The science of ecology is the study of the interactions that determine the abundance and distribution of organisms. In other words, ecology attempts to explain why individuals live where they do and why their populations are the sizes they are. No population, human or otherwise, can grow indefinitely; eventually, some biotic or abiotic variable will begin to limit population growth. This basic ecological principle was first established in 1840 by German chemist Justus von Liebig and has been called the Law of the Minimum. From a human standpoint, this means that all of the world's physical resources are in finite supply. Ecologists also have discovered that all species in an ecosystem interact with one another, either directly or indirectly. A classic ecological experiment illustrates this point very well. American ecologist Robert Paine, working in the rocky intertidal region of the Pacific coast, found stable invertebrate communities dominated by 15 species of animals, including starfish, mussels, limpets, barnacles, and chitons. When Paine removed all of the starfish from the area, the community collapsed, and eventually only 8 invertebrate species were common. Although it was not obvious in the undisturbed regions, the starfish were preying heavily on one of the mussel species and keeping its numbers down. With the starfish removed, the population of this mussel increased, and the mussel was able to outcompete many other species of invertebrates. Thus, the loss of one species, the starfish, indirectly led to the loss of an additional six species and a transformation of the community. Typically, because the species that coexist in natural communities have evolved together for many generations, they have established a balance, and their populations remain relatively stable. Occasionally, when humans introduce a non-native species to an ecosystem, dramatic disruptions occur, often because the natural predators of the introduced species are not present. For example, early sailors routinely introduced goats to isolated oceanic islands, intending for the goats to roam freely and serve as a source of meat when the sailors later came ashore. Free from all natural predators, the goats thrived and, in the process, overgrazed many of the islands. With a change in plant composition, many of the native animal species were driven to extinction. A simple action, the introduction of goats to an island, yielded many changes in the island ecosystem, demonstrating that all members of a community are closely interconnected. In the 1970s the British scientist James Lovelock formulated the Gaia hypothesis, which has attracted many followers. According to this theory, named after the Greek goddess of the earth, the planet behaves like a single living organism. Lovelock postulated that the earth, like many organisms, can regulate its temperature, dispose of its wastes, and fight off disease. Although the Gaia hypothesis serves as a convenient metaphor for the interconnections among living beings, it does not have any particular scientific merit. From a scientific viewpoint, the earth is not a single living organism, but it can be viewed as a single integrated system. The National Aeronautics and Space Administration (NASA), using its expertise in planetary and space sciences, is collaborating with other U.S. governmental agencies in the use of artificial satellites to study global change. NASA's undertaking, begun in 1991, is called Mission to Planet Earth. This project is part of an international effort linking numerous satellites into a single Earth Observing System (EOS). EOS is designed to increase knowledge of the interactions taking place among the atmosphere, land, and oceans; to assess the impact of natural and human events on the planet; and to provide the data that permit sound environmental policy decisions to be made.
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