完形填空 　　Solar energy is called a renewable resource because on a human time scale it is essentially inexhaustible, it is expected to last at least 6.5 billion years while the sun completes its life cycle. 46) A potentially renewable resource can be renewed fairly rapidly (hours to several decades) through natural processes, examples of which include forest trees, grassland grasses, wild animals, fresh lake and stream water, groundwater, fresh air, and fertile soil. 　　47) One important potentially renewable resource for us and other species is biological diversity or biodiversity, which consists of the life forms that can best survive variety of conditions currently found on Earth. Kinds of biodiversity include (1) genetic diversity( variety in the genetic makeup among individuals within a single species), (2) species diversity (variety among the species or unique forms of life found in different habitats of the planet), and ( 3 ) ecological diversity (variety of forests, deserts, grasslands, streams, lakes, oceans, wetlands, and other biological communities). 48)This rich variety of genes, species, and biological communities gives us food, wood, fibers, energy, raw materials, industrial chemicals, and medicines―all of which pour hundreds of billions of dollars into the world economy each year. Earth''s vast list ''of life forms and biological communities also provides free recycling and purification services and natural pest control. 　　Potentially renewable resources, however, can be exhausted. 49)The highest rate at which a potentially renewable resource can be used indefinitely without reducing its available supply is called its sustainable yield. If a resource''s natural replacement rate is exceeded, the available supply begins to shrink―a process known as environmental degradation. 　　Several types of environmental degradation can change potentially renewable resources into nonrenewable or unusable resources. In the United States, one-fourth of the groundwater withdrawn each year is not replenished (filled up again). Between 25% and 50% of the world''s wetlands (55% in the U. S. ) have been drained, built upon, or seriously polluted. Almost half of the world''s tropical forests have been cleared. Thousands of wildlife species become extinct each year, mostly because of human activities. If habitat destruction continues at present rates, as many as 1.5 million species could disappear over the next 25 years―a drastic loss in vital Earth capital. 50)These examples help explain why most environmental scientists believe that over the next few decades, the danger of degradation and exhaustion is greatest for potentially renewable resources, not nonrenewable resources ( except for petroleum and perhaps a few scarce minerals for which we can''t find economically and environmentally acceptable substitutes).