Get reliable rangeland science

It was thought for a long time that equatorial regions had remained virtually unchanged at least since the Tertiary period -- it is only from the Late Tertiary that we can talk of the present disposition of land and sea about the equator. Our teaching has supposed that these regions were the hearth from which general spread to the remainder of the planet's surface by gradual adaptation and speciation. This warm, humid zone has been seen as a refuge for species unable to maintain temperature ranges in the face of stresses and fluctuations to north and south. The diversity of species in rain forest environments was assumed to reflect their essential stability. So indeed it may, but not in quite the manner supposed originally by ecologists. The variety of species present in the various regions of rain forest are greatly different, with African exhibiting a relatively poor list compared with South America. This, as it turns out, may well be a function of disturbance. During the last 25 years and gathering momentum within the last decade a variety of studies have demonstrated that the concept of equatorial biological stasis is no longer tenable. Evidence for equatorial environmental changes is derived from theoretical as well as empirical studies. Regarding the former, various attempts have been made to model the earth's atmosphere at different periods of the Pleistocene and there is now fairly good agreement about the consequences for equatorial regions of a general global cooling, namely a critical drying at lower elevations. The empirical evidence is derived on the one hand from stratigraphic studies of peats and lake muds and also the analysis of ocean cores. It is also derived from studies of the geographical ranges of plant and animal species.