Resurrection ecology is the concept of introducing a closely related species or subspecies of an extinct plant or animal into the environment which the now-extinct species had previously lived in with the hope that the related entity will eventually evolve into something virtually identical to its extinct relative. The reason for doing such a thing is to, in essence, recreate an extinct species (resurrect it) either because it is felt that the species should not have become extinct (if, for example, it was wiped out due to unnecessary human interference) and/or the species' extinction has caused severe damage to the ecosystem the now-extinct species was a part of (for example, without bees and other insects to aid in pollination, many species of flower would likely die out).
Resurrection ecology isn't a concept that all environmentalists feel is necessarily the correct action to take when a species has gone extinct. Extinction isn't necessarily an "unnatural" occurrence. Some people feel that the extinction of a certain species may simply have been meant to happen. Some take this view and combine it with the view that human interference in an environment isn't an unnatural act, even if it leads to such unfortunate consequences: Humans are natural to this planet, therefore humans are a part of nature. This view specifically seems somewhat uncommon though, as humans today often tend to create imbalance in an ecosystem rather than become a part of the existing balance. Resurrection ecology could be considered "playing God" in the same way cloning is. An idea many people, environmentalist or not, are uncomfortable with.
Unlike cloning, resurrection ecology is largely unknown and, as far as I know, has never been suggested as an action to be carried out simply to see if it can be done. The introduction of foreign organisms into environments they wouldn't normally be found in has been shown to have tragic consequences in the past (e.g. disease carried over by Europeans to the Americas wiping out an estimated 90% of the native population, rabbits and other animals devouring massive amounts of crops throughout Australia, etc.), so it's probably safe to assume that most people who would want to transplant an organism in an attempt to restore a natural balance would be wary of upsetting such a balance further.
Fortunately, resurrection ecology wouldn't involve introducing a related species from across the globe to an environment of its now-extinct species. As mentioned earlier, the process involves the introduction of a very closely related (sub)species, which makes the most likely candidates organisms from similar and nearby environs. An important factor in resurrection ecology is, obviously, that the environment the extinct organism lived in still be suitable for such an organism to live in again. If, for example, an organism became extinct because all of its habitat was destroyed and replaced with suburbs, moving similar species into the now-urbanized area will likely only kill the new organisms in the same way the already extinct ones died.
Resurrection ecology should and generally is considered a
last resort approach to repopulating a species. As mentioned earlier, ressurection ecology generally isn't looked at something that should be done just for the sake of seeing if it will work. Confirmation of a species' total demise is hard work. The
Palos Verde Blue butterfly, for example, was
thought to have been extinct for years before a small,
remote group of them was found to have survived in a nearby area. Total extinction of a species should be confirmed before this type of resurrection is attempted.
Resurrection ecology apparently isn't exclusively a human-guided event. A butterfly species in Florida that had become extinct were apparently "resurrected" when some of a related subspecies found in Cuba and the Bahamas made its way into the remaining habitats of the then-extinct species. Over time, the migrated subspecies began to adapt to their new but similar environment and eventually became virtually identical to the species that had died out. Subspecies are where evolution in modern day organisms is most likely to be observed and it is for this reason that they're ideal for resurrection ecology.
British entomologists have (at least*) twice attempted to resurrect extinct species of English butterflies:
In 1847, the English Large Copper butterfly (Lycaena dispar dispar) became extinct due to the fens the butterflies lived in being drained for agricultural use. In response to the elimination of the English Large Copper, Lord Walter Rothschild and other British entomologists founded a committee for insect conservation in 1920s. The committee took steps to set aside the Woodwalton Fen, which was still undrained and a suitable habitat for Large Coppers, and imported German Large Copper butterflies (Lycaena dispar rutila) to the preserved fen in an attempt to reintroduce the species. The German Coppers failed to survive, however, and later the committee gave it another try with coppers from fens in Dutch coastal areas (Lycaena dispar batava). Though the Dutch Coppers survived, those maintaining the fen have had to aid the species' survival in its new home (planting the plants the butterflies lay their eggs on, bringing the larvae indoors in the winter, taking steps to stop the fen from drying out naturally, etc.). The species has not adapted to its new environment, so this shouldn't be considered a successful resurrection.
In 1979, the English Large Blue butterfly (Maculinea arion) became extinct, despite efforts to keep its numbers from dwindling to zero. Despite entomologists attempting to save the species while it was still extant, the species died anyway because its complex relationship with a species of ant native to its habitat wasn't completely understood until after its death. The Large Blue's partially grown caterpillars fall from the plant they feed/grow upon until that point and are carried into an ant colony by the area's naturally occuring ant species. While within the ant colony, the caterpillar begins feeding on some of the developing ant larvae, which the ants tolerate as they milk honeydew from specialized glands on the caterpillar. Eventually, the caterpillar goes into pupation (during which it is in its cocoon) while still within the ant colony. When the pupa has fully transformed into a butterfly, it leaves the cocoon, crawls out of the ant's underground home, and flies away.
Obviously such a cycle of development would not only be hard for humans to figure out but to entice another species to follow as well. Fortunately, the Swedish Large Blue subpsecies (Maculinea arion arion) had a similar (though not as complex) relationship with ants in its habitat. Swedish Large Blues were introduced into a preserved area (with the proper ant species) set aside for the English Large Blues before their extinction and, eventually, the English Large Blue was resurrected: The Swedish Large Blues adapted to their new environment and became virtually identical to the English Large Blues.
Robert Michael Pyle, founder of The Xerces Society (named for the Xerces Blue butterfly, Glaucopsyche xerces, the first North American butterfly to become extinct due to human interference), is currently pushing for related subspecies of the Xerces Blue to be introduced to the Xerces Blue's environment (where survival of the species is still possible) in order to bring back the Xerces Blue.
Sources:
http://www.wholeearthmag.com/ArticleBin/421.html
http://www.edutrek.net/features.new.htm#resurrection
and some passing mentions of the idea on other websites that weren't helpful.
*Both sources cited are excerpts from the same article, "Resurrection Ecology: Bring Back the Xerces Blue!" by Robert Michael Pyle of The Xerces Society. This is the most comprehensive source of information I've been able to find on resurrection ecology and it's possible Pyle may have coined the term. In any event, I haven't found more information on attempts as resurrection ecology other than those mention here. If anyone knows of any more, please contact me.