Invasive Species

Ecologists from the Netherlands Institute of Ecology (NIOO) are investigating the influence of new species on Dutch wildlife. Most plants, animals and microorganisms that successfully establish themselves in the Netherlands eventually become part of the ecosystem. However, these exotic species can also be a nuisance. At this point, we need to ask how this has come about and what course an invasion will take. NIOO researchers are discovering which factors contribute to the invasiveness of exotic species by providing insights into ecological relationships through fundamental scientific research, experiments in the lab and rewilding.

Due to people or climate change

According to terrestrial ecologist Wim van der Putten, exotic species can spread in two ways. “Until recently, species were often introduced to other continents by people, for example, by accident via the ballast water of ships and via bird feed, or deliberately as seeds. Well-known examples of exotic species that have managed to establish themselves in the Netherlands in this way in recent decades or centuries are the red swamp crayfish and the giant hogweed.”

Due to climate change, in recent years, species increasingly often enter our country on their own initiative. Van der Putten: “This expansion of their geographic area is the second form of proliferation by exotic species. Animals and plants in Southern Europe can now establish themselves in Northern Europe because our climate is becoming more favourable for them. Examples of this are spotted knapweed and narrow-leaved ragwort. The latter species was previously an example of a ‘classic’ introduction: it was an exotic species from South Africa introduced to Southern Europe by people.”

However, not every exotic species that flourishes here is invasive. “Invasive means that a species causes a nuisance”, explains terrestrial ecologist Van der Putten. In this respect, species that cause allergic reactions are the major culprits, like, in recent years, giant hogweed, oak processionary caterpillar and common ragweed. In their new habitat, they are not eaten and they can therefore become overdominant.

Eradicate the species or allow it to integrate?

To take into account the influence of all types of organisms in an ecosystem, NIOO researchers examine the entirety of ecological characteristics of an exotic species, including the interaction with possible competitors, collaborators and natural enemies. That makes it easier to estimate whether or not an exotic species will become invasive.

Van der Putten explains the importance of ecological research into invasive exotic species for nature policy: “At the NIOO, we ask ourselves: should you seek to aggressively eradicate invasive species or are there ways of managing these? Eradication is expensive: in 2005, it had already cost the Dutch state more than one billion euros. Furthermore, permanent eradication is very difficult; therefore, it is sometimes better to allow invasive species to develop in a controlled manner to accelerate the naturalisation process.”

Van der Putten’s research centred on the lack of natural enemies in the soil against invasive plant species, such as the wild black cherry, a tree that has been universally used in European forests on poor sandy soil since the early twentieth century. The soil fungi that restrain the indigenous bird cherry in Dutch nature proved less effective for the American sister species that consequently spread so explosively that eradication did not really help. Van der Putten: “Invasive plants have often evaded control by soil life. Then a dilemma arises: should you introduce the soil infections from North America? That was too great a risk for the indigenous bird cherry and, for example, the commercial cherries cultivated in the Betuwe region of the Netherlands.”

Challenge for the development of nature

Invasive species are not only a risk for existing wildlife. Aquatic ecologist Liesbeth Bakker is a specialist in research into rewilding: the development of new nature by allowing areas to become wild again. According to Bakker, that can also go hand in hand with exotic species that establish themselves. “If a plant thrives, then it can rapidly proliferate and replace indigenous plant species. But over the course of time, a species settles: insects adapt and can eat the plants. Therefore, the invasive effects are often temporal.”

Therefore, the challenge with the development of nature is being able to predict whether other species will be able to contain such a plant in its habitat via natural processes and within a reasonable period of time. Bakker: “We are effectively searching for what we call biotic resistance: how can interactions ensure that exotic species do not become invasive? We are trying to understand that via the characteristics of the species.”

Viruses switching between agriculture and nature?

Invisible invasive species also influence our ecosystems and food production systems. For example, agricultural crops can carry exotic viruses, which are kept under control by farmers in the field or in the greenhouse. However, it is not yet known how invasive viruses move between agricultural and natural systems and to what extent these viruses can also cause problems for wildlife.

Microbiologist Mark Zwart often conducts lab tests with the cucumber mosaic virus, a virus that can infect 1200 different plant species, both agricultural crops and wild plants. Zwart: “Wild plants are possibly a reservoir where viruses can hide, so to speak, and this includes invasive species. We would like to be able to monitor the viruses in both wild communities and agricultural systems to establish whether viruses are exchanged between the two. This has not yet been systematically studied.”

A last possible resort for managing invasive species that cannot be eradicated or contained is to examine whether they could also have positive effects. Van der Putten: “Very occasionally, such species can be harvested and the biomass used. That option could, for example, be considered in the case of wild black cherry because cherrywood is a valuable product.”