The Conversation

What is the biggest threat to biodiversity?

Imported from Asia during the Philadelphia Exposition in 1876, kudzu (Pueraria montana var. lobata) is an invasive species that affects the United States.

© Kerry Britton, USDA Forest Service, Bugwood.org, CC BY-NC-ND

Text by: The Conversation

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By Franck Courchamp,

For years, decades even, I have been in the habit of telling my students, explaining in my courses or writing in my work that biological invasions are the second greatest threat to biodiversity on a global level.

These invasions – the process by which certain species are introduced by human activities into regions where they have not evolved, and where they establish themselves, spread and create ecological, health and economic damage – are even the first cause known from recent species extinctions.

That is to say my surprise when, at the plenary of the first global assessment of the IPBES, at Unesco in Paris in May 2019, I heard that a new synthesis placed the invasions in fifth place, behind the habitat destruction and the other three global threats (overexploitation, climate change and pollution).

As part of the CNRS delegation for France at the time, I discussed with other colleagues – in particular delegations from Canada, New Zealand and Senegal – who were all as surprised, even shocked, as I was by this classification. .

Since then, when I mention the second place of biological invasions in the sad list of threats, journalists often respond to me, surprised that the IPBES does not give this order and then ask me what place I should really give to the impacts of invasive alien species.

What is the true global threat ranking.

And what is the worst?

This led me to a lot of reflection, particularly with my colleague Céline Bellard, an expert in biological invasions and who has notably shown – in publications that have worldwide authority – the major impact of this process.

According to his work, biological invasions are also the worst threat to biodiversity in island ecosystems, the very ones that are among the most biodiverse in the world.

Biological invasions: How to fight against species that threaten the balance of ecosystems?

Defining biodiversity and defining threats

So we asked ourselves: have we been on the wrong track all these years?

She, me and thousands of invasion biologists?

Who is right and who is wrong in this disastrous competition to who causes the worst damage?

The fruit of these reflections, joined by her doctoral student Clara Marino, who is doing her thesis on the impact of biological invasions on the different groups of vertebrates, has just been published in

.

They made us realize that this classification was not as simple as that of a running race.

The question "what is the worst global threat to biodiversity?"

involves defining what the biodiversity in question is and how we define this global threat.

Very simply, taking all biodiversity as a whole implies considering all species of plants, animals, prokaryotes, fungi, but also the biodiversity within these species (their genetic diversity) and even within ecosystems.

And this is where it gets complicated.

In fact, not all threat ranking studies really study the same thing to answer the same question.

The Asian hornet, a well-known example in Europe of an invasive species.

Wikimedia, CC BY-NC-SA

Which species to take into account?

To begin with, it is not necessarily the same species that are taken into account.

In an ideal search, we would take all taxonomic groups, but not all species are known, and those that are known are far from all having been assessed for their conservation status.

Even if they were, it is not always possible to attribute a given threat to a threatened species.

In fact, some studies focus on one (or more) specific taxonomic group because the data there are more complete.

But these species are not necessarily the same from one study to another, and therefore we obtain rankings on samples that differ from the start of the evaluation.

Habitat destruction through deforestation in the Colombian Amazon.

© Raúl Arboleda/AFP

For example, according to the most recent list of threatened species from the IUCN (2021), habitat degradation is the first threat for plants but only the third for birds.

We see immediately that it is difficult, just because of the different effects on two major groups, to decide on the place of this threat on global biodiversity.

But it gets complicated.

Because even within homogeneous taxonomic groups, the ecological characteristics of the species will make them sensitive to distinct threats.

If one focuses only on vertebrates as a whole, then the smaller ones are mainly threatened by habitat loss, but the most important threat for the larger ones is overexploitation.

As nothing is ever simple in ecology, the habitat also plays a role.

Within mammals alone, the ranking of threats is totally different between those that live in terrestrial environments and those that live in aquatic environments.

Which indicator to assess the threat?

Then, to study threats to biodiversity, you have to choose a measurement metric, and not all research teams have selected the same one, if only because they focus on biologically very different groups.

It may seem obvious to study species losses, but this metric does not cover all the dimensions of a threat and may even underestimate the danger.

In fact, many metrics are used, and while some are particularly well suited to certain ecosystems, they are not for all.

For example, the percentage of live coral cover, which highlights the threats generated by overexploitation and pollution, is appropriate for some marine ecosystems, but not for others, and therefore the rankings will not be comparable.

Other threat indicators, such as the average size of fish, are only relevant for a limited number of ecosystems, and explain in particular that biological invasions – which have little effect on this metric – can descend into some threat rankings.

Effects that change over time

Finally, it is also important to consider that the importance of the threats, and the importance of their effects evolve over time.

Historically, overexploitation and then habitat destruction have been the main threats to biodiversity.

For several centuries, biological invasions have done the most damage.

Currently, climate change has not yet greatly affected living species, but all the work in ecology predicts that it will probably be the first threat in the decades to come.

Operation to rescue fish, mainly pike, from poor water quality in parts of the drought-affected Siarne River, in Saint-Fraigne, France, August 10, 2022. Captured fish are brought into basins with oxygen before being released on a downstream part of the river, less affected by drought.

© Yohan Bonnet/AFP

We can therefore see that the question of the order of importance of threats to biodiversity is not only more complex than it seems, but makes little sense, because each studies different objects, with different tools. ;

the resulting rankings are often not comparable with each other.

Finally, none of these studies is more accurate than another.

What if we stopped prioritizing threats?

In addition to the complexity and relevance of this issue of hierarchy of threats, our reflection quickly made us realize that it actually presents a danger for the conservation of biodiversity.

If the fault of the scientist is to want to classify and prioritize everything, that of the decision-maker is to prioritize.

If decision-makers are presented with a ranking of the five greatest threats to biodiversity, then their reflex will most likely be to deal with these threats in order of importance.

And since resources for environmental protection are always very limited, regardless of country or political system, the natural tendency will be to deal mainly with the top of the list.

However, as we have seen, certain groups of species, or certain ecosystems, are primarily threatened by processes that are not ranked first overall.

The answer to the question "what is the biggest threat to biodiversity?"

is therefore relatively simple: “it depends”.

Politicians hate this answer, scientists find it hard to live without.

This is certainly one of the reasons underlying the misunderstandings between these two essential links in the preservation of our environment.

The fact remains that if the five major threats to biodiversity are global, they are globally not comparable, not hierarchical, and that there is neither worse nor less serious.

Neither priority nor posteriority.

The preservation of biodiversity must be conceived globally, and carried out globally.

... © axa-research.org/en

Created in 2007 to accelerate and share scientific knowledge on major societal issues, the Axa Research Fund has supported nearly 700 projects worldwide, led by researchers from 38 countries.

To learn more, visit the Axa Research Fund website or follow us on Twitter @AXAResearchFund.

Franck Courchamp is CNRS Research Director at

This article is republished from The Conversation under a Creative Commons license.

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