Exotic insect pests worldwide are many. They are accidental biotic contaminants. Classical biological control (CBC) agents can be considered as deliberately introduced biotic contaminants that, when successful, reduce the overall biomass of contamination and often bring considerable self-sustaining economic relief to farming communites.

Although the introduction of exotic agents would seem to be contrary to conservation philosophy, there are no quantified instances to date where the introduction of arthropod agents has been shown to have harmed a specific conservation programme or has been categorically damaging to native fauna. There is only limited anecdotal evidence that introduced parasitoids may have damaged certain specific native taxa. CBC in some cases actually assists conservation by reducing the level of exotic pests in nature reserves.

As CBC is an important socio-economic method of pest control, especially for tropical farmers, and as the taxonomic groups and life-histories of its targets are so different from those insects of endangered status, the two approaches are not in conflict. But as CBC is virtually irretrievable, it must continue to be carried out carefully and selectively only by truly responsible CBC agencies using appropriate quarantine facilities.

Tourists and general travellers pose a greater threat to native faunas than do the activities of such CBC agencies. It is well known that vertebrate agents and certain invertebrates, especially snails, can be devastating to certain native biotas. Additionally, and in view of the impending world-wide biotic diversity crisis, even traditional agents such as insect pathogens, insect parasitoids, and insect and mite predators, should be viewed with extreme caution—especially when oligophagous, and unquestionably when polyphagous.

Demand of the construction industry for coral rock in the Maldives will soon reach the limit of supply capacity using existing collection techniques. It is estimated that, at the current rate of consumption, the supply of living coral rock from inner atoll ‘faros’ in North Malé will be exhausted within 30 years. Current mining practice has already necessitated the collection of living coral rock from outer atoll ‘faros’ which actually protect the integrity of the islands against the erosive influences of monsoon storms.

Biological surveys of mined sites (compared with controls where no mining activity has taken place) indicate a dramatic reduction in coral variety and abundance and in some cases an almost total depletion of living coral after mining. At intensively-mined sites, diversity and abundance of coral-reef fishes is also markedly reduced, with some reef species commonly used as baitfish entirely absent. The time taken for reefs to recover from mechanical damage has been shown by other research work to be extremely variable, with some cases showing no recovery in some circumstances. Shallow-water reefs that have been colonized by slow-growing ‘massive’ corals, such as those which have been mined in the Maldives, may take a minimum of 50 years to recover to their former state—under optimum conditions.

Observations on mined reefs in the Maldives show that recovery over the last 10 years has been minimal, and therefore suggest that recovery times may be much in excess of 50 years. Indeed many of these reefs may not recover at all unless some attempts are made to re-establish corals at these sites. It is also clear from the study of reclaimed and dredged sites and those where less-intensive mining has taken place, that regeneration of corals is occurring. It is quite probable that the rate of recovery is dependent on the intensity of the disturbance which initially took place.

Alternative building materials are available in the form of concrete blocks. Use of these blocks is more cost-effective than use of coral rock; however, there is a lack of any quality control, which would be needed to promote the widespread use of concrete blocks as a substitute for coral. Recently, recommendations have been approved by the Government of Maldives for the execution of a feasibility study on mining an inner atoll faro which could potentially provide building material for as long as the next 500 years.

Establishment of a planned reservoir at Stiegler's Gorge on the Rufiji River would enable a large amount of highlyvalued energy to be produced. This would represent an important national asset. But although such a large net amount of energy would be realized on a short- to mediumterm basis, its production would be negatively influenced by a variety of reservoir operation options that would be related mainly to irrigated agriculture, fisheries, and water quality, which form trade-offs with hydropower generation.

The most significant effect of the dam would be drastic reduction, by controlled discharge, in the frequency of severe floods in the lower Rufiji valley. Floods in excess of 2,500 cubic metres of discharge per second could be reduced in number from some 14 to 3 in 24 years, or from 167 to 13 during 300 years. The most devastating floods would also be reduced—from once in about 8 years to once in maybe 40 years.

The STIGO Project impact area contains a major wildlife resource in terms of size, density, and diversity. Its accessibility to Dar es Salaam gives it a great potential for tourism development. As an access road to the dam-site would be a necessary prerequisite to implementation of the STIGO Project, it would indirectly help to open up the Selous Game Reserve to tourism, which is currently being hampered by poor communications. Also, a substantial amount of the forest resources identified along this road could be exploited.

No complete populations of animal wildlife would be in danger from direct ecological consequences of river impoundment and dam construction. However, significant proportions of the populations in the STIGO Project impact area of three species (Giraffe, Wildebeest, and Zebra) would be potentially at risk, owing to their need for habitats of restricted range. On the other hand some species, including Crocodile and Hippopotamus, would increase in numbers following creation of the reservoir and improvement of their habitat downstream of the dam (due to swamp drainage).

Not withstanding that the ecological impacts of dam construction are relatively minor, the socio-economic impacts on wildlife and conservation values are potentially great—resulting, for instance, from facilitated access to the heart of the Selous Game Reserve and concomitantly increased conflict between wild animals and Man. These circumstances would reduce wilderness values and disturb animal wildlife, so that, especially, commercially valuable species may be expected to decline unless strict regulations are made and enforced.*

River impoundment would have very negative impacts on floodplain fisheries and agriculture, the latter of which would probably be changed to irrigated agriculture with artificial fertilization, while floodplain fisheries would totally collapse. Some mangrove stands in the Delta would probably be displaced by reeds. Delta fisheries would be very negatively affected, because of changes in the water regime as well as in salinity levels.

Water quality in the planned reservoir and in the downstream area would be negatively affected by the project. The water would often be unfit for human and animal consumption and use, as well as unfavourable for fisheries. The project would also have negative effects on the health of the riparian population, owing to increased potentials for disease vectors.

Overall, a high degree of uncertainty is involved in the project. The effect of the primary project (hydropower) would be negative owing to its drastic consequences for the immediate and more distant impact areas. Its image might improve and probably become positive if development in the Basin were carefully controlled and managed. This would require reliable prediction of the impacts and a thorough analysis of the remedial and/or additional measures to arrive at an integrated development strategy for the Basin. Although several measures are planned to eliminate and/or minimize the negative impacts, their implementation may be difficult in view of the bad economic situation of the country.

Lyon's conclusion (cf. 1919) with regard to the stand reduction dieback on the lower wet slope of Haleakala Mountain led to a forest management policy that saw importation of alien tree species as necessary for protecting Hawaiian watersheds. Unfortunately, this also led to a further demise of native vegetation.

It is possible that the low-canopy species diversity in Hawaii may increase the rate of paludification and swamp (or bog) formation, under wet climatic conditions, over that in a similar environment with a broader spectrum of successional species. However, a boggy landscape can also be a good watershed. Historic evidence comes from the sugar-cane planters on Maui—the supposed beneficiaries of the exotic tree-planting programme—who had objected to it on empirical grounds (Holt, 1983). A more recent study of a canopy dieback in the Hilo watershed showed that extensive tree mortality had no effect on the rate and quality of the water yield from this area. The forest hydrologist, R.D. Doty (1983), who investigated this relationship, attribut ed the outcome to the vigorous undergrowth which remained intact during the dieback event.

Further landscape ageing in Hawaii's shield-shaped mountains involves a process of fluviomorphic dissecting of the gentle slopes lying inland. This results first in amphitheatre-headed valleys and eventually, on the windward sides, in steep residual escarpments with numerous secondary footslopes. Wirthmann & Hüser (1987) derived details of this process from the side-by-side comparison of younger with older Hawaiian mountains. When the waterlogged surfaces of the plateau-like lower wet slopes of shield volcanoes become laterally drained, closed Metrosideros forests can become re-established. That this natural re-establishment process works in the Hawaiian mountains can be seen by going from the shield-shaped slopes of the younger, to the more dissected slopes of the older, high islands. Vigorous Metrosidews forests occur also on the older islands, but they are typically of lower stature than those on the younger mountains.

Unfortunately, most of the secondary slopes formed at the base of the steeply-cut windward mountain slopes of the older high islands have been deforested to make way for crop agriculture. Many of these lower slopes have subsequently been abandoned for economic reasons; but instead of reverting to forests, they have become invaded in many areas by alien grasses and shrubs. Particularly damaging has been the introduced pyrophytic Broomsedge (Andropogon virginicus). This grass has retained a temperate-zone phenology, whereby it undergoes partial dormancy during the winter season when rainfall increases (Sorenson, 1980). As such it has contributed to accelerated erosion in two ways: (1) by preventing recirculation of soil water in areas where excessive runoff is a problem, and (2) by attracting repeated fires and preventing succession to a forest, which would provide much-improved protection against the accelerated soil erosion (Mueller-Dombois, 1973) that is such a common feature on the lower windward slopes of the older islands.