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Thursday, Aug. 2, 2001
Salt tolerance and life's dispersal derby
By MARK BRAZIL
Salt is an interesting mineral. We all need it. It is crucial to the operation of the cells that make up our bodies.
Too much is bad for us, though, and a salt-rich environment is very hard for mammals to cope with. Seals, sea lions, dolphins, porpoises and whales manage to drink saltwater, but the average terrestrial mammal cannot.
Seabirds have salt-extracting glands that enable them to survive drinking sea-water. This capacity, coupled with their natural ability to fly long distances has made them the ultimate colonists. Visit even the remotest oceanic islands -- Ascension or Tristan da Cunha in the South Atlantic, Pitcairn or Easter Island in the South Pacific -- and you will find seabirds. Their unique abilities make them among the first to colonize newly exposed atolls or newly cooled volcanic islands.
Dispersal is crucial to all species. Plants and animals send out spores, seeds, and young in all directions from the parental site. Some species disperse a few centimeters, others meters or kilometers, but the propagules of the mangroves will drift across whole oceans.
Dispersal alone is meaningless without successful colonization of a new site. Imagine, for example, a strong-flying butterfly that makes its way to an offshore island. Successful dispersal, you may think, but if the flowering plants whose nectar it sips have not preceded it, then the butterfly will starve and its colonization of the island will fail.
The same is true of any specialist animal. A flycatcher or an insect-eating warbler will not survive on an island it has dispersed to, if the insects it eats are not already there.
So timing and chance come into play along with skills and natural abilities, when measuring successful dispersal and colonization.
The Japanese islands have at different times been connected, disconnected and reconnected to the Asian continent, allowing a flow of species of plants and animals back and forth over long periods of time. The mixture of species here is balanced.
In contrast, the New Zealand islands separated from the ancient supercontinent of Gondwana so long ago that mammals had not yet evolved. As a result, New Zealand completely lacks native mammals, and the insects and birds that survived there radiated into ecological niches that mammals occupied elsewhere in the world.
Dispersing across saltwater is a problem for mammals. Most species would not survive long in the sea. Few species are even able to survive rafting in trees or vegetation swept away by floods, and most of these are small, such as mice and other rodents. New Zealand was just too far, and no mammals ever made it by natural means, other than marine mammals and a couple types of small bats, no doubt blown across the Tasman Sea from Australia. All other mammals in New Zealand are very recent (and destructive) imports by people.
Visit any remote island group and you will find the same pattern. Plants that have wind-blown seeds, such as ferns and mosses with their light spores; birds; plants that have seeds or berries that are eaten by birds; and aerial insects and wind-dispersing spiders are among the most successful dispersers and colonists.
If land mammals find it difficult to disperse to oceanic islands, then amphibians find it well-nigh impossible. Their delicate, permeable skin makes them even less tolerant of exposure to salt than are mammals. At least mammals can survive a temporary seawater dunking, but for amphibians it is death. Visit a remote island and if you find any kind of amphibian, the likelihood is that it was transported there by people. The toads found in the Ogasawara Islands are a good example of amphibians transported and released by people; naturally they would not occur there.
Reptiles are not quite as successful as birds at dispersing, but they are nevertheless far better travelers than amphibians. By virtue of their special lifestyle they are well able to survive ocean crossings, and have reached some very remote places indeed.
Of the volcanic Galapagos Islands, situated nearly 1,000 km west of the Ecuadorian coast, Charles Darwin wrote, "These islands appear paradises for the whole family of reptiles." He was not wrong, for they are home to several subspecies of giant tortoise, the green sea turtle, the marine iguana, two species of land iguana, several species of lizard (skinks, lava lizards and geckos) and three species of snake.
Other early travelers to the Galapagos were not impressed by the fauna. In particular, the fact that reptiles composed the bulk of the visible fauna did not endear the islands to visitors. Herman Melville, who visited the Galapagos in 1837, just after Darwin, took a bleak view: "Another feature in these isles is their emphatic uninhabitableness . . . Little but reptile life is here found: tortoises, lizards, immense spiders, snakes, and the strangest anomaly of outlandish Nature, the aguano [iguana]. No voice, no low, no howl is heard; the chief sound of life here is a hiss."
A naturalist cannot take such a harsh view. That so many species of reptile occur on the Galapagos Islands is remarkable, a tribute to the unique characteristics of the reptiles as a group. Twenty-one species of reptile are now resident in the Galapagos, and these probably all derive from just seven or eight ancestral colonist species. All but one or two are unique to the islands.
Among other island reptiles that have found fame are the Komodo dragon of the Lesser Sunda Islands in Indonesia (actually a giant monitor lizard) and the giant tortoise of Aldabra in the Indian Ocean. Monitors are strong swimmers and have found their way to almost every island from Malaya to Australia.
Nevertheless, the salt-sneezing marine iguanas with their impish features, their black, verdigris and brick-red skins, and their bizarre habits are the most memorable for anyone who has visited the Galapagos, or even seen one of the many wildlife documentaries about them.
In my next column I'll look at what makes reptiles so successful.