Who's to Blame for Waning Diversity?

Environmental impacts caused by introduced plants, animals and pathogens have a long history, but if the great Zhang Qian were alive today, it would not be so easy for him to bring in all kinds of animal and plant products. Most countries today have legislation to control the entry of foreign species and agricultural products, and carry out inspections at harbors and airports using all kinds of equipment and techniques. But in the past, vigilance towards alien species was focused only on the risk of their carrying infectious diseases or insect pests which would infect livestock, attack crops or directly affect human health. There was rarely any assessment of their impact on wild species and habitats, on biodiversity, or on local gene pools.

The coldest winter for Taiwan's cycas

For instance, an outbreak of introduced scale insects infesting king sago palms (Cycas revoluta), which began late last year in Taoyuan County, has now spread all over Taiwan, causing over 100,000 container-grown king sagos to wither and die. This has resulted in great financial loss to nurserymen, who have repeatedly demanded that the government agencies responsible for agriculture should bale them out financially and act to eradicate the insect. But in fact what is most worrying about the insect-which may have hitched a ride on a seed cone smuggled in by a sago grower, or escaped the gaze of customs officials hidden among the new leaves of a plant-is the fear that as well as attacking the improved, mass-produced king sago, it may also attack the wild Taitung cycas (Cycas taitungensis, a.k.a. the emperor sago). The Taitung cycas, which is able to survive forest fires, is an endemic species which grows wild only in Taitung County, mostly in the Taitung Cycas Nature Reserve. It has evolved in Taiwan over tens of thousands of years, and is a valuable part of the gene pool. But we can only wait and see whether it will survive the scale insect epidemic.

When communities of organisms are separated from each other by mountains, oceans or other geographical obstacles, over time differences evolve in their form and genetic characteristics. But today, "natural separation is losing its effectiveness. For reasons of economic activity, leisure or scientific research, humans deliberately introduce alien species, so that plant and animal populations which formerly would never have come into contact become close competitors in the struggle for survival. In combination with environmental threats such as habitat destruction and global climate change, the problems caused by introduced species have to be recognized as a factor in the global tragedy of species loss." So says a report by the Council of Agriculture's Taiwan Endemic Species Research Institute (TESRI). The UN Convention on Biological Diversity calls on signatory countries to prevent the introduction of alien species which threaten ecosystems, habitats or species, or to control or eradicate them if they have already been introduced.

Flower market

Most alien species that find their way across the oceans to other lands are in fact unable to survive in the wild because of differences of climate, environment and food sources. But some adaptable species, lacking natural enemies in their new home, will reproduce rapidly if given the slightest opportunity. Native species, on the other hand, keep each other's numbers in check over the long term, so it is very hard for them to compete with opportunistic newcomers. "As an island with a unique ecosystem, Taiwan is all the more sensitive and vulnerable to invasions of alien species," comments TESRI director Yan Jen-te.

Many native Taiwanese species are fighting a losing battle to compete with exotic invaders.

Botanists estimate that the number of garden plants introduced into Taiwan already far exceeds the island's 4000-plus native species of vascular plants. Over 200 of these introduced species have spread from parks and gardens into wild places and mountains, whether by deliberate planting or by natural seed dispersal. More than 90 commonly seen exotic species are already displacing native plants.

Busy Lizzy (Impatiens walleriana) from Africa, flossflower (Ageratum houstonianum) from Mexico, angel's trumpet (Brugmansia suaveolens) from Brazil and beggar's ticks (Bidens alba) from Chile make Taiwan's wild places look more like a city flower market. Today, one of the most common flowering plants along river banks in northern Taiwan is angel's trumpet, while on dry river beds in central and southern Taiwan swathes of elephant grass (Pennisetum purpureum) from tropical Africa wave in the wind. Not to be outdone, the water hyacinth (Eichhornia crassipes) from faraway South America crowds wetlands and pools. Exotic species are encroaching on nature reserves and national parks too.

The vigor of some exotic plant species astounds even Peng Jen-chieh, head of TESRI's Division of Botany. "In particular, quick-maturing herbaceous and flowering plants with a short reproductive cycle really are impossible to eradicate. Pull them out and they just spring right back up again." Peng says that several of the native silvergrass species growing wild in Taiwan were rare enough to begin with, but now with elephant grass spreading its seeds everywhere and occupying the dry river beds, pioneer species such as giant Chinese silvergrass (Miscanthus floridulus), formerly the first to colonize bare ground, have become an even rarer sight.

Successful immigrants

There are countless examples of native Taiwanese species being overwhelmed by exotics. Mouth-breeder fish (Tilapia spp.), originally from Africa, introduced to Taiwan in the 1950s, have now taken up residence in almost all the island's rivers. After Tilapia, the growing popularity of aquarium keeping in the wake of the 1980s economic boom brought a new generation of introduced fish to the fore, notably the suckermouth catfish (Hypostomus punctatus). The high levels of pollution in Taiwan's rivers has helped the immigrants, for as the local fish go belly-up in the inky waters, the resilient newcomers find themselves in their element. According to an islandwide survey by TESRI, and other research, alien fish species are resident in 80% of our rivers, and nine of them are commonplace. Even the rivers of eastern Taiwan are at risk, and suckermouth catfish and mosquitofish (Gambusia affinis) have made themselves at home in the crystal waters of the Shuanghsi and Mutan Creeks of northern Taiwan.

The story is much the same when it comes to birds. Surveys of foreign starlings and mynas, also by TESRI, identified 13 non-native members of the family Sturnidae living wild in Taiwan, mainly escaped or released pets and their offspring. "White-vented mynas and common mynas have been recorded in over a third of Taiwan's 361 urban townships, rural townships and city districts," says a survey report by Lin Ruey-shing of TESRI. Are these interlopers also set to take up residence all over Taiwan? The white-vented myna now outnumbers Taiwan's native crested myna by five to two, so that one cannot help but worry about the continued survival of the crested myna. As for the exotic parrots which pop up here and there all the way from Tanshui in northern Taiwan to Kaohsiung in the south, the sulphur-crested cockatoo, Goffin's cockatoo and umbrella cockatoo have all been recorded breeding in Taiwan, and are evidently planning to settle here permanently too.

Alien species have invaded Taiwan's land, rivers and skies in such numbers that birdwatchers, biologists and others who often go out into the wilds have all become aware of the threat they pose to native species' survival. But in the past there was a lack of accurate surveys, and due to its limited personnel resources TESRI is unable to do long-term follow-up studies. Just what is the impact of invasive species on our native birds? How long has such impact been taking place? "That all requires further study," says Lin Ruey-shing, unwilling to jump to conclusions.

Spreading like wildfire

In fact we cannot blame exotic species for prospering in other lands, for it was humans who moved them there from their native habitats. Hence opinions differ internationally as to whether it is worth trying to "eradicate" alien species. All the more so since many introduced species spread like wildfire and reappear at the drop of a hat, so that the enormous cost and difficulty of eradication programs often causes them to be abandoned before they are complete. For instance, even the USA, the country with the largest number of biologists in the world, has not been able to come up with an effective strategy to wipe out introduced tamarisks. The shrubs, which grow on marshland, put down deep, dense, extensive root systems, and produce huge numbers of seeds. Unable to stamp them out, experts can only resign themselves to their presence. New Zealand learned a bitter lesson in its efforts to eradicate the American lodgepole pine (Pinus contorta). Lodgepole pines prefer open ground, and forestry workers discovered that the more of the trees they chopped down, the more grew. The reason was that felling the bigger trees allowed more light into the forest, thus giving the Lodgepole pine seedlings the opportunity to spread faster and further.

The problem of exotic species is a worldwide one, and those nations which take it seriously approach it as if confronting a formidable enemy, not only stepping up their monitoring of farm products, but also imposing ever stricter limits on the introduction of all kinds of organisms. In Taiwan, a Wild Plants Conservation Law is currently being drafted. Peng Jen-chieh explains that the legislation will require people wishing to introduce an organism into Taiwan to submit complete information on the species concerned-including whether it is known to have spread over large areas in an environment similar to Taiwan's-as a basis for deciding whether to allow it in. But sadly, due to limited personnel resources and the likely impact on horticulture and other industries, the draft seems unlikely to become law any time soon.

But no matter how watertight legislation and inspections are, the main reason exotic species always stay one jump ahead of the inspectors is the actions of modern-day Zhang Qians who blithely smuggle in plants and animals. From South American chameleons and African lemurs, to fruit, flowers and even cuttings brought home from visits to European parks and gardens, they leave customs officers at a loss as to which they should confiscate.

Pet owners too are accomplices in the spread of exotic species. The slider turtles which have been described as having "taken possession of the entire Lanyang Plain," are but one example. Such animals as carnivorous American alligators and poisonous snakes from Southeast Asia are being kept by people in Taiwan too, and when they grow too large to cope with their owners often "set them free." More than half the exotic flowering plants in Taiwan's wild places were originally planted there deliberately by well-meaning people, to "beautify nature."

Times change. Zhang Qian's day is long gone, and the global village is upon us. Thanks to the WTO, many citizens of the world can pick and choose from a dazzling array of fruits, vegetables, flowers and seafoods. But as you satisfy your own desires, don't forget to leave a little space for the work of the creator!

p.19

Waterhyacinth filling a wetland area make for a colorful show. But in fact this invasive alien species crowds out native plants. By acting as "vectors," humans have allowed a small number of highly adaptable species to destroy the uniqueness and diversity of local ecologies. (photo by Diago Chiu)

p.20

Introduced slider turtles, golden apple snails (Pomacea canaliculata), suckermouth catfish and parrots have invaded Taiwan's land, waters and skies to become dominant species in the wild.