|Invasive Woody Plants||
|There is probably no place in tropical Africa which can
boast a mixture of historical, geographical and biological
uniqueness that matches that of Amani in the East Usambaras
(Tanzania). These crystalline mountains, part of the Eastern
Arc Mountain chain stretching along the Indian Ocean, are
covered with species rich forests containing a high proportion
of endemic plants (including the African violets) and animals.
The high rainfall and a pleasant climate resulted in the
establishment by the Germans of a botanic garden and
associated research station in 1902. Prior to WWI over 900
species, mainly plants with potential economic value, were
introduced. During the subsequent British rule a number of
introduced species, including animals, were observed to be
spreading, Maesopsis eminii and Clidemia hirta
being the most noticeable examples in the natural forest. After
independence the spread of exotic was favoured by widespread
deforestation and unregulated logging. During the 1990s the Tanzanian
authorities with the help of Finnish aid have successfully conserved
the forest reserves and in 1997 a large nature reserve was established.
This reserve includes the botanic garden which is being rehabilitated
with the help of the Royal Botanic Gardens, Kew.
In the East Usambaras, as is usually the case throughout the world, introduced plant species are only recognized as a problem when they are already widespread. By that stage eradication programmes are totally unrealistic and biological control becomes the only option especially when the invaders threaten the survival of endemic species. In order to eradicate a new invasive plant, the invader must be detected soon after it starts spreading into native vegetation. This, unfortunately, is rarely the case and this is often due to the lack of monitoring and poor knowledge of the local vegetation. Recent observations made in the East Usambaras illustrate the problem.
During the August 1999 Tropical Biological Association field course the natural forest closest to the Amani Research Station was used for teaching. This forest stand was extensively used in 1987 to investigate the ecology of Maesopsis and it was noticed in 1999 that the physiognomy of the forest closest to the forest clearing had changed beyond recognition. Two species, hitherto either not recorded (Pyrostegia venusta) or thought not to be major problem (Selaginella sp.), were seen to be spreading into slightly disturbed forest.
Selaginella sp. (recorded in the 1930s as S. flabellata Spring), a plant with a strikingly iridescent colour (suggesting that it is S. willdenovii (Desv. ex Poir.) J.G. Bak) - an adaptation to low light intensity - was introduced as a greenhouse ornamental prior to WWI. The greenhouse in which it was planted was soon abandoned and along with other species the Selaginella started to spread into the surrounding vegetation. As the area is surrounded on three sides by a river and the other by a main road the species failed to spread further. At some stage during the 1980s some plant material was moved intentionally or as a contaminant to the forest edge. In 1987 it covered no more than 3 m-2 but it now covers over 2000 m-2. The plant spreads vegetatively about 2-3 m per year away from the point of establishment. It produces a ground cover often close to 100% and grows just as well under complete canopy cover or in open treefall gaps. Its canopy height varies from around 50 cm to over 4 m depending on the availability of shrubs and climbers for support. The plant smothers young trees and shrubs and appears to hinder all regeneration.
Pyrostegia venusta was not present at Amani in 1987 and was probably introduced to the area around 1989. It was planted at four locations including the IUCN compound and next to a house in a forest clearing. It was soon realized that the vine was smothering the house and the plant was cut down and the remains were dumped at the forest's edge. Ten years later the vine covers over 6000 m-2 and smothers 30-40 m tall trees. Although it produces numerous brightly coloured orange flowers, observable from a great distance, the vine only appears to be spreading vegetatively at a rate of over 10 m a year. Climbers in the East Usambaras are relatively few and often not very conspicuous and Pyrostegia readily alters the forest's physiognomy. Its profuse horizontal stems running between ground level and a height of 1-2 m hinders human's ability to walk through the forest. However, its main impact on the forest is that of crown dieback as it smothers large canopy trees.
The area covered by both species is still small enough for the species to be eradicated, although the control of the vine may prove harder in view of the thousands of fine stems which cover the ground. It is hoped that the Nature Reserve authorities will quickly address the issue once they are fully made aware of the problem at hand.
In the light of these new discoveries a number of important points which can be made from these observations including:
1. Detection: the two species were only fortuitously detected by the author after the field course was relocated from Uganda to Tanzania. The area currently invaded is only a very small proportion of the Nature Reserve. Most of the reserve was not visited during the one month stay. Does it contain a similar number of new invaders? Casual observations at two other locations indicate that other species, hitherto not thought to be invading, are also spreading into natural forest including two tree species, Castilla elastica and Arenga pinnata, and a bamboo.
2. Prediction: our most reliable predictor of invasiveness is whether a species is invasive elsewhere in the world. This appears to be very unhelpful in our present situation. There are no reports of P. venusta being invasive anywhere else, despite it being widely used as an ornamental. The iridescent Selaginella species is also unreported although a related species is a problem in New Zealand and on some other Pacific islands. Only C. elastica is a known invader and considered to be a serious problem in the Pacific.
3. Time-lag: three of the species reported here were introduced about 90 years ago and despite extensive observation in the late 1980s they have not been reported as invaders until now. In the case of the Selaginella species the time-lag between the introduction and the invasion is due to physical barriers preventing its spread whereas in the other two cases it is unclear what caused it. In the case of P. venusta no time-lag occurred as it started to spread within 2-3 years of its introduction.
4. Species attributes: Both P. venusta and Selaginella species appear to spread purely vegetatively and become dominant, respectively, at ground and canopy level. To date the dispersal agent of all East Usambara invasives have been bird-dispersed and to a lesser extent wind-dispersed. Maybe with the exception of Clidemia hirta, all invaders of the natural forest have been associated with natural disturbance (treefall gaps) and/or human disturbance (pit-sawing gaps or industrial logging) whereas P. venusta and Selaginella sp. spread regardless of the presence or absence of gaps and light intensity.
5. Botanic garden: Probably the main issue relating to invasives in the East Usambaras concerns the Amani Botanic Gardens. These are part of the Nature Reserve, but they contain mainly exotic species many of which are spreading or could potentially spread into the surrounding natural forest which is rich in endemic species. Therefore the obvious question is: should the rehabilitation of the gardens include the maintenance of a large collection of exotic species many of which are a potential threat to the local biodiversity? In view of past and current experiences it is highly unlikely that a large collection of exotics can be maintained in harmony with rare endemics. Thus, swift action is required to remove many of these introduced species and also prevent the introduction of new ones, otherwise the nature reserve will ultimately become a heaven for the garden's exotics
This article was published in 2000 in Aliens 10, 14-15. Aliens is the newsletter of the Invasive Species Specialist Group of the IUCN Species Survival Commission. The ISSG web site provides details of the aims and activities of the group which includes a discussion group dealing with all matters relating to biological invasions.