In its latest research, an international research team including Zoltán Botta-Dukát, scientific advisor at the ELKH Centre for Ecological Research (CER), has investigated how species can be grouped according to their invasiveness and what factors determine the success of the spread of alien species in Europe. The paper presenting their results was published in the journal PNAS on June 1. 

Dimensions of success in invasive species

Alien plant species are species that grow outside their natural range. Of these, species that manage to establish a self-sustaining population in the new environment – known as ‘naturalization’ – can have a significant negative impact on wildlife, the economy and human well-being. However, not all plant species are equally successful in conquering new habitats.

Their study, published in PNAS, describes three distinct dimensions of invasion success: local abundance, geographic range size and habitat breadth in naturalized distributions. Successful species are abundant, occur over large areas and in a wide range of habitats. But only some of the successful species are successful ‘super-invaders’ in all three respects. The researchers examined the success of species on a European scale according to three criteria, which were used to classify most of the native and alien species found in Europe. In addition to the biological characteristics of species, the history of their introduction is also important in predicting the success of a species. Among the species that are successful in all three respects are many fast-growing species that are not native to Europe but were introduced to the continent a long time ago. Such findings not only help better understand the current occurrence of species, but also provide a basis for predicting future invasions.

Are all invasive plants the same?

Ecologists are increasingly recognising that invasive plants – also known as alien plants – do not form a single group that can be defined by a single trait. The different characteristics of a species’ distribution determine whether the invasion of an alien species has been successful, i.e. whether it has become naturalized. To better understand the process, the authors of this article have combined information from two large databases – the Global Naturalized Alien Flora (GloNAF) and the European Vegetation Archive (EVA) – and adapted a three-dimensional system previously used to describe the scarcity of native species to alien species.

The previous system was developed by Deborah Rabinowitz in the 1980s. According to Rabinowitz, a species can be said to be ‘common’ if it is locally abundant, occurs over a large area and in a wide range of habitats. In addition to species that are ‘common’ in all respects, there are seven forms of rarity, where a species is less successful in at least one of the three dimensions. For example, a ‘rare’ species may be one that is locally abundant and geographically widespread, but only occurs in specific habitats.

Just as there are different forms of rarity in Rabinowitz’s model for native species, there are different forms of invasion success for alien species. This more detailed categorization is important because different preventive approaches may be effective for different categories of species. For example, if an alien species proliferates locally, but is not expected to spread over long distances and conquer a wide range of habitats, the focus should be on reducing the existing population, rather than on preventing its spread.

Alien, but not so different

When the framework was first applied to native plant species, ecologists noticed that although examples of all seven types of scarcity could be found, the three dimensions were not entirely independent of each other. Locally abundant species often occur over a large geographical area and are less selective between habitats. “We expected the three dimensions to correlate in alien species, as we saw in native species. After all, all alien species are native somewhere else,” said lead author of the article Trevor Fistoe, explaining the research team’s initial hypothesis.

The study confirmed the hypothesis that invasive plants show a pattern very similar to that of native European species: whichever species is successful in one dimension is likely to be successful in the other. This correlation demonstrates that the occurrence and abundance of native and introduced species are governed by the same plant geography and ecology.

What determines the success of an invasion?

Despite the similarities, there is an important difference between native and introduced plant species: the latter did not evolve here, their evolution took place elsewhere – in other parts of Europe or even on other continents – and they have only recently appeared in their current range. Despite the correlation between the three dimensions of success, the researchers have attempted to identify the decisive factors of species success for each of them separately. In addition to the biological characteristics of the species, the history of introduction was used as a possible explanatory factor.

The researchers found that species that are successful in all three respects tend to come from other continents (Asia or the Americas), while species native to other parts of Europe are not usually successful unless they occurred naturally on that part of the continent. In addition, super-invaders from other continents are often fast-growing but poorly protected against their natural enemies, such as the insects that eat them. This is in line with the ‘enemy release’ hypothesis, which states that, by moving into a new geographical area, the alien species leaves behind the pathogens and consumers that evolved along with it, so that it has less need to defend itself and can devote its resources to growth. The phenomenon is particularly pronounced when the original and invasive area are distant or there are barriers to spread between them, such as high mountains or oceans.

Exceptions that prove the rule

However, there are exceptions where species that are successful in one way are not successful in another. For example, there are many exceptions among species that have recently entered the continent and have not yet reached their final range. This has a very important practical message: if an alien species is successful in only one dimension, but has only recently entered Europe, it can be expected to become successful in other dimensions later. Three-dimensional typification of the species, taking into account the date of introduction, can predict the future success of the species and plan the most effective management strategy.

Publication:

Trevor S. Fristoe et al. (2021). “Dimensions of invasiveness: links between local abundance, geographic range size and habitat breadth in Europe’s alien and native floras”, PNAS