Construction of new roads
The construction of new roads has a profound impact on the forest. Road building is considered to be one of the main causes of deforestation. Between 400 and 2000 ha may be deforested by each kilometre of new road built into intact primary forests (Contreras-Hermosilla, 2000). For example, the Trans-Amazonian highway opened up millions of square kilometres of previously inaccessible forest to colonisation and expansion of the cattle industry. Main arteries were soon followed by secondary roads that penetrated deeper into the forest, eventually producing a wide swath of deforested land on either side of the road [reference needed?]. All roads that are constructed with the purpose of providing better access to less developed regions within a country tend to push up real estate values for non-forest uses and encourage land speculation and deforestation. Logging roads are among the most important types of access roads that facilitate deforestation (Roper and Roberts, 1999)20
Hydroelectric development
Hydroelectric development is another important factor in deforestation. Reservoirs flood forest lands and transmission line rights-of-ways are cut out of the forest to carry the energy to consumers, causing permanent loss of forest cover. (Roper and Roberts, 1999).
Residential and cities development
Forests are also encroached upon by industrial and residential development as populations grow and cities extend outward (Roper and Roberts, 1999).
Mining and oil exploitation
Mining and oil exploration can be an important local factor in forest decline. Large mines, such as those of Carajás in Brazil and the Copperbelt of Zambia, consumed vast quantities of indigenous woodlands to supply fuel to their smelting operations before plantations of fast-growing species were established. The effects of gold mining has been widely publicised, particularly placer mining in the Amazon, but its negative impacts have affected the indigenous peoples and the quality of the water more than the adjacent forests. Oil exploration activities, such as the clearing of the seismic lines in the forests of eastern Ecuador, not only destroy the forests but also open them up to colonisation by subsistence farmers who follow the exploration crews (Roper and Roberts, 1999). Furthermore, oil exploitation often leads to severe pollution due to the leakage of pipelines; for example, several areas of the Siberian forests have been affected [reference needed?].
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Box 5. Extinctions and forest fragmentation
Gilpin & Soule (1986) have grouped extinctions into two kinds: deterministic and stochastic. "Deterministic extinctions are those that result from some inexorable change or force from which there is no hope to escape. A deterministic extinction occurs when something essential is removed, or when something lethal is introduced. Stochastic extinctions are those that result from normal, random changes or environmental perturbations. Usually such perturbations thin a population but do not destroy it. Once thinned, however, the population is at an increased risk from the same or from a different kind of random event."
Aside from outright loss of forest habitat and degradation, forest fragmentation looms as one of the major threats to forest biodiversity. According to the theory of island biogeography (MacArthur and Wilson, 1967) when an area loses a large proportion of its original habitat, especially if the remnants are in fragmented patches, then it will eventually lose some of its species. Species/area curves offer a quantified prediction that the larger the forest area, the greater the number of species that can occupy the forest, and vice versa, e.g. reducing a habitat to 10% of its original size may lead to the loss of about 50% of species. Fragmentation exposes populations to the dangers of demographic or environmental stochasticity. Many populations are committed to extinction in the long term due to this stochasticity, leading to the so-called extiction debt (Hanski, 2000)
However, there is little direct evidence of species extinction in continental forest ecosystems. As an example, the Atlantic forest ecosystem in Brazil has been reduced to around 10% of its original extent over the past century. The theory of island biogeography predicts that around 50% of the species of this ecosystem would have become extinct as a result. A thorough survey of this area has shown that only a handful of species appear to have become extinct, although many are reduced to small populations and are therefore most likely ‘committed’ to extinction (Brown and Brown, 1992?). Another study, though, (Brooks et al., 1999) points to an extinction lag time, e.g. between deforestation and species extinction. Evidence from studies on saproxylic beetles, which are a good indicator group of old-growth boreal forests, suggests that most extinctions have happened in areas with the longest history of forest use and smallest proportion of recent old-growth forests (Hanski, 2000). It is evident that the extinction is a long-term process.
Species which have the biological attributes that enable them to adapt to modified remnant forest fragments, including forest/cleared land ecotone, may increase in frequency in the new, more fragmented and disturbed forest regime. They include pioneer and secondary tree species, those with long distance seed and/or pollen dispersal, species which can better tolerate some level of inbreeding, or those which can coppice. Thus the incidence of ‘weedy’ species may increase with fragmentation.
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Whilst at the broad scale, fragmentation is relatively easily mapped with the aid of aerial photography or satellite imagery, assessment of the long-term impact of such fragmentation on genetic resources of remnant stands is much more problematic. Fragmentation is a very complex threat the impact of which will depend upon many interacting factors including size, shape and location of remnant stands, and extent of gene flow between fragments, which will be affected by the degree of geographic isolation and the presence of forest corridors or scattered trees that act as bridges (‘stepping stones’) for movement and dispersal of species.
For tree species, the study of key evolutionary processes in fragmented populations will help in assessing the likely long-term impacts of fragmentation in given areas for given species. This will involve study of, for instance, breeding systems and gene flow through using isozymes and other molecular markers (microsatellites and RFLPs), Studies on the levels of seed set, seed viability and seedling vigour in fragmented stands may be used to provide early warning of inbreeding. Generally we know that homozygosity increases with fragmentation but that with time this trend reverses to a more heterozygous condition.
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