also occurred in a few sand dune transects, as well as Great Grey Shrike Lanius excubitor and Scrub Warbler Scotocerca inquieta (Fig. 2), which were also present in the acacia habitat type, though at lower abundance. The Great Grey Shrike was also recorded in the few transects carried out in rocky habitats bordering sand dune habitats. These three species formed a small cluster overlapping with the two main species’ clusters.

When comparing the four different habitat types, the Shannon Index was highest in acacia habitats, but the difference was only marginally significant (Tab. 1; Kurskal-Wallis test, p = 0.06). Species richness and avian abundance were significantly higher in the acacia habitat

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Salt marsh Acacia Rocky Sand dunes

Habitat type

Perc

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TreesShrubsDwarf shrubs Herbaceous

Fig. 3. Mean vegetation cover and structural/physiognomic composition in the four natural habitat types (sand dunes: n = 37; Acacia: n = 24; rocky: n = 22; salt marsh: n = 9). compared with all other habitat types (Kurskal-Wallis test, p� 0.001, for both measures). The number of feeding guilds was higher in both the acacia habitat and along the edges of the salt marsh, both characterized by higher shrub and tree density, compared with sand dune and open rocky habitats (Kurskal-Wallis test, p� 0.001).

Relationship between vegetation cover and avian diversity and abundances Different variables related to vegetation cover were correlated to magnitudes of diversity and avian abundance using Spearman Rank correlations. The relationship was tested on all line transects in natural habitats. Two groups of transects were defined by pooling two habitat types, each of similar ground surface structure, but which differed in total vegetation cover and the proportions of the different physiognomic components (Fig. 3), as well as in the species composition of perennial plants (Tab. 2). The first group included therefore the tran-sects carried out in the sand dunes and along the edges of the salt marsh, while the second group included all transects carried out in open rocky habitats and acacia stands, which es-sentially grew on rocky ground, mainly on alluvial fans and at wadi mouths.

A strong correlation was found between total cover and various magnitudes of diversity; in particular species richness in sandy habitats, which in this analysis included the edges of the salt marsh. Avian abundance also increased significantly with shrub cover. In the same group, the number of guilds rose with increasing shrub cover and number of shrub species. Although significant, the relationship between plant taxonomic composition diversity (S(V) in Tab. 3) and the number of bird guilds was relatively low.

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Despite their significance (Tab. 3), the correlation coefficients within rocky habitats in-cluding acacia stands were relatively low. The relationship between perennial cover on one hand and species richness, avian abundance and number of guilds on the other was found to be small. The mean height of trees and shrubs in these transects, related to vertical stratifica-tion of the habitat, apparently had some influence on the number of guilds and, to a lesser extent, on species richness and avian abundance.

Discussion The MDS ordination of bird species, based on Bray-Curtis similarities, confirmed the

presence of different bird communities in relation to habitat variation in Wadi Araba. The ordination produced two main clusters of bird species: the first included species mainly confined to sand dune habitats, and the second species occurring principally in rocky and acacia habitats. The bird community in open rocky areas was accepted to be a subset of the bird community of acacia stands, which grow on the same rocky substrate. The increase in the richness of avian species in relation to shrub and tree cover and mean height was attrib-uted in the rocky habitats to species additions, rather than replacements. New species and even entire guilds (e.g. foliage gleaners: Arabian Warbler) joined the community with an increase in mean height of the vegetation (related to vertical structure) and shrub and tree density. In a previous study, in which species were analysed separately using multiple re-gression analysis, we were able to show that some bird species occur only in rocky habitats with a relatively dense tree and shrub cover (e.g. Arabian Warbler, see AL-SHAMLIH et al. 2005). Moreover birds capable of inhabiting open rocky habitats almost devoid of vegetation are more common in patches with shrubs and trees due to higher productivity and possibly improved nest concealment (e.g. Desert Lark, AL-SHAMLIH et al. 2005).

Similar patterns were observed in the sand dune habitats: a few species, such as the Little Green Bee-eater, joined the sand dune community at sites with a relative dense cover of Haloxylum shrubs. However, a few ground-dwelling species were replaced mainly by scrub species when approaching the edge of the salt marsh. This was attributed to a marked in-crease in shrub cover and possibly also to the replacement of Haloxylum by Nitraria as the dominant shrub. Some of the native sand dune birds also disappeared in proximity (<1 km) to farmland (KHOURY & AL-SHAMLIH 2006), a factor not considered in this paper. Because farms are lined with trees as windbreaks, this indicates that species native to the sand dune desert (e.g. Hoopoe Lark and Desert Wheatear) prefer rather open and flat desert habitats lacking dense shrub and tree cover or other vertical structures, as these may hinder daily movements and vision, which are important for efficient foraging, display and predator avoidance (KHOURY & Al-SHAMLIH 2006).

It can be generally assumed that in arid and sparsely vegetated deserts, vegetation cover and physiognomy affect vertical stratification, as well as horizontal irregularity and produc-tivity levels, and are thus interrelated with avian diversity and abundance. GUTZWILLER & BARROW (2002), TOMOFF (1974) and NEWTON & NEWTON (1997), have previously sug-gested a strong relationship between vegetation complexity and bird diversity in arid envi-ronments.

Bird diversity was found to increase along a scale of increasing vegetation complexity in shrub steppe habitats, primarily through the addition of new bird species or even entire guilds (WIENS & ROTENBERRY 1980). Replacement of one species by another with increas-

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ing vegetation cover is expected to occur only when shrub and tree cover develops to a level at which certain ground-dwelling species adapted to open areas, or shrub-dwelling species adapted to mainly open stands of trees and shrubs, can no longer exist. Extensive stands of shrubs and trees with coverage exceeding 30% occur only locally in arid deserts, e.g. in oases or as riparian vegetation along rivers, both of which usually have different bird assem-blages to the open surroundings (KHOURY 2001).

Although significant, the relationship between the diversity of plant taxonomic composi-tion and number of bird guilds was relatively low; which indicates that plant taxonomic diversity in arid environments is one of several factors affecting bird diversity. ROTENBERRY (1985) and TOMOFF (1974) previously suggested that floristic variation might have a pro-found effect on bird assemblages, as food resources are expected to increase with the taxo-nomic diversity of plants. This was indicated in this study by the increase in the number of feeding guilds with the rise in the number of perennial plant species within the sandy habi-tats, including the edges of salt marsh, where new plant taxa (e.g. Nitraria retusa) emerged.

The sparse and patchy occurrence of annuals was evident during a short period in spring in most transects, although annuals occurred most plentifully on flat sand plains and along waterbeds. The annual vegetation appeared to have some effect on the local distribution and abundance of ground-dwelling species such as Hoopoe Lark and Desert Wheatear in sand dune habitats (AL-SHAMLIH et al. 2005). Annual cover was not however found to be a major factor affecting bird diversity and general avian abundance in the natural habitats of Wadi Araba. Ephemeral annuals of arid deserts occur irregularly, both spatially and temporarily. The annual variation in rainfall makes annual vegetation coverage an unpredictable factor in terms of productivity and food availability (NEWTON & NEWTON 1997). On the other hand, shrub and tree height and cover appear to be reliable cues for many avian species, as they relate to the presence and diversity of food resources and secure nesting sites.

Acknowledgements. We would like to thank Khaled NASSAR and Rami AWAD from the Jordanian Society for Sustainable Development for their assistance during fieldwork.

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