It took some 5 or 6 million years (My) for protohumans to spread from Africa to Asia and then to Europe. The human evolutionary line began in Africa about 7 million years ago (Ma). Understanding the nature and consequences of humans’ environmental impacts – and managing these impacts to protect the well-being of human society and other life on Earth – is humanity’s greatest challenge. As a result, over the past two centuries – barely more than two human lifetimes – humans have disrupted living and nonliving systems everywhere. Moreover, such effects unfold faster and on a scale far greater than any effects of past ecosystem engineers. Human effects since the Industrial Revolution – including many that may be invisible to a casual observer – are recent and outside the evolutionary experience of most organisms. People, in contrast, have become ecosystem engineers on a whole new scale in time and space. Ecosystem engineers and their effects have long been part of evolving ecological systems. In evolutionary terms, changes like these brought about by living things, including ecosystem engineers, have been slow and incremental.
African elephants convert wooded savanna to open grassland by toppling trees as they browse. Beaver dams alter the flow of rivers, increase dissolved oxygen in downstream waters, create wetlands, and modify streamside zones. Some organisms, like beavers and elephants, change their surroundings so much that they have been called ecosystem engineers. Even unusual or seemingly catastrophic events, like volcanic eruptions, are an integral part of the ecological contexts to which organisms adapt over long time spans. Those that survive are molded by natural selection as the environment changes. Over millennia, organisms evolve to contend with changes in their environment. All organisms change their environment as they live, grow, and reproduce.
0 kommentar(er)
