In the 1980s, scientists around the world began to notice something strange: frogs ? ? ? disappeared were disappearing (1). More recent research ? ? ? showed has shown (2) that many kinds of amphibians ? ? ? are declining decline (3) or ? ? ? have become became (4) extinct. Amphibians are animals, such as frogs, that ? ? ? live are living (5) partly on land, and they ? ? ? have been were (6) around for a long time - over 350 million years. They ? ? ? survived have survived (7) three main extinctions, including the extinction of the dinosaur. Why ? ? ? are they dying do they die (8)?
Scientists are seriously concerned about this problem. By studying amphibians, scientists ? ? ? have learned learned (9) about new substances that could be useful for treating human diseases. Further research could lead to many discoveries, but that will be impossible if the amphibians ? ? ? will disappear disappear (10).
The most serious aspect of the amphibian loss, however, ? ? ? goes is going (11) beyond the amphibians themselves. Scientists ? ? ? begin are beginning (12) to think about what amphibian decline ? ? ? means is meaning (13) for the planet. If the earth ? ? ? is becoming becomes (14) unlivable for amphibians, ? ? ? is it also becoming does it also become (15) unlivable for other species of animals and human beings as well?
Scientists now ? ? ? believe are believing (16) that amphibian decline is due to several environmental factors. One of these factors is the destruction of habitat. Amphibians are very sensitive to changes in their habitat. If they ? ? ? will not be able to cannot (17) find the right conditions, they will not lay their eggs.
These days, as wild areas are covered with houses, roads, farms or factories, the arroyo toad of southern California will lay eggs on the sandy bottom of a slow moving stream. There are very few streams left in South Carolina, and those streams are often muddy because of building projects. Not surprisingly, the arroyo toad is now in danger of extinction.
There are a number of other factors in amphibian decline. Pollution is one of them. In many industrial areas, air pollution ? ? ? has poisoned poisoned (18) the rain which then falls on the ponds and kills the frogs and toads that ? ? ? live are living (19) there. In farming areas the heavy use of chemicals on crops ? ? ? has killed killed (20) off amphibians. Another factor is that air pollution ? ? ? has lead lead (21) to increased levels of ultraviolet (UV) light. This ? ? ? endangers is endangering (22) amphibians which seem to be especially sensitive to UV light.