Ecology is the science of how living creatures and plants exist together and depend on each other and on the local environment. Where an environment is undisturbed, the ecology of an area is (71) balance, but if a creature is exterminated(灭绝)or an alien(异类)species (72),then the balance of nature will be (73).
Man is a part of the (74) and has done more to upset the ecology during (75) short span on earth than (76) other living creature. He has done this by his ignorance, his (77),his thoughtless foolishness and his uncontrolled wastefulness. He has poisoned the(78) and polluted both land and water. He has wasted the earth's natural resources with no (79) for the future, and has thought out the most devastating ways of killing his fellow men and every other sort of life at the same time.
Since man has done so much (80),it is (81) to man to try to put matters right — if it is not already too (82).If there is to be any remedy for our ills, that remedy ultimately lies in the hands of the (83),and the sooner they start doing something about it, the (84).
One of the main (85) of the earth's troubles is that the world is overpopulated and that this overpopulation is growing (86) an ever-increasing rate. At the same time we are (87) up our natural resources—fuels and mineral ores — at an ever-increasing speed with no hope of replacing them. For many years the earth has been unable to (88) enough food for these rapidly expanding populations and the position is steadily deteriorating (89) the fertility of some of our richest soils has been lost and vast areas that were (90) fertile lands have turned into barren deserts.