biological invasions

Listen to the trees: a detective work on the origin of invasive species

An attempt to explore the history of the spread of four non-indigenous invasive tree species in one of the most important Hungarian forest-steppe forests of high conservation value.

Guest blog post by Arnold Erdélyi, Judit Hartdégen, Ákos Malatinszky, and Csaba Vadász

Today, almost everyone is familiar with the term “biological invasion”. Countless studies have been carried out to describe the various processes, and explore the cause and effect, and several methods have been developed in order to control certain invasive species. However, one of the biggest puzzles is always the question of how it all happened. It is not always easy to answer, and, in general, the smaller the area, the more difficult or even impossible it is to answer. In the course of our work, we attempted to explore the history of the spread of four, non-indigenous invasive tree species in one of the most important Hungarian forest-steppe forests of high conservation value, the Peszér Forest (approximately 1000 ha). Last week, we published our study in the journal One Ecosystem.

An open formation of the Eurosiberian steppic woods with Quercus spp., a priority natural habitat type of Community interest under the European Union’s Habitats Directive

The Far Eastern tree of heaven (Ailanthus altissima), as well as the North American black cherry (Prunus serotina), the box elder (Acer negundo) and the common hackberry (Celtis occidentalis) are among the worst invasive plant species in Hungary. They are also responsible for serious conservation and economic problems in the Peszér Forest.

Invasion of tree of heaven (top left) and common hackberry (top right) in poplar stands, carpet of seedlings of black cherry (bottom left), and monodominant stand of box elder, regrown from stump after cutting (bottom right)

Historical reconstructions of the spread of invasive species are most often based on only one, or sometimes a few aspects. We used six approaches simultaneously:

we reviewed the published and grey literature,
extracted tree species data from the National Forest Database since 1958,
conducted a field survey with full spatial coverage (16,000 survey units (25×25 m quadrats)) – instead of sampling,
recorded all the largest (and presumably the oldest) individuals for annual ring counts,
performed hotspot analyses on the field data
collected local knowledge.

Cutting down the oldest common hackberry trees in order to count the annual rings from trunk discs

Our results show that each approach provided some new information, and without any of them the story revealed would have been much shorter and more uncertain. We have also highlighted that at the local level, the use of one or two aspects can be not only inadequate but also misleading.

From the literature it was possible to determine the exact place and date of the first occurrence of the tree of heaven and the black cherry. However, in the case of black cherry, for example, it was only possible to piece together the circumstances of the first plantings by combining three different sources. The first occurrences of box elder were found in forestry data. Finally, in the case of the common hackberry, searching for old individuals and determining their age gave the best results.

Common hackberry in the Peszér forest according to the recent forestry data (2016) and the field survey (2017-2019). The difference is clear: in the official forestry database, the tree species is underrepresented several times over

A well-explored story of a biological invasion can go a long way in making more and more people understand that controlling these non-indigenous species can only be beneficial. On the other hand, it can also help to strengthen conservation efforts, for example by increasing the volunteer workforce, which can be a major factor in the reduction of certain species. We hope that our work and the approaches we have taken will serve as a good model for exploring other invasion stories around the world.

Winter snapshot from the Peszér Forest, a diverse forest edge habitat along an inner road.

Research article:

Erdélyi A, Hartdégen J, Malatinszky Á, Vadász C (2023) Historical reconstruction of the invasions of four non-native tree species at local scale: a detective work on Ailanthus altissima, Celtis occidentalis, Prunus serotina and Acer negundo. One Ecosystem 8: e108683. https://doi.org/10.3897/oneeco.8.e108683

Can we predict if a plant species will become exotic?

A new approach compared characteristics of species that succeeded or failed to establish after probably following the same historical introduction route.

Plant species become exotic after being accidentally or deliberately transported by humans to a new region outside their native range, where they establish self-perpetuating populations that quickly reproduce and spread. This is a complex process mediated by many factors, such as plant traits and genetics, which challenges the creation of general guidelines to predict or manage plant invasions. Scientists from Spanish and Australian institutions have now defined a new framework to find the predictors of invasiveness, investigating species that have succeeded or failed to establish abroad after following similar historical introduction routes.

Dr Javier Galán Díaz, University of Seville, Spain, Dr Enrique G. de la Riva, University of León, Spain, Dr Irene Martín-Forés, The University of Adelaide, Australia, and Dr Montserrat Vilà, Doñana Biological Station (EBD-CSIC), Spain, described their findings in a new paper in the open-access journal NeoBiota.

Ancient agricultural landscape dominated by plant species introduced in other Mediterranean regions (Parque Natural de Los Alcornocales, Andalucía, Spain). Photo by Dr Javier Galán Díaz

“While current policies exert strong control on the import and export of living organisms, including pests, across countries, until only a few decades ago, very little attention was paid to this issue. This means that many species were translocated to new regions without any consideration of their potential impacts,” says Dr Javier Galán Díaz.

An example of this is the massive plant exchange among Mediterranean‐type regions as a consequence of European colonialism: crops and cattle were exported, along with tools and materials, potentially bringing along the seeds of many plant species.

Agricultural landscape dominated by exotic species of European origin (Merced Vernal Pool and Grassland Reserve, California, U.S.A.). Photo by Dr Javier Galán Díaz

“So far, most studies on plant invasions have tried to explain the success of exotic species by comparing their traits with those of the native plant communities where they arrive, or by comparing the traits of plant species that have achieved different levels of invasion in the same region. But, if we take into account that the most common plant species from European agricultural landscapes have been in contact with humans and have therefore had the potential to be inadvertently transported to other Mediterranean regions, then only those that have successfully invaded other regions have something different in them that allowed them to establish and spread abroad,” Dr Galán Díaz explains.

Following this approach, the scientists found that, when comparing plant species transported from the Mediterranean Basin to other Mediterranean-climate regions (California, Central Chile, the Cape Region of South Africa and Southwestern and South Australia) in the search of predictors of invasiveness, only those species with large distribution ranges that occupy climatically diverse habitats in their native region became exotic. Also, species with many dispersal vectors (for instance those that have seeds dispersed by animals, water or wind), long bloom periods and acquisitive above- and belowground strategies of resource use are most likely to become exotic. Most of this plant information is readily available or easy to obtain from free and open-access repositories.

Lotus corniculatus, one of the species that the study identified as invasive in other Mediterranean-climate regions of the world. Photo by Teresa Grau Ros under a CC-BY 2 license
Rumex acetosella, one of the species that the study identified as invasive in other Mediterranean-climate regions of the world. Photo by Jakub T. Jankiewicz under a CC-BY 2 license
Carduus tenuiflorus, one of the species that the study identified as invasive in other Mediterranean-climate regions of the world. Photo by jacinta lluch valero under a CC-BY 2 license
Poa pratensis, one of the species that the study identified as invasive in other Mediterranean-climate regions of the world. Photo by AnneTanne under a CC BY-NC-SA 2 license

“Determining the factors that pre-adapt plant species to successfully establish and spread outside of their native ranges constitutes a powerful approach with great potential for management,” the researchers write in their paper. “This framework has the potential to improve prediction models and management practices to prevent the harmful impacts from species in invaded communities.”

“Using the existing information, we can identify the key species to monitor. This is especially encouraging in the era of Big Data, where observations from citizen science applications add to those of scientists, increasing the potential of screening systems,” Dr Galán Díaz says in conclusion.

Research article:

Galán Díaz J, de la Riva EG, Martín-Forés I, Vilà M (2023) Which features at home make a plant prone to become invasive? NeoBiota 86: 1-20. https://doi.org/10.3897/neobiota.86.104039

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North American turtles establish succcessful populations in Germany, possibly threathening ecosystems

For the first time, self-sustaining populations of three non-native species of turtles were identified in south-western Germany by researchers at the University of Freiburg

Original text published by the University of Freiburg

Three species of turtles native to North America have been successfully reproducing in the wild in Germany, report for the first time environmental researcher Benno Tietz and biologist Dr. Johannes Penner of the University of Freiburg, along with Dr. Melita Vamberger of the Senckenberg Natural History Collection in Dresden.

Their results were published in the open-access scientific journal NeoBiota.

The scientists examined a total of nearly 200 animals living in the wild in lakes in Freiburg and Kehl. Their findings suggest that the turtles have established themselves in a new habitat, where they could become a threat to the local ecosystem.

For two species, this is the first evidence of independent reproduction outside of their natural reproductive range. For the third species, this is the northernmost evidence of its presence up to now,
says Penner.

The false map turtle (*Graptemys pseudogeographica*) enjoys the sun’s warmth. Photo: Johannes Penner.

Turtles released into the wild

Invasive species do a great deal of economic damage world-wide. They also contribute to advancing global species extinctions.

Alien reptiles regularly make their way into the wild in Germany. Most often, this is because they have been released by pet owners.

Large numbers of North American pond sliders (Trachemys scripta) were imported into the European Union (EU) in the 1980s and 1990s as house pets. In 1997, their import into the EU was banned. By 2016, the sale of specimens born here was also made illegal. Since then, pet shops have replaced them with other freshwater turtles, such as the river cooter (Pseudemys concinna) and the false map turtle (Graptemys pseudogeographica).

Genetic analyses of specimens of all three species in a range of ages have now demonstrated that they are reproducing independently in local waters.

What’s surprising is that the invasive species have established themselves so far north. In Europe, successful reproduction and self-maintaining populations of Trachemys scripta were only known in the Mediterranean regions and the continental climate zone of Slovenia,
explains Benno Tietz.

Until recently, it had been assumed the turtles being examined couldn’t reproduce in Central Europe due to the colder climate. Especially the false map turtle is actually quite sensitive to the cold,
he says.

A North American pond slider (*Trachemys scripta*) resting on a lily pad. Photo: Johannes Penner.

Consequences for local species unclear

The invasive turtles could become a problem for indigenous species.

The European pond turtle (Emys orbicularis), for example, is now only present in Germany in parts of Brandenburg.

In an experimental setup, the European pond turtle showed weight loss and an increased death rate when being kept together with Trachemys scripta,
reports Penner.

Penner says that could be caused by the larger, alien species forcing the smaller local turtles from places where they sun themselves, leading the local turtles to have problems with thermoregulation. Or perhaps the competition led to them having greater challenges when seeking food.

Beyond that, aquatic turtles could be hosts for viruses and parasites, leading them to play a role in the spread of diseases. This could potentially have a damaging influence on other parts of the ecosystem, including amphibians, fish, or aquatic plants.

On the other hand, in their study, the researchers consider the alien species could assume functions in damaged ecosystems that would otherwise go unreplaced.

Vamberger says these questions urgently need to be explored further.

We need to raise public awareness that people should not release – no matter what kind of species – any animals into the wild in future.”
she insists.

A river cooter (*Pseudemys concinna*) lets itself drift in the water. Photo: Johannes Penner.

Meet the research team:

Dr. Johannes Penner was the scientific coordinator of the research training group “Conservation of Forest Biodiversity in Multiple-Use Landscapes of Central Europe” (ConFoBi) and a lecturer for the Chair of Wildlife Ecology and Management of the University of Freiburg. Currently, he is a curator at the NGO “Frogs and Friends” and a guest researcher in wild animal ecology.

Benno Tietz has completed a Master’s degree in Environmental Sciences at the University of Freiburg. His thesis – finished in the Winter Semester of 2020/2021 – investigated alien turtles. Currently, he is a research assistant at the Freiburg Institute of Applied Animal Ecology.

Dr. Melita Vamberger is a researcher at the Senckenberg Natural History Collection in Dresden.

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The study was supported by the Hans Schimenz Fund of the German Society for Herpetology and Terrarium Science (DGHT) as well as the Academic Society of Freiburg.

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Research paper:

Tietz B, Penner J, Vamberger M (2023) Chelonian challenge: three alien species from North America are moving their reproductive boundaries in Central Europe. NeoBiota 82: 1-21. https://doi.org/10.3897/neobiota.82.87264

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Assessments of alien species impacts are reliable to prioritize resources

Experts are consistent when assessing the economic, health and ecological impacts of alien species, find the scientists.

Original post by EBD-CSIC

An international collaboration led by the Doñana Biological Station (EBD-CSIC) has shown that experts are consistent when assessing the economic, health and ecological impacts of alien species. These assessments are therefore reliable to guide the prioritization of resources invested against biological invasions.

You can find the scientific article published in the open-access, peer-reviewed scholarly journal NeoBiota.

These results have a great impact on the management by national and international institutions, which have limited resources to fight against the growing and worrying increase of alien species invasions and the damage they caused to society and environment.

Biological invasions annually cause huge food losses, disease transmissions, species extinctions and ecosystem perturbations. For these reasons, it is one of the biggest problems that humankind currently faces, and its relevance will alarmingly increase due to the extreme situations that climate change will expose society to.

The seriousness of this problem lies in the limited human resources available to fight against it, that force to prioritize its management. Here is where tools such as impact assessments play a key role. Assessments report the impact of invasive species in different areas, including economy, health and environment, and allow us to rank the most harmful species.

For instance, in aquatic ecosystems like the Ebro Delta in Spain, there are dozens of invasive alien co-occurring species that cause millions of economic losses and irreparable ecological damage.

Such is the case of the Zebra mussel, which affects irrigation; the apple snail that devours rice fields; and the blue crab causing the local extinction and declines of many native species.

“That’s why it is crucial to ensure that the results are not dependent on the assessors and to understand what factors affect discrepancies among experts,”
explains Rubén Bernardo-Madrid, lead author and researcher at Doñana Biological Station – CSIC.

One of the relevant aspects of this study is the quantification of the consistency of responses across assessors for a large number of invasive species of vertebrates, invertebrates and plants. In addition, the researchers have studied multiple protocols focused on different aspects, providing a global view of this problem.

“The study has shown that the great majority of assessments are consistent and therefore valid to aid in decision-making. These results are encouraging as they suggest that these protocols may be useful when facing the worrying forecasts of increasing biological invasions and their damages,”
explains Rubén.

On the other hand, the researchers have observed that discrepancies across assessments might be due to multiple factors, such as the type of impact asked or the linguistic formulation used in the protocols.

The results suggest that there is room for improvement in assessments, but it will require more funding for research, and more multidisciplinary collaborations between ecologists and linguists to develop less ambiguous protocols.

As always, the most effective measure against biological invasions turns out to be prevention.

However, given the incapacity to control every voluntary and involuntary introduction, other tools such as impact assessments are essential to reduce as far as possible the damage caused by these species on human welfare and environment. Its continuous improvement and evaluation, such as the one made in this study, are decisive.

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Research article:

Bernardo-Madrid R, González-Moreno P, Gallardo B, Bacher S, Vilà M (2022) Consistency in impact assessments of invasive species is generally high and depends on protocols and impact types. In: Giannetto D, Piria M, Tarkan AS, Zięba G (Eds) Recent advancements in the risk screening of freshwater and terrestrial non-native species. NeoBiota 76: 163-190. https://doi.org/10.3897/neobiota.76.83028

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