In our changing climates, some introduced ornamental plant species could establish themselves and become invasive. Emily Haeuser and colleagues present a new model for helping assess naturalization risk in their article, European ornamental garden flora as an invasion debt under climate change.

Invasive species can cause significant ecological and economic damage worldwide. They can disrupt ecosystem services, and put threatened and endangered species at risk through competition or predation. Nevertheless, humans continue to trade and introduce non-native species to new regions at very high rates.

Platycodon grandiflorus
Platycodon grandiflorus, a common non-native ornamental species in Europe.

In the case of plants, most non-native species are introduced into new areas via the horticultural trade. Although most non-native ornamental species require intensive management to survive in the areas where they have been introduced, over time, some are able to escape from gardens and establish in local natural environments. These species can produce large negative impacts if they then become invasive. Curbing these escapes is critical for invasive species management, because eradication after escaping can be difficult, if not impossible.

Restricting the import and sale of species can help to limit their potential for establishment in the wild. With few exceptions, most regions of the world typically require detailed evidence of invasion risk before a species can be considered for ‘blacklisting’. These risk assessments can be very costly and time consuming, and it is impractical to conduct them for all of the thousands of ornamental species we regularly plant in our gardens. It is therefore important to look for ways to try to narrow down the lists of potentially-risky species.

One reason some species are currently unable to escape cultivation is that they are not ideally suited to climates in the local environment. These species are capable of surviving and reproducing in gardens only through human mediation. Climate change, however, is likely to alter the level to which many species are locally climatically-suited. For species that may otherwise be predisposed to invasiveness (due to high propagation levels and/or other species characteristics), climate change may alleviate these prior limitations and allow them to become invasive.

In this study, we aimed to identify non-native ornamental species at risk for invasions in Europe under future climatic conditions. To do this, we first developed a model of non-native ornamental naturalization success (that is, successful long-term population establishment, which typically precedes invasion) in Europe under recent climates. This model was created from data compiled for more than 2,000 garden species. It accounted not only for several species characteristics commonly associated with invasiveness, but also recent climatic suitability. By then incorporating future climatic information into the model, we could project the probability of naturalization for more than 1,500 common alien garden species in the year 2050. We identified twenty species with high chances for future naturalization (>70% probability), and we feel that these species warrant detailed invasion risk assessments across Europe.

Workflow
Simplified workflow of analytical methods (squares) and input/output data (circles).

Because weed risk assessments are typically labor-intensive and require the incorporation of expert opinion, countries in which the government bears the costs of such assessments are typically unable to conduct them for large numbers of potential invaders. The model presented in our paper makes use of data that is available for large numbers of species, and we suggest it as a means for identifying species of concern for which more in-depth risk assessments, and potential sales and import bans, may be warranted. Because the species in this study have not yet established persistent wild populations, they make strong candidates for regulation via sales bans.

Some of the species that we identify as risky have already been reported as occasional garden escapees in Europe: Acer palmatum, Dodonaea viscosa, Gomphrena globosa, Perovskia atriplicifolia and Physostegia virginiana (www.europe-aliens.org). To prevent future invasion of these ornamental species we recommend that European governments prioritize curbing their cultivation.

Read the full article, European ornamental garden flora as an invasion debt under climate change in Journal of Applied Ecology.

Read Associate Editor, Rafael Zenni’s take on this paper in this post (also available in Portuguese).

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