¶ … endangered species' means any species which is in danger of extinction throughout all or a significant portion of its range other than a species of the Class Insect a determined by the Secretary to constitute a pest whose protection under the provisions of this Act would present an overwhelming and overriding risk to man." A threatened species "means any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range." (The Endangered Species Act of 1973) People can go into the moral and ethical concerns about ending any evolutionary line that goes back 3.5 billion years (as does our own), some confuse the endangered species with 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.' (Lovejoy)

The Pacific Yew is an example of such a confusion as it used to be considered as a garbage tree until taxol, a compound found in its bark, was discovered to be a powerful drug against ovarian, lung and other cancers. A bacterium that lives in the Yellowstone hot springs is another example. It was discovered that this bacterium had an enzyme that 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. Previously the notion was held that this was a useless object. 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. (Lovejoy)

Every organism, even if or not it has evident practical use to humans, has a functional role in its habitat or ecosystem. Though for many species, these niches appear to be trivial (in terms of total biomass, numerical abundance or relative role in ecosystem metabolism), it should be kept in mind 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. (Clark)

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." (Clark)

Habitat scale modifications often results in loss of rare species and affects a lot more than the one rare species. When a rare specie is lost, it exemplifies many changes of far broader impact, ranging from the loss of habitats (affecting large numbers of species) to large-scale modifications of functions that habitats. As the human population climbs, the technological wonders increase, the humans expand, and the pollution is increased. These cumulative changes will ultimately affect our economies and our well-being, because natural ecosystems perform 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. (Clark)

Quietly, insidiously, over the slowly decading years, the world has been laden with life. Numerous species of plants, animals and microbes have taken up home here, their numbers counting in billions. Many are valuable crops and others are useful plants that humans have carried with them since...

Others are pests that have claimed the habitats of native species, forcing many of them to extinction, causing crop damage and human and animal disease.
The benefits of many imported species are clear. Ninety-eight percent of the U.S. food supply comes from such introduced species as wheat, rice, domestic cattle and poultry, with a value of more than $500 billion a year. But plants such as purple loosestrife; invertebrates like the zebra mussels and gypsy moths; mammal, including rats, feral cats and pigs; and microbes like the AIDS virus are hardly so benign. A team of researchers from Cornell University, headed by ecologist a David Pimental, estimates that their depredations cost at least $123 billion a year in economic losses.

It doesn't take many troublemakers to cause tremendous damage," said Pimentel, when the group presented the findings at the recent annual meeting of the American Association for the Advancement of Science in Anaheim, Calif. If anything, the estimates made in the damage assessment study, Environmental and economic costs associated with non-indigenous species in the United States. are conservative. More than 40% of species on the U.S. Department of the Interior's endangered or threatened species lists are at risk primarily because of non-indigenous species -- and a pricetag cannot be placed on their loss.

Precise economic costs associated with some of the most ecologically damaging alien species are not available. The brown tree snake, for example, has been responsible for the extinction of dozens of bird and lizard species on Guam, yet only minimal cost data are known. In other cases, such as the zebra mussel and feral pigs, only control cost data are collected. Some well-known invaders, such as the kudzo vine, have been overlooked for lack of economic information. "If we had been able to assign monetary values to losses in biodiversity, ecosystem services, and aesthetics, the costs of destructive non-indigenous species would undoubtedly be several times higher," the researchers say.

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 would 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." (Lovejoy)

Even so, the rather sketchy results are sobering. The English sparrow was intentionally brought to the U.S. In 1853 to control canker worms. Instead, the hardy little bird became a pest by eating crops, displacing some native birds, and harassing others, carrying 29 diseases that affect humans and domestic animals. Canker worms still bedevil gardeners.

Similarly, the mongoose was introduced into Puerto Rico and Hawaii in the late 1800s to kill rats in sugarcane plantations. The islands still have rats, but the mongooses are preying on native ground-nesting birds and on amphibians and reptiles that could be beneficial for pest control. The extinction of at least 12 species of reptiles and amphibians in Puerto Rico and other islands of the West Indies is blamed on mongooses, which also carry the pathogenic organisms for rabies and leptospirosis.

Meanwhile, rats, which probably arrived in the continental U.S. As stowaways on ships, now have an estimated population in the billions. On farms each rat is estimated to destroy grain and other goods worth $15 a year. In urban and suburban areas there is roughly one rat for every human. These rats cause fires by gnawing on electric wires, polluting foodstuffs and carrying diseases such as salmonellosis and leptospirosis.

Our domesticated predators -- dogs and cats -- also have a share in the mayhem. America's 63 million domestic cats and 30 million feral cats are believed to kill some 200 million birds a year. Wild dogs running in packs in Florida, Texas and other states cause an estimated $10 million a year in livestock losses, rivaling or exceeding the damage from wolves and other indigenous canines.

Many plants that once delighted gardeners have also run amuck when they escaped into the wild. Purple loosestrife -- a European native popular as an…