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Invasive Plants in Tropical Forests: Warnings from the Amani Botanic Gardens, Tanzania

Volume 2 Number 3 - May 1994

D. Sheil

The Amani Botanic Gardens, sited amongst the rich forests of the East Usambara Mountains, Tanzania, have previously received attention in this magazine. The garden, arboretum and research station were for several decades in the early part of this century a major centre for research in the cultivation of tropical plants. The many exotics subsequently left at Amani have created a long-term experiment in the process of plant naturalisation. Given the high conservation value of the East Usambara forests (Hamilton & Bensted Smith, 1989; Iversen, 1991) the question of how to control and prevent habitat deterioration due to naturalised exotics is important.

Continental evergreen tropical forests have not been generally recognised as vulnerable to exotic plant invasions, and it is inferred that such problems are confined to oceanic islands. A recent review of naturalised plants in the forests of the East Usambaras in Tanzania has indicated that the threat may be much greater than was previously thought (D. Sheil, in press), and suggests that collections of exotic species such as are found in botanic gardens can pose a considerable threat to the species composition of even closed tropical forests. This study indicated that more than 40 species are already naturalised within the forest outside of the site of original cultivation. It is possible to speculate that the derelict plots of the Amani Botanic Garden may ultimately be responsible for even more species "escapes", as it is known that plants may reveal invasive tendencies only after considerable periods at a given location (Moody & Mack, 1988). The original introductions were often well-documented, providing a unique opportunity to examine the story of individual introductions.

A well-researched example of the development of an invasive plant and its consequences is provided by the fast-growing tree Maesopsis eminii Eng. (Rhamnaceae). M. eminii was introduced to the East Usambaras from central Africa early in this century. The tree, the seed of which is dispersed by hornbills and monkeys, now dominates large areas of the forest, despite initial observations which suggested that it would not regenerate naturally. This species has been the focus of several recent reports (Binggeli, 1989, 1990; Binggeli & Hamilton 1990). The invasion remains unconfronted on the ground as any practical control would now require truly massive intervention.

Binggeli (1990) places the example of M. eminii in the context of other recognised examples and emphasises the historical role of botanic gardens in the accidental introduction of exotic species into tropical forests, giving several, often anecdotal, examples. The data from the Usambaras suggests many species were introduced through the Amani collections. The siting and management of botanic gardens that incorporate large exotic collections in areas of conservation importance needs to be carefully reviewed.

While there have been initiatives seeking to regulate species introductions (IUCN, 1987) these have lacked widespread recognition and utility in most poorer countries. Naturalisations and plant invasions remain generally undocumented and the risks involved are often unrecognised or played down. There is a need for an easily accessed reference on known "invasive" species and genera.

The opportunity to learn from experience is significant. M. eminii has already been seen to regenerate naturally in Fiji (Marten, 1980). Improved and informed regulation is needed to control and evaluate the transfer of exotic species generally - the added bureaucracy will be an unavoidable cost, but the exercise will provide significant long-term benefits. In the case of derelict collections such as those at Amani, guidelines are needed on the necessary species eliminations that are unfortunately but inevitably required.

Cronk & Fuller (in press) provide a detailed appraisal of invasive species and issues involved. The general picture supported by evidence from Amani suggests a serious and growing problem.