Alien species in the Netherlands, 1990-2020

Number of alien species increasing

The Dutch flora and fauna are constantly changing. Since 1990 the number of alien species in the Netherlands has increased. Alien species are plant or animal species that have not entered the Netherlands by natural distribution, but have been brought to the Netherlands from their original range by the direct or indirect agency of humans and are able to reproduce here in the wild. The main pathways of introduction of alien species into the Netherlands are the trade in plants and animals, as contaminants and as 'hitch-hikers' in cargo shipments, for example in the ballast water of ships. Some alien species have been deliberately brought to Europe or the Netherlands for production purposes, such as the Japanese oyster for oyster farming and the muskrat for fur farming, while others have been deliberately released for recreational or professional fishing. Other species , such as the brown rat and the Atlantic Jackknife clam, arrive here inadvertently as 'hitch-hikers' in cargo or the ballast water of ships.

The biggest pathway of introduction is the trade in exotic plants and animals. Sometimes these exotic species escape from captivity, are released (e.g. the Egyptian goose and rose-ringed parakeet) or are accidentally discharged into water (e.g. the giant salvinia). Following the completion of the Rhine-Main-Danube Canal in 1992 there was a large increase in the number of fish species and macrofauna (very small but visible invertebrates) in Dutch waters (e.g. the Caspian mud shrimp). These species were previously unable to reach the Netherlands, but the construction of the canal has allowed them to spread from the Danube into the Rhine catchment.

Most species come from elsewhere in Europe or from North America. Most marine species are from the North Pacific and North Atlantic oceans.

Invasive alien species harmful to humans and nature

An alien species is invasive if it spreads rapidly and becomes a pest. The number of invasive and potentially invasive alien species is also increasing, and this increase has been faster during the past four decades than ever before. These invasive alien species can be very harmful to human health and public safety. For example, the pollen of common ragweed can cause violent allergic reactions, and just coming into contact with giant hogweed can under the influence of sunlight lead to blistering of the skin. The muskrat digs burrows in dikes and other water-retaining structures, which makes intensive control necessary.

• Muskusrat, 1950-2023

Invasive alien species can also be harmful to native species and ecosystems. They can outcompete, predate and infect native species or recombine with them to form novel species mixes and hybrid ecosystems. They can have negative impacts on native species by competing with them for food or niches. An example is the Japanese oyster, which displaces the native European flat oyster because it completely covers all the available substrates. The Atlantic jackknife clam and compound sea squirt are considered to be harmful because they are present in huge numbers and displace other molluscs.

In addition to displacing native plant and animal species, alien species can also cause damage by transmitting diseases. The virile crayfish damages water plants, eats fish eggs and larvae and is sometimes infected with crayfish plague (a water mould) against which the native European river crayfish has no resistance. As a consequence, the latter species, which is listed in Annex V of the Habitats Directive, is now almost extinct in the Netherlands. The multicoloured Asian lady beetle, which was introduced for the biological control of aphids, is an invasive alien species. It eats native species of ladybirds, so where the multicoloured Asian lady beetle occurs the native ladybirds are absent. The released pumpkinseed or pond perch has a negative impact in mire pools, because this fish species eats amphibians and dragonfly larvae. In the pools where the fish is present in large numbers there are demonstrably fewer dragonfly larvae and amphibians, and the biodiversity is declining.

The fathead minnow has been present in the Netherlands since 2007. This released species of fish presents a risk to other fish because it carries a bacterium that causes a deadly disease. For this reason the fathead minnow is actively controlled.

Financial loss

Alien species can cause enormous financial damage. The exact sums are not known, but Van der Weijden (2005) estimated the total annual cost of alien species to be 1.3 to 2.2 billion euros. This includes the cost of damage and controlling exotic contagious diseases. Of the groups of animals shown in the figure only a few alien species are known to cause considerable economic damage. For example, it costs the regional water authorities around 35 million euros to control the muskrat and 1.8 million euros to control the floating pennywort (NVWA, 2018, 2019a). Floating water plants such as the pennywort, parrot's feather, swamp stonecrop and water primrose cause problems locally in waterways. The negative impacts are oxygen-depleted water covered by a mat of floating plants, blocked and stalled pumps and a decline in ecological water quality. The water authorities have to clear away the rampant growth of water plants to ensure a free flow of water, which costs them about 2 million euros each year in extra maintenance work. These water plants are still spreading, despite the statutory prohibition and intensive control efforts.

• Invasieve plantensoorten zijn in de laatste decennia sterk toegenomen in Nederland

Policy objectives

There is considerable attention for alien species in international nature policy. The European Union (EU) and the Convention on Biological Diversity (CBD) both have policies for invasive alien species. According to the definitions of the EU and the CBD, these are non-native species that present a risk to native biodiversity. However, the 2020 targets of the EU and CBD on preventing introduction have not been achieved; the number of actual and potentially invasive alien species is still increasing. The targets and measures have therefore been tightened up.

On 1 January 2015 a new EU Regulation (No 1143/2014) on the prevention and management of the introduction and spread of invasive alien species came into force to control the damage they cause to biodiversity and ecosystem services. Central to this Regulation is a list of invasive alien species: the Union list. EU member states are obliged to control and prevent the establishment of the invasive alien species on the Union list. From 3 August 2016 it has been illegal to import, breed, trade in or possess plants and animals on the Union list. Pet shops and garden centres may no longer sell the plants and animals on the list. Should any invasive species nevertheless become established, the animals should be removed as quickly as possible and the plants destroyed. If invasive alien species can no longer be caught or destroyed, further spread must be prevented. The Netherlands Food and Consumer Product Safety Authority (NVWA) has developed a strategy for the elimination and control of the species on the Union list. On 2 August 2022 the EU added 18 animal species and 4 plant species to the Union list, bringing the total number to 91.

The new CBD target for 2030 is: Eliminate, minimize, reduce and or mitigate the impacts of invasive alien species on biodiversity and ecosystem services by identifying and managing pathways of the introduction of alien species, preventing the introduction and establishment of priority invasive alien species, reducing the rates of introduction and establishment of other known or potential invasive alien species by at least 50 per cent by 2030, and eradicating or controlling invasive alien species, especially in priority sites, such as islands. The EU Biodiversity Strategy target for 2030 is: A 50% reduction in the number of Red List species threatened by invasive alien species.

In 2004 the International Maritime Organisation drew up a convention (International Convention for the Control and Management of Ships' Ballast Water and Sediments) to reduce the unintentional introduction of alien species in the sea via the discharge of ship ballast water. The essence of this agreement is that marine ships must be equipped with approved treatment installations to remove organism form the ballast water. The Convention came into force on 8 September 2017, when it was signed by more than 60 member states, representing more than 70% of world trade tonnage. The Netherlands signed the Convention in May 2010.

Policy implementation

The Office for Risk Assessment & Research of the Netherlands Food and Product Safety Authority (BuRO-NVWA) studies the risks and advises the Ministry of Agriculture, Nature and Food Quality. The NVWA also supervises the trade in and possession of invasive alien species. Under the nature conservation regulation (Regeling natuurbescherming) responsibility for the elimination, control and restoration measures for a number of invasive alien species was transferred to the provincial governments on 1 January 2018.

Several species have been eliminated in recent years. Since 2011 the American bullfrog, which had established itself in two ponds in the province of Limburg, has been actively controlled and the population has now been successfully eliminated. A population of Pallas's squirrel was eliminated from the region of Weert and in 2012 the government decided to eradicate the population of house crows in the Netherlands, which then consisted of 27 individuals. Between 2019 and 2022 there were some incidental sightings of a few Pallas's squirrels and house crows (waarneming.nl). The Asian hornet was observed for the first time in the Netherlands in 2017 and since then the number of sightings has increased. The nests are tracked down and eradicated as far as possible. The grey squirrel has so far only been occasionally sighted. Possession and trade in this species has been prohibited since August 2016 and the species is on the Union list of invasive alien species. The American bullfrog, Pallas's squirrel and Asian hornet are also on the Union list of invasive alien species.

For species that can no longer be eliminated, the only approach is management. Since 8 July 2016 commercial fishing of Chinese mitten crab and alien crayfish has been permitted in the Netherlands (under the fishing exemption scheme) as a management measure for these species.