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Aliens attack: Predicting the spread of marine invasive species


Seebens, H., Schwartz, N., Schupp, P. J., & Blasius, B. (2016). Predicting the spread of marine species introduced by global shipping. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1524427113


Marine invasive species are plants and animals that originate from one location and become introduced to another area. You probably have heard of some invasive animals like lionfish or green crabs, but there are many others, spanning a wide range of size and shape (5 invasive species you need to know). Invasive species can be introduced naturally (i.e. transportation by wind or ocean currents) or through human interactions such as releasing aquarium pets into the wild.

Most invasive species are transported across the ocean by ships, often hitching a ride on ship hulls or stowed away inside ship ballast. Large ships have tanks that are filled with seawater to counterbalance cargo weight. Ballast water is water taken on to a ship in one port to help increase weight (when cargo is offloaded) and then is released in another port to lighten the load when more cargo is brought on. Marine life can be carried in this water from port to port, getting released in new, foreign territories (Fig. 1A). It is estimated that some 45,000 cargo ships move more than 10 billion tons of ballast water around the globe each year. Thus, marine species have plenty of opportunities to “hitch-hike” to foreign territories that were once inaccessible to them.

Figure 1: (A) Ballast water being drained from a ship and (B) Caulerpa taxifolia, a marine invasive alga, frequently transported by ballast.

Problems can arise when invasive species outcompete natives for the same resources (i.e. food, sunlight, nutrients or shelter). Furthermore, invasive species rarely have predators like the natives do, meaning they can often grow unchecked. You may think this is just “survival of the fittest”, but in reality, this type of ecosystem shift can have cascading effects. An example is the invasive marine algae, Caulerpa taxifolia, also known as the “killer algae”. This plant can completely cover portions of the seafloor, blocking out light for other algae and corals (Fig. 1B), which can devastate fish populations who rely on corals and the fisheries who exploit commercial fish. As a result of their rapid spread, it is next to impossible to remove an invasive species like C. taxifolia from its new habitat. What if we had the power to predict where invasive species were most likely to land next? This predictive power may finally offer a solution to global species invasion.

The model:

In order to predict future marine invasions, Seebens and colleagues set out to create a simple, yet efficient statistical model. They used data on shipping traffic and environmental factors (i.e. temperature and salinity) to predict where species were most likely to invade, given their starting habitat range and environmental requirements. The first order of business was to test if their model actually worked; to test the model they used 40 random marine species (ranging from fish to algae) already known to be invasive. Next, they focused solely on marine algae. The tested model was applied to 97 previously-known invasive marine algal species, to try and predict their future spread.


Does the model work?

Yes, this model worked very well and was able to accurately predict marine invasions that had already occurred. The presence/absence of an alien species was correctly predicted for 77% of marine invasions outside the species native habitat. With proper validation, it was time to put this predictive model to the test using marine algae.

Invasive algae predictions

The majority of invasive algae tested in this study were native to temperate-subtropical marine regions in Europe and East Asia (shown in red in Fig. 2A). These same regions had the highest probability of future invasion by marine algae (Fig. 2B). It makes sense that algae would be highly transported between the East Asian and Northern European Seas because they share similar environmental conditions and are well connected by ship traffic. More ship traffic would lead to more species “hitch-hiking” in ship ballast.

Figure 2: (A) Native habitat ranges of 97 marine algal species with a known invasion history and (B) probabilities of further spread of species into previously unoccupied regions. Color scales on the side represent number of observed species (A) and predicted probability of invasion (B).

Incorporating future ocean warming (water temperatures predicted for 2040-2060) revealed a north-south gradient in the change of algae invasion probabilities. Many algal species used in this study are originally from colder, temperate regions. If water temperatures increase in the future, these same species will invade further north to find more appropriate habitats. For example, the model predicted a decrease in invasion probabilities in the tropics (near the equator) and an increased invasion in temperate regions, particularly in North America (Fig. 3).

Figure 3: Predicted changes in invasion probabilities of 97 marine algal species due to climate warming.


It is very hard to control the spread of invasive species in the ocean as a result of many environmental factors and extensive global ship traffic. Invasive marine algae are of greater concern because they are easily transported in ballast water (over 400 invasion cases reported) and some can form toxic blooms in areas that were previously unaffected (more on harmful algae). This model offers a huge first step in reducing species invasion, by determining hot spots of future invasion and the species most likely to invade those ocean hot spots. Even better, this model can be easily applied to other animals and plants in the ocean or on land.

Regulations on the transport of ballast water are loosely defined and not enforced by all shipping nations. Thus, while these predictive models are helpful, they become useless if strict regulations on ballast are not enforced. Go out, raise awareness and start a conversation about species invasion and the harm of ballast water. Although this study primarily examined the spread of invasive species via ballast water, that is by no means the only way invasive species can be introduced into an area. One of the more common ways species are introduced is through people releasing aquarium pets into the environment (more on pet trade invasions). You can help prevent these types of invasions by making sure you properly take care of your aquarium (or terrestrial pets). With an expanding population, increased shipping on the seas and a fast-changing climate, it is imperative we understand how species in the ocean may spread in the future.

3 thoughts on “Aliens attack: Predicting the spread of marine invasive species

  1. I have learnt that invasive species are very hard to remove once the invasion has begun.An invasion might be easy to predict with the right measures of temperature and salinity as well as other environmental factors. I have also learnt that humans mostly cause other animals to be transported into a habitat where they don’t belong such as releasing aquarium animals into any marine environment which they may not belong in.What steps have been taken to end invasion of marine animals which had already taken place in parts of the world? Are there any solutions that will completely end invasive species? Is there a way to transport invasive species back to their own habitat without hurting the environment or the species as well? Is it possible to repair the damage caused by the invasive species?

  2. This was a great article as I learned many new interesting facts about predicting the spread of marine invasive species. I had no idea that invasive species was a global problem and also I didn’t know that invasive species was spreading??!!! I learned several types of invasive species such as lion-fish, green-crabs, and killer algae are all invasive species. I learned the various harmful effects on invasive species can have on the world. For example, I learned that invasive species can harm other species in the same environment as they can out compete each other for resources of food and nutrients. Also, I learned that invasive species are usually at the top of the food chain, and I learned that they don’t have that many predators who will eat them. In addition, I learned that some invasive species like the killer algae can rob other animals from nutrients of sunlight. Also, I learned that invasive species can harm the economy as if species like killer algae steal nutrients of sunlight that other corals need to survive then this can harm the economy as it harms fish populations that rely on corals and fisheries to exploit commercial fish. Furthermore, I learned the various ways invasive species can travel/spread to a new area/environment. For instance I learned that invasive species could be introduced and transported to an environment naturally and through human interactions as they could be be moved by the wind or ocean currents or be moved by ships and from zoos/aquariums to the wild. I learned the most common way that invasive species are spreading is that they are being transported through ships by catching a hitch hike as they will be put away inside ballasts of ships or ride ship hulls. Moreover, I would like to know examples of the different types of models that are being used to predict the spread of invasive species? Also, how does the model collect data from shipping traffic and environmental factors like salinity and temperature? In addition, besides using models, is there any other way to predict the spread of invasive species? Also, instead of limiting human interactions that cause the spread of invasive species like transportation of ballast water, is there any other solutions to killing/stopping invasive species? Also, just out of my self being curious, since I’ve learned that invasive species are very harmful to the world/ecosystems, do they have any positive impact and are there any unexpected benefits that invaders like invasive species could provide to an environment? Also, is there anything besides sea shipping and releasing aquarium pets into the wild, that humans are doing to contribute to the spread of invasive species? Lastly, I would like to know if creating innovative models to predict the spread of invasive species be considered expensive/harmful to the economy, as wouldn’t it cost a lot of money to get materials/resources to build/create the models? In conclusion, if it doesn’t hurt the economy, what are some of the reasons why it isn’t hurting the economy? There were many unique and interesting things I learned in the article, but also there are many interesting things I still would like to know more about.

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