Consider this familiar scene. A shopping centre with all the characters we know: shoppers, shop assistants, maintenance men and security guards. Everyone knows his role well and does what he needs to do: buy from this shop, sell to shoppers and keep the place running well and safe.
We've developed this system over many years, shed roles and actions we don't need, and kept those we do, so we can have a functioning shopping centre.
Then one day, a security guard in a pink uniform turns up. He scares off our security guard who's not used to his ways. He still keeps the place safe so nobody notices anything for a while. More pink-clad guards start to appear. Suddenly one of them snatches food from a shopper. He tries to sell shoes. Scared shoppers run outside for safety. Upset maintenance men stop fixing things. Confused sales assistants stop selling.
Some shoppers can handle this strangeness and continue with their day. But the pink guards have changed the characters and roles, and soon the shopping centre becomes unrecognisable. It no longer provides the services it was meant to provide, and may eventually crumble in a state of disrepair.
Some shoppers flee to other shopping centres, but each centre can hold only so many shoppers and some are left outside with nowhere to go.
In this scene, the shopping centre is our ecosystem and the characters our native species. Singapore's native flora and fauna have evolved alongside one another for many years, with each character playing its own important role in the ecosystem upon which others rely.
The shoppers which flee are our more sensitive forest species. The pink security guards are the invasive exotic species. Often more aggressive than native species, they can push native species out of the ecosystem.
Unfortunately these natives are usually species that are more shy, sensitive to disturbance, rare and threatened. With increasing invasive exotic species, the original ecosystem no longer provides the services it was developed to provide, and may become so disrupted it collapses.
A species is native if it occurred in a location naturally, while a species is exotic if it's living outside its natural range.
Exotic species appear as a result of deliberate or accidental human intervention, and are also called non-native, introduced, alien, feral or pest species.
The presence of exotic species is not necessarily bad. Some appear to have little effect on native ecosystems and biodiversity, filling an empty niche in the urban environment and getting along nicely with their new neighbours.
The banded bullfrog is one such species, now a permanent resident in Singapore. Listen carefully at night and you can hear its low moans coming from drains all over the island.
When species invade
BUT exotic species become invasive when their presence causes environmental or economic damage, and we do not know when an apparently harmless exotic species becomes invasive.
This is the case of the Javan mynah. We do not yet know if this exotic species is invasive in Singapore, although its rapid expansion (and the decline of the common mynah) suggests it might be. In our analogy, the Javan mynah could be a pink guard as it has displaced the native common mynah (as recently suggested by the Nature Society of Singapore bird census).
It may be ironic to consider that some native species which we fight to protect are considered invasive exotic species in other countries. Our pretty common mynah is the No. 1 invasive exotic species (that is, the naughtiest pink guard) in Australia, beating the cockroach and rat, while the Javan mynah is becoming rarer in its native habitat.
Invasive exotic species can have devastating effects on native flora and fauna, sometimes leading to species extinction and permanent ecosystem loss. This can happen through competition for food and shelter, predation, introduction of exotic diseases, seed dispersal of exotic plants, modifying the habitat so other species cannot survive as well as affecting ecological processes such as pollination, connectivity and succession, to name a few.
For example, introduced rats are notorious for devastating island ecosystems by preying on native reptiles, birds and mammals, leaving a once diverse island full of "pink guard" rats. Their ability to adapt and the lack of natural predators contribute to exotic species' success in foreign lands.
But what about people? Across the world, invasive exotic species have also affected people. The effects include disease transmission (caused by rats), crop destruction resulting in annual losses of millions of dollars (insects, birds), contaminated food stores (mice), property damage (pigs), native fish stock reduction (fish), and loss of ecosystem services such as clean air and water. So in the long run, controlling invasive species benefits humans as well as wildlife.
What does it mean to our native ecosystems that the Javan mynah has displaced the common mynah? The answer may simply be fewer common mynahs. Or perhaps there are more serious effects which may be direct or indirect, on a scale that is specific to one species or affects the entire ecosystem, and with effects that may be apparent only in the long term - effects which we have not yet understood, but which we should perhaps devote more research to.
Protecting pristine forests
WHAT impact do exotic species have on humans in Singapore? Not much directly, other than a diminished experience of the beauty of nature's diversity.
Like economies, ecosystems are dynamic and ever-changing, albeit at a much slower pace. Species experience boom and bust patterns in their population sizes as the environment and species composition change.
An exotic species may be the most abundant species this year, but may be displaced by another (exotic or native) 50 years later. What matters more is maintaining the diversity of native plants and animals, because diversity in nature, like in our shopping centres, contributes to the overall health of our environment.
By nature of the prevailing human disturbance, cities are almost always going to support a certain number of exotic species. And perhaps these urbanised areas are so modified by human interference already that having more exotic species does not really matter. What's important is to ensure exotic species do not enter our more pristine forests, where our rarer species live. With fewer or no pink guards, the other characters can continue with their roles and interactions, and contribute to a well-functioning, successful native ecosystem.
There is already some resistance from our more pristine forests such that exotic plant species have not been able to take hold there.
But for how much longer?
With forested areas being developed, disturbed and exposed areas become attractive to exotic pink guards, pushing our sensitive shoppers deeper into the forest, which is already low on space.
A robust and healthy ecosystem with high native biodiversity can naturally exclude exotic species, but maintaining that robustness and health is the tricky bit, especially when we live in a state of ceaseless development.
What we do depends on the importance we, as a society, place on our native ecosystems and biodiversity. And it also depends on the type of country we want to live in. A country with some development, native biodiversity and healthy forests, or a country with pervasive development, widespread exotic species and degraded areas?
If we want healthy ecosystems, we can do so by refusing further development of our more pristine forests, and managing exotic species when they become invasive.
The writer is principal ecologist at Ecology Matters, an environmental consultancy providing ecological advice and biodiversity studies for environmental impact assessments.