IUCN: The World Conservation Union Education Unit

BIOINVASION: Threat to Ecology and Economy
Surayya Khatoon¹ and Qadeer M. Ali²
1Department of Botany, ²Marine Reference Collection and
Resource Center, University of Karachi

According to a recent news item, the attack of American Bollworm (Heliothis armigera) on Pakistan’s cotton crop has been more severe this year than ever before. Although the claims about the yield and extent of damage may be conflicting, one thing is certain: Heliothis has become a new challenge for cotton researchers after developing immunity against pesticides.

American Bollworm is an aftermath of the introduction of Deltapine variety of cotton form America in late 1960s. This is just one example of the hazards of importing exotic plant and animal materials without giving a thought to the long-term consequences.

The spread of the species of plants, animals and micro-organisms from one part of the world to other through intentional or unintentional human effort is now recognised as one of the most serious threats to Earth’s Biodiversity and ecosystems. It is variously termed as "Bioinvasion" or "Biopollution". Some scientists consider it as more disastrous than the chemical pollution, because organisms can evolve and proliferate while chemicals cannot.

The deliberate introductions include the crop plants, farm animals, fish and shrimp for aquaculture, decorative plants, pets, biological control organisms, etc., while accidental introductions may be as contamination with various trade commodities or passengers’ luggage, and most importantly through ship ballast which daily transfer millions of aquatic organisms throughout the world. In 1991 the outbreak of cholera in America, which killed about 10000 people, is believed to be due to the ballast water dumped into a Peruvian harbour. Historically, the human pathogens rapidly evolved and spread after the change in human lifestyle form nomadic to permanent settlement in the wake of agriculture about 12000 years back. And now it is globalisation, which is turning the world into a global village where the human pathogens and crop pests are exploring new dimensions of evolutions and spread.

Once on a new land, an alien species may either perish or flourish. In the later case it generally exerts profound effects on the biodiversity and ecosystems of the land. The alien plant or animal species which establish themselves in the natural or seminatural ecosystems unusually outcompete the native species, diminishing them or causing the extinction of more vulnerable and rarer ones. Such exotics are termed as invasive species or invaders. They usually proliferate exponentially depriving the native species of space, nutrients, moisture, etc., thus modify the entire structure and functioning of the ecosystem. The modified structure of flora deprives the native animals of food and shelter, as well as the exotics may bring with them new pests and pathogens or allergens. In some cases they can hybridise with the native species and disturb their genetic integrity. The effects of the invasive species are, in most cases irreversible.

All introduced species do not become invasive, but even the small proportion of invasive wreaks havoc on the ecology and economy of the region. According to a recent review, just 79 invasive species in the USA have caused $97 billion losses from 1906 – 1991and another 15 potentially high impact species could cause $134 billion loss in the future. It is almost impossible to predict whether a new introduction would become invasive or not. An alien may take decades, even centuries, before coming invasive. Generally it needs a long period of acclimatisation. In 1984-85, Passiflora Ligularis started invading forest gaps in Indonesia, although it existed there since 19^th Century.

The factors, which promote invasiveness, include the human-induced habitat disturbance, the magnitude and frequency of introduction, the inherent qualities of species for fast growth, profuse proliferation, efficient seed dispersal, etc. However, one factor is common to all aliens; they do not have natural predators and competitors in the new land; and the natives don’t have the defence mechanism against the aliens because they did not evolve together in the same place.

Many introduced species have benefited humanity like crop plants and farm animals. However, the overall damages caused by the exotics out-number the benefits. Many species brought for certain purposes like decoration, soil conservation, aquaculture, aquaria and house pets, escape and establish in the natural ecosystems. Even the biological control, generally considered environmentally sound, is not without risks. The organism introduced to control certain pest may go haywire, attacking other species besides the target species.

Lessons from abroad

In 1960, Nile perch (Letes niloticus) was introduced into Lake Victoria, the largest tropical lake, to enhance the fish catch. The proposal was strongly opposed by the scientists but their warning was disregarded. Within few years, Nile perch being a voracious predator wiped out the endemic cichlid fish. About 200 species became extinct and another 100 or so diminished to a remnant. The absence of herbivorous cichlids resulted in increased algal growth reducing transparency of water. This is worst ever-recorded example of vertebrate extinction. The native fishermen with their primitive equipment were unable to catch the large Nile perch, neither they liked its taste. The country as a whole got the benefit from Nile Perch export, but those living around the lake were devastated and Lake Ecosystem destroyed irreversibly.

The Zebra mussel (Dreissena polymorpha), native to Russian lakes invaded America in mid 1980s, probably carried in the bilge of ships flushed into the Great Lakes. By 1994, the Zebra mussel had spread throughout the Great Lakes and entered eight major rivers. Each female can produce one million eggs a year. Being colonial, as many as 500 000 mussels may be attached as a solid mass on each square meter of substance, encrusting and clogging various utilities like the intake ducts and pipes of power stations and industries, drains, etc. Removing this pest is expensive. The estimated cost to industry, shipping and sport fishing could be $5 billion in the Great Lake region alone by the year 2000. The mussels are already threatening the bio diversity.

In 1906, Melaleuca tree was introduced from Australia to help drain Everglades, Florida. Being fast growers they are effective in lowering water table. In 1940s, the priority shifted towards saving Everglades as a national treasure. Melaleuca proved to be highly invasive. In 1987, 19000 hectares were pure Melaleuca. Its colonisation replaced breeding grounds for herons, ibis, spoonbills etc. The single species stands have almost no wildlife value. The cost of removing this tree would be enormous.

• In 1984, Caulerpa taxifolia, a decorative alga of aquaria, escaped into Mediterranean Sea. In 1989, it occupied one hectare, which increased to 30 hectare in 1991, 1300 hectares in 1993 and more than 3000 hectares in 1996. A native of warm seas, it surprisingly acclimatised that cold habitat. It is strongly competing with native endemic Mediterranean species, mainly due to its high content of toxic metabolites. It could cause the greatest transformation of the Mediterranean ecosystem in the last several thousand years.

• Signal crayfish from N. America introduced into Britain for fish-farming brought the fungal infection of "crayfish plague" which eliminated the native European crayfish to an alarming level after the escape of Signle crayfish from farming. It is itself immune to the disease.

• The weevil Rhinocyllus conicus was introduced into N. America to control invasive Eurasian thistle, but now it also attacking the narrow distributed native endemic thistles in national parks.

• Casuarina equisetifolia (originally from Australia), a favourite sand-binding tree in many parts of the world (including Pakistan), releases large amount of pollen in the air which can cause human respiratory problems.

The home scenario

In Pakistan, any comprehensive study on invasive species is non-existent. According to a rough estimate, about 700 plant species are introduced aliens, compared with about 4500 natives. Most of these introductions could be traced back to the colonial era. One such exotic is Mesquite (Prosopis juliflora) – native of Mexico, known as Devi in Sindh), introduced in 1878 by the British to afforest the deserts.

About 100 years later the species had invaded almost every type of habitat in Sindh. The worst impact has been on the riparian forest of Acacia nilctica. Like many invasive, Devi has toxic metabolites which keep away the herbivores as well as indigenous plant species, thus forms almost pure stands. It has caused severe damaged to native bio-diversity. Land bulldozing in the tree-plantation drives has tremendously promoted this species, because the scouring of its seed in soil enhances their germination. Water hyacinth (Eicchornia crassipes), a native of S. America is a notorious weed of water bodies in Sindh and Punjab. A cosmopolitan obnoxious aquatic weed, Salvinia molesta has recently appeared in Keencjhar Lake. Till now it has shown the full magnitude of its devastation. Another potential disaster is Eucalyptus (native of Australia). Although originally introduced about a hundred years back, the magnitude of introduction has increased several folds in recent years. The tree harbours flying fox whose population has increased in recent years and already causing serious damage to the orchards. The litter from this tree covers the soil and inhibits the growth of indigenous herbaceous species both physically and chemically. Due to the high content of volatile oils, both litter and tree itself are highly inflammable. It is especially risky for Sindh, where high summer temperature, low humidity, and strong winds can prompt fires through friction, which is common feature in its native home. Eucalyptus has already started to show the signs of naturalisation with spontaneous seedling here and there.

The information about introduced animals is still insufficient. The deliberate introductions include sheep and cow from Australia and New Zealand, fin and shell fish for fish farming and aquaria like silver carp, china grass carp, tilapia, rainbow trout, etc., from America, Africa, China, Japan, Malaysia, Philippines etc., and bees from Australia for apiculture. The accidental introductions include various insect pests of fruits and other crops. Any information on the invasiveness of these introductions is not available. The China grass carp introduced into Haleji Lake in 1970s for controlling weeds has reportedly deprived the native herbivorous fish of their food. Any data are not available about the organisms carried to our shores in ship bilge and ballast.

What the world is doing:

The problem of invasive species was recognised by the scientists as early as in 1950s, but the widespread awareness has emerged only recently. The Article 8(h) of the Convention on Biological Diversity (CBD – 1992) calls upon the parties "to prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats, or species". In July 1996 an international Conference on Alien Species was held at Trondheim, Norway, at which experts discussed the threats to biodiversity from alien species. World Conservation Congress (Montreal October 1996) also highlighted this problem. In 1997, the conference of the South Pacific Regional Environment Programme also gave attention to invasive species. In March 1998 IUCN Global Initiative on Invasive Planning Workshop was held in California.

What we are doing:

In spite of Pakistan being a party to the CBD, the Biodiversity Action Plan of Pakistan very superficially covers the problem of alien species. There is just one sentence in the entire BAP, i.e. Action 6.6, which reads "Take measures to control invasive alien species of fauna and flora, and to prevent further introduction". At the government and administrative levels there seems to be hardly any awareness. The tree-plantation drives mostly involve exotic species planted by bulldozing the natural vegetation. Despite the known negative impacts of Eucalyptus, its relentless plantation is still continuing. Prosopis juliflora is being planted on Baluchistan coast for sand-dune stabilisation instead of exploring the potential of indigenous species. Sometimes back seeds of Seabuck throne (Hippophae rahamnoides) were imported from China to check soil erosion, although the specie is naturally found in Pakistan also. According to some recent news items, 15700 palm oil seedlings from Malaysia have reached Karachi and a delegation of Pakistani officials is going to China to acquire Chinese fish species for fish farming in Balochistan.

In this era of the globalisation of free trade, it is almost impossible to put a ban on the movement of species, nor the planners, policy makers and politicians generally appreciate this problem. Government decision-making about the risks of exotic species may vary from country to country depending upon the value placed on the short-term benefits of "free-trade" versus the long-term consequences of biological damage. The scientists would have to work hard for effective dissemination of knowledge on the risks of alien species.

Since it may take decades to centuries for a species to become invasive, the alien species on a land can best be regarded as a time bomb with unknown time setting. The attitude of "guilty until proven innocent" needs to be adopted when it comes to exotic introductions.

(The authors are thankful to Prof. S.I. Ali former VC, University of Karachi, for providing some of the material used in the preparation of this article).