To whatever degree human activity is narrowing the variety of life, scientists say, it is doing so on all levels. Conservationists and biologists offer a number of answers. On the practical level, wild species provide the raw material for medicine and food. Acting together, wild species support the human economy by providing an array of services like water purification, soil formation, pollination, flood control and, in our age, outdoor recreation. But without insects and microbes, humans would disappear quickly.
Some experts say natural systems are intrinsic to psychological well-being.
Scientists believe that in many ecosystems, a few keystone species hold everything together, and that if they are removed, the system will collapse and all the other species in it will disappear. The problem is that in most instances, no one is sure which are the crucial species. What's more, scientists have found, the greater the variety of plant species in an ecosystem, the more productive it is over all. So, conservationists argue, it is necessary to save as many species as possible. They have their work cut out for them, since the catalogue of imperilment is long.