Invasive Alien Species: Ecological Disruption and Conservation Challenges

What are invasive species? An alien species is a species which is exotic and non-native to an area where it has been introduced, intentionally or unintentionally. As the name implies, it does not belong to the ecosystem in which it has been introduced. When we add term ‘invasive’ with it, it clarifies that now this species would exhibit negative impacts in the area of its introduction. There appears a sharp trend of discussions and publications on the issues of invasive alien species (IAS) over the last few years. Consequently, the term alien species or IAS have been described in different ways by several agencies/ experts.

An alien species refers to a species, subspecies or lower taxon, introduced outside its natural past or present distribution; including any part, gametes, seeds, eggs, or propagules of such species that might survive and subsequently reproduce. Kolar and Lodge point out that invasive species is a non-indigenous species that spreads from the point of introduction and becomes abundant. According to di Castri, a biological invader is a species which is usually transported inadvertently and spreads into new territories. Similarly, according to IUCN, invasive alien species are plants or animals that are introduced by man, accidentally or intentionally, outside of their natural geographic range into an area where they are not naturally present.

A similar sense of understanding has been proposed by the Birdlife International which defines IAS as species of animals and plants that are introduced accidently or deliberately into a natural environment where they are not normally found. The definition proposed by ISSG underlines the impacts of invasive species also along with defining the meaning of invasiveness. In other words, according to ISSG, invasive species is a species that has established and spread or has the potential to do so outside of its natural distribution range, and which then threatens ecosystems, habitats and/or other species, potentially causing economic and/or environmental damage, or harm to human health. In a nutshell, IAS are exotic species that may have detrimental impacts on the ecosystem in which they are introduced.

Which taxonomic groups are more invasive? It is very difficult to predict that which group of organism would show more, less or no invasiveness. In other words, every species may have the potential to become invasive if it is being removed from its native environment. This can be understood by the fact that IAS are found in all taxonomic groups, including, mammals, aquatic and terrestrial plants, fish, birds, insects, amphibians, molluscs, reptiles, mosses, ferns, algae, fungi and viruses.

Alien species do not always pose a threat as in most cases they find their survival difficult in new ecosystem/conditions and thus, their introduction becomes unsuccessfu. However, studies propose that about 1 out of every 1000 organisms introduced into a new environment has great potential to thrive and become invasive. Interestingly, in their native range IAS are kept under control by environmental factors and predators; whereas, in new environments lack of such controlling factors gives them sufficient opportunities to thrive and proliferate.

Characteristics of Invasive Alien Species and Causes of Invasion

As mentioned above, there is no specific methodology to predict the potential of any species to become invasive. Nevertheless, there are few characteristics which may increase our understanding about a species to determine it invasiveness. Common obvious characteristics of IAS may include broad native range, rapid reproduction and growth rate, strong dispersal and competitive capabilities, ability to adapt physiologically to new conditions, large reproductive output and ability to survive on a wide range of food types with broad tolerance under diverse environmental conditions.

According to McNeely et al., trade, travel, and transport (3Ts) are the major drivers of biological invasion. Initially, there were natural barriers in the form of oceans and mountains that prohibited the movement of species from their native range to elsewhere. However, human progress in the form of seaborne global trade opened the avenues of the movement of species across the world. Furthermore, globalization considerably increased its rate.

According to World Resources Institute and World Bank, globalization of the economy is demonstrated by the increase in the value of total imports from US$192 billion in 1965 to US$5.4 trillion in 1998, a 28-fold increase in just over 30 years. Consequently, globalisation has shifted the trend of trade where even the most developed country is now economically dependent on other countries for goods and services. Nevertheless, globalisation has become a source of a new challenge in the form of movement of species outside their native range.

Experts find sea/water route potentially responsible for the issue of IAS due to various pathways such as shipping, navigational canals, aquaculture and aquarium trade. Out of various factors responsible for the introduction of IAS, ballast water has been held frequently responsible as much global trade is seaborne.

Two Types of Introduction of IAS: Intentional and Unintentional

Intentional Introductions

The introduction of alien species in their new ranges finds a close linkage to ever-existing nature of human movements across the globe. Besides food, there are several definite purposes for which humans introduce exotic species such as aquaculture, forestry, fisheries, agriculture, game species, biological control agents, preventer of soil erosion, research subject etc.

There are cases when alien species do not have surety of survival unless they are taken care of. Therefore, humans put their best efforts in ensuring the greatest chances of their proliferation. Consequently, there are some species which do not behave as per the expectations of their care taker. For example, the purposeful introduction of African tilapia went wrong into Lake Nicaragua in the 1980s as it exhibited unfavourable consequences in the form of destruction of native fish populations along with other impacts. Other causes include the instances when alien species are deliberately released from captivity such as zoological gardens, or botanical gardens. Besides, smuggling of endangered or least concern species increases the risk of introduction of IAS in newer environments.

Unintentional Introductions

Under this category, we consider the cases where alien species are introduced to newer areas around the world as a by-product of human activities such as trade, travel and transport. In other words, two or more countries are often connected to each other in the form of financial affairs, disaster relief projects or military operations, which become the cause of unintentional introduction of IAS. Escape of exotic species from the captivity also fall under this type of introduction. Besides, climate change can also facilitate invasion by making habitats more challenging for native species and more hospitable to IAS.

Effects of Invasive Alien Species

The graveness of the disastrous impacts of IAS can be understood from the notion of the IUCN, which cites the impacts of IAS as immense, insidious, and usually irreversible. In the present scenario, the problem of IAS is being faced globally and almost every country is paying its expensive cost. Apparently it looks that the impacts of IAS can be measured in terms of economics but the issues of unemployment, ecological imbalance, shortage of food, biodiversity decline, disasters, epidemics, and loss of lives are closely related to IAS and therefore, can not be ignored.

Environmental Impacts

The disastrous impacts of IAS on the recipient environment are well-documented. For example, IAS have been found responsible for the extinctions of a number of native species along with upsetting ecological balance, altering prey-predator, competitions, nutrient dynamics, community structure and ecosystem services, spreading pathogens, causing irreversible loss to the native environment across the globe. According to Villamagna and Murphy, the thick mats of water hyacinth lead to a decrease of phytoplankton, an increase in water turbidity and a decrease of dissolved oxygen.

There are examples where introduction of predators such as rats, mongooses, snakes or fishes has led endemic species of birds, reptiles, snails and aquatic insects towards extinction or local extinction. The Convention on Biological Diversity emphasized biological invasion as one of the major driver of biodiversity decline and the second biggest threat after habitat destruction and ecosystem degradation. The Millennium Ecosystem Assessment has also recognized biological invasions as one of the five main causes of declines of biodiversity. According to BirdLife International, in the last 500 years, species like rats, cats and mice have driven more than 70 bird species to extinction.

Economic Impacts

The economic impacts of the IAS are very costly. For example, according to Birdlife International, IAS cause around 12.5 billion Euros worth of damage each year in Europe alone. One study for the USA estimates costs of $137 billion per year due to invasive species. The costs of IASs to agriculture in the USA have been estimated at over US$120 billion per year, and for invasive insects globally at over US$70 billion per year. According to Bright, the losses to agriculture have been estimated at between US $55 billion and nearly US $248 billion annually globally.

Also, a nation faces economic losses, when its tourism is affected by IAS. There are reports where IAS may cause the degradation of different habitats which are important for recreational activities in the form of snorkelling, Scuba diving and recreational fishing. It is surprising to note that the impact and management costs of a single IAS can cost even in millions. For example, the Golden apple snail (Pomacea canaliculata), introduced from Latin America as a high protein food source, caused losses to Philippine rice crops during the 1980s of approximately US $1 billion.

To understand the patterns and processes involved in the issue of IAS, governments has to invest substantial money is research activities that further adds economic burden to the national funds. For example, the European Union has funded forty nine major projects to address different aspects of biological invasions since 2000.

Diseases/ Health Issues

Some alien species have been reported to transmit diseases to native populations. Vibrio cholerae and some other microorganisms that can cause harmful algal blooms, are relocated and released in the ballast water carried by large ships. Gaffkaemia, caused by Aerococcus viridans has been introduced in European waters by American lobsters (Homarus americanus) and has infected European lobsters (H. gammarus).

There are instances when IAS become dangerous to humans too. For example, the African honey bee Apis mellifera has invaded parts of southern USA, where due to its aggressive and fatal nature it has killed a number of people and animals so far and is popularly known as killer bee. According to Firehun et al., malaria is one of the major fatal diseases resulted from the impeding effect of water hyacinth. This weed has been linked to an increase in the instances of diseases such as cholera and typhoid. Besides, chemicals such as pesticides used to kill IAS later find their path in the food chain and pose their harmful impacts on humans, animals and environment.

Control and Management of Invasive Alien Species

It is true that all alien species do not turn into invasive species. Nevertheless, if it happens, it exhibits multi-scale harmful impacts on the environment, economics, human health, tourism, culture and other important aspects. Once an IAS is established, it becomes very difficult or economically high-priced to eradicate it. Goals for addressing the problem of IAS include prevention, early detection, eradication and control of the IAS. Following are few methods that have been suggested for the control and management of IAS along with other strategies:

Mechanical control includes the physical and direct removal of IAS by hand or other machinery. However, it requires substantial investment of time and human resource and poses the risk due to the presence of animals such as snakes and crocodiles, if the target IAS is an aquatic weed. Mechanical control has been identified as an effective way for the removal of many IAS including Water hyacinth (Eichhornia crassipes). For example, the use of machinery to remove water hyacinth from water bodies has been found an effective and non-polluting control method. Its main advantage includes the simultaneous removal of nutrients and pollutants from the water body.

Chemical control involves the use of different chemicals such as herbicides or insecticides. For example, glyphosate and 2,4-D [(2,4- dichlorophenoxy) acetic acid] have been considered as efficient and secure herbicides to control the proliferation of water hyacinth. However, chemical method is a costly affair and demands repeat applications along with the potential development of resistant strain of the IAS over time and the threat of biological magnification.

Biological control involves the intentional introduction of natural enemies of the target IAS in the form of predator or parasite. It often brings the population of IAS to a tolerable level and is viewed as environmentally friendly and cost effective method. For example, in the case of water hyacinth, the method of biological control using a natural predator (the weevil species Neochetina eichhorniae, N. bruchi or the moth species Sameodes abligullatis) or pathogen (the fungus Alternaria eichhornia) has been found effective. There are studies indicating the beneficial use of certain alien species too to control other IAS. However, the complete eradication of IAS by this method is quite time-taking.

Conclusion

The Convention on Biological Biodiversity (CBD) has recognised the need for the compilation and dissemination of information on different aspects of IAS and calls for further research on their impacts on biological diversity. Since substantial information about different aspects of IAS is now available, its promotion can be helpful in the management of IAS. For example, data from countries or locations where an alien species has turned into IAS can provide useful information on its invasion rate, impacts and management approaches. The Invasive Species Specialist Group (ISSG) organized by the IUCN’s Species Survival Commission is one of the wonderful examples in this connection that aims to reduce threats to natural systems and native species by increasing the awareness of IAS and suggests methods for their control and management. 

References

• Simberloff, D., & Rejmánek, M. (2011). Encyclopedia of Biological Invasions. University of California Press.
• Pimentel, D. (2002). Biological invasions: economic and environmental costs of alien plant, animal, and microbe species. CRC press.
• Richardson, D. M., Pyšek, P., & Carlton, J. T. (eds.). (2011). Fifty years of invasion ecology: the legacy of Charles Elton. John Wiley & Sons.
• Mooney, H. A., & Hobbs, R. J. (eds.). (2000). Invasive species in a changing world. Island Press.
• Vilà, M., Hulme, P. E., & editors. (2017). Impact of biological invasions on ecosystem services. Springer.
• Mack, R. N., Simberloff, D., Lonsdale, W. M., Evans, H., Clout, M., & Bazzaz, F. A. (2000). Biotic invasions: causes, epidemiology, global consequences, and control. Ecological applications, 10(3), 689-710.
• Pyšek, P., & Richardson, D. M. (eds.). (2009). Fifty years of invasion ecology: the legacy of Charles Elton. Wiley-Blackwell.