Species Without Borders


A whirlwind final semester left me with little spare time for blogging, but I have now officially graduated with my Master of Science in Marine Biology! In the next few posts, I’ll share some stories about the work I’ve done over the last months. 

My research has taken me to countries like Norway and Croatia, which are just a short flight away from my home in the Netherlands. Today, our world is more accessible than ever, and with worldwide travel increasing, we are becoming an increasingly mixed global community.


With increasing globalization and ease of travel, the world is at our fingertips. But this means that plants and animals are traveling more too – and when they cross borders, it can spell trouble.

However, humans are not the only species crossing borders. Globetrotting has opened passageways for unintentional hitchhikers: alien species. An alien species is a species that is introduced outside of its native range. An alien species is not inherently bad or harmful, and many don’t even survive in an unfamiliar habitat. But sometimes an alien species becomes invasive – and that is bad. An invasive alien species is an alien species that has established itself in the new habitat, and aggressively outcompetes native species, posing a threat to biodiversity; they may also cause harm to human health or the economy.

Species are introduced for many reasons, either accidentally or purposely. Maybe you came back from a trip with a hitchhiking seed lodged in the sole of your shoe. Maybe live animals were being transported for sale as pets, but escaped, or were released into the wild by a well-intentioned owner. Ships often carry and release alien species through their ballast water. Some species are introduced to create fisheries, like the Pacific oyster that I studied in the Wadden Sea, and which you can read about in my previous posts


History can blur the line between what species are considered “alien” and “native.” Tulips are now seen as quintessentially Dutch, but they actually originate from the Ottoman Empire (which we now call Turkey). They were brought to the Netherlands in the 1500s where they became popular ornaments, and were bred into the varieties available today, now on display at the Keukenhof gardens. 


People come to the Netherlands from around the world to admire the veritable sea of tulips and other flowers (luckily these didn't become invasive). 


Although species move naturally throughout time, humans have caused the rate of introductions and invasions to rise exponentially. The consequences of these invasions can be severe.

For example, the algae Caulerpa taxifolia, a popular aquarium plant native to the tropical Pacific and Indian Oceans, was accidentally released into the Mediterranean Sea in the 1980s and quickly grew out of control. Caulerpa rapidly spread and gained a reputation as the killer algae. Caulerpa has smothered thousands of hectares of other algae and plants, and contains a toxin that makes it inedible for herbivorous fish and invertebrates. As a result, Caulerpa also causes fish abundance to decrease, and ultimately reduces catches for fishermen. Eradication of small colonies is possible (as was demonstrated in California), but in the Mediterranean there is little hope that the Caulerpa invasion can be controlled. 


Pretty for your aquarium, yes, but Caulerpa the killer algae has smothered vast swaths of the Mediterranean. Photo by Richard Ling.

Another infamous marine invader is the lionfish. This striking spiky beauty may have been released into the wild by aquarium owners who grew tired of their pets. Now it is regularly seen along the southeast coast of the US and in the Caribbean. Large predators prey on lionfish in its native Indo-Pacific waters, but in the Atlantic, the voracious lionfish has no native predators. Its appetite and abundance threaten fisheries and the balance of the ecosystem. With depleted herbivorous fish, algae grow more quickly, which poses a threat to the already-pressured coral reefs of the Caribbean. Efforts to control the lionfish include a campaign to fish and eat them – a topic which Carl Safina covered here – to limit their numbers.   


A lionfish is covered in colorful venemous spines - so fishers and chefs who want to make a meal of this invader must work carefully! Photo by Michael Gäbler.


You might be wondering, what can we do to help stop the spread of invasive species? Whether you’re coastal or landlocked, there is a lot that you can do to help prevent new invasions:

  • DON’T release your pets into the wild. You would be surprised how much havoc a goldfish can wreak in your local pond.
  • DON’T catch and transport plants or animals and release them into other areas.
  • DO clean and disinfect all of your gear carefully to remove species that might become invasive. You can follow the Check, Clean, Dry technique.
  • DO garden with native species (you’ll save water and support local wildlife too!).
  • DO support efforts to protect native habitats and species – a damaged ecosystem can be more susceptible to invasion.
  • DO report unfamiliar species. Join a citizen science program to help keep track of new invasions. For example, download the What’s Invasive app, or check out iMapInvasives, TexasInvasives, the Midwest Invasive Species Information Network, or another group in your area.   
  • DO learn more about the issues and what species are problematic in your area, and spread the word!

Scientists and policymakers are also beginning to implement further action to prevent and manage invasions, both in the ocean and in other habitats. For example, the European Union has passed new measures requiring member-states to develop management plans for certain species of concern. Scientists gathered this year in Croatia to share their latest findings about invasions and how to battle them. Techniques for eradicating invasive species have slowly improved, and some countries, such as Norway, have logged impressive successes in eradicating invasives and restoring native aquatic species.

Stay tuned, because in upcoming blogs, I will discuss these topics and how my research took me deeper into the world of aquatic invasions! 


Making the most of life in the Netherlands: basking in flowers! Photo by Agnes Tonkes. 

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