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The introduction of non-native plants can lead to invasions, which can have a harmful effect on biodiversity, humans and the environment. These potential effects could be predicted with far greater accuracy, according to Thomas van Hengstum.

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Van Hengstum bases this conclusion on the results of his PhD research project, a study on the ecological effects of plant invasions. The doctorate conferral ceremony will take place on Wednesday, 11 September at the University of Amsterdam.

Risk analysis

In the Netherlands – as in many other countries around the world – non-native plant species (especially ornamental plants) are often allowed to enter the country without any form of inspection. In a relatively small (but growing) number of cases, this results in invasions with potentially serious effects on the environment, biodiversity and even economy. Van Hengstum introduces three new tools that can be applied in the risk analysis of non-native species, thus reducing the risk of invasions and the associated environmental effects. These tools will allow us to predict the potential distribution of non-native plants, the native species likely to be affected in terms of pollination success and the types of insects sensitive to the toxins carried by some non-native species.  

Problems caused by invasive plant species

Invasive plants are plants that have  been introduced into a new area (generally) as a result of human actions and subsequently dispersed rapidly. They can have a major impact on the area's biodiversity and ecology. For example, some invasive plants may outcompete native species, while others may affect sedimentation processes and the chemical composition of the soil. Some invasive plants are weeds and cause problems in the agricultural sector. In the Netherlands, the non-native plant species Hydrocotyle ranunculoides (water pennywort) causes considerable economic damage by clogging up entire water systems. Genetically modified plants also pose a potential risk; they may ‘escape’ and potentially become invasive following feralisation or hybridisation with a wild relative.

Pollination, herbivory and biodiversity

Van Hengstum focuses on assessing the effects of plant invasions on pollination, herbivory (damage caused by insects and other organisms) and invertebrate communities. He demonstrates that invasive plants can have both a positive and a negative impact on the insect visitation frequency to nearby native flowers. Despite these effects, he did not identify any consequences in terms of seed set of the species included in the study. 

In most cases, plant invasions were found to reduce the abundance (number of individuals) and taxonomic diversity (number of species) of invertebrates. The opposite effect was found to occur on the edges of the invasion areas, probably due to greater diversity of food sources as a result of the presence of the invader. The plant invasions did not affect the intensity of herbivory affecting nearby native plants.

Lower diversity of species

Areas with one or more exotic (non-native) species have a lower plant diversity than areas with exclusively native species. Van Hengstum suggests that non-native species find it easier to invade an area with a lower diversity of species. In comparison with native species, exotics are more prevalent in areas with abundant shade, nutrients and a warmer, more continental climate. If we take into account that warming, over-fertilisation and vegetation encroachment are common phenomena in the current landscape, this observation might help explain the success of invasive species in the Netherlands.

Publication details

Thomas van Hengstum, Ecological Effects of Plant Invasions. Professor P.H. van Tienderen is the doctoral thesis supervisor. J.G.B. Oostermeijer is the co-supervisor.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).

Time and location

The graduation ceremony will take place on Wednesday, 11 September at 10 am. Location: Agnietenkapel, Oudezijds Voorburgwal 231, Amsterdam.