Research Projects

Global Naturalized Alien Flora (GloNAF)

Anke Stein, Arne Erpenbach, Noëlie Maurel, Mark van Kleunen

Due to human activities, many plant species have spread to and become established in regions outside their native range. Although some of these alien species pose a threat to ecosystems, biodiversity, and human health and economy worldwide, our understanding of the patterns and drivers of naturalization and invasion is still limited.

In an international, collaborative effort, we have compiled an extensive database on the distribution and characteristics of naturalized alien plant species worldwide, which we continuously expand and update. This database allows us to address a large variety of research questions regarding the patterns and processes of naturalization and invasion at a global scale. Among other things, we investigate how macroecological and socioeconomic factors interact with plant traits and phylogenetic relatedness to influence naturalization success. Moreover, we analyze the main flows of plant introductions among continents and regions, and whether plant naturalizations have led to a taxonomic and phylogenetic homogenization of the floras worldwide.

For further information on the project and project partners, please visit the GloNAF website.

Naturalization of ornamental alien plant species

Noëlie Maurel, Mark van Kleunen

Humans have intentionally and accidentally introduced tens of thousands of plant species from their native ranges into new territories, and over 13,000 of them have successfully established wild populations somewhere in the world (see project GloNAF). Of all introduction pathways, ornamental trade is the largest source of naturalized and invasive plants. However, the reasons for this are still largely speculative.

In this context, we are interested in the general question of how important historical aspects of plant introductions are for the naturalization of ornamental alien species, relative to other intrinsic and extrinsic factors. An advantage of the ornamental pathway is that there are data about introduction history (when, where and how frequently a species has been introduced), for both those species that have then become naturalized and the ones that have failed. Our approach is to compile, for a given region, such historical data with data about species origin, species traits, and geographical and environmental factors such as climate. This allows us to address and disentangle the factors that underlie the success or failure of naturalization among ornamental alien plants.

Determinants of invasion success

Zhijie Zhang, Mark van Kleunen

We study the determinants of invasion success, with a strong emphasis on the different invasion stages. We integrate different plant invasion hypothesis into a single framework, test the effect of different diversity metrics (species diversity, functional diversity and response diversity) on invasibility, and apply trait-based measurement to Darwin’s naturalization conundrum.

Common garden experiment with inclusion cage for herbivory arthropods

Darwin’s naturalization hypothesis: phylogeny, enemy release and multi-trophic interactions

Eva Malecore, Mark van Kleunen (in collaboration with Mialy Razanajatovo)

Phylogenetic distance can be used as an approximation of ecological similarity of species, with more closely related species behaving in a comparable way. How the success of alien plants in a new range can be linked to its phylogenetic relatedness to the native community has been addressed in several studies, but no clear pattern has emerged yet. In this project, we are interested in how phylogenetic distance could influence the success of an alien plant, by affecting different aspects, like competition, interactions at different trophic levels and pollination.

Darwin expressed the hypothesis that exotic species more closely related to the native community would be more likely to naturalize, since they would share preadaptations to the local environment. However, he observed the opposite pattern, reasoning this was due to a strong competition imposed by the closely related resident species, because of high niche similarity.

Alien plants more closely related to the native community could attract more herbivores, but on the other hand, they could also draw more predators and mutualists. The success of the alien plant in the new range would thus depend on the strength of each interaction.

Another aspect related to phylogenetic distance is heterospecific pollination: the fruit and seed set of a plant can be negatively influenced by the presence of heterospecific pollen depending on the relatedness of the other species. Thus, an alien plant could both be affected negatively by the native community or be hampered by it, depending on the relative strengths of the effects.

We are addressing those hypotheses, focusing on grassland communities, by both reanalyzing previous plant introduction experiments and by performing common garden experiments.

Modified leaf-vacuum to collect arthropods from the grassland community

The role of epigenetic inheritance in rapid evolutionary adaptation of invasive plants

Jasmin Herden, Marc Stift and Mark van Kleunen (in cooperation with Silvia Eckert and Jasmin Joshi at the University of Potsdam)

Some of the best known examples of rapid evolution are provided by invasive organisms, but given that most invasions started from small populations with little genetic variation, the speed at which these adaptations arose is quite surprising. Consequently, the mechanisms that allow for such rapid adaptation in invasive species are still poorly understood. One non-genetic mechanism that has been suggested to play a role in rapid adaptation is epigenetic inheritance such as genomic cytosine methylation. In two field experiments,and a molecular marker study, we are testing for local adaptation in invasive plant species, and how local adaptation might be modified in the same plants treated with the demethylation agent zebularine. Unravelling the mechanisms of rapid evolutionary adaptation will not only contribute to a better understanding of the drivers of biological invasions, but could also provide us with insights into how native species may respond to global change.

Admixture effect in invasive plant beyond first generation heterosis

Yan Li, Mark van Kleunen, Marc Stift

Admixture could enhance progeny performance, and has potential importance in driving invasiveness of alien plants. The few studies that tested it were restricted to admixture effects in the F1 generation. Therefore, we made crosses between plants of Mimulus guttatus from native populations (western North America) and invasive populations (Scotland and New Zealand) to create plants equivalent to the parental (P) generation, and F1 and F2 admixture between populations in each range and between invasive and native ranges. We assess the performance of the resulting plants in a greenhouse experiment, in which plants are grown under benign and drought-stress conditions.

The importance of phenotypic plasticity under global change

Yanjie Liu, Mark van Kleunen

Adaptive phenotypic plasticity

It is frequently assumed that phenotypic plasticity can be very advantageous for plants, because it may increase environmental tolerance (fitness homeostasis). This should, however, only hold for plastic responses that are adaptive, i.e. increase fitness. Numerous studies have shown shade-induced increases in specific leaf area (SLA), and there is wide consensus that this plastic response optimizes light capture and thus has to be adaptive. However, it has rarely been tested whether this is really the case. We did a meta-analysis to test this.

Interactions between plant invasions and other global environmental changes

Invasive alien plant species threaten native biodiversity, disrupt ecosystem functions, and cause large economic damage. It has been suggested that these impacts may further increase under ongoing global environmental change. Numerous case studies involving local comparisons of a single or a few pairs of alien versus native plant species for differences in performance under global environmental change (i.e. under altered mean levels of precipitation, temperature, atmospheric CO2 concentration or nitrogen deposition) have given mixed results. We perform a phylogenetically-controlled meta-analysis to establish a general pattern of invasive and native plant performance under global environmental change.

Environmental variability and plant invasion

Global environmental change not only affects mean environmental conditions but also their variability. It is frequently assumed that habitats with a high variability of resource availability will generally be more easily invaded than habitats with less variable resource conditions. This is because alien plant species are more likely to outperform native plant species under highly variable than under less variable resource availability. We do multi-species experiment in the greenhouse to test whether nutrient variability favors alien plants more strongly than native plants, and whether differences in plasticity of root traits drives the performance differences. In addition, we do multi-species experiments to test whether invasive alien plants have a higher nitrogen-uptake efficiency than non-invasive alien plants.

Identifying future invaders

Emily Haeuser, Mark van Kleunen

Preventing the invasions of new alien species is an important environmental and economic priority, however identifying probable future invaders remains a challenging task. While new potential invaders continue to arrive in Europe every year, little attention is paid to the many alien ornamental species that are already widely planted across Europe and which may also have the potential to naturalize and become invasive. Many such species may not yet be naturalized in Europe due to their less-than-ideal suitability to current European climates. However, in the midst of ongoing climate change, some of these species may be more suited to future European climates, and may find themselves well-poised to naturalize in Europe under those future climate conditions. Using both experimental and modeling multi-species approaches to address factors such as future climate suitability, propagule pressure, global naturalization success, and associated species traits, we attempt to delimit which of these species will be more likely to naturalize in Europe in the future.

This project is a part of the BiodivERsA project WhoIsNext, which involves collaborations with Oliver Bossdorf, Madalin Parepa and Svenja Block at the University of Tübingen, Stefan Dullinger, Franz Essl, Dietmar Moser, Iwona Düllinger and Günther Klonner at the University of Vienna, Wilfried Thuiller, Marta Carboni, Matthew Taluto, Tamara Muenkemueller, Sebastian Lavergne and Luisa Conti at the University of Grenoble, and Wayne Dawson at the University of Durham.

Climate change and biodiversity effects on invasive alien species

Carola Dostal, Emily Haeuser, Mark van Kleunen

In this Master-thesis project, we test how establishment success of invasive alien plants is affected by the diversity of the native community and how this may change under climate warming. In a large experiment, we introduced seeds and seedlings of invasive alien plants into artificial communities of different diversity, and combined this with a climate warming treatment.

Impacts of climate and land use change on spread of non-native plant species across Germany

Aleksandra (Sasha) Kosanic, Mark van Kleunen

Recent climate change and land use change have a major impact on non-native plant species intrusions and changes in plant species geographical distributions. These impacts are visible globally, but the magnitude and direction of the change differs locally and regionally. Studies so far have mainly analysed effects of anthropogenic factors (i.e. climate change and land-use change) on non-native plant species intrusions at continental or global scales, and these effects were investigated separately. In order to have a better understanding of anthropogenic factors on success of non-native plant species invasions at the local and regional scale, it is necessary to analyse combined and interactive effects of these anthropogenic stressors. For this, it is crucial to have multiple historical records (i.e. instrumental weather records, vegetation records and land-use records) with a good spatial and temporal coverage. This project aims to analyse non-native species intrusions across regional and local scales, in protected and non-protected areas taking advantage of the good coverage of such records in Germany. To better understand more individualistic non-native invasive plant species response to these anthropogenic factors, Ellenberg ecological indicator values will be used as they were proven to be a good proxy for environmental change. Final step in this research will be to investigate if there is any relationship between the change in distribution of non-native plant species intrusions and distributions in native species that are important for provisioning and cultural ecosystem services, and regional identity. This could inform conservation strategies and help to maintain key elements for the regional economy.

Adaptive ecology of annual and perennial plants to environmental changes

Marti March Salas, Mark van Kleunen, Patrick Fitze (Spanish National Research Council)

We test the effects of changes in precipitation variability by the manipulation of watering regimes stochasticity in phenological response and plants trait selection not only taking into account the current conditions but also past experiences.

The early stages of mating system evolution

Samuel Carleial, Yan Li, Mark van Kleunen, Marc Stift (in collaboration with Barbara Mable, University of Glasgow)

The long term consequences of the evolutionary transition from outcrossing to selfing are relatively well known, mainly by systematic comparisons of closely related species with contrasting mating system (for example the outcrossing Arabidopsis lyrata with the selfing model species A. thaliana). Genetically, selfers have low genetic diversity and heterozygosity. They normally have attained an annual life cycle and are successful as weeds and in ephemeral habitats. As selfers do not depend on pollinators, they often have flowers with a so-called 'selfing syndrome': small flowers lacking adaptations to promote outcrossing. However, relatively little is known about the short term consequences of the evolutionary transition from outcrossing to selfing. Our model system Arabidopsis lyrata is useful in this respect, because it has variation in the reproductive strategy among populations. In principle the species is an obligate outcrosser with a strong molecular self-incompatibility system. However, some populations exist where plants have lost the ability to reject self, and have become self-compatible and inbreeding. We compare life history traits of inbreeding populations with normally outcrossing populations. In particular, we are looking at the effect of inbreeding on growth and tolerance/resistance to herbivores. We will also look if the investment in floral advertisement decreases with the shift to inbreeding, as pollinators are no longer required for pollination.

It is also not well understood why selfing evolves in some systems, but not in others. The main barrier to the evolution of selfing is inbreeding depression due to expression of otherwise masked recessive mutational load. We investigate the role of inbreeding depression in A. lyrata by generating selfed progeny from normally self-incompatible plants by self-pollination in a CO2 rich environment. We then look at inbreeding depression in different life-history stages, and test if this inbreeding depression decreases over multiple generations of selfing (purging).

Plant selfing ability and establishment on islands

Mialy Razanajatovo, Mark van Kleunen (in collaboration with Patrick Weigelt and Holger Kreft, University of Göttingen, and the other GloNAF members)

Species with selfing ability should have an advantage over the ones that do not have this ability when establishing after long distance dispersal. When seeds accidentally arrive on islands, they are disconnected from their source populations by oceans, and mates are likely to be limited. Therefore, species with selfing ability should be more likely to establish on islands. On the other hand, the relatively high frequency of self-incompatible and dioecious species in the floras of some islands challenges this hypothesis. This apparent contradiction might be the result of subsequent evolution of self-incompatibility or dioecy of species following establishment. Because the establishment of alien species on islands is relatively recent compared to that of the natives, the potential for evolution of traits post-arrival on islands should still be small for alien species. Using a database study, we test the benefit of selfing ability for the establishment of native and alien plant species on islands.

Novel plant-pollinator interactions

Mialy Razanajatovo and Mark van Kleunen (in collaboration with Eva Malecore)

Biological invasions offer unique real-time opportunities to study the assembly of communities, as many alien organisms have integrated into native resident communities, and novel biotic interactions have been established. There are two apparently contradicting major concepts on the roles of novel plant-pollinator interactions in the assembly of invaded communities. First, alien plant species that can use the same pollinators as the native plant species should more readily attract pollinators and establish successfully (pollinator facilitation or filtering). Second, alien plant species that can use different pollinators from the natives should more readily attract pollinators in the local community, and thus form novel plant-pollinator interactions (pollinator-mediated competition). We test the importance of phylogenetic distance and dissimilarity in functionally important floral and flowering related traits between alien and native plant species in novel plant-pollinator interactions involving alien plant species and native pollinators. To this aim, we use data collected from trait databases and botanical gardens, field and greenhouse experiments.