The Kingdom of Rarities (28 page)

The flora and fauna on islands recently connected to the mainland—as with Sri Lanka to India or Trinidad to South America—are similar to those on the nearby mainland, so newly arriving species must cope with predators and herbivores they encountered on the mainland. But species that established themselves on islands that were never connected to the mainland or were otherwise far from continental coasts typically evolved in the absence of predators. The “naive” resident fauna lacked appropriate escape behavior, even birds that could simply have picked up and flown away at the first hint of danger when predators, including
Homo sapiens
, were introduced. As a result, most vertebrate extinctions during the past several centuries have occurred on islands. Such ecological naïveté extends to plants, too. As we shall see in the case of Hawaii, plants that evolved over time in the absence of large herbivores lost their chemical defenses and physical structures such as thorns and prickles; they suffer terribly when exposed to introduced goats, deer, sheep, and rabbits. An often overlooked group of exotic, invasive organisms are invisible to the naked eye—bacteria, viruses, and protozoans that cause outbreaks of disease. In some areas, introduction of disease-causing organisms has been even more deadly for rare natives that have had no previous cause to develop antibodies than have been creatures with backbones. Hawaii has been hard hit on the microscopic front as well.

So much attention has been given to recent invasions as part of the contemporary biodiversity crisis that we lose sight of the fact that some ecologically catastrophic invasions happened long ago. The first biodiversity crisis in Hawaiian history occurred when the Polynesians arrived more than 1,000 years ago, bringing with them slash-and-burn agriculture, pigs, and stowaway rats that scurried to shore. Combined with the unwelcome ark of more recent invaders that Europeans introduced—sheep, goats, cattle, deer, mongooses, and mosquitoes—these foreigners now plague an archipelago with
perhaps the highest concentration of rarities on Earth. Ironically, while the suppression of malaria played a significant role in the decline of the rhinoceros in Nepal, the introduction of malaria in Hawaii has decimated a group of native birds, the honeycreepers.

That some of the spectacular native species are still holding on, such as the woodpecker-like
‘akiap
ō
l
ā
'au
, demands our attention. Many evolutionary biologists, such as Stanford University's Peter Vitousek, a native son, view the string of South Pacific islands collectively called Hawaii as America's Galápagos Islands. If Hawaii's multitude of endemic species has often been decimated by invasives, this cradle of evolutionary exuberance, like Darwin's living laboratory, is also a showcase for how natural selection and isolation have combined to foster new species, many of them rare. Hawaii supports the highest percentage of plants and animals found nowhere else. For example, of the nearly 1,000 species of flowering plants found in Hawaii, 90 percent are restricted to the islands. This unusual level of endemism and such narrow ranges means that if we lose a piece of wild Hawaii, we lose many species forever.

In chapter 2, we looked briefly at the process of adaptive radiation as it occurred among birds of paradise in New Guinea. Evolutionary biologists have also discovered in Hawaii an extraordinary array of species created through adaptive radiation, such as honeycreepers, ferns, tarweeds, and the two most prolific but least heralded radiations, that of the land snails and the fruit flies. At least fifty-one species of Hawaiian honeycreepers, the main protagonists in this chapter, evolved from a single species of primeval finch originating on the American mainland. Eventually some honeycreepers, such as the
‘i'iwi
, evolved a long, decurved bill to feed on nectar from species such as
Lobelia
that have appropriately curved flowers. Other finches evolved a facility for eating fruit or for gleaning insects from bark. Still others stayed with the ancestral finch behavior of cracking seeds. Some of these evolutionary variations never spread beyond a single island. Members of some bird species, though, managed to make it to another island. The ‘i'iwi,
for example, populated several islands. Many of these species are now quite rare, and at least twenty are extinct.

E. O. Wilson has shown how the arrival of a founding species—such as the prototype of the honeycreeper—is typically the opening to an evolutionary play told in four acts. In act 1, a species arrives and colonizes the entire archipelago, expanding in range and numbers. By act 2, populations on the different islands have become distinct from the founding father and mother, and on some islands the populations become extinct. By act 3, most of the original birds to settle on various islands in the archipelago have gone extinct, and those that remain have evolved into distinct species, as different as the ‘akiapōlā'au is from the ‘i'iwi. Finally, in act 4, only a single population remains as an endemic species limited to a single island. The legacy of a single, widely dispersing species colonizing many islands in an archipelago is the generation of many new species with very narrow ranges.

This process of adaptive radiation is common on island archipelagoes because radiation from a single ancestor and formation of new species require barriers to gene flow between forming species, often provided by water gaps among islands. There probably would have been no radiation of finches in either Hawaii or the Galápagos if there had been only one island of a size equal to the sum of current islands. Such avian speciation did happen in New Guinea, a single island, only because New Guinea is so large and the steep mountain chains are sufficiently isolated from one another that they served as barriers to commingling, just as the currents and deep water separating archipelagic islands often do.

When it comes to organisms with more limited mobility and range, it's a different story. Even the main island of Hawaii is large enough to witness the explosive radiation of two diminutive and relatively weak dispersers, fruit flies and snails. Speciation in birds, and even in land snails and fruit flies, requires geographic isolation of populations for long enough that they accumulate sufficient genetic differences that they fail to interbreed if and when they subsequently
come into contact. One size does not fit all, however. What is a dispersal barrier to a snail or a fruit fly is not a barrier to a bird.

On isolated island chains such as Hawaii, then, one can witness both the results of incredible Galápagos-like radiations from a single ancestral species and the catastrophic effects of some more recent introductions by humankind. The outcome of this clash, the first evolved over eons, the other more recent, and its effect on rarity has application to oceanic islands everywhere.

I had a chance to see honeycreepers during a visit to Hawaii's Big Island in July 2001. Ecologist David Wilcove, a friend who had also accompanied us on part of the journey in the Cerrado, was with me. Our main purpose was to attend an international conference on conservation biology taking place at the University of Hawaii's Hilo campus. The real attraction, however, was a chance to risk burning our retinas by looking straight at a molten-red ‘i'iwi, perhaps the most beautiful of all Hawaiian honeycreepers, and to meet scientists in charge of saving the extant half of this endangered family.

At dawn on the first day, David and I headed up the road to the pass between Hawaii's reigning natural monarchs—the volcanoes Mauna Kea and Mauna Loa. Joining us was conservation planner John Morrison, a colleague from the World Wildlife Fund; later that afternoon we would rendezvous with another, ornithologist John Lamoreux. We ignored the danger posed by the drivers hurtling by at high speeds and instead took pleasure in listening to one another mangle pronunciation of the local bird names. If we could spot even one of the birds whose names we were trying to master, such as the ‘i'iwi (pronounced e-
e
-vee), we joked, our struggles with the Hawaiian language would disappear. We stopped at a popular birding spot, K puka 21—a
k puka
being a forest patch that has become isolated by a recent lava flow. Native koa and
‘ōhi'a
trees make up these fragments of mature forest, the ‘ōhi'a sporting flowers
resembling the bottlebrush blossoms of eucalyptus. The bright red color of the flowers acted as a magnet for the nectar-feeding ‘i'iwi and
‘apapane
.

Map of Big Island (Hawaii) and its position within the Hawaiian archipelago

We picked our way over treacherous crumbles of hardened lava to reach the forest edge. Within moments we saw what at first glance looked like a vermilion-colored ‘ōhi'a flower. Suddenly the flower started to move. The dense flowers and foliage only partially concealed the scarlet and black plumage of the gorgeous bird that, once fully exposed, was ignited by the bright mountain sunlight like a flare—an ‘i'iwi. ‘I'iwis also occur in native forests on Maui, Kauai, and Molokai, but they are now almost extinct on Oahu; today fewer than five are left. A visitor could spend a day, as I once did in the early 1990s, driving around the lowlands of Oahu, unsuccessfully trying to spot a single honeycreeper or native plant. Had I read more about Hawaiian ecology in advance, I would have saved some gasoline. Many native Hawaiian birds and plants that are not yet extinct are fugitives, finding refuge on islands such as Oahu only in mountain strongholds. On the Big Island, the birds are more accessible.

We continued in search of the palila, another honeycreeper species that resembles in its behavior and profile the pine grosbeak of North America. Having emerged from the rain forest, we were now about 66 kilometers from Hilo, in the Big Island's dry forest zone, or what is left of it. Cattle ranches lined the road; such pastures are widespread in tropical dry forest areas globally. The climate and soils are more conducive to raising livestock than are the pastures carved out of true rain forest and abandoned a few years later. The spread of cattle on the Big Island has turned a continuous belt of dry forest composed of native
māmane
, a leguminous tree, and naio, a kind of sandalwood tree, into a savanna with an understory of introduced grasses. Today only about 1,200 palilas remain in the wild, and the species has become the subject of a US Fish and Wildlife Service recovery program. We spent the next four hours walking along the road on the slopes of Mauna Kea where, according to
our birding guide, the palila was supposed to be, searching māmane branches for a sturdy yellow-headed honeycreeper. No luck.

The endangered palila population is an example of what can happen when a rare species becomes too dependent on a particular food plant. The māmane is manna for the palila, and when the trees disappear—as a result of pigs browsing the seedlings or the woodlands being cleared for cattle ranches—the birds vanish. The fate of the māmane is a prime example of the potential ecological repercussions of introducing large plant-eating mammals, such as cattle or pigs, onto an island where they never occurred naturally. The local species of plants were unprepared for the invasion and had not evolved chemical defenses to deter herbivory. There are also larger evolutionary processes at work here: the birds are so specialized on māmane in part because the Hawaiian flora has become impoverished and in part because the palila does not venture beyond a narrow range at midelevation on the Big Island.