Introduction

Although much of East Africa is arid savanna, about 2% of Tanzania contains rich tropical rain forest. The majority of this is montane forest, found on a chain of ancient (80 million years old) block-fault mountains; these mountains, which stem from the Pare Mountains in the north to the Southern Highlands in the south, have been termed the Eastern Arc Mountain Chain (Lovett, 1985). Most mountains in the chain have been isolated from one another since the Pleistocene (Hamilton, 1982; Kingdon, 1989) by a sea of arid woodland savanna. The isolation of the montane forests, in conjunction with a relatively stable climate (Hamilton, 1982), has resulted in significantly fewer overall species of nearly all taxa examined when compared with the more continuously distributed Guineo–Congolian forest (Stuart, 1981; Rodgers, Owen & Homewood, 1982; Lovett, 1985; Lovett, Bridson & Thomas, 1988; Kingdon, 1989 and several chapters in this volume). [The Guineo–Congolian forest is the main forest block extending across Central Africa, from Lake Victoria to Liberia: White, 1981.] The isolation of the Eastern Arc mountains also has produced high rates of endemism in almost every major taxonomic group (Stuart, 1981; Rodgers et al., 1982; Hamilton, 1988; Lovett et al., 1988; Kingdon, 1989 and several chapters in this volume). This endemism is particularly striking among the plants: over 25% of the 2000 plant species found in Tanzanian forests are endemic (Lovett, 1985).