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In the mid-eighteenth century, the Swedish botanist Carolus von Linné (1707-1778) better known as Linnaeus, invented the classification scheme of the living world that we still use. Early scholars such as Aristotle and Pliny had also invented such classifications, some of which still hold. For example, Aristotle first classified dolphins as mammals! Pliny (23-79 AD) wrote a 37-volume Natural History, classifying all reported living beings! Linnaeus' scheme gave every living being two names. The first is its genus, the group to which it belongs, and the second the species, describing the subclass in the genus. Thus the present species of humans are Homo sapiens, others like Homo erectus and Homo habitis being extinct. The genus and species have Latin names, with the genus term written capitalized.
Several members of the same species in a particular area at the same time constitute a population, and the area is called the habitat of the species. Different species may live together in a habitat, forming a community. The different species in a community might interact through a food web or exist in symbiosis. Symbiosis is a state in which members of different species live in physical contact, mutually benefiting from each other's presence. Lichens that occur on exposed rocks throughout the world are a wonderful example of symbiosis: They are usually a fungus and an algae (or bacterium) living symbiotically. The photosynthetic algae provide nutrients for the fungus. The fungus seems to provide support and the ability to extract essential minerals from the rock. Because of this pairing, lichens can colonize extreme environments where the fungi or algae alone cannot exist. These include the rocks of Antarctica and of Donegal, Ireland. The lichens scraped off from rocks in Donegal is used to color the woolen material called Donegal tweed.1
Biomes are the several habitats that co-exist in a particular climatic area. Tropical rainforests and coniferous forests are examples of biomes. Biosphere is the general term for the highest organizational level in which life exists, ranging from the very depths of the oceans to several thousand meters into the tropospheric region of the atmosphere, and including land masses.
Ecosystems and Ecological Balance
Ecosystems are living and nonliving components of an area that include the habitat and the physical and chemical environment. The classic definition of an ecosystem was stated in 1953 by Odum: any unit that includes all organisms (i.e., community) in a given area interacting with the physical environment so that a flow of energy leads to a clearly defined structure, biotic diversity, and materials cycles. [INDIRA - I don't have a reference for this, because it came from Sharon's modules. Should we just take it out?]
What do we mean by "ecological balance," "balance of nature," or "ecosystem stability"? Balance and stability in this context are different from a static condition in which there is no change. Nature is continuously changing, and especially over periods of thousands of years, changes substantially. In his book, Discordant Harmonies, Daniel Botkin writes, "...every thousand years a substantial change occurred in the vegetation of the forest, reflecting in part changes in the climate and in part the arrival of species that had been driven south during the ice age and were slowly returning."2 The forest he is referring to is in the western region of northern Minnesota and southern Ontario, which Botkin studied in detail.
Recognizing this difficulty of defining balance, and the fact that balance or stability occurs over different time scales, ecologists talk of "ecological stability" or "resilience." For each of these terms, one may focus on one or two species and their change over time. Most ecologists study population ecology or community ecology. In general, the stability of 10 to 100 species over time scales of 10 to 1000 years is considered when talking of stability. Over this time, populations may remain in an equilibrium. Population resilience is defined as the rate at which the population return to equilibrium after it is disturbed.
Figures 1A-D show a representation of the progress of the Earth's ecosystems as we progress from prehistoric times, through hunter-gatherer societies and agricultural societies, to an ecosystem in which industrial activities dominate. In Figure 1A, the different levels of the ecosystem depend upon the plants, the primary producers of nutrients from H2O and CO2 using the sun's energy. As we go from 1A to 1D the role and impact of humans increase. In Figure 1D, human industrial activity and pollution dominate. As the human-dominated fraction of the system increases, we see the shrinking of the other levels, representing loss in biodiversity and even species extinction.3
McFadden, Johnjoe. Quantum Evolution, Norton: New York, 2000.
 Botkin, Daniel. Discordant Harmonies, Oxford University Press: New York, 1990. p. 62
 Clark, Mary E.. Ariadne's Thread, St. Martin's Press: New York, 1989.