For most of human history the influence of human beings on biophysical processes, ecological systems, and evolutionary change has been relatively limited, as compared with the influence of "natural" (nonhuman) processes. Ecological and evolutionary change has generally been attributable to natural variation in energy and material flows and to natural selection by parasites diseases, predators and
competitors. Today however humans affect Earth's ecosystems at extraordinary rates through land and resource consumption, alteration of habitats and species composition, disruption of hydrologicapl rocesses, and modification of energy flow and nutrient cycles. Humans now use approximately 40% of global net primary production and more than half of accessible freshwater runoff. At least half of the world's forests have disappeared as a result of human activity, and three-quarters of that total have disappeared since 1700.
Human activities fix amounts of nitrogen and sulfur comparable to those fixed by all nonhuman causes. Humans have radically revamped Earth's carbon cycleE arth'sc arbon and freedi nto the environment vast quantities of naturally occurring trace materials (e.g., cadmium, zinc, mercury, nickel, arsenic) and exotic new anthropogenic substance (se .g.,polychlorinated diphenyls, and chlorofluorocarbon.
Humans also influence evolutionary processes. Natural selection is more and more directed by human behaviour. This was taken up in the 1980s by Madhav Gadgil who opened up a discussion of what makes a stable culture. He saw the answer in the adoption of prudent ecological behaviour. VAnimals often behave in a profligate fashion and decimate the populations of plants and animals they depend upon. They may, however, evolve prudent behaviour under special conditions, namely when such prudence greatly enhances the success of populations that are not too prone to be invaded by profligate individuals. Cultural evolution in human societies can also lead to the adoption of prudent practices under similar conditions. These are more likely to be realised in stable environments in which the human populations tend to grow close to the carrying capacity, when the human groups are closed and when the technology is stagnant. These conditions probably prevailed in the hunter-gatherer societies of the tropics and subtropics and led to the adoption of a number of socially imposed restraints ion the se of plant and animal resources. Such practices were rationalised in the form of Nature-worship. The Indian caste society became so organised as to fulfil these conditions and gave rise to two religions, Buddhism and Jainism, which emphasise compassion towards all forms of life. The pastoral nomads of the middle east on the other hand, lived in an environment which militated against prudence, and these societies gave rise to religions like Christianity, which declared war on nature. As the ruling elite and state have grown in power, they have tried to wrest control of natural resources from the local communities. This has sometimes resulted in conservation and prudent use under guidance of the state but has often led to conflicts with local populations to the detriment of prudent behaviour. Modern technological progress has also often removed the need for conservation, as when availability of coal permitted the deforestation of England. While modern scientific understanding has led to a better appreciation of the need for prudence, the prevailing social and economic conditions often militate against any implementation of the understanding, as seen from the history of whaling. However, the imperative for survival of the poor from the 'third world' countries my finally bring about conditions in which ecological prudence may one again come to dominate human cultures as it might once have done with stable societies of hunter-gatherers.