The Exotic Species of Singapore

Wednesday, November 17, 2004

Northern snakehead in Maryland

Habitatnews article

The Northern snakehead Channa argus, an exotic species that wreaked havoc in some U.S. states, reappears in Maryland.

What are the impacts of exotic species?

Native species of animals are much more vulnerable to alien predators because they have been thriving without the introduced species prior to the latter’s entry into the habitat. Coupled by a slow reproductive rate that was sufficient to maintain the species, alien predators can wipe out native species extremely effectively. Entire species of plants and animals that are endemic to the area, which is defined as species that are found only in the particular geographical location, can perish very quickly as a result. Endemism is a crucial factor in extinction as global extinction is said to have occurred if an endemic species on an isolated habitat is exterminated (Primack, 1998). For example, the introductions by the Polynesians in the 5th century into Hawaii, together with the clearing of forests for agriculture, caused the extinction of an estimated 50 of the 98 endemic bird species in the archipelago (Olson, 1989; Pimm et al., 1995). Following that, the animals introduced by the Europeans brought the extinction toll up to about 70% of Hawaii’s endemic bird species, with the remaining not far from extinction.

One of the most dramatic events in species decimation occurred in the late 19th century at Stephen Island, an atoll off the coast of New Zealand. A single cat brought into the island by the lone lighthouse keeper killed the entire species of a native rare flightless bird, now known as the Stephen Island Wren (Diamond, 1984). This could well be the only documented elimination of an entire species by a single animal predator, but this surely highlights the grave danger faced by native animals in any habitat in the face of alien animal introductions.

Herbivore Grazing
Native plants are vulnerable to the introduction of herbivores. Of the many exotic herbivores documented to cause extinctions or near-extinctions of plant species, goats have been found to be the most voracious alien grazers. One of their many undesirable effects have been the grazing and reduction to near-extinction of an endemic shrub, Hebe breviracemosa, in the Kermadec Islands off New Zealand (Atkinson, 1989).

Exotic species compete with native plants and animals primarily for food and space. Competition occurs when there are overlaps in the requirements and preferences for food and space, along with other factors such as water source and also light, especially in the case of plants and immobile organisms. Resources have to be split among the two species to reduce the overlap and the native species is eventually forced to compromise its dietary and spatial requirements (Schoener, 1974). If this is partition is not achieved, the competition may result in decreased reproductive capability and reduced growth in either the exotic or the native species, or even both. The social wasp of European origin, Vespula germanica, introduced into southern Australia in the late 1970s (Spradbery and Maywald, 1992), was found to have an adverse effect on the previously predominant native wasp, Polistes humilis, due to a food overlap. This overlap is only slight but because the native species has a more specific diet preference, this gives the exotic wasp the advantage as it can utilize both the exclusive resource as well as the shared resource, leading to its invasive success (Chase, 1996; Kasper et al., 2004). This example highlights the detrimental ecological effects of invasive competition due to the highly delicate interspecies relationships in the ecosystem.

When closely related species are present in the native habitat, the introduced species can hybridize with native species to give rise to hybrids that will result in the mixing of genetic information between the 2 related but distinct species. The genetic information of the native population, also known as the gene pool, becomes diluted by the alien gene pool and this leads to the elimination of the unique features present in the local population. The populations of the Apache trout (Oncorhynchus apache) and the Gila trout (O. gilae), both of which are residents of the rivers of Arizona in southwestern U.S., have been significantly reduced by the introduction of the rainbow trout (O. mykiss). Hybridization with the alien sport fish has been found to be an important cause of this population and range reduction (Dowling and Childs, 1992; Loudenslager et al., 1986).

Diseases and Pests
Pathogens introduced by alien plants and animals can have devastating effects on the ecological environment as well as on human health. In the example of the native birds of the Hawaiian archipelago, it has been found that avian malaria also had a part to play in the reduction and extinctions of native bird species. The disease was believed to have been brought into the region by alien birds introduced from Asia (van Riper et al., 1986). Disease-bearing insects have also found entry into Singapore by way of international aircrafts landing at the Singapore Changi Airport (Goh et al., 1985). If not monitored closely, the exotic pathogens and insects may cause outbreaks of infectious diseases across the country. Physical injuries may also result from the bites of exotic killer bees, fire ants, sandflies and tropical biting midges. It has been predicted that the increase in the incidence of diseases and injuries caused by these exotic pests can lead to the diminishing appeal of the outdoors and the subsequent development of biophobia that might undermine current research efforts in biological conservation (Soulé, 1990). Eventually, the greatest lost to the nature-loving community will be the strong supporters of biological conservation. What lies beyond is seriously unthinkable.

How are exotic species introduced?

These are species that were transported into their nonnative habitats by accident. They get a free passage unnoticed by humans because of their small size and inconspicuous nature. The most common animals that have been circulated throughout the Earth are also the ones that cause the greatest losses in capital and extinction of species – the rats (Hunter, 2002). Disease organisms can also take a free ride on the bodies of travellers, typified by the European explorers centuries ago and even the SARS carriers that arrived on the shores of Singapore recently (Fleischauer, 2003). With the development of a seamless worldwide transportation system, this form of alien introduction could become ever more common and a much greater challenge to control.

Commerce and Subsistence
This is the deliberate introduction of species into an area for food and farming. The Polynesians who first inhabited the Hawaiian archipelago were one of the earliest to practise this form of introduction. Following that, the European colonists in the 1778 arrived with more animal species, including rats, goats, cattle, the domestic cat, the Indian mongoose as well the barn owl (Primack, 1998). This form of transportation also occurs in the other direction. As colonists send their ships back for more supplies, organisms from the colonized lands hop on and are also transported to other parts of the world.

Although game hunting is not allowed in Singapore, the introduction of exotic mammals, game birds, game fish and smaller fish as food for the game fish has been an important source of alien species for many western countries. More than 50 mammalian species has been introduced into the Texas ranches to allow game hunters to hunt for nonnative mammals without having to travel beyond Texas (Bolen and Robinson, 1999). Fishing, despite being a popular pastime in Singapore, has not led to the large-scale introduction of exotic game fish by anglers as is occurring in the U.S., since it is against the law to release animals into the wild. Under the National Parks Act (Cap. 198A), any person found bringing any animal and releasing it into a nature reserve can be fined up to S$10,000.

Pet and Wildlife Trade
Pet keeping is an activity that is enjoying an increase in popularity. In Singapore, there are rules and regulations to ensure responsible pet ownership and to prevent the endangering of wild animals. Under the Wild Animals and Birds Act (Cap. 351), wild animals and birds are not allowed to be killed, caught or kept. Importation of wild animals and birds are also illegal in Singapore. The animals that can be kept as pets, as listed by the Agri-Food and Veterinary Authority (AVA) in its website, are dogs, cats, rabbits, guinea pigs, hamsters, gerbils, mice, chinchillas, red-eared sliders, and many types of birds and fish. However, there still exist many cases of wild animals being kept illegally. One of the recent ones was a student who kept 22 wild and endangered species in his home. The collection included of pythons, other assorted reptiles, and even mammals known as sugar gliders. The offender was fined S$5,600 (Straits Times, 13th April 2004).

These exotic species, if released into the wild, can cause devastating effects. An example is seen in several U.S. states, where numerous species of the voracious snakehead fish have, over the last few years, been terrorizing the inland waters. These exotic pets were easily available and their ability to wriggle short distances on land means that they could invade many aquatic habitats quickly (Bergguist, 2003; Miller, 2002; Watson, 2002). In Singapore, the highly popular luohan (flowerhorn cichlid) trade and hobby could lead to similar effects if these aquatic pets are released into local waters, either by accident or on purpose, as some environmentalists fear (Ng, 2002). Presently, the compassionate release of animals by religious groups has been a source of alien introductions in Singapore. Every year, many Buddhists and Taoists mark Vesak Day by releasing turtles, fish and birds at temples, inland waters, parks and beaches as a symbolic act of repentance, though illegally (Dawn Tan, 1995; Joann Tan, 2004; Wee, 2001). Under the National Parks Act (Cap. 198A), any person found bringing any animal and releasing it into a nature reserve can be fined up to S$10,000.

Research and Biological Control
Scientists may keep living specimens in their laboratories for biological research and these species may escape into the wild, and sometimes even into areas which are not in their native ranges (Carlton, 1989). At this age when gene alterations techniques are used intensively in research laboratories, such accidental introductions may pose an even greater danger to the ecosystem.

Exotic species may also be deliberately introduced by scientists to control other exotic species that are already present due to other factors. Although this is considered a salvage method, there are cases where this has led to success in the control of exotic species. This method of biological control involves the searching of a predator of the exotic species in its native range and introducing this exotic predator into the area where the alien species has been causing harm to the ecosystem (DeBach and Rosen, 1990). However, backlash may also result if such introductions are inadequately planned and managed. Euglandina rosea, predators of the introduced giant African snail in Moorea, an island in the southern Pacific, were brought into the island to control the African snail, but unfortunately, the African snails were not affected. Instead, 7 native species of viviparous tree snails were decimated by the introduced predators as they found easier prey in the former (Cowie, 1992; Murray et al., 1988).

Environmental Factors
In some instances, species may expand their ranges or move into an area in which they would become exotics because of changes in the environment (Hunter, 2002). Such changes can possibly be a result of human activities. The clearing of the forests in the eastern part of the United States for agriculture may have led to many plant and animal species moving in from the western part of the country as they are now allowed to expand their ranges eastwards (Brothers, 1992). The global climate is also a crucial factor in determining the geographical range of species. Changes in the global temperatures in recent years, especially of the warming trends, have caused expansion in the range of many species (Crozier, 2003; Lingren et al., 2000; Thomas et al., 2001). This can eventually lead to exotic species appearing at the range margins as global temperatures continue to rise. The amount of global warming that is attributable to human activities is, however, still an ongoing debate.

What is an exotic species?

An exotic species, otherwise known as alien species, is an organism that is not indigenous to an area. It could have been transported there from its natural habitat by humans, human activities or other agents (Oxford, 2000). This act of transportation alters the ecological balance that has been established due to the environmental and climatic barriers limiting the geographical range of the particular species. Examples of environmental barriers are mainly climatic in nature, and in specific terms, oceans, rivers and deserts are some of the various boundaries that restrict the movement of species (Primack, 2000).

These barriers have long been transcended since the pre-industrial era, when humans migrated with cultivated plants and domesticated animals to establish new farming locations and settlements. For example, land mammals of North America, in nature, are incapable of crossing the Pacific Ocean to get to Hawaii. However, the Polynesians who first reached and inhabited the archipelago introduced a rat species, the domestic dog, the domestic pig, as well as cultivated plants to the area (Primack, 1998).

Exotic species do not always establish a self-sustaining wild population in the area which they are introduced. This is due to an unsuitable environment in which they cannot naturally thrive, or simply an adverse environmental or climatic condition which renders their survival impossible. However, some of these nonindigenous species do establish self-sustaining wild populations and are hence naturalized. Previously domesticated animals such as the domestic dog that has naturalized are known as feral animals (Oxford, 2000).

Many exotic species can go on to cause harm to the ecosystem, with populations expanding dramatically at the expense of indigenous, or native, species. These undesirable alien organisms are aptly termed as invasive species.