Abstract

1. Introduction
Coastal zones of the arid tropics are defined as semi-continuous coastal areas where arid land exists in proximity to mangrove and coral reef areas of the tropics/ sub-tropics. They stretch from the East and Northeast African, Red Sea, Arabian and Gulf countries to the India sub-continent, along seashore edges of the Afro-Eurasian continents. In these zones, mangrove ecosystems with Avicennia marina as the dominant species and coral reef ecosystems of fringing reefs interact with each other (Nawata, 2005a) (Fig.1). From the viewpoint of plant and animal net production and biomass, the coastal zones of the arid tropics are characterized as zones where the most unproductive land ecosystem and the most productive sea ecosystem come into contact with each other (Nawata, 2013) (Fig. 2). Mangrove ecosystems in the coastal zones of the arid tropics can be an important source of energy for sur- rounding terrestrial ecosystems. Mangrove productivity (litter-fall) is as much as ten times higher than in the surrounding desert zones (approximately 1 ton d.w. ha -1 year 1 ) (Whittaker & Likens, 1975; Myers, 1984), and part of this high productivity can flow from the man- groves into the adjacent terrestrial ecosystem through livestock feeding (Flores-Verdugo et al. , 1993). In most situations, as more energy and materials flow through detritus food webs than through grazer food webs (Schleyer, 1986; Mann, 1988), the use of mangrove foli- age for livestock feeding is an interesting phenomenon in terms of material circulation between land and sea. However, not all livestock species can feed on mangrove foliage. Plants contain variable amounts of secondary metabolites as chemical defenses against browsing and grazing of herbivores. In general, man- groves are designed to resist herbivores. This is reflected in their high tannin content and leathery texture, which may even involve extensive development of idioblastic sclereids (notably Rhizophora, Sonneratia and Aegialitis) (Tomlinson, 1986). It is reported that herbivores may reject tannin-rich plants because they cause internal ma- laise, or the tannins may inhibit the digestion of protein and fiber (Lindroth, 1989; Provenza et al. , 1991; Gihad & El Shaer, 1994). A researcher explored the possibilities of processing mangrove leaves of Rhizophora mangle for cattle feed, as he had observed that wild deer browse in mangrove swamps in Florida, U.S. At first the cattle did not eat the mangrove leaf meal, so some farmers tried mixing it with other feeds. Unfortunately, it ended with the termination of the trials (Morton, 1965). On the other hand, it has been reported that the leaves of Sonneratia caseolaris are used in the Lamu area as camel fodder (Dale, 1938). Such cases show that it is necessary to clarify factors affecting the livestock palatability of a given plant species. In assessing the ratio of soluble tannins to soluble non-tannins in African mangrove barks, those of Rhizophora mucronata , Bruguiera gymnorhiza and Sonneriatia alba are very high (3.5, 3.6 and 1.2, respec- tively), but those of two species of Verbenaceae, Avicennia marina and Avicennia germinans are compar- atively low (0.3 and 0.7, respectively) (Chapman, 1976). Probably therefore, Avicennia officinalis is actively used for livestock species: goats and cattle in Tanzania, and dromedaries and cattle in Kenya (Dale, 1938; Grant, 1938). Paying attention to Avicennia marina , it is confirmed that dromedaries, goats, sheep and cattle feed its foliage in Iran, Qatar, U.A.E., Oman, Yemen, Saudi Arabia, Sudan and other areas (Kogo & Takatsuki, 1980; Takatsuki, 1980; Mohamed, 1984; Field, 1995; Miyamoto & Al- Wetaid, 1996; Hogarth, 1999) (Fig. 3). However, only N: normal form (clipping free), P: pruned form, CP: complex pruned form (the central part is beyond the reach of clipping), U: umbrella form (pruned by camel browsing, not found on bars) dromedaries can go into intertidal zones and feed on its foliage by themselves, while other livestock species are provided it by humans. That is why the human role needs to be considered in social ecosystems in relation to mangrove foliage diets of livestock. So far, I have shown that the most distinctive socio- ecological characteristic of the human-livestock-plant relationships in the coastal zones of the arid tropics is the pastoral system of camels that rely on halophytes and mangroves for food, that the raised coral reef islands are utilized as grasslands and shrublands that are available only for camels, and also as gathering and fishing sites where the people target gastropods and fish in mangrove and coral reef-associated ecosystems, and that the physical environmental factors, such as coral reef topo- graphy and tidal conditions, determine the principal types of use of the islands by camels (Nawata, 1997, 2001a, b, 2002a, b, c, 2003a, b, 2004, 2005a, b, 2006a, b, 2012a, b). In this article, based on the former of these basic data, I analyze resource patch accessibility and availability on the Sudanese Red Sea coast, focusing on the human- camel relationship among the Beja. Consequently, I also try to reconsider a framework for sustainable human utilization of biological resources in the coastal zones of the arid tropics. Finally, I examine whether over- exploitation of resources in the coastal zones of the arid tropics has been limited. This article tries to illustrate the human-dromedary relationship in terms of sustainability of human resource use, as the key interspecific relationship in arid tropical mangrove ecosystems.
2.  Study Subjects and Survey Area
The Beja live in the area surrounded by the Red Sea, Nile River, Ethiopian highlands and Eastern Desert of Egypt, stretching over parts of three countries: Egypt, the Sudan and Eritrea. The Beja's economic production depends mainly on their herds of camels, cattle, sheep and goats. Many people practice some cultivation of sorghum and pearl millet, and some also engage in fish- ing on the seashore (Nawata, 1997, 2001a) (Fig. 4). The area the Beja consider their home range includes the Red Sea coastal region in eastern Sudan. The Red Sea region can be classified into three general physical units: the coastal plain, the Red Sea Hills, and the western plain. The Red Sea Hills, which represent the western edge of the Great African Rift Valley, extend for approximately 500 km in a northwest-southeast direction along the coast of the Red Sea. Their altitudes range between 900 and 1,200 m above sea level. The western plain is located on the western side of the Red Sea Hills and the coastal plain, on its eastern side. The coastal plain is a narrow strip ranging between 20 and 50 km in width, and extending from the shore to the foot of the Red Sea Hills (Abd el Ati et al. , 1996). The very small freshwater input from rivers or rainfall permits well-developed fringing reefs, which are found semi-continuously along both coasts of the Red Sea (IUCN/UNEP, 1988), and particularly in the Sudan, the seaward edge of the fringing reef may be over 1 km from the shore, with a substantial 10 m deep lagoon in between. The Red Sea shores also contain sandy beaches, mudflats and mangrove swamps (Head, 1987) (Fig. 5). The survey was conducted in the area around 'Agetai village on the coastal plain. The village covers an area about 3 km long by 5 km wide. The estimated population is about 2,000. Following the subsistence de- pendence put forward by Murdock (1967), rated 0 to 9 with respect to the relative dependency of the society, 'Agetai villagers depended 0 (0%-5%) on gathering, 0 (0%-5%) on hunting, 1 (5%-15%) on fishing, 7 (66%-75%) on animal husbandry, and 2 (16%-25%) on agriculture. Fieldwork was carried out for thirteen months during the period of 1992-1997: in August 1992; December 1992 to January 1993; November 1993 to June 1994; and January 1996 to March 1996 (Fig. 6). Tables 2 and 3, in particular, are based on interviews with five key informants between January and March 1996. Climatically the area exhibits a semi-arid subtropical Mediterranean type of climate in general (Bakhit, 1988). Annual rainfall means decreased from over 400 mm in the late nineteenth century to less than 150 mm after the 1970s as the process of aridification intensified (Kassas, 1957; Abu Sin, 1991; Musa, 1991). The mean tempera- ture of the area is around 30° Celsius. Strong winds blow, especially during the summer season (Ali & Mohamed, 1991). The vegetation is classified as semi-desert grass- land and shrubland that forms the eastern fringe of the Sahelian transition zone. Some data suggest that aridifi- cation has caused considerable ecological degradational changes in the vegetation. Thus, in fact, the present- day vegetation of this area seems to be coastal desert that is said to cover the area north of Port Sudan. The coastal vegetation is characterized by drought-resistant halophytes (White, 1983; Manger et al. , 1996) (Figs. 7, 8 and 9).
3.  Resource Patch Accessibility and
Availability, Focusing on Human-Camel
Relationships
I studied resource/land use on the coastal plain and access routes to the coral reef islands by camels (Fig. 10) (Nawata, 1997; 2001a, b; 2002a, b, c; 2003a, b; 2004). I made schematic overviews that showed the physical environment, biological environment and livestock graz- ing zones in a three-dimensional figure (Fig. 11). I attempted to summarize some characteristics along an environmental inclination, in order to easily make the relative importance or interaction between physical and biological elements of the coast easier to understand, and further to enable correlation between livestock grazing zones, accessibility to coral reef islands and resource patch availability. For illustrating discontinuity (non- homogeneity or heterogeneity) of biological resources, that is resource patches, I set out a horizontal axis (X axis), vertical axis (Y axis), and third axis (Z axis).
3.1   Physical Environment
The physical environment consists of a series of components including the Red Sea Hills, coastal plain, salt-marshes, lagoons, coral reef islands, shallow sea and open sea from landward to seaward. Inside each zone, remarkable resource patches have formed. For example, as for the land area from the shoreline of the mainland, surface runoff of seasonal streams and flooding of the deltas in which these terminate clearly differ among locations. As for the sea area, coral reef islands form closed spaces as islands because of the effects of seaward and landward winds, seasonal tidal level changes and daily tidal movement, though these are connected with the land area through the coral reef flats (Fig. 12). In such locations, there are heterogeneous discontinuous re- source patches. There are four raised coral reef islands with heights of less than ten meters. In Fig. 11, for later discussion, I put labels on and gave the abbreviations Space 1 (S1), Space 2 (S2), Space 3 (S3) and Space 4 (S4) to Umm al-Karāsī, ʿAyrī, ʿAbīd, and Hemān islands, respectively.
3.2   Biological Environment
Plant communities and animal communities form habitats on the basis of the structure of the physical environment (Figs. 10 and 11 and Table 1). As for Leguminoseae shrubs, they are scattered from the base- ment hills to the coastal plains and seaward edge of the
salt-marshes. Acacia tortilis is especially well-developed along seasonal streams inland and in hill escarpment. There is an open shrubland dominated by Acacia nubica, a type which lies inland to that of Acacia tortilis generally, but a population of dwarf Acacia nubica also happens to be found intensively in shrublands in which the surface runoff of seasonal streams terminates. Next, Gramineae grasses are widely spread in non-salty soils of the coastal plains and the coral reef islands. Gramineae perennial grass Panicum turgidum plains constitute the largest and most uniform type of coastal plain, especially those which consists of sheet-like loose sand deposits. The Gramineae grass Echinochloa colonum grows on silty plains of alluvial origin, particularly in neglected agricultural fields on flooded plains. The perennial halophyte Gramineae grass, Aeluropus lagopoides , is often found in areas behind coral sand dunes along shores with alluvium deposits. On the other hand, the community type dominated by the Chenopodiaceae succulent shrub Suaeda monoica forms sand hummocks reaching one meter in height in the shrublands located at the inland margins of the salt marshes. Various halophytes, mainly Chenopodiaceae plants such as Arthrocnemum glaucum and Atriplex farinosa , dominate the communities of salt marshes near the shoreline. Salt marshes are also developed on the coral reef islands. Shallow lagoons protected by coral reefs and inlets, called marsas, provide a favourable habitat for the growth of mangrove vegetation of small Verbenaceae trees, Avicennia marina (Figs. 13 and 14). The shorelines of the islands are also fringed with various types of mangrove communities. Seagrass beds are located in lagoons and moats cut off from the open sea by coral reefs and also adjacent to mangrove communities. In short, the species composition of plant communi- ties consists basically of zonation that changes along with the distance from the shoreline, but also has patches that have comparatively abundant flora such as locations where the surface runoff of seasonal streams terminates feeding a camel (Patch 6) (February, 1996). and mangrove communities in the intertidal zone (Fig. 10 and Table 1). Such plant communities have direct relationships with the distribution of animal communities. For exam- ple, the largest of the Strombidae, Strombus tricornis , is common on vegetated fringing reefs and grazes upon a variety of algae, with Sphacelaria a major item, but also including Ulva, Enteromorpha, Caulerpa, Polysiphonia, as well as blue-green algae (Taylor & Reid, 1984). The typical habitat of Strombus tricornis is the vast seagrass beds formed by the spermatophytes Halodule and Halophila (Mastaller, 1987). Such seagrass beds have developed on the sandy flats in lagoons of fringing reefs. Therefore, Strombus tricornis is abundant in the inshore habitats of fringing reefs but absent or rare on the off- shore reefs (Taylor & Reid, 1984) (Fig. 15). As for fish, dory snappers Lutjanus fulviflamma are common throughout the year and are usually found around the inner edge of the fringing reefs (Reed, 1964). They occur by day in relatively inactive schools which disperse at nightfall to forage as solitary individuals. They form schools which shelter near overhangs or in gullies near the reef crest, often where there is some surf and wave surge (Ormond & Edwards, 1987; Sheppard et al. , 1992). Yellowfin bream Rhabdosargus sarba in- habit the shallow channel inside the fringing reef and also most of the marsas along the coast (Reed, 1964; Ormond and shells left on the shore at Site N (Patch 8) (February,
& Edwards, 1987). Red Sea houndfish Tylosurus choram live near the surface and also inhabit the fringing reefs and marsas (Reed, 1964; Randall, 1983) (Fig. 16 and Table 2). I put labels on resource patches from Patch 1 (P1) to Patch 10 (P10) for mangrove and seagrass communities under different conditions (Fig. 11). Patches P1, P2, and P6 are mangrove communities along island shorelines in shallow sea. P3 is a mangrove community along island shorelines facing open sea. P4 and P5 are seagrass com- munities along island shorelines facing open sea. P7 is a seagrass community on coral reef flats connecting islands facing open sea. P8 is a mangrove community along the mainland shoreline in shallow sea (Fig. 13). P9 is a mangrove community in the middle of a shallow sea area. P10 is a mangrove community near open sea. These labels from P2 to P8 correspond to fishing and gathering sites that I previously described (Nawata, 2001a), with patch P2 corresponding to Site A, P3 to Site C, P4 to Site D, P5 to Site G, P6 to site H, P7 to Site I and P8 to Site N.
3.3 Livestock Grazing Zones
Both the physical and biological environments men- tioned above strongly influence the feeding behavior of livestock on coastal vegetation and inter-species varia- tions of pasture utilization, that is, livestock grazing zones (Figs. 10 and 11). Donkeys, cattle and sheep are so-called grazers and depend on various grasses of grasslands on the coastal plain and shrublands in which the surface runoff of sea- sonal streams terminates. More specifically, the grazing range of donkeys is limited to only around the village
(human living) area. On the other hand, cattle may be brought to other areas only in the summer and given roughage (straw of sorghum and pearl millet) or concen- trates. Sheep, goats and camels are herded only in this area. Goats that can browse on shoots and leaves of acacia Leguminoseae shrubs often use the shrublands at the inland margins of salt marshes and shrublands with leg- umes where the surface runoff of seasonal streams terminates. In this area, however, the herding patterns of goats as well as sheep and cattle show that they mainly graze on Gramineae grasses and partly browse on Leguminoseae or Chenopodiaceae half-shrubs. Goats do not prefer Chenopidiaceae halophytes, except for the perennial grass halophyte, Aeluropus lagopoides (Tables 2 and 3). On the other hand, camels rely on half-shrub halo- phytes, mainly Chenopodiaceae plants, and the foliage of the evergreen mangrove, Avicennia marina. Camels ex- hibit not only a broad feeding range for every family of plants, but also high-level palatability toward almost all species of halophytes. Especially in the summer, camels frequently feed on halophytes and mangroves (Table 3). The most distinctive feature of the livestock grazing zones, however, is that the camels' grazing zone includes the space extending to coral reef islands which other livestock cannot access through the coral reef flats. The dromedary is excellent at moving not only on the loose sandy soils of the desert but also on the irregular and complicated topographies of coral reef flats. It is possible to reach the coral reef islands without a boat by riding on a camel. No livestock other than camels (cattle, sheep, An old camel herder leading a camel herd (Livestock 5) Fig. 17 from  'Ayrī Island (Site 2) to the mainland (February, 1994). goats and donkeys) find it possible to walk on the bad terrain of both soft substrates (sandy beaches, mudflats and salt marshes) and hard substrates (coral reef flats and raised coral reefs) in the littoral and sublittoral zones, because of their anatomical structure. Other livestock may fall into spaces between coral reef colonies, because their feet are smaller than camels', despite their weight being less. Camels also have enough height to withstand a certain degree of strong wave action and currents that would sweep away other livestock or humans (Figs. 17, 18 and 19). The raised coral reef islands are utilized as grasslands and shrublands that are available only for camels, and can therefore be called an exclusive habitat or an exclusive niche for camels (Fig. 20). (b) the camel's distinguishing anatomical structure of height with long legs in the littoral zone, in comparison with cattle, goats and sheep (after Nawata, 2001b). I give abbreviations for livestock as follows: Live- stock 1 (L1) for donkeys, L2 for cattle, L3 for sheep, L4 for goats and L5 for camels.
3.4 Accessibility to Coral Reef Islands and Resource
Patch Availability
The physical environments, especially coral reef topography, tidal conditions and existence of water points, determine resource patch availability and human and camels' accessibility to coral reef islands (Table 4). There are islands of different types such as frequently accessed islands for long stays (Space 2), frequently accessed islands for short stays (Space 4), infrequently accessed islands (Space 3), and inaccessible islands (Space 1). Based on these conditions, types of resource patches are divided into three categories of resource patch availability: accessible and available resource patches with the use of camels (Patches 2, 3, 4, 5, 6 and 7), accessible and available resource patches even without the use of camels (Patch 8), and inaccessible resource patches even with the use of camels (Patches 1, 9, and 10). I look closely at these in detail below. First of all, the raised coral reef island Space 2 is the longest distance from the mainland, and the water level of its coral reef flat reaches the highest among the corri- dors between the mainland and the islands. However, this island is most frequently accessed by camels and humans, because hand-dug shallow wells make long stays possi- ble (camels have a high ability to tolerate drinking salty water) (Table 5). The camels are herded for more than several months on this island, which has the largest area among the islands (Fig. 20). The foliage of mangrove communities hemming the island shoreline is easily cut. Some of the goals of gathering and fishing activities utilizing camels include collecting gastropods, finding driftwood and fishing by handline. In the mangrove communities along island shorelines in shallow water (Patch 2), the people target gastropods such as the stromb Strombus tricornis, whose typical habitat is the vast seagrass beds formed by the sperma- tophytes Halodule and Halophila . This gastropod is uti- lized for many purposes. The flesh is a source of animal protein in the people's diet, and their operculum provides a cash income as incense and perfume. It is known that the gastropods of Strombidae use their claw-like opercu- lum for locomotion, digging for food and also defense (Mastaller, 1987). This operculum is a resource for daily life materials. That the people use these gastropods not only as a nutritional resource but also for daily life materials and as a means of subsistence is worthy of special mention. The muscular feet of gastropods are also used as bait
for fishing. The ways in which the people utilize them as a means of subsistence can be illustrated by some interesting fishing activities that make use of character- istics of this ecosystem. They target fish whose habitats are in coral reef, seagrass beds and at the surface, when fishing from the coral reef flats. Dory snappers, common throughout the year and usually found around the inner edge of the fringing reefs, are a member of mangrove ecosystems essentially as a predator of medium-sized invertebrates, such as Crustacea and small echinoids, and of small fish (Ormond & Edwards, 1987). Yellowfin bream, abundant on fringing reefs, as well as dory snap- pers are caught near mangrove communities along island shorelines in shallow sea (Patch 2) and mangrove com- munities along island shorelines facing open sea (Patch 3). Red Sea houndfish are not only the target of boat fishing by local fishermen but also caught frequently in seagrass communities along island shorelines facing open sea (Patch 4). Space 4 is used for camel pasturage, like Space 2, and is generally easier to access than Space 2 in terms of coral reef topography and tidal conditions. However, the peo¬ ple cannot herd camels for long periods there since there are no shallow wells. They gather and fish in the man- grove community along the islands' shorelines in shallow water (Patch 6) and seagrass communities on the coral reef flats connecting islands facing open sea (Patch 7). In particular, it is observed that they catch Chicoreus virgineus, which is comparatively abundant on the coral reef flats facing open sea (Patch 7). Conversely, Space 3 is an island to which the people have had limited opportunities to access and which they have infrequently utilized. Though it is a place that catches plenty of driftwood because it faces open sea, and it attracts many gastropods and fishes, the people do not bring their camels to feed there (Patch 5), because the channel between Space 4 and Space 3 is narrow resulting in a very fast flow of sea water. Furthermore, Space 1 cannot be accessed with camels at all (Fig. 21). The reason is the camels find it impos- sible to walk on the coral reef flats to Space 1. Therefore, Patch 1, the mangrove community along the islands' shorelines in shallow water, is not used. There are also mangrove communities that are inaccessible because they are located in the middle of shallow sea (Patch 9) or near open sea (Patch 10). These resource patches are inaccessible even with camels. Mangrove trees attaining a height of approximately more than four meters (measured by eye) seem to be rich in such mangrove stands without disturbance from feeding camels. There are few opportunities for the people to approach Patches 1, 9 and 10 with small boats to cut some mangrove foliage there, because it takes much time and great effort. The 'Agetai villagers own three small boats whose length is about six meters and a couple of which have a sail. Approximately twenty people engage in net-fishing with these boats. They occasionally find dolphins or dugongs and try to catch them with a few boats together (Nawata, 2002a). The owners of these boats manage to go to the Space 1 island at a close distance from the main- land to cut foliage of mangroves, but it is almost certain that they do not go to Patches 9 or 10, the mangrove communities in the middle of shallow water or near open sea, to obtain mangrove foliage. In contrast to these patches, there are resource patches that are accessible and available even without camels, one of which is Patch 8, a mangrove community along the mainland shoreline in shallow water. These, however, are also still approached not on foot but by camel, be- cause when packing mangrove foliage into hemp bags, the people need to load them on the camels' backs and bring them to their village. They bind the camels' legs so as not to let them approach the mangrove stands and eat the foliage because of poor footing in the intertidal zone. In such places, foliage is cut down by humans and carried to the shore for the camels' forage. They don't dare give it to cattle, even though they recognize that cattle may eat mangrove foliage. The trunks of the mangroves are used as materials for building settlements with driftwood and other kinds of wood. As a side note, concerning the dispersal ranges of mangroves, most (approximately 78%) of Avicennia marina propagules were stranded within 2 km of their starting point (Clarke, 1993; Hogarth, 1999), so we can regard at once that a certain source of propagules of viviparous mangrove species is guaranteed and the reproductive basis of these is kept at least level. Certainly, the surrounding tidal conditions and seedling conditions must be stable. Factors such as the reproductive mecha- nism of mangrove forests as a whole, a minimum area for maintaining their community and population, the stabil- ity of population density, changes in community diversity, and so on also need to be cleared up. What things are definite, however are that nearly undisturbed mangrove communities exist along coral reef islands or in the open sea where camels cannot go, and that people cannot use the biological resources in such locations inasmuch as they engage in foraging activities using camels.
4. Discussion
I have elucidated the role of dromedaries with an outline of resource patch accessibility and availability, by constructing a relationship web among human beings, livestock, coral reefs (the physical environment), and resource patches (the biological environment) (Fig. 22). H: humans; L1 (Livestock 1): donkeys; L2 (Livestock 2): cattle; L3 (Livestock 3): sheep; L4 (Livestock 4): goats; L5 (Livestock 5): camels; S1 (Space 1): Umm al-Karāsī ; S2 (Space 2): 'Ayrī ; S3 (Space 3): 'Abīd ; S4 (Space 4): Hemān ; P1 (Patch 1), P2 (Patch 2), and P6 (Patch 6): mangrove communities in shallow seawater along island shorelines; P3 (Patch 3): a mangrove community along an island shoreline facing the open sea; P4 (Patch 4) and P5 (Patch 5): seagrass communities along island shorelines facing the open sea; P7 (Patch 7): a seagrass community on coral reef flats connecting islands facing the open sea; P8 (Patch 8): a mangrove community in shallow seawater along the mainland shoreline; P9 (Patch 9): a mangrove community in the middle of the shallow sea; P10 (Patch 10): a mangrove community near the open sea. Resource patch utilization of P2 to P7 depends on human accessibility to the coral reef islands of spaces S2 to S4 using camels. The camels also play an invaluable role in appropriating and carrying biological resources in P8 along the mainland shoreline. On the other hand, P1 on the coral reef island of space S1 cannot be accessed by camel, but it can by boat. Boats are also used in accessing P9 in the middle of shallow water and P10 near open sea. Concerning the role of livestock, other livestock (L1 to L4 but not L5, camels) are certainly related to resource patch utilization in terrestrial zones, but not at all in coastal zones. In this way, I can explain that the role of camels would be centered on resource patch accessibility and availability in the coastal zones of the arid tropics. As I pointed out above, camels' feeding on mangrove foliage forms a grazer food web between productive mangrove ecosystems and less productive desert ecosys- tems. The relationship between camels and mangroves, however, cannot be determined only by the correlation between plant chemical defenses such as tannin content and livestock physical mechanisms such as palatability of these species. They are strongly related to resource patch availability that is affected by both the physical environ- ment of coral reef topography and tidal conditions, and the biological environment in reflecting all aspects of human resource acquisition activity. It is clear that dromedaries have an immeasurable significance con- cerning the relationships among humans, mangroves and many factors of whole coastal ecosystems. For example, of further interest is the degree to which the organic matter in excrement of camels is provided to the food web, because I observed camels discharging when they walked on coral reef flats in the intertidal zone. At the same time, the impact of human activities, including camels' feeding, must be examined carefully and assessed cautiously in terms of biological resource preservation in coastal ecosystems. It is very important to see what is appropriate, but many aspects are still unclear because of the lack of quantitative data. Salm and Clark (1984) express the view that all harvest activities such as grazing by various species on mangrove vegetation; grazing on mangrove vegetation by camels, goats, and cattle and in salt marshes by livestock; and harvesting of vegetation as fodder can be sustainable uses. On the other hand, Clough (1993) states that direct grazing in man- grove areas is not recommended owing to the difficulty of controlling overgrazing and to the considerable dam- age that can be done to the soil surface, even at low stock- ing densities. Instead, he suggests, it would be preferable to cut the foliage for hand feeding outside the mangrove areas. Yoshikawa (1998) also pointed out that camels step on mangrove roots, causing disturbance to mangrove forests and furthermore leading these to die off. Reef walking inevitably causes some physical damage (Salm & Clark 1984). Therefore, appropriate management of human and camel activities on coral reefs may be neces- sary. In any event, this case study of coastal resource utilization among the Beja on the Sudanese Red Sea coast leads to interesting observations concerning capturing human effects on the coastal environment qualitatively. In conclusion, I have demonstrated that to the degree that people use camels to access coastal resources, the camels can restrain overexploitation of these biological re- sources. In such a sense, I suggest that resource patch accessibility and availability are determined by camels' intervention so that overexploitation in the coastal zones of the arid tropics has been limited consequently. There- fore, it can be interpreted as a restraint on overexploita- tion inclusive to the mechanism or process of human resource utilization under natural conditions.
5. Concluding Remarks
Arid tropical mangrove ecosystem areas are rich in biodiversity, and may serve for seafood and pastoral food production by reforesting mangroves for fish nurseries and safe foraging sites. One of the most interesting as- pects of food habits along the coastal zones of the arid tropics is the local dependence on hunting, gathering, and fishing of sea products (fish, shellfish, dugongs, dolphins, and sea turtles). Therefore, in terms of arid land food production, we should consider the potential of sea product development as a principal element of future diets (Nawata, 2006b). On the other hand, the conversion of sea water to fresh water in coastal zones presents a large development frontier. It may, however, also lead to environmental degradation as highly concentrated saline water is re- leased into the sea. Many coastal towns and cities have developed solar-powered desalination plants, which have made agriculture and forestation possible in remote areas. I am examining this issue and compiling information to counteract new environmental problems (Nawata, ed., 2013). Going forward, there is a need for a series of studies on relations between humans, livestock and resource patches, comparing other cases from the standpoint of seeking possibilities for sustainable utilization of bio- logical resources in arid tropical mangrove ecosystems. Further study, particularly with site-specific quantitative data on the relationship between humans and drome- daries and its impact on mangrove communities in arid tropical mangrove ecosystems would help us to evaluate the sustainability of that form of resource utilization. Abd el Ati, H., O.R. Vetaas and L. Manger (1996) The natural environment of the Red Sea Hills: Lessons in variability. In : L. Manger, H. Abd el Ati, S. Harir, K. Krzywinski and O.R. Vetaas, eds., Survival on Meagre Resources: Hadendowa Pastoralism in the Red Sea Hills, 37-58, Nordiska Afrikainstitutet, Uppsala. 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Clark (1984) Marine and Coastal Protected Areas: A Guide for Planners and Managers , International Union for Conservation of Nature and Natural Resources, Gland. Schleyer, M.H. (1986) Decomposition in estuarine ecosystems. Journal of the Limnological Society of Southern Africa , 12: 90-98. Sheppard, C., A. Price and C. Roberts (1992) Marine Ecology of the Arabian Region: Patterns and Processes in Extreme Tropical Environments , Academic Press, London. Takatsuki, S. (1980) Summary of an ecological survey on the mangrove forests at Queshm Is., Iran and Miani Hor, Pakistan, 1979. In: Japan Cooperation Center for the Middle East, An Eco- logical Survey on the Mangrove Forests of Persian (Arabian) Gulf and Pakistan , 23-68, Japan Cooperation Center for the Middle East, Tokyo. Taylor, J.D. and D.G. Reid (1984) The abundance and trophic classification of molluscs upon coral reefs in the Sudanese Red Sea. Journal of Natural History , 18: 175-209. Tomlinson, P.B. (1986) The Botany of Mangroves , Cambridge University Press, Cambridge. White, F. (1983) The Vegetation of Africa: A Descriptive Memoir to Vegetation Map of Africa , UNESCO, Paris. Whittaker, R.H. and G.H. Likens (1975) The biosphere and man. In : H. Lieth and R.H. Whittaker, eds., Primary Productivity of the Biosphere , 305-328, Springer-Verlag, New York. Yoshikawa, K. (1998) Challenges for Combating Desertification , Chuokouron, Tokyo. (in Japanese) Hiroshi Nawata is an Associate Professor at the Research Institute for Humanity and Nature (RIHN) in Kyoto, Japan. He is the RIHN project leader for "A Study of Human Subsistence Ecosystems in Arab Societies: To Combat Livelihood Degradation for the Post-Oil Era" (2008-2013). He received his Ph.D. in Human and Environmental Studies (Cultural Anthropology) from Kyoto Uni- versity in 2003. He was an Assistant Professor at the Division of Comprehensive Measures to Combat Desertification, Arid Land Research Center, Tottori University from 2004 to 2007. His major fields of interest are camel pastoral systems, Muslim trading networks, and indigenous (traditional) knowledge about rural development in the Middle East and Africa. (Photo: the author at Kulanaib, Sudanese Red Sea coast, December, 2011) (Received 4 March 2013, Accepted 16 June 2013)