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The new threat from ocean plastics - invasive species

Plastic waste in the ocean could be an even bigger problem than we knew

We already know that the amount of plastic in the ocean is causing serious environmental problems, including marine animal entanglement, plastic  ingestion, smothering, and ‘ghost fishing’ as well as  aesthetic issues both on- and off-shore. Increasingly, however, scientists have are concerned with another problem with plastic in the ocean, as it appears that debris is carrying invasive species across the seas. This unexpected consequence of plastic in the ocean has been seen in the US, in New Zealand and the South Pacific 

This isn’t new, as  floating plant matter has been carrying seeds, insects, marine life and occasionally birds and mammals across oceans for millennia (Van Duzer), but the hard surfaces of plastic marine debris, and the extent of the marine pollution problem, may mean an increase in the volume and frequency of alien species arriving on our coasts.

In this post, we look at why invasive species are a problem, consider the effects of plastic pollution on the ocean on the issue, and consider what can be done.

Van Duzer C. 2004 Floating islands: a global bibliography. pp. 204p, Los Altos Hills, CA: Cantor Press

Gunn C. R., Dennis J. V. 1999. World guide to tropical drift seeds and fruits Malabar, FL: Kreiger Publishing Company 

Gregory, M. 2009 Environmental implications of plastic debris in marine settings—entanglement, ingestion, smothering, hangers-on, hitch-hiking and alien invasions, Philos Trans R Soc Lond B Biol Sci.

Why are invasive species a problem?

Invasive species are the second most significant threat to an area’s  biodiversity, after habitat loss, according to the World Conservation Union. Most ecosystems have at least one example of an invasive species that has become permanently established in its new environment to the detriment of native species. Often the invasive alien species has a higher rate of reproduction, is well adapted to the natural environment, and lacks natural predators.

“The introduction of invasive species can have a dramatic effect on our natural resources, human health, and economy. When non-native species are introduced into an ecosystem in which they did not evolve their populations sometimes explode in numbers.”

US Fish and Wildlife Service

It doesn’t necessarily follow that a new species being introduced to an ecosystem, will have negative consequences, to either natural resources of habitats, human health, or the economy. Often the species may not be able to adapt and thrive to its new environment and will quickly die out, and sometimes the species will adapt to its new surroundings but without negative consequences (or can even have positive effects, as with the example of certain fish species introduced to new environments for sports fishing –

In any natural environment, native species have evolved together over an extended period of time and this evolutionary process has created an ecosystem with natural checks and balances that keep populations of different species in balance. With many species competing with one another and taking their place in the food chain, a natural equilibrium can be created and maintained – invasive species threaten this balance.

The consequences of an invasive species entering a habitat when it has not evolved there can range from minor to catastrophic, and are difficult to predict. Any disruption to an ecosystem can upset delicate balances, reduce diversity and resilience, and change habitats. Many of the most notable instances involve larger animals (cane toads in Australia, for example) but there are a number of marine and litoral examples as well, including:

  • Tamarisk (saltcedar) – a native plant to Eurasia and Africa that is spreading along waterways in the United States, causing localised flooding
  • Zebra mussels – since arriving in the UK from the Caspian Sea in the nineteenth century has badly displaced native freshwater mussels
  • Giant salvinia, alligatorweed, Brazilian elodea, water hyacinth, hydrilla, Eurasian water-milfoil, water lettuce, (US) – all aquatic plants which can quickly cover an entire body of water when unchecked. This blocks sunlight, which harms other plant growth and phytoplankton and quickly depletes oxygen levels in the water.
  • Comb jellyfish, orginally from the Americas, reached the Black Sea in Europe in the early 1990’s, probably in a ship’s ballast, and has caused the collapse of the local anchovy fishery (which was once a thriving industry worth millions of pounds each year)
  • Water hyacinth has spread from the Amazon basin to many areas around the world, and in some places unchecked growth has had severe impacts, including Lake Victoria in Africa where entire communities have been displaced in order to find clear water.

On the whole, then, the introduction of non-native invasive species is something that should be avoided.

How do these ‘invasions’ happen?

As already noted, migrating species is a natural process that has taken place for millenia,  but the human contribution to the issue is relatively new. Deliberate transporting of animals and plants has been happening since at least the Roman empire (just think of the poor animals in the Colosseum!) as has accidental transport like rats or mice travelling round the world on ships. 

Anthropogenic plastic is rapidly increasing the scope of this problem, however, and is entirely unpredictable. 

“Floating plastics can also transport marine animals, plants, and microbes long distances across the oceans through ‘ocean rafting’. While floating in nearshore environments, local marine species, such as mussels, amphipods, and barnacles, can settle on plastic litter. Because plastics are exceptionally buoyant and slow to degrade, floating plastics in coastal waters can be carried out to sea by offshore ocean currents, where the debris and the attached organisms can circulate for years, in some cases, traveling far enough to reach new shores. Marine organisms hitchhiking on plastics can thus be deposited hundreds of miles from their native ranges, increasing opportunities for non-native species introductions.”

Linsey Haram

Postdoctoral fellow, Smithsonian Environmental Research Center

A 2017 report considered the impact of marine plastic on invasive species, and is well worth a read: The authors acknowledged that the issue is complex and under-researched, but reached four interesting conclusions;

  1. That marine debris may be able to extend the current range of a species to a particular coastline, fueled by factors such as the impacts of climate change
  2. That invasive species that are already established in an area due to another pathway (such as shipping) can use local pathways (such as marine debris) to extend their range even further. 
  3. That large debris items in the open ocean can carry large numbers of organisms and species to new locations.
  4. And that  reoccurring arrivals of small biofouled debris items from a particular locale may be important in leading to multiple inoculations of the same species, overwhelming the local population and introducing genetic constraints, as opposed to the introduction of any one small isolated item. 

Can species really travel on plastic pollution in the ocean? A case study: the effects of the 2011 earthquake and tsunami in Japan

The 2011 tsunami in Japan unfortunately provided researchers with an extensive opportunity to study the effects of rafting and marine pollution on invasive species. Thousands of tonnes of debris was washed into the Pacific, and eventually some of it washed up onto beaches in the US, where scientists were able to study the species clinging to it. They found that  Japanese mussels, barnacles, and sea squirts had not only survived for up to six years clinging to marine debris as it crossed the ocean, but were able to reproduce in their new habitat.

The effects of the tsunami and this research has prompted scientists to look more closely at the impact of marine debris on species migration, and in particular to start to look at how increasing amounts of hard plastic in our oceans are likely to impact on the number of invasive species that we are likely to see.

What can we do?

Eradicating an invasive species once it has established itself is extremely difficult. Efforts can be made through:

  1. Physical or Mechanical Control – This type of control involves physically removing the invasive species (i.e. harvesting) or using barriers or traps to prevent their spread or to capture them. For invasive plants, mowing is another example of physical control.
  2. Chemical Control – This type of control involves all sorts of pesticides (herbicides, insecticides, fungicides, piscicides, etc.) Although chemical use can be very effective, they can be very dangerous to other species or to the ecosystem in general and must be used in an environmentally sound manner. The key is to choose chemicals that are low-risk yet effective and that can be applied when the pest is at its most vulnerable
  3. Cultural Management – Cultural management is the manipulation of the habitat in ways that increase the mortality of the invasive species or reduce its rates of increase and damage. Cultural management that can affect invasive species including: selection of pest resistant varieties of crops, mulching, winter cover crops, changing planting dates to minimize insect impact, burning, flooding, crop rotations that include non-susceptible crops, moisture management, addition of beneficial insect habitat, or other habitat alterations that help the native species compete better against the invasive ones.
  4. Biological Controls – This type of control is the purposeful use of an invasive species’ enemies (predators, parasites, and pathogens) – in other words other exotic species – to reduce the invasive species populations. This option involves much research and testing to be sure the species to be used preys only on the target invasive species. (US Fisheries dept)

More importantly though, is to stop invasive species actually reaching new habitats, and an important part of that will be to reduce the amount of plastic in the oceans. Of course, as we have written before, it will take a global commitment if we are going to make a difference. However, the more waste we can stop from entering the ocean, the better, particularly in terms of plastics which once they are in the ecosystem can be there for a long time. This is why we are actively supporting anyone who wants to improve their local marine environment including inland waterways.

Here at Water Witch, we hope to play our part by providing fully electric debris collection and trash retrieval boats to Ports, Marinas and Waterways Authorities around the world. Our range of tried and tested workboats have been developed to offer users a versatile, multi-purpose craft that can perform a wide range of duties in addition to efficient aquatic trash and debris removal.

Water Witch partner with Marina and Harbour Authorities, Waterway Authorities and tourist resorts to provide peace of mind, deliver efficient management and maintenance of your marine environment that will improve your own green credentials and ensure your visitors keep coming back.


If you need more information about our workboats and innovative solutions for marine debris and waterway cleanup, please contact us.


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