Having described the nature of the incident radiation flux presented to the aquatic biosphere and the influences to which the light is subject within the water, we shall now consider the kind of light field that results. We shall begin, in this chapter, by examining the ways in which the properties of this light field are studied. The physical definitions of these properties are given in Chapter 1.

Irradiance

Irradiance meters

The most frequently and easily measured property of the underwater light field is irradiance. Knowledge of this parameter is valuable, not only because it provides information about how much light is available for photosynthesis, but also because irradiance plays a central role in the theory of radiation transfer in water. An irradiance meter, since it is meant to measure the radiant flux per unit area, must respond equally to all photons that impinge upon its collector, regardless of the angle at which they do so. With any given meter this can be tested by observing the way in which its response to a parallel light stream varies with its angle to that light stream.

As the angle of the radiant flux to the collector changes, the area of the collector projected in the direction of that radiant flux changes, and the proportion of the flux intercepted by the collector changes correspondingly (Fig. 5.1). Thus, the response of an irradiance meter to parallel radiant flux (wider than the collector) should be proportional to the cosine of the angle (θ) between the normal to the collector surface and the direction of the flux. An irradiance collector which meets this criterion is known as a cosine collector.

Introduction

Colobus monkeys belong to the family Cercopithecidae, subfamily Colobinae. The actual classification of red colobus at specific and subspecific level is equivocal (Verheyen, 1962; Dandelot, 1968; Rahm, 1970; Kingdon, 1971; Struhsaker, 1975). For instance, the Zanzibar red colobus is considered a distinct species by Verheyen (1962) and Dandelot (1968), while others classify it at subspecific level (Rahm, 1970; Kingdon, 1971; Struhsaker, 1975). This problem arises as a result of the allopatric distribution and high affinities of the red colobus. The classification adopted in this chapter follows that of Rahm (1970), Struhsaker (1975, 1980), Kingdon (1971, 1981) and Rodgers (1981), in which all the 14 red colobus types are maintained as subspecies of Colobus badius Kerr.

The 14 subspecies of red colobus occur in a patch-like allopatric distribution across tropical Africa from Senegal to Zanzibar (Rahm, 1970; Kingdon, 1971; Struhsaker, 1975). Most populations are primarily adapted to mature low and medium altitude rain, riverine and groundwater forests (Struhsaker, 1975; Rodgers, 1981). However, some, such as Colobus badius temminckii (Kuhl), also occur in savanna woodland (Struhsaker, 1975; Starin, 1981). The Zanzibar red colobus are also found in mangrove forest, thickets of secondary growth and sometimes in cultivated areas: habitats that are also reported for the Gambia colobus C. b. temminckii (Starin, 1981).

Five allopatric subspecies of red colobus occur in East Africa. Three of these are found in eastern African forests (Figure 12.1).

Introduction

Most visitors as well as residents of eastern Africa are unappreciative of the variety of amphibians and reptiles found there. With the exception of some people who value snakes for mystical and curative properties, the reptiles are generally great feared and killed whenever encountered, and even amphibians such as frogs and toads are regarded with some suspicion.

Although the layman may be familiar with some of the larger, more conspicuous amphibians and reptiles in the open habitats such as woodlands, and those species which can survive in and around man's dwellings, very few people spend enough time inside natural forest to identify the various amphibians and reptiles found there. These are often cryptically coloured, and some may be found only high in the forest canopy, or hiding inside rotting logs and leaf litter of the forest floor. Thus, all but a very few of those people who live near the forest and work in it are unfamiliar with some of its most interesting inhabitants. Among the amphibians, there are brightly coloured treefrogs which are able to change their colours: a frog, Leptopelis uluguruensis with a colour pattern which resembles a small patch of fungus; and a toad, Bufo brauni which closely resembles a dead leaf on the forest floor.

Abstract

Knowledge of the montane diplopod fauna of East Africa dates back only to the collections made by Sjoestedt on Mount Kilimanjaro in 1905–6 and described by Attems in 1909. Little was added to that beginning until the onset of explorations since about 1964 by personnel from the universities of Dar es Salaam and Copenhagen. Since the great majority of species discovered in the Tanzanian Eastern Arc mountains are undescribed endemics, it is not possible to present a comprehensive chorographic analysis. Only the species of Oxydesmidae have been worked out (Hoffman, 1990) for the entire region, and of the various geographic units only the fauna of the East Usambaras is at all well known. Nonetheless it is possible to indicate some generalities of interest: (i) with few exceptions most of the genera occurring in these mountains are endemic, so that lines of affinity with other regions must be sought at the level of tribe or higher; (ii) in most cases such genera appear to be the result of local derivation from formerly widespread ancestral stocks; (iii) the postglacial condensation of montane forest to higher mountains surrounded by seasonally arid savanna or scrub forest has resulted in profuse local speciation on individual ranges or close clusters, involving, so far as can be deduced, both sympatric and allopatric isolating mechanisms. These constellations of local species may thus be classified as neoendemics in the East African fauna.

Introduction

In his classic paper on the vegetation of what is now Tanzania, the geographer Clement Gillman (1949) recognised the importance of the disjunct arc of mountains in the east of the country as condensers of moisture brought inland from the Indian Ocean. At this they are remarkably efficient: on the eastern side of the mountains rainfall can be well over 2000 mm per year; yet in the rain shadow only a few tens of kilometres to the west, it can be below 500 mm per year. In his list of Indian Ocean condensers, Gillman included the northern volcanic mountains of Meru and Kilimanjaro, and then ran the arc southwards along the disjunct crystalline block-faulted mountains to the Kipengere range above Lake Nyasa. Here he pointed out that high rainfall in the great amphitheatre of volcanic and crystalline mountains surrounding the northern end of Lake Nyasa was largely attributable to convection from the lake surface.

The volcanoes of Meru and Kilimanjaro are geologically recent, having been formed within the last million years. However, the crystalline block-faulted mountains of the Eastern Arc are very old, with initiation of faulting dating from 290–180 Myr BP (million years before present) and reactivation of the faults creating the modern mountains during the last 7 Myr BP (Griffiths, Chapter 2). Indications are that the Indian Ocean climate was comparatively stable during Pleistocene climatic fluctuations (Lovett, Chapter 3).

Introduction

The spider fauna of East Africa is not particularly well known, but one group of spiders, the sheetweb weavers or dwarf spiders (family Linyphiidae), is much better known than any other group. This is quite strange since these spiders are small (most spiders are less than 3 mm in total length) and do not have any remarkable body colours. The family Linyphiidae is one of the largest spider families (second only to the jumping spiders, Salticidae) with some 3600 species described in 403 genera (Platnick, 1989).

The common English names for these spiders (dwarf or sheet-web spiders) are not very appropriate, since linyphiids are not smaller than spiders in several other families and sheet webs resembling those of linyphiids are also built by members of other spider families such as the Theridiidae, Cyatholipidae, Diguetidae, Pholcidae, Araneidae and Uloboridae (Eberhard, 1990). Furthermore, sheet webs are built only by some linyphiids, particularly representatives of the subfamily Linyphiinae.

An impressive amount of information on the East African linyphiid fauna has been published during the last 30 years, especially on the montane fauna. In East Africa, many mountains have now been explored at least once and this has revealed a relatively rich linyphiid fauna. However, ‘rich’ should be understood in an African context, as the diversity is low if one compares the number of species on each mountain in East Africa with the number of species on mountains in the northern hemisphere.

Introduction

The African climate has been far from stable. Equatorial rainfall is created by oceanic solar heating generating the intertropical convergence zone (ITCZ), so rainfall and hence forest growth is in part dependent on the relative position of the solar equator. The continent was south of its present position at the end of the Cretaceous with the equator running through the present-day Sahara, suggesting that the rainfall was high in what is today a desert. The amount of rain carried inland by the ITCZ is dependent on oceanic currents which are in turn modified by positions of continents; for example, Antarctica creates the cold Benguela current which brings aridity to southwestern Africa, and Madagascar casts a monsoonal rainshadow on southeast Africa. Ocean currents are also affected by periodic variations in the Earth's orbit, the Milankovitch cycle, which is thought to be responsible for repeated Pleistocene global cooling and warming (Imbrie & Imbrie, 1980). Finally, rainfall on the continent is affected by positions of mountains and lakes which create their own weather systems within the overall ITCZ and oceanic current climates.

Climatic fluctuations are partly responsible for eastern African moist forest biota distribution patterns. The three main patterns to elucidate are: apparently ancient links to the Guineo–Congolian forests of western and central Africa; apparently recent links to the western Guineo–Congolian forests; and high degree of endemism within the eastern African forests.

The tropical rain forests of Africa are divided by a corridor of arid land that runs from the Horn of Africa, through Kenya, Tanzania, Zambia, Zimbabwe and Botswana to the Namib Desert of Namibia (Werger, 1978). The arid corridor reaches the sea on the east coast of Africa in Mozambique, where Madagascar casts a rain shadow, and in the north where the corridor covers much of Somalia. A narrow strip of relatively high rainfall lies between the deserts of the Somalia coast in the north, the Madagascar rain shadow in the south and the woodland of the central African plateau to the west. It is this humid area of eastern Tanzania, Kenya and southern Somalia that we define here as eastern Africa (Figures 1.1 and 1.2).

The area of tropical rain forest in eastern Africa is not large; it is approximately 10000 km2, a mere 0.1% of the estimated 10 million km2 of tropical rain forest in the world (Mabberley, 1983). Unlike the vast west and central African forests, the forests of eastern Africa are highly fragmented – discrete islands associated with localised areas of high rainfall, surrounded by a sea of comparatively arid woodland. Contrasts between these wet and dry areas are pronounced. For example, on the eastern slopes of the Uluguru Mountains of Tanzania there is a per-humid climate where more than 100 mm of rain falls each month of the year and the annual rainfall exceeds 3000 mm.

Abstract

This chapter examines the geological and geomorphological processes that have shaped the present-day landscape of the eastern part of East Africa. The underlying rock types, their age, environment of formation and interrelationships are described. The structural evolution of the landscape is traced from the Karroo c. 300 million years before present (myr bp), encompassing the Karroo Rifting, the breakup of Gondwanaland, the relative ages of the block mountains, volcanic mountains, plains and plateau, the East African Rift Valley system, and the vertical movements affecting the continental margin of East Africa. Reference is made to the general geology and topography of West Africa for comparison and the debate about the pre-drift position of Madagascar is briefly reviewed. Variations in soil types in relation to parent rock, age, climate, organic material and relief are considered.

Introduction

A wide variety of rock types are found in East Africa representing all three of the major groups: igneous, metamorphic, and sedimentary. They range in age from over 2000 myr BP to unconsolidated sediments accumulating at present, but represent two geological time spans: the Precambrian (> 2000–570 myr BP) and the Karroo–Recent (290–0.01 myr BP) (Figure 2.1 and Table 2.1), separated by a major break in the geological record. These two groups of rocks were formed under entirely different geological conditions, resulting in distinct properties.

Introduction

Merely understanding the biology of eastern Africa's closed forests is not sufficient if we wish to maintain the resource as a functional natural community for posterity. Conservation needs action: it needs management inputs into both the resource itself and the human populations who depend on the resources for their livelihood and in so doing often degrade the resource. There is still a need for biology, however, especially for a resource as complex as the tropical forests, where even the identification of the component species remains problematical.

We are witnessing the loss of forests and loss of forest species all over the tropics; eastern Africa is no exception, but here we have had only a small and fragmented resource base of forest to start with. These forests are important for water catchment and timber, and their area is coveted for agricultural development. Pressures on the forest land are growing and are often incompatible. It is a sad paradox that now, in the 1990s, we often know how we could achieve conservation and how we can share the ‘cake’, but we often lack the political and financial will to do so.

This chapter traces the history of land use in the forest areas in eastern Africa, and examines the causes of incompatibility between land usages, and some theoretical and practical implications for conservation.