100 Under 100

by Scott Leslie

  One of the big worries for conservationists who are trying to save the Catalina mahogany is that it’s hybridizing with the very closely related and much more abundant mountain mahogany. The seven endangered trees are outnumbered by its more populous cousins 10 to 1. The mountain mahogany looks poised to genetically swallow up the rarer tree. How does this work?

  Species adapt to their changing environments through the continual process of natural selection through mutation. Genetic mutations usually have no real effect, and sometimes they’re just plain bad, but every once in a while they are beneficial. For example, maybe a chance mutation of one or more genes results in a plant producing offspring that have slightly stronger roots. If this gives them a better chance of making it through the yearly windy season, relatively more of them will survive to pass this revised genetic code onto their offspring, and so on. Eventually the less fit, weak-rooted trees lacking the mutation will be replaced by the strong-rooted ones. A new species is born, an old one disappears. In time, another mutation will come along and the species will evolve again. But there’s another way trees evolve: through hybridization.

  The mixing of genes by breeding with a closely related, sexually compatible species is common among plants. Sometimes, whatever physical differences the genetically distinct offspring hybrid has compared with its two parent species will be beneficial. This results in a new species that out-competes both its parent species and eventually replaces them. So hybridization may end up dictating the fate of the Catalina mahogany. (Hybridizing is relatively rare among animals. When it happens, it usually results in sterile creatures such as the mule, the offspring of a horse and a donkey. There are exceptions. North America’s eastern coyote, likely a recent hybrid between wolves and western coyotes, is a highly successful, fertile, and ecologically fit species.)

  The dilemma is this: Should the mountain mahogany be removed from the vicinity of the endangered tree, leaving the last seven Catalina mahoganies to struggle on their own (something they’re apparently not very good at)? Or, should the two species be allowed to mix their genes and create a completely new species? With hybridization, the Catalina mahogany will at least have some of its genes survive, enfolded into the genetic code of the mountain mahogany.

  Though few other trees can claim to be as rare as either the Catalina mahogany or the round-leaf birch, three of them are found on one tiny island in the South Atlantic.

  THE SHE-CABBAGE, HE-CABBAGE, AND EBONY TREES OF ST. HELENA

  Most oceanic islands have never been part of a continental land mass. Instead, they are usually formed when an underwater volcano emerges from the sea. The ones that have been around for millions of years can be real factories of evolution. What they usually lack in land mammals, reptiles, amphibians, and often trees they make up for with birds, bats, and flying insects, along with plants with lightweight or salt-resistant seeds. All of these island-dwelling organisms have one thing in common: they’ve made it across open ocean from the mainland.

  Such arrivals find empty niches on newly formed islands and over time evolve to fit them, creating new species. Unlike organisms living on continents that tend to have widespread populations, those on islands have much smaller ranges, so they are much more impacted by local human activities. This is why so many living things on islands are endangered.

  Few islands fit this principle better than St. Helena, a British territory since the 1600s. A mountainous island of volcanic origin, it is located in the middle of the tropical South Atlantic, 2,000 kilometres from Africa. There are few places on earth more remote. Pondering what this seclusion meant for the biodiversity of the island, biologist E.O. Wilson wrote: “St. Helena was nearly a closed ecosystem, a biosphere functioning in great isolation, one step removed from a satellite colony in space.”11

  Aware of the near impossibility of escape, England took advantage of St. Helena’s far-flung location by exiling Napoleon there in 1815, the Zulu king Dinizulu in 1890, and 6,000 Boer prisoners in the early 1900s. No longer in the exile business, today about 4,000 Saints, as they are known, make the island home.

  St. Helena’s human history is surpassed by its natural history. Many remarkable endemic species, from the beautiful to the bizarre, inhabit its 122 square kilometres. Take the St. Helena giant earwig. At eight centimetres long, it’s the world’s largest earwig and just one of 200 insects and other invertebrates on St. Helena that are found nowhere else on the planet. The wirebird—named for its skinny legs—is a species of plover exclusive to the island, and the national bird. The bug and the bird are critically endangered; the earwig hasn’t been seen in decades and may already be extinct, while just a few hundred wirebirds survive. That said, St. Helena is especially well endowed with endangered botanical riches. Thirty-six of its plant species are found nowhere else.

  Overgrazing by goats first introduced to the island in the early 1500s, the logging of native forests, and the introduction of alien plants over centuries of human occupation have decimated the once spectacular botanical life of St. Helena. Several species are already extinct: the dwarf ebony, the St. Helena heliotrope, and the St. Helena olive, to name a few. The bastard gumwood is down to one known individual. And much of the rest of the island’s plant biodiversity hangs on by a thread.

  SHE-CABBAGE TREE

  Free from competition with larger shrubs and trees, plants of the aster family (also called the sunflower or daisy family) have evolved into what are essentially “tree flowers” on St. Helena. This is exactly what the curiously named she-cabbage tree and he-cabbage tree are. In essence giant flowers, they’re among the most endangered plant species anywhere.

  The she-cabbage tree is named for its large smooth leaves, as opposed to the hairy he-cabbage. Like a long-stemmed flower, a slender, branchless trunk is topped by a clump of leaves and rosettes during one stage of its life cycle. Growing to only about seven metres tall, the once common tree was logged for its straight trunks, which were used for building houses. Invasive plants and the clearing of forest for pasture also took a toll.

  The she-cabbage was thought to be extinct until 1976, when three mature trees were discovered along with a few seedlings on a high ridge. Another group was later found some distance away. These two subpopulations are considered to be genetically isolated—they’re far enough apart that they can’t reproductively mix under natural conditions. As a result, their ability to adapt to changing environmental conditions, pests, or disease may be compromised by what amounts to inbreeding within each separate population.

  The she-cabbage isn’t alone in its struggle to survive: once covered by lush tropical forests, today only about 10 percent of St. Helena is covered by trees. Now there are just a few wild she-cabbages left, all of them mature or in old age. Even nursery-raised trees cultivated to conserve the species number fewer than 50. Because it is so short-lived and enough new trees aren’t growing in the wild to replace older ones, it’s feared the she-cabbage could soon become extinct.

  HE-CABBAGE TREE

  Despite its similar name and similar appearance to the she-cabbage tree, the he-cabbage tree is a surprisingly distant relative and belongs to a different genus.

  Found at the highest elevations of St. Helena, in Diana’s Peak National Park and on High Peak, the he-cabbage tree was probably never common. An already scant population of fewer than 100 in the 1990s has shrunken even further due to a deadly infestation of moth larvae and competition from introduced plants. To stem this decline, invasive species such as New Zealand flax have been cleared from the tree’s habitat, and between 100 and 200 cultivated he-cabbage seedlings were reintroduced in Diana’s Peak National Park. Many of these have survived, but none is yet mature enough to produce seeds of their own. Despite these efforts at recovery, the population is very small and its habitat fragmented. Moreover, the percentage of viable seeds is low (many simply refuse to sprout), limiting the number of seedlings available for repopulation. Today, there are estimated to be fewer than 50 wild mature tr
ees surviving on the island.

  ST. HELENA EBONY

  Once the dominant species on parts of the island, the St. Helena ebony was thought to be extinct for over a century. In 1980, two of them were discovered literally clinging to life on the face of a cliff near a geologic formation known as the Asses Ears. The ebonies survived because they were out of reach of the hungry goats that had wiped out the rest of their kind, along with many other botanical treasures. Two of the ebony’s close relatives weren’t so lucky: the St. Helena dwarf ebony is extinct, and the St. Helena redwood is extinct in the wild.

  A member of the mallow family of flowers, the St. Helena ebony was historically described as a small tree of about five metres in height. The surviving wild plants, however (the only ones known in modern times), hug the ground as low bushes, its long branches covered in heart-shaped dark green leaves. Up to three beautiful white and purple flowers can bloom on the tree at any time of year if there’s enough rain. The St. Helena ebony’s wood is very dark, very hard, and very dense—so much so that it doesn’t float, not unlike the commercially exploited, though unrelated mainland ebony. Demand for its fine wood helped push the island species toward extinction.

  Although no additional wild plants have been found since the rediscovery of the species three decades ago, thousands of seedlings grown from root cuttings of the two survivors have been reintroduced to various wild sites on the island, as well as in local gardens. Nevertheless, because of the way these seedlings have been produced from cuttings, they are all identical clones of the two wild trees and are subject to the same problem that comes up again and again with tiny remnant populations: inbreeding. Such limited genetic diversity could result in susceptibility to diseases and pests in the future. To tackle this, scientists from Kew Gardens in the United Kingdom—also active in several other conservation projects on the island—are working with conservationists on St. Helena to develop seedlings with more genetic diversity. Seed banks are also being developed both on and off the island to ensure that the wild genes of the species will be available for future reintroductions.

  NORFOLK ISLAND’S PHREATIA ORCHID AND PHILLIP ISLAND WHEATGRASS

  Although on the opposite side of the planet, Norfolk Island, like St. Helena, is an extinct volcano hosting a number of extremely rare, endangered plant species. The small 35-square-kilometre Pacific island, lying between Australia, New Zealand, and New Caledonia, is best known for its most famous export, the distinctive Norfolk pine, a species occurring naturally nowhere else.

  With its classic Christmas tree shape and wispy fronds, this beautiful evergreen was long ago cultivated and has been introduced to warmer coastal areas around the world. It’s a close relative to the rare Wollemi pine of Australia. The Norfolk pine isn’t the only unique evolutionary product of the island, though. An auspicious combination of climate, terrain, soil conditions, and isolation from other terra firma has blessed Norfolk with a diversity of endemic life. It is home to 15 species or subspecies of birds that are exclusive to the island (6 of which have already become extinct in modern times, including most recently the white-chested white-eye, a beautiful green and white warbler-like songbird, which hasn’t been officially recorded in decades). But it is Norfolk’s vegetation that is especially varied and fragile. Nearly 50 plant species grow here that are found nowhere else. About one-fifth of them have total populations in the wild of fewer than 50 individuals, making it one of the most endangered botanical assemblages in the world. The trouble is habitat destruction and invasive species—a familiar refrain on many of the world’s far-flung oceanic islands.

  NORFOLK ISLAND PHREATIA ORCHID

  The Norfolk Island Phreatia orchid is the rarest of the world’s 22,000 or so members of the family Orchidaceae living in the wild. Only five plants survive, all within the island’s national park. These few tiny orchids eke out a life in Norfolk’s last fragment of natural rainforest, which covers only about 500 hectares. Phreatia grow on the branches of trees as epiphytes, or air plants, where they take their water and nutrients from the air. It’s a common lifestyle among tropical plants: about 24,000 species of the earth’s plants are epiphytes. Phreatia limenophylax are only about five centimetres tall, with two-centimetre-long clusters of tiny greenish-white flowers; they’re not the flashy, vividly coloured orchids we normally think of. Whether it was ever abundant isn’t known, but it’s a good bet the flower is so exceedingly rare today largely because of the destruction of 90 percent of the island’s natural forest for farming and grazing, and for materials to build houses.

  Living within the national park, the phreatia is well protected from direct human disturbance. Nevertheless, its tiny population makes it highly vulnerable to extinction. A disease or an unusually severe windstorm could take out the last five orchids in one fell swoop. As well, invasive plant species competing with the native trees on whose branches the orchid grows could have an impact on whether its population will increase. As insurance, the Norfolk Island Botanic Gardens stores Phreatia limenophylax seeds and is working on artificially propagating the plants. As well, there are ongoing efforts to identify and control invasive plant species, critical not only for the survival of the orchid but also for the world’s most endangered grass, which lives on a little island just off Norfolk.

  PHILLIP ISLAND WHEATGRASS

  Of all the plants on earth, none is more ubiquitous than the grasses. From the equatorial rainforest to beneath shallow seas, from the high Arctic to the Antarctic (where only one native species grows, using a special protein to keep it from freezing), members of the 10,000-strong Poaceae family are everywhere. One grass must necessarily be the rarest, and given the perilous state of the Norfolk Island Archipelago’s native vegetation, it isn’t surprising that it’s found here, on a tiny island six kilometres south of Norfolk.

  Formerly covered by luxuriant vegetation, Phillip Island’s 190-hectare ecosystem was rendered a treeless lump of volcanic basalt with little vegetation and heavy erosion by pigs, goats, and rabbits introduced when it was a penal colony during the 18th and 19th centuries. The removal of the pigs and goats in the early 20th century and finally the rabbits in 1988 allowed the island to undergo some natural regeneration of its flora. But for the Phillip Island wheatgrass, it was almost too late. This tufted, metre-tall perennial grass was thought to be extinct on the island until a handful of individual plants were discovered in the late 1980s living on cliff faces, where they had been inaccessible to the hungry introduced grazers of the past. Today, fewer than 50 Phillip Island wheatgrass plants survive here, with a small handful on Norfolk Island and another few dozen living about 1,000 kilometres away on Lord Howe Island.

  Although the grass remains rare, Phillip Island has experienced a green renaissance. Many native plant species have made a comeback since the grazers have been removed. The island is also being reforested with Norfolk pines. This is not all good news for the Phillip Island wheatgrass subspecies, however. Improving growing conditions have also made it easier for non-native invasive species to become established, including kikuyu grass and buffalo grass, aggressive colonizers that compete with the native grasses for the best growing areas. Nevertheless, revegetation to prevent erosion and weed control are part of a continuing program to help the recovery of not only Phillip Island’s endangered grass but its entire natural ecosystem.

  10. Donald Culross Peattie, Flowering Earth (New York: G.P. Putnam’s Sons, 1939), 4.

  11. E.O. Wilson, The Diversity of Life (New York: W.W. Norton, 1993), 104.

  PART TWO

  LIVING BY THE GRACE OF HUMANITY:

  GONE FROM THE WILD BUT NOT EXTINCT

  There’s a truism that says “extinction is forever.” You can’t argue with that. But, as you’ll soon see, extinction in the wild is not necessarily for all eternity. There are a few living things that have been erased from the natural world completely, with populations of zero in the wild, that still survive in small numbers under our captive care in zoos, aviaries, botani
cal gardens, laboratories, and breeding facilities, and within protective fenced-in habitat enclosures; they’re like intensive care units for nature. And like a proper ICU, species or “patients” are given whatever acute attention they need to survive in the short term, while improving their situation enough so they can completely heal on their own after leaving. The primary goal of such ex situ (Latin for “out of place”) species conservation is to increase their typically tiny populations back to some semblance of health, for future reintroductions back into the wild. And therein lies an even bigger challenge: there has to actually be a wild to return them to, a natural place where they can sustain themselves indefinitely, without our help. So protecting or restoring habitats is also critical for success.

  NORTHERN WHITE RHINOCEROS

  The northern white rhinoceros, a subspecies of the white rhinoceros, once lived across much of central Africa. Its former range reads like a who’s who of war-torn countries: Uganda, Chad, Sudan, the Central African Republic, and the Democratic Republic of the Congo all hosted populations of the animal in the past. It’s an irresistible target: two tonnes of meat on the hoof sporting a horn practically worth its weight in gold living in countries saturated with firearms on a continent suffering widespread hunger. The outcome is painfully predictable. In 1960, there were 2,000 northern white rhinos left in the wild. Today, there are none. The last few animals, which survived in Garamba National Park in the Democratic Republic of the Congo, were gone by 2007. There was a report of three northern white rhinos in southern Sudan in 2008; though encouraging, it was unconfirmed. What’s surprising is that the northern white rhino lasted as long as it did.