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Islands: biodiversity gems at risk

Posted on 9 October 2025 by Violette Silve

The theory of evolution took its very first steps on an island – and that is no coincidence. In 1835, during his journey to the Galápagos, Charles Darwin carefully observed finches. It was the differences in the shape of their beaks from one island to another that put him on the trail of evolutionary mechanisms (read the key to understanding Darwinian evolution).

Islands are true jewels of biodiversity. Their geographic isolation, combined with unique ecological conditions, encourages the emergence of unique, often endemic species. That is why, although they cover only 7% of Earth’s land area, islands host nearly 20% of global biodiversity. This richness also comes with extreme vulnerability: three-quarters of recorded modern extinctions have occurred on islands.

Key percentage comparisons between islands (blue) and continents (green). Sources detailed in Fernández-Palacios et al. 2021.

This contrast between exceptional biodiversity and extreme vulnerability is precisely what the FRB-CESAB RIVAGE group explores, proposing a specific framework for assessing island ecosystems (Bellard et al. 2025). But before diving deeper, let’s take a moment to understand: How can islands, so rich in life, be so fragile?

The extraordinary biodiversity of islands is explained by their isolation, but it is precisely this trait that makes their ecosystems fragile. Scientists call this the “island syndrome.” Many island species have evolved in environments without certain predators or competitors, leading them to lose defenses that were no longer needed (think of the famous dodo of Mauritius or the kagu of New Caledonia, both birds that lost the ability to fly).

Small territories also mean small populations, which greatly limits genetic diversity and therefore their ability to adapt to disturbances. Finally, an isolated environment implies reduced mobility. When threatened, individuals cannot escape to safer territories.

Dodo of Mauritius (extinct) and kagu of New Caledonia (IUCN status: endangered). © Violette Silve.

On top of this intrinsic vulnerability, external pressures are intensifying:

Climate change, including rising global temperatures and sea level rise — a particularly serious issue for islands,
Land-use changes — from urbanization and intensive agriculture to tourism development — which further reduce and fragment available habitats,
Growing human activity, leading to resource overexploitation and pollution,
And finally, the intentional or accidental introduction of species from elsewhere. Predators, herbivores, or competitors disrupt ecological balances by preying on species that never developed — or lost — the ability to “resist.” These are called invasive alien species, causing biological invasions.

Biological invasions: a global threat to terrestrial vertebrates

In a study conducted just before the creation of the FRB-CESAB RIVAGE group (Marino et al. 2024), scientists cross-referenced data on 1,600 terrestrial animal species (birds, mammals, and reptiles) with those of 304 invasive alien species known to harm them. They estimated that at least 38% of Earth’s land area is already affected by these invasions — a figure likely underestimated, since the study considered only 10% of invasive species recorded worldwide.

But exposure does not necessarily mean danger. To refine their analysis, the scientists also considered how sensitive native species are to these threats. This approach allowed them to produce global maps of vulnerability to biological invasions. The result is clear: islands emerge as the most fragile zones, especially for bird populations. Some areas appear spared from invasions, but this may reflect gaps in data collection — a worrying blind spot for global biodiversity conservation.

And yet, even though island biodiversity plays a crucial role, it remains largely overlooked in global assessments, which tend to focus mainly on continents and climate. For instance, in the Convention on Biological Diversity (CBD), only 1 of the 23 targets explicitly mentions islands as a conservation priority.

In this context, it has become urgent to better assess the vulnerability of island ecosystems. This is precisely the goal of the FRB-CESAB RIVAGE group, which proposes in its first article a brand-new evaluation framework specifically adapted to island biodiversity (Bellard et al. 2025).

This framework assesses vulnerability across three dimensions:

Exposure: the degree to which species or their environments face pressures (in terms of intensity, scale, or frequency),
Sensitivity: the extent to which species respond to pressures, depending on their biological and ecological traits,
Adaptive capacity: their ability to adjust to new conditions through rapid ecological changes. For animals, this depends on mobility. For plants and immobile organisms, it relates more to persistence (e.g., high fertility and seed dispersal). Adaptive capacity can also depend on external factors such as habitat quality, availability, protection, or accessibility.

Overall biodiversity vulnerability can thus be defined as the sum of exposure and sensitivity, minus the adaptive capacity of species.

Unlike other approaches, which often consider just one pressure at a time (most commonly climate change, or more recently biological invasions with Marino et al. 2024), the framework proposed by RIVAGE integrates multiple pressures simultaneously, factoring in their intensity and scale, while also accounting for species-specific traits and adaptive capacity. This tool complements broader indicators, such as the percentage of species threatened on the IUCN Red List. The aim is not to reproduce the same results, but to highlight differences between approaches, refine analyses, and guide action where it is most urgently needed. A detailed comparison between the RIVAGE index and existing indicators is presented in their upcoming article, already available as a preprint (Marino et al. in review).

By proposing this index adapted to the specificities of island ecosystems, the scientists of the RIVAGE group are calling for action to put islands at the heart of conservation priorities.

Learn more about the FRB-CESAB RIVAGE group:

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COREIDS | Predicting community resilience to invasions from diversity and network structure

As the 4th cause of biodiversity loss in the world, invasive species raise important environmental issues. The COREIDS project aimed to better understand the behavior of these species in ecosystems, to better determine their impacts and, ultimately, to devise strategies to predict and limit them. The earth has entered a new era since man has extended his grip on nature. This new era is called the “Anthropocene”. It is a change as important as those that delimit the great periods of the geological and biological past of our planet. One of the most striking actions of man is to move animal and plant species from one place to another, often even from one continent to another, intentionally or not. Some of them adapt well to the environment where they are transplanted, and multiply and spread, often to the detriment of local species and sometimes human economic activities. They are called invasive species. To understand their impact, it is not enough just to understand the links that these species create with the local species present before the arrival of the invaders. Indeed, the local species are interdependent with each other through networks of interactions: for example, one constitutes the food of the other (trophic relation), or one is useful to the other for its reproduction, such as insects that pollinate plants. In such networks, the arrival of an invasive species will not only directly affect the species with which it interacts, but also have an effect that will spread in a chain to the entire species community.

Understanding how these interaction networks work is essential for predicting the impact of an invasive species within an ecosystem. These networks are complex objects that are difficult to describe and study in nature. The objective of the FRB-CESAB COREIDS project was to gather the available knowledge on how invasive species interact with these networks to better understand their impacts and ultimately to devise strategies to predict and limit them if possible.

This document summarizes in a few pages the group’s context and objectives, the methods and approaches used, the main findings, as well as the impact for science, society, and both public and private decision-making.

GASPAR | A general approach to Species-Abundance relationships in a context of global change, reef fish species as a model

The invasion of non-native species is considered one of the main threats to biodiversity, leading to major changes in the structure and functioning of ecosystems. In the context of globalization, research on invasive species has become increasingly important due to the growing number of species being relocated through human activities—such as transport, trade, and others. Invasive species often cause significant problems as they compete with local flora and fauna, frequently disrupting the abundance and quality of native species. This results in a loss of biodiversity which, in the long term, can lead to substantial reductions in ecosystem services, such as lower fishery yields. Since the opening of the Suez Canal in 1869, the Eastern Mediterranean has undergone an unprecedented transformation in species composition, with the massive arrival of non-native species—known as Lessepsian migrants—from the Red Sea. These species already account for more than a quarter of total catches in the Eastern Mediterranean, and sometimes even more. Some of them are now spreading as far as the Western Mediterranean.

The FRB–Cesab Gaspar project specifically investigated the mechanisms that enable a species to adapt to a new environment, in the context of biological invasions and species adaptation to climate and global change. For the first time, the Gaspar project conducted a comprehensive study of all species inhabiting adjacent systems, in order to identify the processes that determine which ones become invasive. Such information is rarely available, and this approach had never been applied before.

This document summarizes, in just a few pages, the project’s context and objectives, the methods and approaches used, the main findings, and the impacts on science, society, and both public and private decision-making.