Many organisms create or alter resource flows that affect the composition and spatial arrangement of current and future organismal diversity. The phenomenon called ecosystem engineering is considered with a case study of the mound building termite Macrotermes michaelseni. It is argued that this species acts as an ecosystem engineer across a range of spatial scales, from alteration of local infiltration rates to the creation of landscape mosaics, and that its impacts accrue because of the initiation of biophysical processes that often include feedback mechanisms. These changes to resource flows are likely to persist for long periods and constrain the biological structure of the habitat. The value of ecosystem engineering is discussed as a holistic way of understanding the complexity of tropical ecology.

A study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre‐existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.

A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion.

In Botswana, tourism is the second most important economic activity after diamond mining and trading. The Okavango Delta in northern Botswana is the largest single tourist centre in the country. This study estimates the total economic value of tourism in the Okavango Delta and compares this value to that of other sectors in the economy of Botswana. The results are compared to results of similar studies for tourist destinations elsewhere in the world, and the policy implications of the findings are highlighted. The study uses secondary data to estimate the direct consumptive and non-consumptive use value, and a survey among tourists to determine the existence value of the Okavango Delta. JEL O13, Q20

The Okavango Delta is a large wetland system situated in northern Botswana. The hippopotamus forms an integral part of this dynamic ecosystem, as it exerts a catalytic effect on geomorphological change. In the distal reaches of the wetland, regular movement of hippos to feeding grounds results in the development of incised channels, which are kept clear of vegetation and act as nodes for swamp expansion. Hippos maintain pathways in backswamp areas, which lead to the development of new channel systems during channel avulsion. They create breaches in the vegetation levees which flank channels in the permanent swamps, causing diversion of water and sediment to backswamp areas. Their paths often lead to lakes in the permanent swamps. During channel avulsion, diversion of a channel into lakes may occur via these paths, which can result in lake closure.

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Summary Vegetation on islands in the perennial swamps of the Okavango Delta exhibits a marked zonation pattern. Island fringes are generally characterized by a broadleaf evergreen riparian community of Syzigium cordatum, Ficus verruculosa, F. natalensis, F. sycamorus, Phoenix reclinata, Garcinia livingstonei and Diospyros mespiliformis. This gives way towards island interiors to a community dominated by Acacia nigrescens, Croton megalobotrys and Hyphaene ventricosa. The central regions are characterized either by short, sparse grassland dominated by Sporobolus spicatus or are completely devoid of vegetation with sodium carbonate (trona)‐encrusted soil surrounding a central pan of extremely high conductivity. An indirect gradient analysis indicated the important determinants of vegetation distribution as being not only elevation, depth to groundwater and distance from the water source, but also as groundwater chemistry (conductivity and pH) and soil chemistry (sodium concentration). These studies were supported by a direct gradient analysis approach with species tolerances to depth to groundwater, groundwater conductivity and soil‐surface sodium concentrations having been established. Feedback interactions of vegetation and groundwater elevation and chemistry, as well as soil chemistry, are evident and appear to be responsible for the vegetation dynamics in the study area. Résumé La végétation des îles dans les marais permanents du delta de l'Okavango présente un schéma de zonation marqué. Le bord des îles se caractérise généralement par une communauté riveraine à larges feuilles toujours vertes de Sizigium cordatum, Ficus verruculosa, F. natalensis, F. sycamorus, Phoenix reclinata, Garcinia livingstonei et Diospyros mespiliformis. Celle‐ci cède le pas vers l'intérieur à une communauté dominée par Acacia nigrescens, Croton megalobotrys et Hyphaene ventricosa. Les zones centrales, soit se caractérisent par une prairie courte et clairsemée où domine Sporobolus spicatus, soit sont complètement dénuées de toute végétation, avec un sol encroûté de carbonate de soude entourant un centre où la conductivité est très élevée. Une analyse indirecte du gradient indiquait que les facteurs déterminants pour la distribution de la végétation ne sont pas seulement la hauteur, la profondeur de la nappe aquifère ou la distance par rapport à une source, mais aussi la composition chimique de l'eau (sa conductivité et son pH) et celle du sol (concentration en sodium). Ces recherches ont été confirmées par une approche directe de l'analyse du gradient, de la tolérance des espèces face à la profondeur de l'eau, sa conductivité et les concentrations en sodium de la surface du sol ayant étéétablies au préalable. Les interactions entre la végétation d'une part et le niveau et la composition chimique de la nappe aquifère d'autre part, ainsi que celle du sol sont évidentes et semblent être responsables de la dynamique de la végétation dans la zone étudiée.

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