By understanding the effects humans have on land cover, we can develop strategies to reduce negative impacts and create more sustainable long-term land use.
Beyond the noted associations between land use and cover change, the human dimension of land cover change is multidimensional and complex. At any one time and place there can be both negative and positive feedbacks in operation. For example, suppose an area of forest was cleared for agriculture and then later developed into an urban complex. Urban sprawl ensues and more and more forests are felled to grow crops to feed the growing population. Accelerating feedback is associated with the increased wealth and opportunities in the area, encouraging immigration. Growth will progress away from the center as long as resources permit the expansion. The core may tend towards decay while resources and wealth are concentrated at the fringe. A braking feedback will slow and eventually halt the expansion, as populations at the urban core become dependent on the wealthier fringe populations, leading to resource redistribution and a more stable situation.
These pressures and cycles are in part natural, and as with many natural events they can lead to environmental rejuvenation and net benefit instead of degradation. However, when uncontrolled, these human interactions often turn negative due to a lack of commitment. Review and community resolve are keys to sustainability, and in order to examine the main reasons for land use change, we need to separate the usually obvious direct factors (proximate) from more subtle but pervasive underlying factors (ultimate).
In our review we risk the possibility of focusing resources on factors that are not the most important (proximate) vs. those that are critical (ultimate). Proximate causes of change include the actual clearing of land by settlers for farming, and the handling of the cleared land for crops or pasture. Ultimate causes of change include the economic incentives that drive the migration of settlers to regions where forest clearing is possible, including land ownership, international influence, national to local government policies, economic and social development, and so on. Many of these factors affect the demands that will be placed on the land, whereas others (such as technology) affect the intensity of exploitation.
A comprehensive discussion of these many interacting factors is beyond the scope of this chapter; instead, the following sections will focus on three of the most commonly noted causes of land cover change, one ultimate (population growth) and two proximate (deforestation and agriculture).
Globally, population is expected to grow by about 50% from 1995 to 2025. This is a serious concern since most of the conversion of land can in some way be linked to population growth, typically via increases in population density. While these pressures are a global phenomenon, they are tied to land cover change in sometimes subtle ways according to the cultural milieu. In agricultural economies an increase in population leads to labor surplus, and can result in an increase in the area under cultivation -- this is called extensification. An alternative or parallel process is intensification whereby agricultural systems are enhanced to increase production via inputs of nutrients and pesticides.
The traditional way to gather population data is to take a census. Although this can give reliable results, data reliability should not be assumed for all areas around the world. Also, in the more loosely administered areas these data often do not indicate where populations are concentrated. For this reason, remote sensing techniques are sometimes used to estimate population and locate population concentrations.
Detection of city lights with US Department of Defense (DoD) satellites is one technique used to locate population concentrations. Under the Defense Meteorological Satellite Program (DMSP) the platforms are near-polar orbiting, Sun-synchronous satellites that monitor the global environment through the use of seven different sensors. Products from this program include daily nighttime images which can detect visible light that is emitted from population centers such as cities. Figure 8.04 shows an image of the United States. In this image, the white spots indicate city lights and the larger bright spots are metropolitan areas. Smaller and less bright spots often indicate small cities or towns.
The same DoD data are used to detect forest and grassland fires. This is done by overlaying images and removing the bright spots that are constant from image to image (population centers). After the bright spots that consistently appear in all images are removed, the remaining bright spots represent features that are only temporary in existence. In many cases, these are fires (see Figure 8.05).
Deforestation is an excellent example for showing population pressure as an ultimate (underlying) cause of land cover change. A graphic phenomenon and an obvious story, deforestation has been widely covered by the media, yet is also of great concern to those dealing with sustainable use of resources. It is difficult to assign blame for deforestation, although in many cases rural people in economically stressed countries are considered to be the culprits. This is an oversimplification because the root of the problem can often be traced to ultimate causes, such as government policy (or lack thereof), or production demands from people outside of the region undergoing deforestation. In Brazil over half of the deforestation in the Amazon basin was done by small farmers, and as the soils turned unproductive the areas were grazed by cattle, largely for export. Both the settlement and grazing were subsidized by the Brazilian government.
The detection and monitoring of deforestation can be done using remote sensing change detection techniques. Yet to point to trees being cut down is only the beginning of a long process of educating the host culture in a vertically integrated manner about the implications of these changes. A more difficult problem is documenting the long-term consequences of deforestation. The effect of habitat fragmentation on plant and animal populations is currently being researched.
2.3.1 Habitat fragmentation -- Habitat fragmentation is the loss or subdivision of a habitat as a result of land cover change. In addition to reducing the total area of a habitat type, fragmentation divides the landscape into patches that are similar to islands because they are surrounded by a matrix (or sea) of different habitat conditions. Because of fragmentation, large populations are divided into several smaller independent populations. This leads to genetic isolation and a loss of diversity as gene flow is severely restricted and the smaller populations become inbred. Extinction occurs when local populations are no longer viable, that is, if they cannot adapt to the changes or migrate to other areas.
There are a number of opportunistic plant and animal species that thrive in highly fragmented habitats. Often many of these species were not present before the land cover was fragmented, and they may be able to outcompete those species that were established before the fragmentation occurred. This greatly alters the species composition of an area, and in most cases results in impoverishment of the local flora and fauna.
Globally, agricultural expansion accounts for most of the area associated with land cover change. Much of this agriculture related change is due to the demand for increased quantities of agricultural products, in ways which tend to encourage a modification or abandonment of traditional practices, and introduce practices that are unsustainable without inputs of fertilizers and pesticides. This, in turn, creates a situation where there are cycles of dependency, for example, where the soil is degraded to the point that it cannot sustain crops without special intervention. Modern practices and improved technology are used to meet the growing need for food production but they, too, often are selected with a focus on only short-term benefits, resulting in negative impacts on the longer term environmental trends. These impacts can take the form of erosion and associated channel siltation, flooding, pollution, destruction of soil structure, leaching of nutrients, decreased microbiological activity, and alteration of the aquatic biota that provide important foodweb links for local populations. Thus, modern improvements should be confirmed appropriate to the task, and undertaken while keeping issues of sustainability in mind.
2.4.1 Traditional practices -- Traditional and sustainable agricultural practices in most parts of the world have benefitted from the use of long fallow (unused) periods, and a reliance on natural systems to replenish the nutrients removed by the cropping process. Unfortunately, some of these traditional practices have been modified to improve production with unexpected and undesirable results. An example of this is swidden agriculture with a short fallow period which does not allow land to recover adequately.
Swidden agriculture, also referred to as "slash and burn," is a practice that involves clearing the forest by cutting and then burning the trees to prepare the land for crops. Burning the forest in situ (in place) kills many pests and provides an abundant supply of nutrients that will benefit crop growth. The downside of swidden is that the land does not remain fertile for a long period of time. Typically the nutrients are depleted in just a few years, especially in the tropics where temperatures, extended growing cycles and soil ecology are particularly predisposed to nutrient loss. In the past, this style of agriculture was used successfully when the fields were left fallow for long periods, but in recent decades there has been for a great many areas a trend towards shorter and shorter fallow periods until crop product becomes unsustainable. Then more forest is cleared to meet the needs of the farmer.
A growing set of institutions are now encouraging farmers to use traditional techniques in conjunction with appropriate modern practices. Some cases have demonstrated that traditional agriculture may increase both the productivity and the biodiversity of an area, through mulch and tree crop mechanisms of microhabitat moderation and nutrient recycling, and through the mixing of multiple crop species (the flora) and native species that are drawn as pollinators and secondary consumers (the fauna). This agroecosystem approach is fostered by some of the more progressive programs oriented toward sustainable land use.
2.4.2 Modern practices -- Modern agricultural practices depend on continuously improved technology to develop and maintain productivity. The goal of these forms of agriculture usually is to achieve the greatest output per unit area of soil at the least short-term cost. Modern practices are predominant in industrialized nations, although they are rapidly being introduced in nonindustrialized countries.
Monoculture is a practice where large areas of a singular crop species or variety is grown, often year after year. This common form of farming has several disadvantages. A susceptibility to disease and pest infestation results in rapid spread of outbreaks over large areas because all of the crops are genetically similar. Depletion of soil nutrients is also accelerated by a lack of varied pathways, and by repeated demands upon the same fractions of nutrients and microbial fauna. To offset these problems, frequently enormous expenditure is required for inputs of nutrients and pesticides.
Historically, traditional farming has been focused on food crops and designed to support a family or village. More recently, there is an increase in farming designed to support markets around the world. As described above, this is adding tremendous pressure on farmers to raise food and other cash crops for profit rather than for sustenance, with new consequences of growing period loans and complex implications of crop shortfall. While farming for profit does not necessarily carry higher risk, it creates situations where farmers look for a short-term gain without regard to the long-term consequences. Not surprising are the manifestations of a more pervasive threat of malnutrition in those areas which have recently made the transition from subsistence food to cash crop agriculture.
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