Deforestation, whatever its causes and motivation, is the most powerful direct threat to forest biodiversity. Some support the view that the conservation of biodiversity requires halting deforestation and keeping commercial timber production out of forests (Leslie et al. 2002). This is the principle underlying the creation of totally protected areas (TPAs). However, few countries are willing or able to place all of their natural forests in TPAs. Most countries, under present economic conditions, have no choice but to continue encouraging the harvesting of timber growing in their natural forests.
Other opinions hold that biodiversity can also be conserved in production forests under sustainable management regimes; total conservation should be the first strategy and SFM is only preferred in cases when the alternative land use is wholesale logging or dramatic land use change; in some cases it is better to allow one-time conventional logging, followed by complete protection of the area (Reid and Rice 1997). However, this seems to be quite impractical given that there is no guarantee that the logged area can be effectively protected. In addition, regeneration following conventional logging may be good in some types of forests but not in all types (McRae 1997).
In order to maximize the diversity of species that are conserved, it is essential to address biodiversity conservation at the landscape level. Some species require large areas to conserve a viable population. The question of scale is important when considering the impacts of disturbance on forest biodiversity and when designing strategies for biodiversity conservation. There are different measures of biodiversity on different scales. For tropical forests greater than 106 km2 in size of Africa, Asia, and the Americas, overall or "gamma diversity" (or diversity found across large geographical regions) varies from perhaps 30,000-120,000 species of flowering plants. Smaller forest plots ranging from 0.001-0.01 km2 in area contain from 30-300 tree species ("alpha" or "within-habitat diversity"). Less information is available on "beta" or "between habitat diversity", which describes how species composition varies from one area to another. How diversity varies among plots of similar sizes in different forests and with distance among plots is a question relevant to the design of protected areas. For example, Condit et al. (2002) found that regions in Panama and the western Amazon that are 104 km2 in area support 3,500-5,000 tree and shrub species, yet at smaller scales (0.01 km2), the western Amazon forests support two to ten times as many species as do Panamanian forests. This raises the question, is it more worthwhile to preserve an Amazon forest than a Panamanian forest of similar size? Numbers may not be the only factor that matter when considering the preservation of species; the answer may depend on which species are present - their rarity, scarcity, and value (known and potential but unknown) to humans.
There are difficult issues relating to the size and spatial arrangement of protected and managed areas. The general conclusion is that protected areas should be as large as possible, but that ultimately their value as refuges depends more on their integrity and optimal distribution across the landscape than on their absolute size (Boyle and Sayer 1995). Several studies have shown that forest fragment sizes and degree of isolation are the prime determinants of species loss (Boyle and Sayer 1995). For example, Laurance et al.
(1997) reported that rain forest fragments in central Amazonia experienced a dramatic loss of above-ground tree biomass that was not offset by recruitment of new trees. These losses were largest within 100 m of fragment edges, where tree mortality was sharply increased by microclimatic changes and elevated wind turbulence. Permanent study plots within 100 m of edges had lost up to 30% of their biomass in the first 10-17 years after fragmentation. Habitat fragmentation affects the ecology of tropical rain forests by altering the diversity and composition of fragment biotas and changing ecological processes like nutrient cycling and pollination (Laurance et al. 1997). Therefore a strong case can be made for the maintenance of corridors of undisturbed forest linking refuge areas.
The conservation of species in isolated fragments of TPAs will be enhanced if these areas are surrounded by areas of modified, but biologically diverse, buffer zones, transition zones, and corridors (Boyle and Sayer 1995). ITTO's Guidelines for the Conservation of Biological Diversity in Tropical Production Forests suggest that there will be some degree of biodiversity loss in tropical production forests that would be mitigated by a comprehensive and integrated TPA network. The function of production forests in biodiversity conservation would be to allow the persistence of a large portion of the original biodiversity within a buffer zone around the TPAs, and to provide corridors that allow the free flow of genetic material among the forested areas (Leslie et al. 2002).
Some innovative plans, such as the UNESCO Man and the Biosphere (MAB) program's worldwide network of biosphere reserves, have begun considering the needs of local people by incorporating biophysical and socioeconomic factors into the management plans of protected areas (Khasa and Dancik 1997). A typical UNESCO MAB Reserve includes a core area under complete protection surrounded by buffer zones where a variety of human activities are possible. These include SFM, agroforestry, ecotourism, and even human settlements in the outer limits of the buffer zones. Similarly, extractive reserves, in which local people are responsible for forest management, have been developed for timber products in Quintana Roo, Mexico, and for NTFP extraction (mainly rubber and Brazil nuts) in Brazil.
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