Land Use

Changes in how the land is used are probably the principal contributor to the current decline in biodiversity. About 1 to 2 percent of the land surface of the Earth is now devoted to urban use, but other changes in land cover and land use far exceed the direct impact of the small fraction that is paved or developed for homes and factories and other buildings. Homo sapiens has already converted about a quarter of all the land surface to agricultural uses. By some estimates we now appropriate directly or indirectly about 40 percent of what biologists call the primary production of the Earth's biota (the products of photosynthesis on which all other life depends), and the percentage that comes under our control in this way is increasing.

The pressures on terrestrial resources and land depend very much on population growth and the demands of early stages of economic development. Moreover, land acquisition, especially for agriculture and forestry, focuses initially on those areas with the most fertile soils and equable climates, which are often the areas of greatest biological diversity.

Deforestation in the humid tropics is probably the best-known current example of rapid land-use change. During the decade of the 1970s, vast areas of tropical forest in South America, Africa, and Southeast Asia were cleared and converted to agriculture and other uses. In the middle-to-late 1980s, the rates of deforestation in South America slowed dramatically, largely due to economic and tax policy changes in Brazil, but the pace of cutting in Africa and Southeast Asia, though poorly quantified, remains high. Globally, the rate of loss of tropical forests for the 1980s has been estimated at about 1 percent per year, but there is still considerable uncertainty. The rates of extinction of local species that accompany these rapid changes in land cover may soon be far in excess of what is found today, reaching as high as 10,000 times the natural background rate.

In the industrialized nations of the Northern Hemisphere the most rapid and widespread conversion of forest to other uses took place over the last several hundred years. In this time, much of the northeastern U.S., for example, was deforested at least once, in connection with the rise of agriculture and timber industries. But as regional and national economies changed, many previously cleared areas were left to return to their natural vegetation. As a result, forests have reappeared in parts of the Northeast, and indeed the country as a whole has probably gained forested land over the last several decades.

The current trend of most concern with respect to tree-cover in the U.S. is a shift to smaller parcel sizes. What once were continuously forested landscapes are now a quilt of small patches of trees, criss- crossed with roads, subdivisions, agricultural tracts, and a variety of different land-uses and land-covers: a scene that is familiar to anyone who has looked out an airplane window. The average size of tree-covered parcels is smaller than was the case twenty, fifty, or a hundred years ago, resulting in a landscape that is highly fragmented and partitioned.

The difference in terms of the natural world is great, and several studies now point with concern to the biological impacts of the shift to less continuous landscapes. The known consequences of these changes are reduced numbers of both plants and animals and a greater possibility of the outright loss of some of them--when in effect, they are painted into a corner with nowhere left for them to go. The interweaving of favorable and unfavorable habitats also curtails the ability of organisms to disperse, and makes recolonization of distant areas more difficult.

An analogous pattern of fragmentation can be found in parts of South America where deforestation was previously extremely rapid. Although the amount of new cutting appears to have fallen from that of previous decades, it seems to be increasing again in the rain forest of the Amazon, and the deforested, newly colonized regions now have their own distinctive appearance. Patchworks of active fields, orchards, abandoned fields, second growth forest, and primary forest are the norm. But the scene is ever changing through an interplay of active use by initial colonizers, abandonment, partial recovery through natural processes, and as then often happens, subsequent re-use. Analyses of potential impacts on biodiversity that are based on simple measures of deforested area can provide little more than very general conclusions.

Deforestation is not the only land-use change of interest or concern. Another with broad implications for biodiversity is the intensification of agriculture and grazing on those lands that have been traditionally devoted to these purposes. Of particular importance for biodiversity are the secondary impacts of intensive agriculture. Heavy applications of fertilizers and pesticides have the potential of creating additional environmental problems as well as affecting the abundance and viability of the other plants and animals and micro-organisms in the same or adjoining areas.

The adverse effects of non-point-source pollution due to the run-off of pesticides and herbicides from intensively-used fields are well- known. In addition, because of the understandable tendency to put the best land into production first, the expansion of agriculture into less fertile areas typically requires heavier applications of chemicals, more extensive site preparation, and other forms of more intensive management. The typical result is increased chemical run-off to the landscape, and with ensuing degradation, additional pressure for expansion, and so on. It is such a cycle that has led to widespread desertification in some parts of the world, primarily through overgrazing that can be compounded by naturally occurring droughts.