Land use change has obvious direct effects on the world’s ecosystems through habitat destruction and alteration of competitive relationships but also indirect effects through alteration of fire frequency and nutrient and water balances (Mooney and Hofgaard, 1999). Land use change is a major contributor to the introduction and spread of alien invasive species which, in turn, are the second most important threat to biodiversity, behind habitat destruction.
Activities of the forest sector can contribute to the introduction and spread of alien invasive species through forest utilization practices and the intentional introduction of species for commercial forestry, agroforestry and other purposes. Trade in forest products is also a contributor as was discussed in the section on trade in Chapter 3.
Forest utilization, in particular practices such as timber harvesting, extraction of non-timber products, the construction of logging and transport roads and facilities for logging camps, and the conversion of natural forest to plantations, can have direct and indirect negative impacts on the ecological functions of forests and on forest biodiversity by promoting the invasion of alien species. For example, logging was found to be the prevailing factor influencing the establishment of alien plants areas in a national park in Madagascar and due to the dominance and persistence of the invasive species, the logged sites never recovered native species diversity (Brown and Gurevitch, 2004). As a result, the structure and diversity of the forest remained altered even long after logging operations has ceased. Clearcutting practices have also been shown to lead to the replacement of native ant communities with invasive nonindigenous species (Zettler et al., 2004).
Forest roads provide essential access for timber extraction, management and monitoring of forest resources and as such are an important requirement for sustainable forest management and use. However when poorly designed and maintained, forest roads are often the cause of a variety of environmental problems associated with forest harvesting operations. Under some circumstances, forest roads may also initiate or accelerate the invasion of non-native species that ultimately displace native species. In addition, the increased levels of human activities in previously inaccessible areas, as facilitated by forest roads, cause many environmental problems including the possible introduction of alien species.
Land use changes can promote the emergence or re-emergence of infectious diseases which degrade human health and that of other species (Morse, 1995; Vitousek et al., 1997; Bryan, 1999; Western, 2001). Infectious disease agents often, and perhaps typically, are alien invasive species since they are invaders over most of their range (McNeely et al., 2001). Activities of the forest sector contribute to the spread of infectious diseases in a variety of ways.
Forest activities, such as clear-cutting and road building, may increase exposure of workers to infectious diseases such as human immunodeficiency virus (HIV), Ebola hemorrhagic fever, Marburg hemorrhagic fever, yellow fever, leishmaniasis, malaria and Ross River virus disease (Morse, 1995; Daszak, Cunningham and Hyatt, 2000; Chivian, 2002). The index case for the Ebola epidemic in the Democratic Republic of the Congo in 1995 is thought to be a charcoal maker that worked in the forest outside Kikwit (Sanchez et al., 1995). Logging can change the abundance, extent and quality of aquatic larval habitats for the Anopheles mosquito vectors of malaria and by disturbing the forest floor, creating depressions that catch and hold water, thus creating new sites for the development of more mosquitoes (Chivian, 2002).
Deforestation can result in humans coming into closer contact with the vectors for such diseases as leishmaniasis, yellow fever, trypanosomiasis (both African sleeping sickness and Chagas disease), and Kyasanur forest disease (Morse, 1995; Wilson, 1995; Chivian, 2001; Chivian, 2002; PAHO, 2004). Similarly, some animal reservoir hosts increase in abundance near the edges between forests and human settlements thus increasing the risk of human exposure to pathogens.
The destruction of forest habitat may result in the removal, replacement or eradication of dominant vector species and sometimes, the replacement species are more effective vectors of disease as observed with loa loa (tropical eyeworm) and onchocerciasis (river blindness) (Chivian, 2002). Deforestation and desertification may also be accompanied by changes in the distribution of vectors such as ticks, blackflies, tsetse flies and Anopheles mosquitoes (Chivian, 2002). For example, forest degradation in the United States has led to the emergence of Lyme disease as a result of the loss of some predator and competitor species of white-footed mice which increased population densities of the mice in remnant forest fragments (Chivian, 2002). Higher population densities of the mice, which carry the bacteria (Borrelia burgdorferi) that cause Lyme disease, increase opportunities for ticks to acquire such infectious agents.
Reafforestation activities can also affect the population dynamics of vectors and reservoirs which can promote the emergence of infectious diseases. Such activities in the United States and Europe have resulted in an increase in the population of deer and hence the deer tick, which is a vector of Lyme disease (Morse, 1995).
The forest sector itself is a major source of alien invasive species. Many of the tree species used in commercial forestry in many parts of the world are alien or non-indigenous to the area. Hundreds of species have also been widely and successfully planted for a variety of purposes including afforestation, desertification and erosion control, and for the supply of fuelwood and other forest products. Such intentional introductions however, can bring about many unintended and costly consequences when these species escape cultivation and invade natural ecosystems.
Alien tree plantations can have negative effects on the biodiversity and water resources of afforested areas (van Wilgen et al., 2001). Introduced species that spread from plantations to natural and semi-natural areas, and also into areas set aside for conservation and water production, have considerable impacts on ecosystem properties and functions (de Wit, Crookes and van Wilgen, 2001).
In 2000, forest plantations covered 187 million hectares (FAO, 2000b). Pinus spp. (20 percent) and Eucalyptus spp. (10 percent) were the dominant species planted worldwide, followed by Hevea spp. (5 percent), Acacia spp. (4 percent), Tectona spp. (3 percent), other broadleaved species (18 percent), and other coniferous species (11 percent); the remaining percentage was unspecified. Globally, broadleaves made up 40 percent of forest plantation area with Eucalyptus the principal genus while coniferous species made up 31 percent of which Pinus was the principal genus; the remaining percentage was unspecified. Among the most widely planted trees, Acacia and Pinus species are the most prominent on weed lists and in reviews of invasive tree species (Richardson, 1999). This prominence is perhaps due to the fact that many species in both genera are highly capable of adapting to a wide variety of environments. Richardson and Rejmánek (2004), in a summary of data on conifer taxa from 40 countries, found that 80 alien taxa were known to be naturalized (13 percent of species) and 36 taxa were known invasives (6 percent). Twenty-eight of these known alien invasive conifer taxa belong to one family (Pinaceae) and 21 of these to one genus (Pinus). The Pinaceae has the highest proportion of invasive species than any other angiosperm family comprising mainly trees and shrubs.
Considerable attention has been given to the effects of alien invasive trees from plantations in the southern hemisphere, in particular in South Africa. Plantations cover over 1.5 million ha of land area in South Africa and are an important part of the economy contributing US$300 million to the gross domestic product (GDP) and employing over 100 000 people (Richardson, 1998; FAO, 2000b). The forest sector has been one of the country’s major sources of alien species infestation; a large proportion (38 percent) of the area invaded by woody alien plants in South Africa is occupied by species used in commercial forestry, in particular Pinus and Acacia species (Richardson, 1998). Rouget et al. (2002) analyzed the distribution of forestry plantations and invasive stands of Acacia mearnsii and Pinus spp. in South Africa and discovered that these two taxa account for 60 percent of the area under commercial plantations and 54 percent of the area invaded by alien trees and shrubs. Black wattle (Acacia mearnsii), introduced into South Africa from Australia in the mid-19th century, has been widely planted in the country and forms the basis of a small but significant industry. A highly invasive species, A. mearnsii has spread to an area of 2.5 million ha in South Africa where it negatively affects water resources, biodiversity, and the stability and integrity of riparian ecosystems (de Wit, Crookes and van Wilgen 2001). Due to their ability to fix atmospheric nitrogen, Australian Acacia species (A. cyclops and A. saligna), have radically altered nutrient-cycling regimes in nutrient poor fynbos ecosystems in South Africa (van Wilgen et al., 2001). The invasion of these species and some Pinus species into large areas of the tree-poor fynbos have caused many ecosystem level changes by altering factors such as biomass distribution, plant density and vegetation height, leaf-area index, litterfall and decomposition rates (van Wilgen et al., 2001). Such changes radically alter habitats resulting in major changes in the distributions of many animals. Invasive alien pines and acacias in South Africa can also reduce water tables (McNeely et al., 2001).
Many countries are also reporting significant problems with introduced eucalypts from Australia which can be particularly harmful environmentally because their leaf litter contains chemical compounds that prevent other species from growing (McNeely et al., 2001). This can have significant consequences for the forest sector.
Other notorious examples of invasive trees that negatively affect ecosystems include melaleuca and black cherry. The melaleuca or paper bark tree (Melaleuca quinquenervia), native to Australia, was introduced from South America into southern Florida in the United States as an ornamental in the early 1900s. Decades later, the population began to expand and the species is currently spreading at a rate of 11 000 ha a year throughout forests and grasslands of Florida’s Everglades National Park where it damages native vegetation and wildlife (van Wilgen, Cowling and Burgers, 1996; Crooks and Soulé, 1999; Pimentel et al., 2000; McNeely, 2004).
The North American black cherry, Prunus serotina, was initially introduced into central Europe as an ornamental and over the years had been planted to produce valuable timber and to function as windbreaks, firebreaks and shelter for game (Starfinger et al., 2003). By the mid 1980s, the species was considered an aggressive invasive species, particularly in Germany, that formed dense thickets which threatened plant diversity and natural regrowth of indigenous forest tree species.
Of the 2000 or so species used in agroforestry perhaps 10 percent are invasive (Richardson, 1999). While only one percent is considered highly invasive, these include many frequently planted species such as Casuarina glauca, Leucaena leucocephala and Pinus radiata (McNeely, 2004). The spread of introduced species from agroforestry plantings however are less of a threat than those from commercial forestry plantations since plantations are typically planted in much greater numbers over larger areas, afforested areas often adjoin natural vegetation, and spreading agroforestry trees will likely be held in check by agricultural practices and human use (Richardson, 1999).
With the increase in afforestation and land use changes over the past few decades, the magnitude of such detrimental consequences has increased significantly (Richardson, 1998; de Wit, Crookes and van Wilgen, 2001). Generally, the species that cause the greatest problems are those that have been planted most widely and for the longest time and the areas most affected tend to have the longest histories of intensive planting (Richardson, 1998).