From deep in the cypress swamp comes word that the ivory-billed woodpecker is no longer extinct; no thanks to the Endangered Species Act — yet.

How does a species go extinct? Consider this: Every individual has a genealogy, but not every individual has progeny.

Each species derives from some pre-existing life form; but instead of numbering in the days, years or decades of an individual's life, species' life expectancies stretch out thousands to as much as 10 million years.

Some species become extinct when a new one emerges (or several branch off), replacing the "parent." Extinction is also the fate of dead-end species that no longer have the ability to adapt to new conditions and die off before a descendant appears.

Paleontologists estimate some 2 billion species have come and gone since life first appeared on Earth. Ninety-five percent of all species that ever existed are extinct. Extinction is part of a natural process — as waves of new life forms replace earlier ones — that has been going on long before the appearance of man a brief time ago, geologically speaking.

At present more species (biodiversity) populate the Earth than in any previous time. They number between 8 million and 10 million (excluding microorganisms) but only about 1.8 million have been catalogued to date. Beetles number some 400,000 species — quite an extended family!

This large uncertainty highlights the difficulty of counting all the plants and animals that inhabit our planet in bewildering numbers.

Estimates (the opinions of experts) diverge widely because locating and cataloguing the myriads living in intricate webs (ecosystems) in the tropical rain forests and at great ocean depth is a difficult task, to say the least.

Recently biologists reported a peculiar "new" rodent, called the "kha-nyou" (rock rat) by the people of Laos. Local villagers actually had no idea this staple of their diet might be an endangered species and were unaware of its significance owing to its limited range and very unusual taxonomy. No doubt, other "exotic" animals and plants await discovery, although it's widely agreed that no more big surprises lurk among the large land animals, "Bigfoot" not withstanding.

Lacking an accurate species count, biologists find agreement on a meaningful estimate for the rate of extinction problematic.

A vast majority of the past life-forms presumed to have existed once, have not been discovered in the geologic record. Fossil remains of comparatively few of these species were fortuitously buried in sediment and now lie preserved in sedimentary rocks with their images morphed in stone.

Surface erosion obliterates geologic formations over time, and deep burial makes systematic hunting for fossils of unknown species impractical. Consequently, many species that once lived may never be discovered.

By observing the succession of fossils from older to more recent formations, paleontologists can estimate the numbers of species living in the distant past and by employing statistical methods, arrive at effective extinction rates.

The "natural" rate is estimated to be about 0.0001 percent per decade, admittedly a very rough estimate, averaged over long periods in the Earth's history.

Typically, sharp rises in extinctions follow dramatic changes on land and oceans. These result from geologic-scale events: extensive volcanic activity, the aftermath of a giant meteor impact or a period of superglaciation referred to as the "Snowball Earth."

During longer stable periods, inferred from the known fossil record, certain species flourished for tens of millions of years.

But prolonged quiescence is always punctuated by genetic instability.

Great bursts of new species followed every major die-off, leaving a wake of extinct species behind. If there is to be another mass extinction, triggered by human activity or not, a similar pattern of speciation would subsequently unfold despite the dire predictions being ascribed to global warming. Life is resilient after all.

We lament the loss of recently extinct mammals and avians, which live in our collective memory — the passenger pigeon and the Carolina parakeet to name just two. We watch as the whooping crane and California condor teeter on the brink of extinction and delight when an individual ivory-billed woodpecker is again seen and heard in an Arkansas swamp — 50 years after being officially declared extinct.

We hope a pair of the birds will be discovered, making its reprise from a past echo possible.

Our faltering efforts to save threatened birds and animals can hardly be declared worthy of the memory of Aldo Leopold — not yet.

(William Balgord is a consultant and writer who heads Environmental & Resources Technology, Inc. in Middleton, Wis.)