seconds to millennia, but also slow or punctuated evolutionary changes in biota are a continuing phenomenon. Similarly, climate as well as landscape and vegetation has fluctuated greatly during the brief interval of human occupance of the earth (Table 1). Thus the impact of population numbers or of population and technological change cannot be evaluated in the absence of some knowledge of the behavior of the ''natural'' scene. Understanding the vicissitudes of the natural system is particularly important in evaluating efforts at remediation of human impacts and in assessing the degree to which particular impacts are likely to be manageable if not reversible within varying periods of time. Temporal and spatial scales, however, are interrelated. For example, as small parcels of land are changed within a forest, upon abandonment the surrounding forest may readily provide seed for regeneration. In contrast, extensive cutting of forests for agriculture may leave only small refuges of original plants and animals, reducing the likelihood of regeneration of some of the biota and increasing the duration of transformation. Human beings have altered the land at varying rates and over vastly different areas. Many of these changes can be seen in the historical record.