Chapter Summary

Certain conditions are essential for life on earth, including the availability of required chemical elements, a steady influx of solar energy, mild surface temperatures, the presence of liquid water, and a suitable atmosphere. Energy is the capacity to do work. Matter is the substance of which the universe is composed. It exists in three interchangeable phases: gas, liquid, and solid. Matter is made up of atoms, which are composed of particles called protons, neutrons, and electrons.

The energy that holds atoms together forms the basis for energy transfers in the bodies of living organisms and, therefore, in the biosphere. Chemical reactions occur when bonds joining atoms break and new compounds or molecules form. Energy is released when bonds are formed. Ionic compounds form between charged metals and nonmetals. Covalent bonds form when nonmetal atoms share electrons.

Solar radiation provides the heat and light energy needed to support life in the biosphere. Water, which covers approximately three-fourths of the earth’s surface, is a remarkable substance. Because of its unique characteristics, it stabilizes the biosphere’s temperature and provides the medium in which life processes occur. The earth’s atmosphere provides gases necessary for life, helps maintain surface temperatures, and filters out dangerous radiation.

Physical laws, including the principle of conservation of matter and the first and second laws of thermodynamics, govern ecosystem dynamics. The recycling of matter is the basis for the element cycles that occur in ecosystems. Unlike matter, energy is not cycled. Energy always flows through systems in a one-way process in which some energy is converted from a high-quality, concentrated form to a lower-quality, less useful, dispersed form. We describe this increase in disorder as entropy. Primary producers, which synthesize organic compounds from solar energy and carbon dioxide, capture nearly all energy in ecosystems. Resulting compounds provide chemical energy and materials to consumers.

A species is all the organisms of the same kind; often this means organisms genetically similar enough to breed in nature and produce live, fertile offspring. The populations of different species that live and interact within a particular area at a given time make up a biological community. An ecosystem is composed of a biological community together with all the biotic and abiotic factors that make up the environment in a defined area.

Matter and energy are processed through the trophic levels of an ecosystem via food chains and food webs, and biogeochemical cycling. At each energy transfer point, less energy is available for work because of the laws of thermodynamics, so ecosystems require a continuous supply of energy. The relationships between producers and consumers in an ecosystem, often depicted as pyramids, demonstrate this principle. Most of the energy that enters an ecosystem comes, ultimately, from the sun. Living organisms constantly use and reuse elements. Carbon, nitrogen, sulfur, and phosphorus, for instance, recycle through complex biogeochemical paths.