Why are Marine Invasive Species Posing an Increasing Threat?

Invasive species are a very real but often forgotten threat to our oceans as plastic pollution, overfishing and climate change all appear more pressing and devastating dilemmas. Yet species are constantly undertaking trans-ocean voyages with some surviving the journey and settling in new homes. Many of us simply couldn’t tell our European lobster from our American lobster or our wakame kelp from our knotted wrack seaweed, meaning we often don’t even realise the threats to local biodiversity that are lying just past our doorsteps. So let’s delve into some of the more recent invasions and why they are taking place.

First off, an invasive species, also known as a non-native or alien species, is a group of organisms that have been introduced to new areas outside their normal geographic range. Often, this leads to not only ecological damage but economic and social consequences too. Once humans began building boats and setting sail across the world, it became much easier for species to move around the world as well as they latched onto the hulls of boats or snuck into the ballast waters of ships and entered new territories. In the 21st century however, the potential for marine species to move around the planet has grown enormously and it’s down to our human waste. In the last 20 years, plastic floating in the ocean has increased one-hundred-fold. In the past, species could only hop onboard clumps of seaweed or driftwood but today, millions of new vessels are available in the shape of tangled fishing nets, plastic bottles and empty crisp packets. ‘Rafting’ on plastic trash is becoming a new way for species to travel and it poses a large and mostly unknown danger to all the world’s oceans.

Gyres are thought to be particularly troublesome areas of our ocean because they form a meeting place for both powerful ocean currents and synthetic waste. The currents can easily sweep lightweight plastic fragments away from the gyre and to various coastlines around the planet, many of which are remote. For example, the South Pacific Gyre is thought to be responsible for the glut of plastic waste which is washing up on the remote shores of Easter Island. The island has a small population of 5,000 people yet their shores are teeming with multicoloured pieces of plastic which have swept in on the current. To add to this disaster, 34% of debris on the island was found to carry organisms which were non-native such as foreign species of crab and stony coral. Easter Island is not alone either and there are hundreds of stories similar to this around the world, such as the giant Pacific oysters and Australian barnacles which have clung onto pieces of rubbish and sailed halfway around the world and landed in the Bay of Biscay near France and Spain. It seems that as the world globalises, we are not the only ones who are becoming more connected.

So what impacts can these alien creatures have? The largest impact is often on the local marine ecosystems which can be massively altered by some of these new species. Although it is worth bearing in mind that some species can slot into an existing ecosystem very easily whilst others will not be able to survive in the new conditions they find themselves. Yet the odd few species will find that they thrive in their new environment due to a lack of predators or a larger food source. The lionfish is one of the most obvious examples.

Lionfish typically live in the warm waters of the Indian and Pacific Oceans as well as the Red Sea. However, the creation of the Suez Canal has allowed not only ships to travel between the Red Sea and the Mediterranean Sea but also marine creatures. The increasingly warm seas in the Mediterranean have also attracted lionfish which are swept with the prevailing northward current through the Suez Canal and can now be found off the coasts of Cyprus, Greece and are closing in on Italy. These bright stripy fish are very good at being invasive creatures as each female can lay two million eggs per year and when you combine this with a lack of natural predators in the region, their population has quickly boomed. Yet these carnivorous fish are wreaking havoc under the waves as they eat large numbers of reef fish, crustaceans and molluscs. As they eat large numbers of herbivorous fish, this means there are fewer small fish to eat algae from coral reefs and so algae growth booms which negatively impacts corals and can lead to lower levels of biodiversity in these areas. For the Mediterranean, as well as the Caribbean where lionfish have also invaded, this could be harmful to the popular diving and tourism industries which rely on thriving oceans to attract visitors. To worsen the matter, lionfish actually have venomous spines which pose a nasty hazard to divers and swimmers. Managing lionfish numbers have proven to be a difficult task whilst marine protected areas pose a complex problem on their own. Marine protected areas are designated to protect native species, yet we cannot stop invasive species crossing their invisible border and once inside, these damaging alien animals also become protected by the safe haven where they can feed, grow and reproduce. Specially trained divers and citizen scientists are working together to remove lionfish, yet many live in the deep ocean, too deep for divers to reach them. This means that whilst they can remove lionfish from an area, new individuals may simply rise from the deep later on and continue to pose biological problems. Researchers in the Mediterranean are now calling for better protection for larger predatory fish such as sharks, grouper and some eels which may form a predator to lionfish and help naturally keep their population in check. Meanwhile, others are trying to promote lionfish as a new seafood delicacy whilst some call for tighter biosecurity measures in the Suez Canal which is thought to have let over 450 alien species into the Mediterranean, such as nomad jellyfish and long spined sea urchins.

One region which has one of the most stringent biosecurity measures in the world is the Galapagos Islands. These unique islands are thought to be at an increasing risk of invasive species which can travel to their shores not only by debris, such as plastic, but also by the increasing number of tourist boats docking in the islands. The Galapagos Islands are totally unique with many endemic species which are found nowhere else in the world. Marine iguanas, giant tortoises and Darwin’s finches which he used to establish his theory of evolution all live in the archipelago and have been allowed to evolve with very few predators and a limited number of diseases. Due to this, native creatures have developed few natural defence mechanisms and could be under a massive threat from more pervasive foreign species such as crabs, mussels and sea worms. In order to protect their islands, every international vessel is inspected by divers for animal immigrants whilst cargo hubs mean that all maritime traffic docks in two specific places, limiting the number of areas which are at risk. Yet those drifting in on plastic are harder to detect and as the oceans warm under climate change, previous barriers to travel and survival are breaking down.

Nowhere is this clearer than in the Antarctic. Recently, non-native kelp with living organisms clinging onto it has been found in the Antarctic region. This is the first time foreign species have been found alive in this harsh climate, something scientists previously thought was not possible. Yet as the seas slowly warm and ice melts, it seems the Antarctic could be becoming a little less hostile for some creatures. The increase in maritime traffic in the area, from scientists to tourists to fishing vessels, is also increasing the risk of invasive species entering this incredibly fragile area. Many species in this region are unique and have once again evolved in relative isolation. If new species bring an increased predation risk, more competition for food and new diseases to the area, our unique Antarctic wildlife could start disappearing before we have even had a chance to understand them fully.

There are countless other examples of invasive species which are slowly changing native ecosystems around the world. From green sea fingers displacing native seaweeds in Canada to snowflake coral outgrowing harder coral species in Hawaii. Even the Scottish lobster industry is under threat as American lobsters invade their waters, bringing a dangerous blood disease to European lobsters whilst interbreeding between the two species has been found to lead to infertile male offspring, threatening future populations.

However, most of the invasive species in the world are still hiding under the radar with scientists only understanding the potential impacts of 10% of all alien species. Whilst fish and molluscs are well studied, we still know relatively little about the impacts of species such as sea stars and sponges despite their potential to alter habitats, unbalance food chains and add to the pressures of overfishing and climate change, all of which can have a knock on impact on our marine industries such as fishing. There are several different approaches which are being taken to help reduce the spread of invasive species, with researchers developing special monitoring systems using satellite imagery to track both species and the rafts which they use to travel. Scientists are also aiming to better understand currents, helping us detect where alien creatures may land before they get there but there is one thing we can all do to help reduce this often invisible threat to our local underwater habitats. If we turn off the marine litter tap by reducing plastic use, halting ocean dumping and campaigning for ‘polluter pays’ policies, we can take away a large chunk of the rafts these creatures are using to set sail to new horizons.

By Neve McCracken-Heywood