Environmental Problems, Their Causes, and Sustainability 1

5Evolution and BiodiversityBiodiversityCASE STUDYEarth: The Just-Right,Resilient PlanetLife on the earth (Figure 5-1) as we know it needs acertain temperature range: Venus is much too hot andMars is much too cold, but the earth is just right. Otherwise,you would not be reading these words.Life as we know it depends on the liquid water thatdominates the earth’s surface. Temperature is crucialbecause most life on the earth needs average temperaturesbetween the freezing and boiling points of water.The earth’s orbit is the right distance from the sunto provide these conditions. If the earth were muchcloser, it would be too hot—like Venus—for watervapor to condense to form rain. If it were much fartheraway, its surface would be so cold—like Mars—that its water would exist only as ice. The earth alsospins; if it did not, the side facing the sun would betoo hot and the other side too cold for water-based lifeto exist.The earth is also the right size: it has enoughgravitational mass to keep its iron and nickelcore molten and to keep the light gaseous moleculesin its atmosphere (such as N 2 , O 2 , CO 2 , andH 2 O) from flying off into space.On a time scale of millions of years, theearth is enormously resilient and adaptive. Duringthe 3.7 billion years since life arose, the averagesurface temperature of the earth has remainedwithin the narrow range of 10–20°C(50–68°F), even with a 30–40% increase in thesun’s energy output. What a great temperaturecontrol system.For several hundred million years oxygenhas made up about 21% of the volume of earth’satmosphere. This is fortunate for us and mostother forms of life. If the atmosphere’s oxygencontent dropped to about 15%, this would belethal for most forms of life. If it increased toabout 25%, oxygen in the atmosphere wouldprobably ignite into a giant fireball. And thanks to thedevelopment of photosynthesizing bacteria more than2 billion years ago, an ozone sunscreen protects usand many other forms of life from an overdose of ultravioletradiation. In short, this remarkable planet welive on is just right for life as we know it.We can summarize the 3.7-billion-year biologicalhistory of the earth in one sentence: Organisms convertsolar energy to food, chemicals cycle, and a variety ofspecies with different biological roles (niches) has evolved inresponse to changing environmental conditions. Perhapsthe two most astounding features of the planet are itsincredibly rich diversity of life and its inherent abilityto sustain life.Here is the essence of this chapter. Each specieshere today represents a long chain of evolution andplays a unique ecological role (called its niche) in theearth’s communities and ecosystems. These species,communities, and ecosystems also are essential for futureevolution as populations of species continue toadapt to changes in environmental conditions.Figure 5-1 Natural capital: the earth, a blue and whiteplanet in the black void of space. Currently, it has theright physical and chemical conditions to allow the developmentof life as we know it.NASA