Many aloes are regarded as endangered species. Various threats exist, and they can be placed into three main categories: over-collection of plants for cultivation, destruction of plants in harvesting leaf exudates and destruction of natural habitats.
Some species, especially the miniature species of Madagascar and smaller-growing plants of South Africa, are possibly over-collected by people supplying the nursery trade (Newton and Chan, 1998; Oldfield, 1997). The rising popularity of 'field trip holidays' by amateur growers as a new kind of tourism, also results in some collection of wild plants. An individual might collect only one or two plants of each species and may feel that little or no harm is being done to the population. However, as the numbers of such travellers grow the result can be a major depletion of well-known populations to which access is easy.
There is also a lucrative trade in leaf exudates, required mainly for medicinal and cosmetic purposes, and these are frequently harvested from wild plants. Much of this activity is well organised, but there is also a large unofficial exploitation of wild plants (Newton, 1994; Newton and Vaughan, 1996). In South Africa 'aloe tapping' is a well-established industry, going back for over 200 years. The main species used is A. ferox, with export records dating back to 1761. The species is widespread in Western Cape and Eastern Cape Provinces. Newton and Vaughan (1996) estimated that a total of about 700 tons of crystalline bitters is harvested each year from about 17 million plants, 95% of which are in the wild. Much of this harvesting and export is illegal or undocumented. However, they concluded that the harvesting is carried out on a sustainable basis, thanks to traditions established in the communities of aloe tappers. In contrast, the harvesting of leaf exudates is more recent in Kenya, with local people being paid by 'outsiders' with no attempt to ensure sustainability. With no traditional or other controls in place, various species may be harvested without regard to chemical composition. In some areas this harvesting might be done on a sustainable basis (Newton, 1994) but cases are known where whole populations are destroyed in the process.
A third threat is the destruction of habitats. One problem is overgrazing. Many people in arid areas have herds of domestic animals in numbers far greater than the carrying capacity of the land, and the land becomes increasingly denuded of vegetation. In many countries where aloes are native, the rise in human population levels results in an increased demand for land to use for agriculture, building, etc. This has led to wholesale clearing of natural vegetation. In some areas, the continued expansion of human populations is forcing people to move into arid areas, where many aloes occur.
Attempts to protect aloes as endangered species have been made at two levels — national and international. Many countries have signed various international agreements on the conservation of biodiversity, though the will to act, which is implied by the signature, is not always translated into action. Most countries have national legislation aimed at protecting endangered species, animals and plants and at preserving habitats in selected localities, such as in national parks. Unfortunately, enforcement of the legislation is poor or wanting in most African countries, because of a lack of enforcement personnel and a need to concentrate national effort on solving enormous economic and social problems. Only South Africa has strong enforcement activity, and even there much illegal activity is known to occur (Newton and Chan, 1998).
The most effective protection attempt at the international level is provided by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), started in 1976. This convention aims at controlling the movement of endangered species and derivatives between countries, prohibiting trade in some species (listed in CITES Appendix I) and requiring official documentation for numerous other species (listed in CITES Appendix II). Currently, 22 species of Aloe (mostly Madagascan) are listed in CITES Appendix I and the rest are in Appendix II, with the sole exception of Aloe vera, which is free of restrictions. The documentation requirement provides an opportunity for monitoring the international trade in the listed plants, or at least the legal trade. Data for the period 1983—1989 suggest that the most heavily traded aloes are not the species regarded as threatened in the wild, and much artificially propagated material is involved (Oldfield, 1997). However, there is concern about the demand for rarer species of Madagascar and South Africa. To meet the demand in the horticultural trade many nurserymen and individual growers propagate their stock plants by seeds or by vegetative means. This is positive action to reduce the pressure on wild populations. There are also attempts to re-introduce some rare species of succulent plants to their natural habitats, using material propagated in cultivation. One such scheme involves plans to plant out seedlings of Madagascar's largest aloe, A. suzannae, raised in South Africa from field-collected seeds, though the bureaucratic formalities have proved to be formidable (Smith and Swartz, 1997—1999). The most promising development is the use of tissue culture techniques, by means of which thousands of plants can be produced within a few months. Relatively few aloes have been propagated by tissue culture, but there are reports of success (Barringer etal., 1996; Fay and Gratton, 1992; Fay etal., 1995). It is to be hoped that future work in this field will ensure the survival of these fascinating plants.
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