Changes in terrestrial ecosystems are often divided into two broad categories -- changes in land cover and changes in land use.  Land cover denotes the physical state of the land including the quantity and type of surface vegetation, water, and earth materials (Meyer and Turner 1994).  Land use describes the human employment of the land, including settlement, cultivation, pasture, rangeland, and recreation, among others (Meyer and Turner 1994).

    The rate and magnitude of human-induced change of terrestrial ecosystems have increased through time, although European colonial expansions, the industrial revolution, and the post-World War II era stand out as periods of particularly high rates of human-induced terrestrial change.  These periods of pronounced change are associated with, but not limited to, cropping and livestock expansion, land-use intensification, and urban influences in general.  Table 5 illustrates the changes in several terrestrial ecosystems, including forests and woodlands, grasslands and pastures, and croplands, for North America and the world.

    These global trends, of course, mask major variations by time and place.  Richards (1990), for example, suggests that from the beginning of the modern era “agricultural expansion, forest clearing, wetland drainage, irrigation of grasslands, expansion of human settlement, and similar processes have traced a spiraling arc that is determined, for the most part, by European political and economic control.”  Richards has thus illustrated the extent to which terrestrial change is determined, in part, by global and regional economic and political structures.  More recently we might trace variations in the trends of terrestrial change to the vagaries of international markets (production and consumption) and the locations of large-scale population growth.
 

    Changes in terrestrial ecology are important in their own right.  They affect the biodiversity of an ecosystem, that is to say, the number of plant and animal species that can live in an ecosystem.  Changes in the structures and functions of ecosystems can affect their fragility and resilience and thus the sustainability of our use of land resources.  Changes in terrestrial ecosystems also have important links to climate change because land cover acts as a source and a sink of greenhouse gases.  Land cover and land use also determine the amount of reflected versus re-radiated energy the atmosphere receives.  (These links to climate change will become clearer in Unit 3.)

    Tropical deforestation for ranching and cropping illustrate these complex linkages.  When deforestation occurs in tropical areas, the most biotically diverse land cover on earth is replaced with biotically impoverished ones.  The cutting and burning of the forest to clear the land for human use releases carbon dioxide into the atmosphere, contributing to the greenhouse effect (see Unit 3), while cropping itself releases carbon stored in the soils.  In addition, soil moisture and the quantity of evapotranspiration in the area are changed, affecting precipitation levels and soil and air temperatures.  Changes in soil carbon, local precipitation, and temperatures, in turn, affect the productivity and costs of ranching and cropping.  Finally, the newly created ecosystems disrupt the habitats of some species and often provide opportunity for invasive species to thrive.

    There is yet one more link to consider -- terrestrial ecosystems are also affected by global climate change.  Vegetation and climate are closely linked -- so much so that climate classifications sometimes infer vegetation types.  For example, one of the most widely used climatic classification systems is the Köppen System, developed by Wladimir Köppen in the early twentieth century.  Though it is an empirical classification based on average temperature and precipitation statistics, Köppen’s climatic regions were formulated to coincide with well-defined vegetation regions (Gabler, Sage, and Wise 1991).  His climatic classifications make the links between climate and terrestrial ecosystems clear.  If climates change, so too will vegetation patterns and the pattern of ecosystems across the earth.

    The diagram in Figure 4 illustrates some of the complex links that exist between terrestrial change and other forms of environmental changes, including climate change.