The complex of physical, chemical, and biological factors that act upon an organism or an ecological community and ultimately determine its form and survival. The pace of environmental change quickened dramatically with the introduction of agriculture from 7000 years ago onwards: forests were cut down and cultivation led to soil degradation and erosion. New species were introduced, both as crops and weeds, and the relentless growth of population ensured that man's activities made an ever-increasing impact on the landscape.