Biological Invasions

Who would expect to encounter a lion, a polar bear, an Andean condor or a Sequoia tree while taking a walk in Hong Kong (except maybe within a zoological or botanical garden)? No one, right? And the reason for this, is that species are not found everywhere or randomly across the surface of Earth. Instead, most species are restricted to a particular part of the world, sometimes to just a few square kilometres, where they have evolved and co-existed with other species for thousands to millions of years.


In the past few millennia and even more so in the most recent centuries, however, things have changes… With humans colonizing and exploring lands and oceans all over the world, species that once were restricted to a particular region, have been transported and introduced into new regions. Dogs, cats, rats or cockroaches for instance that accompany humans in their journey, voluntarily or not, have thus reached new areas that once were inaccessible to them.



Nest Casting of Fire Ants


This phenomenon known as “biological invasions” now concerns thousands of species that have been introduced voluntarily or accidentally all over the world. In Hong Kong, you are most certainly familiar with several of these species, like the Red Imported Fire Ant (Solenopsis invicta), the Apple snail (Pomacea canaliculata), the Yellow Crested Cockatoo (Cacatua sulphurea), the coqui frog (Eleutherodactylus planirostris), the mosquitofish (Gambusia affinis), the red-eared terrapin (Trachemys scripta elegans), or the Brazilian Water Hyacinth (Eichhornia paniculata), to name just a few of these introduced species. Because of its geographic position, history and main trading activities, Hong Kong has over the years become a hotspot for invasions globally, with hundreds of introduced species now established.


Why is it important?


While one may enjoy this extra-biodiversity brought to a territory, the reality in nature is often more complicated, and sometimes gloomy. Indeed, invasive species are considered as the second causes of species extinctions globally, after…. you have it… habitat change (e.g. deforestation)! Yes, you read well, the introduction of new species into new areas is the second cause of global extinctions of other species. While a plethora of mechanisms are responsible for this, which would take too long to expose here, suffice is to say that native species (those originally present in a region) are often not adapted to these newly introduced species that can invade their habitats and push them away.


In addition, if invasive species can disrupt native species directly (e.g. through predation), they can also completely modify habitats and sometimes particular disturbance regimes, such as fire. Some introduced plants for instance, will be storing fuel chemicals to increase fire frequency and intensity; which they require for their own germination! The advantage for them is that once fire has removed all or most of other native species, the space is free for them to germinate and spread across the landscape. Ultimately, and fire after fire, the landscape will transition to a monoculture of these fire-adapted species, while native species will be restricted to edges too wet or protected for the fire to spread. And as plant composition changes, so does the diversity of insects, birds and other species dependant on local species.


Fortunately, only a small subset of introduced species is becoming invasive to the point to modify habitats and the species living therein. The problem though for scientists and managers is that we do not know exactly which ones do and those who don’t for two main reasons.


First, studies on biological invasions remain very limited, especially in Asia, and thus the potential impacts even for some widespread species remain unknown.


Second, an invasive species can be “dormant”. Let me explain. In some cases, now well documented, species were introduced and established for several decades or even over a century into a new region to suddenly “wake up” and become harmful to other species and the environment. Phenomenon of micro-evolution or interaction with a changing environment may sometimes trigger this sudden change. The problem ultimately is that it makes predictions very difficult to control these species, as the removal of introduced species is not possible, nor desired (as the majority are harmless).


You may wonder thus what you can do?


First, the best thing is to prevent any behaviour that may increase the likelihood of invasive species to occur. As a result, do not purchase exotic species, plants or animals, for raising or gardening them. If you do (or are doing it), be absolutely certain of not releasing them in the wild, and prevent any potential escape.


Second, when you see these species in nature, do not feed them… or you risk to increase their population size and thus the damage that they could cause. Just like nobody would go and provide food to a colony of fire ants, do not feed other exotic species (even if they look pretty).


Third, you can be on the lookout for the detection of exotic species. Indeed, the most efficient way to prevent ecological, but also huge economic or human health impacts is to detect these species early and then implement strict control measures. That may sound familiar to you, if you think about an epidemic. The first period is the most critical to detect and suppress it, while when it has run wild (e.g. a pandemic) you fall into casualties and mitigation with huge costs involved.


In a few days, we will introduce a project in which YOU can all help to collect information about the spread of exotic and native ant species within Hong Kong. We hope that many of you will contribute to this new and fun project. Stay tuned!