What is the role of invasive species in biodiversity loss?

Invasive alien species (IAS) are often defined as plants or animals introduced by man, accidentally or intentionally, outside of their natural geographic range. We could add to this definition that they often have a negative impact on the ecosystems in which they arrive. The Asian hornet, which came to France inside Asian pottery and has spread all over Europe since then, is a good example of IAS. It kills thousands of bees in France every year, raising a major pollination issue (i).

Figure 1. Spread of the Asian Hornet in Europe since its arrival in 2004. Source: https://nurturing-nature.co.uk/wp-content/uploads/2016/10/Spread-of-the-Asian-H

Within the scientific community, as a matter of fact, the worry about invasive alien species (1) is high to the point that, in its 2019 report, IPBES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) declared that IAS are one of the five principle drivers of biodiversity loss (ii). However, it appears that while scientists generally agree on the concern about invasion of natural areas by introduced species, they keep debating the role played by IAS in causing decline and extinction of native species communities (iii). 

At the heart of this debate lies the question of whether IAS are drivers, passengers or back-seat drivers of ecosystem change. According to the driver model, IAS are the direct cause of biodiversity loss. Namely, IAS are blamed for exploiting the natural resources of the invaded ecosystem by competing with native species, which thus are weakened. As a consequence, IAS tend to proliferate while native species start declining. Conversely, the passenger model implies that IAS benefit from pollution, altered habitats and climate change, which are considered the main and direct causes of biodiversity loss. In this case, IAS are treated as the consequence and not as the cause of native communities’ decline. In fact, IAS manage to spread because they are pre-adapted to habitat degradation, whereas native communities decline as the ecosystem they were adapted to has been compromised (iv). Finally, the back-seat driver model is a middle-ground between the other two models. It entails that ecosystem changes and IAS act synergistically, consequently, biodiversity loss is the result of the action of several stressors (v).

Figure 2. Graph for the three different models describing the place of invasive species.

The choice of which model is the most appropriate to handle IAS in a given context should be made by analyzing scientific data of the studied ecosystem. This is the only way to develop an effective management and restoration strategy (vi). However, these models are not only ecologically determined. They should also be regarded as alternative communication models, which can impact public perceptions and willingness to take action. Indeed, it was demonstrated that the driver model fosters public action, as IAS are thought to be the main driver of biodiversity loss; in contrast, the passenger model lowers general public involvement. So, decision-makers could define different policies to deal with IAS according to the model which scientists better promote (vii). By putting forward the risks connected to IAS and the causal responsibility of human beings, it would be easier, for instance, to support awareness-raising campaigns and recruit volunteers, who often are an important tool in monitoring and managing invasive species. 

Figure 3. Islands are places where many threatened native species live. Here is a view of Molokai, a Hawaiian island. Source: Tor Johnson, Hawaii Tourism Authority. https://www.usatoday.com/story/travel/destinations/2016/10/31/molokai-hawaii/92894528/

It is therefore of the utmost importance not to overestimate nor underestimate the role of IAS in biodiversity loss. In the first case, the risk is that the use of overly extreme measures will be justified to fight against them. For instance, the use of gene drives in conservation policies has been denounced by several scientists (viii). In the second case, the risk is to avoid taking measures which could be essential to eradicate IAS and allow native species to grow back. For example, it is known that most of the threatened native species live on islands, which are fragile ecosystems; hence, by prioritizing IAS eradications on islands, it could be possible to assure better native biodiversity conservation.

To conclude, there is much at stake when it comes to biodiversity loss and, as a consequence, the complexity of this issue should not be neglected. The more specific our understanding is of the nature of threats, the more specifically we will be able to address the mitigation of those threats (ix).

This article was written by Pierre Gousset, Master’s student in the “Quaternary, Prehistory and Bioarchaeology” specialization and Camilla Di Maulo, Master’s student in the “Society and Biodiversity” specialization, both programs at the Muséum.

Bibliography:

1. Several definitions have been used to define IAS; the IUCN has established two main features to identify them. First of all, they are plants or animals which can be found outside of their natural geographic area. Secondly, they have been introduced by men, intentionally or accidentally. IUCN website. Invasive species. https://www.iucn.org/theme/species/our-work/invasive-species [May 17, 2020]

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ii. IPBES. (2019) Global assessment report on biodiversity and ecosystem services of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. E. S. Brondizio, J. Settele, S. Díaz, and H. T. Ngo (editors).

iii. DIDHAM, Raphael & al. (2005) Are invasive species the drivers to ecological change?. Trends in Ecology and Evolution, vol.20 n.9, p. 470-474.

iv. MACDOUGALL, Andrew S. & TURKINGTON, Roy. (2005) Are invasive species the drivers or the passengers of change in degraded ecosystems?  Ecology, 86(1), pp. 42-55.

v. SHACKELFORD, Nancy & al. (2013) Finding a middle-ground: the native/non-native debate. Biological Conservation, 158, pp. 55-62.

vi. GUREVITCH, Jessica & PADILLA, Dianna K. (2004) Are invasive species a major cause of extinctions? TRENDS in Ecology and Evolution, vol. 19, n°9, pp. 470-474.

vii. HART, P. Sol & al. (2014) Communicating About Invasive Species: How « Driver » and « Passenger » Influence Public Willingness to Take Action. Conversation Letters, vol. 7, n°6, pp. 545-552.

viii. ETC group website. A Call for Conservation with a Conscience: No Place for Gene Drives in Conservation.http://www.etcgroup.org/files/files/final_gene_drive_letter.pdf [May 17, 2020].

ix. HART, P. Sol & al. (2014) Communicating About Invasive Species: How « Driver » and « Passenger » Influence Public Willingness to Take Action. Conversation Letters, vol. 7, n°6, pp. 545-552.