Evolved for Extinction?

Research Blogging / by Dave Munger /

Could the novel evolutionary adaptations of animals like the Galapagos tortoise and the Komodo dragon actually leave these species more vulnerable to extinction?

Charles Darwin published his Voyage of the Beagle in 1838, more than 20 years before he proposed his celebrated theory of natural selection in On the Origin of Species. Yet even then there were hints of the much grander theory to come, based on his observations of the finches of the Galapagos:

Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.

The 14 different species of finch on the islands seem to have arisen from a common set of ancestors and subsequently evolved to fill other environmental niches. Their adaptation continues to the present day, as key features of their beaks can be observed changing over successive generations.

Many species in the Galapagos are found nowhere else on Earth, from the equatorial penguins thousands of kilometers north of their nearest relatives to the enormous tortoises that bear the islands’ name. The Galapagos tortoises, sadly, are vulnerable to extinction after evolving without predators or competition for millennia. When humans came to the islands, they preyed on the tortoises and other unique species and introduced “alien” species, such as goats, which competed with the tortoises for food. These pressures pushed some Galapagos species, including the tortoise, to the brink of extinction. If the tortoises can’t survive on the Galapagos, they probably won’t be able to survive in the wild anywhere else.

Similar processes can be seen all over the globe, but they are most dramatic in places that are physically separated from similar regions. Islands are perhaps the most obvious spots: Witness the gigantic Komodo dragon, the unique flora and fauna of Hawaii, and the Tasmanian devil and now-extinct Tasmanian wolf. But unique species can arise in other isolated places: peninsulas, ponds and lakes, mountaintops, and geothermal hot spots. If these solitary spaces rapidly change or are invaded by non-native competitors, the resident, uniquely adapted species are in danger of extinction.

Zen Faulkes, a biologist at the University of Texas–Pan-American, recently discussed research about “giant” nine-spined sticklebacks (a species of fish), which can grow to a length of 7 centimeters in isolated ponds. While that may not seem very big, it’s double the size of most sticklebacks. For comparison, this would be like discovering a new variety of dog the size of a small horse.

How do these sticklebacks get so big? They’re only found in ponds where no three-spined sticklebacks or trout exist. Without competition from larger rivals, the nine-spined sticklebacks grow bigger to fill in that niche. But when trout are introduced to a pond, the giant sticklebacks disappear.

The adaptations of animals to these isolated environments can be remarkable—but also precarious. Christie Lynn, a graduate student of molecular biology at the University of Hawaii, took note of the difficult situation faced by a beautiful Hawaiian bird, the akepa. Over centuries, this bird has evolved a unique breeding strategy: Males hatch earlier in the year, while females mostly hatch later. This strategy worked well for the birds because strong males were necessary to support “harems” of females.

But when a competing species, the Japanese white-eye, was introduced in 1929, the situation changed for the akepa. White-eyes have a shorter breeding season, confined to the latter half of the akepa’s extended breeding season, when their competition takes its toll on breeding akepas. So while plenty of male akepas are fledged in the beginning of a breeding season, by the end of the season competition becomes so fierce that very few females hatch and survive. From 2000 to 2006, in one studied population, only four female akepa chicks survived to maturity, while 27 males survived, leading to a dangerous sex imbalance. It now seems likely that the akepa’s clever survival strategy may perversely drive it to extinction.

Corey Bradshaw, a biologist who blogs at ConservationBytes.com, was a co-author on a study published in PLoS ONE demonstrating that the akepa’s situation is not unique. An estimated 32 percent of the amphibians worldwide are in danger of extinction, Bradshaw says, and his study found that by far the most important reason for species decline is habitat loss. The twin threats of human development and climate change contribute to shrinking habitats, and as these habitats decline, so do the amphibians’ chances of survival.

In another post, Bradshaw points to two studies demonstrating that building narrow corridors to connect isolated habitats doesn’t appear to solve the problem. When a species begins to decline due to habitat loss, making pathways for “trickling migrants” to go elsewhere won’t always restore the species. It may be preferable to boost local populations by increasing the availability of resources like food.

As Tim Flannery pointed out in his book The Weather Makers, one of the first impacts of global warming may be found in alpine settings. As the Earth warms, mountain species will be forced to higher elevations to find appropriate environments. The geometry of the situation isn’t in their favor: Progressively less area will be available, and mass extinctions will be inevitable. The unique adaptations these organisms evolved over tens of thousands of years will be useless in a new, changed climate—indeed, they may contribute to their own demise.

For more about evolution and extinction, you can continue the discussion at ResearchBlogging.org.

Originally published October 14, 2009

Tags ecology genetics research

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