Thomas E. Lovejoy of the Smithsonian Institution in Washington, D.C., offers the following thoughts:

"While I could go into the moral and ethical concerns about ending any evolutionary line that goes back 3.5 billion years (as does our own), what I would prefer to address is the operative phrase 'no practical use to humans.' What it really means is something more like 'no known practical use given our current state of knowledge.'

"One example of how this opinion can change is the Pacific Yew, which was considered a trash tee until taxol, a compound found in its bark, was discovered to be a powerful drug against ovarian, lung and other cancers. Another example is the bacterium that lives in the Yellowstone hot springs. This bacterium might have seemed quite worthless before it was discovered to have an enzyme that drives the polymerase chain reaction, a biochemical process that won the Nobel Prize in 1993 and that is now responsible for billions of dollars of economic activity annually. The point here is that like books in a library, species have value (some of it practical) that may become apparent only when they are studied closely.

"A second point is that as elements of ecosystems, species contribute to valued ecosystem services: they may help regulate the watershed, generate soil fertility, pollinate crops and contribute to the cycling of water, energy and nutrients. These are important contributors to human welfare, the value of which is becoming more recognized. For example, New York City recently discovered that it will be 10 times cheaper to buy key parts of its watershed and manage them appropriately than to build new water treatment plants. Likewise, Costa Rica has recognized that its protected forests contribute water for power generation that is worth $104 million per year (in other words, that is how much it would cost to import enough fossil fuels to produce an equivalent amount of energy). Each species in that ecosystem is contributing to those services, though that contribution has not always been appreciated.

"There are other arguments regarding the value of biodiversity, but these are a good initial two to ponder."

Kerry Bruce Clark, a professor of biological sciences at the Florida Institute of Technology in Melbourne, Fla., adds:

"Every organism, whether or not it has direct practical use to humans, has a functional role (or 'niche') in its habitat or ecosystem. Though many species appear to have trivial niches (in terms of total biomass, numerical abundance or relative role in ecosystem metabolism), we should remember that the relative effects of various organisms in biological systems are seldom static, and minor species can sometimes become very important as systems fluctuate. Each species also represents a unique genetic library. Our genetic technology is only beginning to tap the vast potential benefits of these libraries, and seemingly 'minor' species are typically the most specialized organisms; we can expect that ecological specialists will often turn out to have the most unusual genes and hence represent potential resources that we should preserve for our future needs.

"Additionally, minor species often have functions that we may not understand but that may be ecologically or evolutionarily important, often involving complex interactions of many other species, some of which may in turn be ecologically or commercially important. The dodo and the Carolina parakeet were important dispersers of seeds, and their loss has permanently affected forest structure in their habitats; rare insects are often highly specific pollinators whose loss affects the reproduction and survival of other plants. On evolutionary time scales, we know far less about the effects of extinction of rare species, but we do know that evolution can amplify the effect of a species over time through its interactions on survival of other species. In most cases, we simply do not know enough about the biology of a rare species to predict the effects of its extinction. But once the species is lost, we can never provide a perfect substitute.

"Loss of rare species often results from habitat-scale modifications that affect far more than the one rare species. When we lose one rare species, it actually symbolizes many changes of far broader impact, ranging from the loss of habitats (affecting large numbers of species) to large-scale alterations to the functions of those habitats. As the human population climbs, these cumulative changes will ultimately affect our economies and our well-being, because natural ecosystems perform--free of charge--many functions which we take for granted, such as purification of our wastes, production of harvestable resources, regulation of our climate, and restoration of the oxygen that we breathe."

Marianne Robertson, an assistant professor of biology at Millikin University in Decatur, Ill., gives this pithy reply:

"Even animals that humans deem insignificant because they cannot provide us with medicine, food, etc., play a big role in the food chain. The glamorous 'feathers & fur' animals that the public supports being placed on the Endangered Species List are generally the top predators or higher herbivores. But the less aesthetically pleasing invertebrates also play crucial roles in the base of the food chain, in nutrient recycling, energy flow, and so on. Without them, we would not be here!"

And Catherine Carter, an assistant professor of sciences at DeKalb College in Atlanta, Ga., makes a plea for humility:

"My concern is expressed to every group of biology students that I meet. I find it sad that we humans have so much self-importance that judge the value of anything based on its use to us. The world was not created for humankind only. Every organism, no matter how small or unattractive, has its place in the ecosystem. When our species begins disrupting the ecosystem by destroying those organisms that we do not consider important, we are assuming the role of creator. Humans are too dumb for that job.

"Consider the nurse sharks that were exterminated along the coast of Australia just because some people wanted to be considered heroes by making the beach appear safe. These sharks were not dangerous. But now they are gone, probably never to return. If I do nothing else in this world I hope to help some of my students realize that we humans are only one small, and perhaps unimportant, part of this whole world. Unfortunately, we are causing much of its destruction.