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Aliens among us

How introduced species affect native flora and fauna

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I have just returned from a visit to Chiang Mai, where I ventured into the forest looking for snakes, gibbons and other interesting wildlife. While there, a guide pointed out green tea plants, originally from India, planted by the local tribes. When grown, the leaves will be sold to make tea. 

Critics of alien strains are concerned that the lucrative cassava industry could soon look like this cassava plantation in Nakhon Ratchasima if the alien African wasps, introduced into Thailand last month, get out of control or fail to cull the mealy bug population.

I was also alerted to coffee bean plants, originally from Brazil, which the tribes also grow and sell. I obviously understand their need to earn money to live, as the world in which they were once very adept has changed dramatically and they, like any successful species, have adapted in order to survive.

But I got to wondering what effects the introduced, or alien, species are having or have had on the native species in the area. Certainly the affects are visibly localised as native plants have been cleared off the slopes in order to grow the invasive alien species. But what are the long-term effects on, and what other introduced species are causing problems for, Thailand's native flora?

Alien species

An animal or plant species in its native habitat is not an alien, and it has evolved alongside all other lifeforms in its particular habitat. That means that its number is naturally controlled and, depending on what it is, it eats or is eaten by other things around it.

When you take a species out of its habitat and put it into a foreign habitat, the rest of the life around it has not evolved with it and often cannot cope with it. If it is a plant, it can out-compete native species by having a more efficient root system or it may grow bigger or faster than the natives and nullify their place in the ecosystem.

Animals, like plants, can possibly out-compete native species, eating plants more quickly than the natives and breeding faster or, if they are predatory animals, they can hunt native species often to extinction, or they may be poisonous to native predators, in effect generally ruining the local ecosystems due to the natives not evolving adequately to cope with these new creatures.

As a previous resident of Australia, I am very familiar with the damage an alien species can do to native plants and animals. The water hyacinth has damaged many aquatic ecosystems and continues to do so. The dingo has driven the Tasmanian tiger and Tasmanian devil to extinction on the Australian mainland, and the infamous cane toad has poisoned many native predators to local extinction.

Thailand's aliens

There are over 1,500 alien species of plants, animals and microorganisms in Thailand!

Flora, ranging from exotic and aquatic plants, trees, climbers and grasses were introduced for various reasons, such as for medicinal purposes, for food or for their decorative properties.

Water hyacinth plants are the floating green masses that cover much of the Chao Phraya River and clog up canals. It was introduced from Indonesia generations ago for its ornamental beauty, but now its out of control.

Of the world's top 100 worst invasive alien species, the water hyacinth is one of 38 present in Thailand. Many of the other 37 are plants that out-compete natives for space and nutrients; others are obnoxious to herbivorous animals.

Thailand's worst aliens include insects, snails, fish (especially the ever-persistent carp), the ferocious Indian mynah bird, and many mammals we may not have considered as aliens, including cats, goats, mice, rats and pigs, feral populations of which wreak havoc on native Thai ecosystems.

Biological control

Last month, in the Bangkok Post there were several articles - such as "African wasps let loose to save cassava" Main Section, page 1, July 18 and "[DOA director-general] Somchai [Charnnarongkul] launches a stinging attack", Main Section, page 4, July 21 - on the introduction of the African wasp species (Anagyrus lopezi) that is now being used to control our local pink mealy bug (Phenacoccus manihoti), population. The mealy bug sucks the sap of cassava until it is dry and shrivels up.

The series claims that Thai mealy bugs are out of control, and that they cause great damage to local cassava plants, resulting in substantial economic losses.

Following research by Thai entomolo-gists (insect specialists), scientists released 10,000 wasps in Khon Kaen province to prey on mealy bugs. According to one article, the scientists stated that, "Based on our trial, we have found that the wasps do not harm native insect species, such as the lacewing bug, the ladybird beetle, and 10 other species. The wasps eat pink mealy bugs only".

How they could confirm this is a mystery to this animal specialist. And how can they control such a study, as the wasps must surely be able to fly out of the control area?

As well as preying upon mealy bugs, the female wasps are parasitic and lay their eggs in the mealy bugs, meaning that when their eggs hatch, the young wasp larvae have a meal immediately available to them.

What bothers me about this plan, however, is that there are very few animals with such a specific, non-variable diet. Even the panda bear will eat honey, eggs, fish, yams, shrub leaves, oranges, and bananas when available.

And this is the point. When the wasp numbers climb above the mealy bug numbers, if they genuinely do eat only mealy bugs, then the number of wasps will decline and an undulating balance will ensue.

Indeed, nature teaches that for any species to be successful (and African wasps obviously are), they must be adaptable. Therefore, if mealy bugs suddenly become unavailable, the wasps will adapt to eating other invertebrate species native to Thailand, including species that may not have been discovered by science.

In any regard, the plan is to release a million of these wasps over the next few months, pending the results of the more than 200,000 wasps released to date. Wasps are also expected to be similarly used in neighbouring countries to control other mealy bug infestations.

I'm sorry, but I simply foresee problems. The wasps may spiral out of control, or the wasps may be preyed upon by native animals, possibly leading other native species to exponentially breed out of control. I have a real problem with biological control, and I don't believe it can be tested to the point of being foolproof with guaranteed success. I would hate to look back in 15 years and see the repeat of another cane toad-type disaster.

Balancing act

To be fair, there is no doubt that some of the introduced species in Thailand do not have a major adverse effect on native species. And you could argue that many alien species are beneficial, from a cultivable or profitable point of view. And perhaps some of the biological limitations aren't spiralling out of control just yet.

But in reality, introduced species are considered one of the major threats to global biodiversity. The problem is only exacerbated by negligent humans but, ironically, humans are the only "alien" species that can temper and hopefully rectify the problem.

Dave Canavan has an MSc in Behavioural Ecology and is the principal of Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You may contact Dave at

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