"What we found is that with the extinction of a bird, or a mammal or a plant, you aren't just necessarily wiping out just one, single species," said Dr. Heather Proctor from the University of Alberta's Department of Biological Sciences. "We're also allowing all these unsung dependent species to be wiped out as well."

Proctor and a research team lead by Lian Pin Koh of the National University of Singapore and Robert Dunn from the University of Tennessee, calculated the expected levels of co-extinction across a diverse selection of host and associate systems. Their research is published in the current edition of the journal, "Science."

The team first compiled a list of 12,200 plants and animals currently listed as threatened or endangered. They then looked at the diverse selection of insects, mites, fungi and other organisms that are uniquely adapted to the threatened host.

The researchers found that at least 200 affiliate species have already been lost through co-extinction and that a further 6300 should be classified as "co-endangered."

"What we wanted to learn was, if the host goes extinct, how many other species will go with it," said Proctor.

"It would be easy if there were always a one-to-one relationship with a host and its affiliate; however, not all parasites, for example, are restricted to a single host species. The trick was in trying to determine how many other species could act as hosts and factoring that degree of dependence into the study."

The researchers believe these processes have been largely overlooked in the past because some of the most susceptible organisms are uncharismatic parasites, but other more popular animals are also at stake.

Proctor cites a host plant vine that became locally extinct in Singapore, taking along with it a species of butterfly, Parantica aspasia, that was dependent on the vine for survival.

"When we lose this vine, this beautiful butterfly dies off with it, and we'll never see it again except in photographs at museums," said Proctor. "And when that happens, it can never be recovered."

While this new research has implications for theoreticians who calculate endangered species, the moral issue is even more significant and should suggest more efforts to maintain the original species, said Proctor.

The loss of species through co-extinction represents the loss of irreplaceable evolution and co-evolutionary history, say the researchers, and should have immediate implications for local conservation and management decisions.

According to the World Wildlife Fund, the Earth is losing species at a rate not seen for 65 million years, since the extinction of the dinosaurs.

The study of biodiversity and astrobiology share the common thread of viewing the planet as a whole and attempting to see its future by examining its past.

The present moment in history has been characterized as the first time in which one species- humans- are in one way or another 'responsible' for the entire biosphere: changing it, maintaining it, and of course, possibly extinguishing it.

As Harvard professor of evolutionary biology , Andrew Knoll, remarked: " [For astrobiology] everything we know about life in the universe comes from life on Earth. In a sense, putting current diversity at peril for those who would like to understand biology as a planetary phenomenon is like burning a library."

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