Degraded Habitats Push More Species to Extinction

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Kihansi spray toadThe latest global assessment of biodiversity ruled yesterday that an additional 11 species are either fully extinct or extinct outside of captivity. 

The International Union for the Conservation of Nature (IUCN) updated its Red List of Threatened Species, considered the authority on the status of the world's species, to an all-time high of 17,291 species threatened with extinction.

The annual index has become a consistent indicator of how environmental change is altering natural habitats worldwide. As climate change, invasive species, and habitat destruction place greater pressure on wildlife, more species are disappearing at rates faster than conservationists can react to ensure the species' survival.

"A serious extinction crisis is mounting," said Jane Smart, director of IUCN's biodiversity conservation group, in a statement. "We're rapidly running out of time."

The Red List compiled the latest scientific data on 47,677 species across the globe. The findings indicate that 36 percent of assessed species are threatened with extinction and 2 percent of assessed species are extinct or extinct outside of captivity.

Plant species, frequent victims to land use changes and shifts in local and regional temperatures, are particularly imperiled. The new data reveal that 70 percent of assessed plant species are threatened.

Among assessed animal groups, 37 percent of freshwater fish species, 35 percent of invertebrates, 30 percent of amphibians, and 28 percent of reptiles are threatened. In addition, 21 percent of known mammals and 12 percent of known birds are under threat.

Although the data is the most reliable thus far, insufficient studies of species abundance prevented the Red List from categorizing 14 percent of known species, and many species remain understudied, said Craig Hilton-Taylor, manager of the IUCN Red List Unit.

"These results are just the tip of the iceberg," Hilton-Taylor said in a statement. "There are many more millions out there which could be under serious threat."

The Kihansi spray toad is among the species that IUCN now considers extinct in the wild. Native to a two-hectare region of the Udzungwa Mountains in southern Tanzania, the population numbered as many as 21,000 in June 2003 before effectively collapsing.

A hydropower project built upstream of the toad's habitat three years prior led to the rapid decline. The dam cut off 90 percent of the water flow to the gorge near the Kihansi Falls. Dry conditions, a fungal disease outbreak, and possible pesticide contamination led the toad population to crash in 2003. By mid-January 2004, scientists spotted only three individuals and heard the croaking of only two males.

A Wildlife Conservation Society-led breeding program is making progress on recovering the species. The Tanzanian government invited WCS scientists in 2000 to transfer 499 of the toads to the Bronx Zoo in New York. The zoo announced last year that the program raised a population of 300 toads. The Toledo Zoo has since advanced a similar captive breeding program.

IUCN also listed eight tree snail species from French Polynesia and Seychelles as extinct, as well as two similar snail species as extinct outside of captivity. Each snail species was adapted to living on a particular island, forest, or tree type, but invasive species, deforestation, and climate change pushed them to extinction.

The introduction of African giant land snails to the Pacific islands led to the eventual demise of seven of the native snail species. The introduced snails provided a food source for local communities, but when the snails began to eat area banana plants, residents decided in the 1970s and 1980s to substitute them with another non-native species, rosy wolf snails. The carnivorous new snails had an appetite for the native Polynesian tree snails, eating many of them to extinction within less than a decade.

The Aldabra banded snail, listed now as extinct, was abundant on Aldabra, a 150-square-kilometer coral atoll in the Seychelles, in the 1970s. But the snail lives in a dormant state during prolonged dry seasons, reducing the likelihood that reproductive individuals find mates. After a series of dry years in the 1980s and 1990s, the population crashed. The last living individual was spotted in 1997.

An estimated 125 different tree snail species once lived across French Polynesia. Now, at least 50 tree snail species are extinct and another 24 are alive in captive breeding programs across Europe and North America. The programs have produced mixed results. Three species have been released into a reserve on the French Polynesian island of Moorea. Attempts to breed one of them, Partula labrusca, ended in 2002.

Two Polynesian tree snails formally listed as extinct, Partula affinis and Partula taeniata, have been rediscovered and are know listed as critically endangered.

The Miles' Robber frog, a species native to Honduras, has also been upgraded from "extinct" to "critically endangered." IUCN declared the frog extinct in 2004, but last year scientists rediscovered an individual in Cusuco National Park.

Ben Block is a staff writer with the Worldwatch Institute. He can be reached at

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