Convention on biological diversity
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AgricultureConversion of forests to agricultural landThe major causes of deforestation are the expansion of subsistence agriculture and large economic development programmes involving agriculture. The conversion of forests into agricultural land has been the major historical cause for deforestation in Europe and still is a major driving force in the tropical and sub-tropical areas. The agents vary from small farmers practising shifting cultivation or clearing forests for subsistence needs to large agricultural concerns that clear vast tracts of forest lands in order to establish cattle ranches or agro-industrial plantations such as soya beans in Latin America and oil palm in Indonesia/Malaysia. Dismantling of agro-forestry systemsAn emerging and rather insidious threat to biological diversity and tree genetic resources is posed through the dismantling of agro-forestry systems, i.e. the removal or failure to plant trees in agricultural and horticultural systems. This is usually associated with intensified, often monocultural, agricultural and livestock husbandry practices that eliminate trees from rural and urban agricultural areas. In Tonga, especially on Tongatapu, successive phases of unsustainable cash cropping have led to the elimination of trees in agro-ecosystems. In parts of Africa many useful tree species now exist only as scattered individuals or highly fragmented non-viable populations in agro-ecosystems, and are likely to disappear within the next few decades [reference needed]. Trees in agro-ecosystems may disappear either directly through cutting and clearing, or through the establishment of a hostile environment for regeneration and recruitment of remnant tree species. OvergrazingOvergrazing is increasingly a major threat to biodiversity in both tropical and temperate forests. The main impacts are damage to the topsoil, destruction of understorey vegetation and/or its replacement with a narrower range of unpalatable species and selective browsing of regenerating tree species, which may eventually result in the elimination of particular species.
Poor forest utilisation practices remain widespread in all forest types, despite well-documented studies and widespread demonstrations of the environmental and economic benefits of sustainable management methods such as reduced-impact logging (RIL) practices (Putz et al., 2000) or close-to-nature forestry. Poor logging practicesPoor logging practices are those that lead to a longer-term reduction in the actual and potential economic, ecological and social functions of the forest. For example, large clear cuts using heavy machinery can lead to soil compaction and nutrient washout. Another form of poor logging practice is the destruction of specific habitats, such as rivers and streams. Poor logging practices adversely impact on biodiversity values of forests by degrading them in various ways, and sometimes make it more likely that the areas will be converted to an alternative land use at a later date. Several more sustainable logging practices exist. Reduced impact loggingReduced impact logging systems are currently being developed in several tropical countries in response to concerns over the ecological and economic sustainability of harvesting natural tropical forest stands. RIL systems use an array of best harvesting techniques that reduce damage to residual forests, create fewer roads and skid trails, reduce soil disturbance and erosion, protect water quality, mitigate fire risk, help maintain regeneration and protect biological diversity (Holmes et al., 2000). Ecological damage is minimised by taking an inventory of trees before harvesting and cutting only mature trees. Putz et al. (2000) have examined the various possible reasons why RIL practices are not being more widely applied. These authors conclude that the main reason for low adoption rates is that, under certain conditions, RIL adds to logging costs rather than delivering savings. These conditions include situations in which compliance with logging guidelines restricts access to steep slopes and/or prohibits ground-based timber yarding on wet ground. Putz et al. (2000) conclude that widespread adoption of RIL may require financial incentives, such as those which may be provided due to enhanced carbon sequestration in carefully logged forests c.f. conventional logging (e.g. Andrewartha and Applegate, 1999). Over-intensive forest managementIn temperate and boreal forests, several aspects of over-intensive forest management negatively impact on forest biodiversity. These include: the replacement of natural and semi-natural forests by monospecific and even-aged plantations, large scale clear-cutting in areas without natural large scale disturbances, the planting of inappropriate species, the removal of important forest structures such as dead and decaying wood, the destruction of key habitats and the disturbance of drainage patterns. “Close-to-nature” forestryHowever, in many countries of the temperate and boreal zones, forest management methods that try to maintain a high level of biodiversity in managed forests (e.g. “close-to-nature” forestry) have been increasingly applied in recent years. These management methods mimic natural disturbances regimes, use local and site adapted species, increase the diversity of species, ages and structures in the forest stands, maintain important micro-structures such as dead and decaying wood, protect important key habitats and limit the use of pesticides, fertilisers, drainage and damaging machinery. The Pro-Silva movement, an association of European foresters that advocates close-to-nature forestry, has developed specific principles and guidelines related to sustainable forest management, the conservation of ecosystems, the protection of soil and climate, the production of timber and other forest goods and the recreational, amenity and cultural aspects of forests (Box 6). According to the German National Forest Programme15, close-to-nature forestry has been developed over the past few years in all states (Länder) largely relying on the use of natural processes and self-steering mechanisms. In many boreal forest countries (Canada, Finland, Norway, Sweden), private companies, state forest services and forest owners increasingly apply specific forest management techniques such as the retention of particular trees and habitats in order to mimic natural forests, they also use controlled burning to mimic natural fire disturbance regimes. Many forest certification standards require such forest management methods to be applied16.
Fuelwood and charcoal collectionExcessive fuelwood collection from forests will often have negative consequences for FBD for several reasons. In areas where fuelwood remains plentiful then particular preferred fuelwood species may be targeted and these can decline and eventually disappear. For instance, fire-stick harvesting from Pinus merkusii in north-east Thailand, a small-scale but highly destructive practice, is likely to lead to the disappearance of pine and mixed pine-dry evergreen forest associations in the region. In areas where fuelwood is in high demand, especially around major population centres, then forest communities may be degraded through excessive cutting and/or removal of biomass and nutrients. For example, extensive removal of fallen leaves for fuel in Eucalyptus and Casuarina plantations can break the nutrient cycle, lower productivity and lead to soil erosion. Specific fuelwood plantations can take the pressure off native forests to provide this commodity, while involving local villagers in thinning and pruning operations in existing plantations (in which the wood is salvaged for fuel). [Need references and would be much better to have an example from native forests than from eucalypts and casuarinas] Harvesting of non-timber forest products18Harvesting non-timber forest products (NTFPs) can strengthen sustainable management and conservation of forest biological diversity through the provision of direct benefits to people. There are exceptions however, e.g. resin tapping of important commercial timber species, including many dipterocarps in South east-Asia, may considerably weaken or kill trees if practised intensively over a long period of time and accordingly resin tapping has been banned in some countries. In Central Africa, several localised high valuable timber species, Baillonella toxisperma, Milicia excelsa and Pterocarpus soyauxii, are also important sources of valuable and irreplaceable NTFPs. These species may be heavily depleted by logging and accordingly there is a high conflict between their utilisation for timber and their potential to provide NTFPs (Laird, 1999). Furthermore, extraction of NTFPs can have major ecological impacts, including adverse consequences for forest biological diversity. Such situations usually involve commercial extraction, including export to areas remote from the production area, rather than subsistence/traditional uses. They may also be associated with identification of new medicines, e.g. taxol and taxine from Taxus baccata, Himalayan yew (Sharma, 1999), or new markets for traditional NTFPs, e.g. yohimbine from Pausinystalia johimbe, yohimbe (Sunderland et al., 1999), or with increasing pressure on a diminishing resource base, e.g. Securidaca longipedunculata (‘mother-of-medicine’) in Niger and Alphitonia zizyphoides (toi) and Tarenna sambucina (manonu) in Tongatapu, Tonga. Prunus africana is already over exploited in parts of its natural range for its medicinal bark (Dawson et al., 2000). Reductions in forest area and associated reductions in availability and access to traditional NTFPs, coupled with increased human pressure, are factors often associated with unsustainable extraction rates for NTFPs. Genetic impacts of harvesting of NTFPs can be quite considerable (see: Namkoong et al., 1997; Peters, 1996; Boyle, 1999). In cases where whole plants are harvested, the effects of reduced population size may be genetically significant. For a small number of particularly valuable species, entire populations have already been lost or severely depleted through over-exploitation of NTFPs. Examples include aquaje palm (Mauritia flexuosa) in Peru (Vasquez and Gentry, 1989), Aquillaria (agarwood) species in South and South-east Asia, sandalwood species19 in the south-west Pacific (Corrigan et al., 1999) and rattan species in parts of South-east Asia (Dransfield, 1989). Large-scale harvesting of the reproductive structures of plants (flowers, fruits, nuts and seeds), will directly reduce the effective size of the pool of reproductive parents and reduce genetic diversity in subsequent generations. Selective commercial harvesting of these parts can also adversely affect the genetic composition of the tree species and populations being utilised (Peters, 1990, 1996). In such cases, harvesting the ‘better’ fruit may remove particular, advantageous genotypes and may result in a population dominated by trees of marginal economic value with much less value as a genetic resource. In cases where a high proportion of flowers and/or fruits of a particular species is harvested on a regular basis, the most important long-term ecological and genetic impact will be a reduction in seedling regeneration and recruitment, possibly leading to eventual extinction of the population. One approach to management and utilisation of NTFPs in the Brazilian Amazonia has been the establishment of specific “extractive reserves”(see e.g. Kageyama, 1991). Such reserves are managed by the local communities that depend on the forest for a significant component of their livelihood. It was anticipated that their establishment would result in sustainable management of the natural resources in the reserve, e.g. rubber (Hevea brasiliensis) or Brazil nuts (Bertholletia excelsa), and concomitant conservation of forest genetic resources and biological diversity. However, such export commodities are very susceptible to external market forces. Therefore, when the price of natural rubber was more than halved in the 1990s, production from Amazon forests collapsed (Assies, 1997). In more recent times, competition from cheaper Bolivian rain forest sources has contributed to a sharp decline in the export of shelled Brazil nuts from Brazil (Assies, 1999). Peters (1992) has suggested that where market-orientated extraction of NTFPs is the objective, then it would be best to focus on tropical forests dominated by only one or two useful species (oligarchic forests), rather than species-rich ecosystems. Peters (1992) gives a number of examples, mainly palm species, from Amazonia.
OverexploitationOverexploitation can be the main factor causing extinction in the case of a relatively small number of species, principally certain valuable larger animals and tree or other plant species. While numerically small, such losses are of great concern, because they may be irreplaceable species at/or near the top of the food chain and/or keystone species and also because they often represent the loss of a component of biodiversity that is more highly valued by humans. Overexploitation typically involves the hunting of larger mammals for subsistence and sale in local markets and, more destructively, for long-distance trade and export. Bush meat hunting and trade has in many areas lead to the phenomenon of “empty forests”, i.e. the total extinction of wildlife, even in natural forests (Carey, 1999) so that in many tropical forest areas, bush meat hunting has become the single most important conservation problem (see Chapter IV, Box 9.). The Convention on International Trade in Endangered Species of Fauna and Flora (CITES) has contributed substantially to the reduction in across border trade of endangered animal and plant species, but tougher penalties and more coordinated enforcement may be needed to reduce black markets in the products of some highly-valued species. In order for CITES to be fully effective it needs to be appropriately complemented by well-implemented domestic regulations in the countries where the threatened species occur naturally. Introduction of invasive alien species and genotypesThe ever-accelerating rate of biotic invasion of alien species and genotypes is a major element of human-induced global change. There are well-documented and numerous examples of invasive alien species and diseases causing tremendous damage to FBD. [give examples and references; e.g. elm disease]. The damaging species may come from any taxanomic group (i.e. from micro-organisms to algae to vertebrates), as well as being from all function levels, (for instance, predators, herbivores, primary producers, parasites). Island forest ecosystems and species appear particularly vulnerable, e.g. the brown tree snake (boiga irregularis) introduced into Guam, in the northern Pacific, has devastated the island’s birds [reference needed]. Invasive alien plant species, in particular, pose a well-confirmed and increasing danger to ecosystem integrity of many forest ecosystems. For example in the Pacific Islands there are a number of weedy trees which are having a major negative impact on FBD [reference needed for the a-d below?]: (a) Leucaena leucocephala has taken over vast expanses in drier forest associations in the Pacific. It was deliberately broadcast on several war-disturbed islands after World War II (e.g. Guam) and more recently has been introduced for agro-forestry plantings. (b) Miconia calvescens (purple plague), a small tree from tropical America, is the biggest invasive species problem in French Polynesia. It now covers 60% of Tahiti where is forms monospecific stands and totally shades out native vegetation. It has become established on other Pacific islands, including the biodiverse island groups of Hawaii and New Caledonia. (c)
(d) Cordia alliodora is a major invasive tree of native forest in parts of Fiji, Tonga and Vanuatu where it had been introduced from Central America for forestry trials and plantings. |