Mahale, Tanzania

Invasive Plant in Chimpanzee Habitat at Mahale

by Linda A. Turner Kyoto University

(E-mail: turner@macaca.zool.kyoto-u.ac.jp)

Approaching the Mahale Mountains National Park in western Tanzania from a boaton Lake Tanganyika, the contrast between the deforested hillsides to the north of thepark boundary and the green sweep of forest on the western slopes of the mountains tothe south is remarkable. Also remarkable are the dense, shady groves of a small, slimtree which crowd down to the lakeshore and whose crowns are covered in the rainyseason with striking displays of golden-yellow flowers. Unfortunately this prolific andattractive tree is unwanted. Senna spectabilis DC (Irwin & Barneby), a native of tropicalAmerica which was introduced to Mahale in 1967 as a source of shade, has becomecompletely naturalized in the park and for the past several years has been perceived asa threat to the regeneration of local plant communities, due to its apparent ability todominate native flora.

When S. spectabilis was introduced to the Kasoje area of what is now the MahaleMountains National Park, most of the lowland primary forest had been destroyed due to along history of slash-and-burn cultivation. The forests at Mahale have been in theprocess of regeneration for about two decades now, but unfortunately areas of Kasoje,which were probably once covered with wooded grassland and coastal forest, are nowdominated by the exotic species. S. spectabilis has proved an aggressive colonizer ofopen land and continues to spread in the lowland area. The tree has shown resistance toefforts at control through cutting and girdling: in 1991 the Park Warden ordered that alltrees in the vicinity of the Kansyana research camp (about 1 km inland from thelakeshore) and the worker's village of Kasiha be cut down but the trees quickly regrew.Moreover, chimpanzees which use the lowland forests have not been observed to eatany part of this species and preliminary investigation suggests it is of little use to otherwildlife.

Study of the density, distribution and ecology of S. spectabilis has been underwaysince July 1994 as part of a larger investigation of chimpanzee habitat use at Mahale. Mr.C.K. Ruffo, a Tanzanian botanist with the National Tree Seed Programme in Morogoro,visited Mahale for three days in February, 1995 at the invitation of Tanzania NationalParks management to positively identify the exotic species, conduct some preliminaryinvestigations regarding its invasion and make suggestions for its control. Futuremultidisciplinary research into the tree's ecology and effect on wildlife is planned withcolleagues in Japan, Canada and Tanzania.

S. spectabilis certainly merits further study. Early analysis has shown it to beequipped with an impressive chemical arsenal: parts of the tree were found to contain anallelochemical which may inhibit the growth of other species, an enzyme which interfereswith protein digestion, an insecticide and medicinal compounds including anantischistosomal agent. The tree appears to have antimalarial properties as well.

To the human inhabitants of the Mahale of the past, S. spectabilis was an excellentsource of shade, firewood and building material as well as crop protection due to itsability to form living fences. Now, however, the unchecked spread of this species hasserious implications for wildlife conservation and tourism due to its apparent lack ofutilization by most animals, especially the chimpanzees (Pan troglodytes schweinfurthii)which the park was established to protect. A thorough understanding of the ecology of S.spectabilis and implementation of management strategies for its control are required iffurther disruption of the native biota at Mahale is to be avoided.

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