SCIENTISTS LAST WEEK reported the creation--there is no better word--of a new bacterium, capable of metabolizing the toxic herbicide 2,4,5--T. What does that mean? It means there could be a big payoff: if the bacteria perform as expected, they will be able to decontaminate waste dumps and agricultural areas where the herbicide has been heavily sprayed. It should be possible to use this new organism together with the herbicide to gain the weed killer's useful impact without its harmful side effects. And the method for creating the bacterium should be usable in creating any number of others with equally unique appetites.

The achievement illustrates why it is so important to stop the world's accelerating loss of species. Scientists can find genes in nature's extraordinary diversity that are capable of doing almost anything. Using the techniques of genetic engineering, these can be selected and combinedinto useful new organisms. But scientists cannot design the genes themselves: for this they have to rely on nature. Nature's raw material--the 5 million to 10 million plant, animal and microbial species that inhabit the earth-- is being lost at a ruinous rate.

Scientists calculate that one species becomes extinct every day. By the end of this decade, the rate is expected to reach 10,000 per year. By the year 2000, fully 20 percent of the species now on earth will have disappeared. There are many biologists and a few policy makers who are beginning to believe that this loss is the most dangerous trend on the already overcrowded list of global problems.

The species loss means the loss of genes with unlimited industrial, agricultural and medical applications. One species recently saved by the merest luck is a type of corn that is perennial--it will produce for years from a single planting. If that characteristic can be transferred to cultivated species of corn, the advantage should be immense. Yet it was almost lost: only a few thousand plants are known to exist, on a single hillside in Mexico.

Also being lost are the genes that confer resistance to disease, genes that permit adaptation to climate change and genes that would provide new food supplies, new drugs and resistance to ever-changing pest populations (remember the Medfly). The aggregate loss of thousands of species also means the eventual loss of entire habitats. This in turn, biologists believe, threatens the planet's environmental stability and therefore its ability to support life. Slowing the species loss will require an expensive international effort and a high level of cooperation between the developing countries--where the greatest number of species are found and where most are being lost--and the industrialized world. The importance of taking these steps hardly seems open to argument.