EVOLUTION OF BIODIVERSITY IN EUROPE

While not exhaustive regarding the overall state of biodiversity in Europe, this work aims, through the analysis of extensive, time-series datasets, to provide key insights for approaching an understanding of its evolution.

It is structured into four chapters: definitions and context derived from [1–7], the evolution of species diversity based on data from [8], the monitoring of a biological indicator reflecting the state of biodiversity using data from [9, 10], and the evolution of forest ecosystems, studied using data from [11–14].

To address the notion of biodiversity, it is necessary to define what life is.

In the broadest biological sense, one proposal [1] describes a living organism as a dissipative structure capable of autocatalysis, homeostasis, and learning.

A structure is termed dissipative if it maintains an ordered state by drawing energy from its environment. Autocatalysis, for its part, is the capacity for growth and reproduction. Homeostasis allows for the maintenance of an internal balance despite external disturbances, whilst learning is a means to increase its chances of survival through the storage and exploitation of information about its environment.

These theoretical criteria apply to life on Earth as follows: the elementary building block is the cell. It contains DNA, which allows for the storage of the genetic information necessary for learning, and proteins responsible for all cellular tasks (metabolism), which allow — by consuming energy — for the cell's survival, the maintenance of internal balance, particularly during external disturbances, development, and reproduction (cell division).

A living organism on Earth thus possesses all of these properties. However, the mechanism of biological evolution, through genetic mutations, has generated a genetic diversity that manifests as morphological, anatomical, and behavioural diversity, amongst others.

Today, phylogenetic classification criteria allow individuals to be separated into groups. A clade (phylogenetic group) contains all descendants of their last common ancestor. The basic unit of this classification is the species, which thus groups together all individuals capable of interbreeding and producing viable and fertile offspring.

Biodiversity refers to the diversity between and within species, that of ecosystems and the communities of species living within them, and that of the interactions between these different elements. This study relies on indicators to assess the evolution of biodiversity over time.

The first is the estimated number of living species, which is a subject of debate among biologists, the number having evolved significantly since the advent of genetics, varying today between 10 million and 1 trillion [2–7].

A recent study [2] includes in its calculation cryptic species (those with the same morphology but distinct genetically) and host-specific species. Each insect species would thus host, on average, one unique associated mite species, and both would each be associated with a unique nematode species.

This work collates data gathered from several types of European ecosystems, including forests and grasslands, but Europe is composed of numerous other ecosystem types such as heathlands and peatlands, amongst others. To study the evolution of biodiversity, a first step consists of looking at the diversity of known species.

To measure the evolution of species diversity, the IUCN [8] assesses a portion (approximately 8% as shown previously) of the described species, monitoring the evolution of their population.

It assigns each assessed species an extinction risk level, ranging from Extinct in the Wild (EW - score: 5), Critically Endangered (CR - score: 4), Endangered (EN - score: 3), Vulnerable (VU - score: 2), Near Threatened (NT - score: 1), Data Deficient (DD) to Least Concern (LC - score: 0).

The risk level is based on criteria including the evolution of the geographic distribution, of the size of the population, and the estimation of the probability of extinction.

To measure the change in this risk, the IUCN constructs the Red List Index (RLI) defined as: $$ \text{RLI} = 1 - \frac{\sum{\text{S}}}{\text{N} \times \text{S}_{\text{EW}}} $$ where S is the species score, N is the number of species assessed, and S_EW is the maximum score (S_EW=5). The closer the index is to 1, the greater the survival probability of the species.

A change of scale allows verification of this trend through the study of a species group whose properties make them a key biological indicator: butterflies.

More than half of the described species in the world are insects [8]. They contribute significantly to global biodiversity and provide numerous ecosystem services that help preserve it, such as pollination.

Many European Union (EU) indicators are related to gene or species diversity but do not directly capture the state of biodiversity. Certain characteristics make butterflies a key biological indicator.

They have a short, fragile lifespan and are thus highly and rapidly sensitive to environmental changes. Their wide array of colours and forms make them popular and more easily identifiable than other insect species. Finally, they represent a broad diversity of insect species.

For these reasons, amongst others, butterflies have benefited from rich documentation derived from numerous observation studies. Along with birds and bats, they are the only groups for which harmonised data are available across Europe.

Measurement campaigns since 1991 have made it possible to obtain standardised data [9] by following the protocol below: the observer follows a path (transect) of 0.2 to 3 km at a constant speed, whilst counting and identifying butterflies present in a 5-metres wide corridor around them.

The Grassland Butterfly Indicator (GBI) is constructed from the observations of 17 common butterfly species across the 27 EU member states [9].

The loss of grassland butterfly habitats stems from the intensification of agricultural grasslands, which involves the use of fertilisers, pesticides, and monocultures to artificially maximise production, in particular from nitrogen depositions — used to fertilise soils — which contaminate and transform the ecosystems [9].

A series of warmer and drier summers due to climate change is causing an additional decline. In short, the protection of remaining semi-natural grasslands, the reversal of the habitat fragmentation trend, and the transformation of intensive agricultural practices can contribute to restoring grassland biodiversity and allowing for the benefits of its ecosystem services.

In Europe, another ecosystem covers a large part of the territory, and has evolved significantly during the Holocene — the current interglacial period which began approximately 12,000 years ago — with a climate conducive to its development: the forest ecosystem.

A forest ecosystem is a system composed of a physical environment with trees, communities of plant, animal, fungal, and microbial species, and all the interactions amongst these organisms and with their environment.

Forests provide numerous ecosystem services, such as the production of renewable material, the storage of CO2, soil protection, and the development of biodiversity, amongst others.

As these services may compete with one another, an understanding of the balance of the entire ecosystem is necessary to allow for sustainable management.

In this work, the study of the evolution of forest ecosystems in Europe begins with the observation of recent forest cover changes [11], based on field measurements taken during National Forest Inventory (NFI) campaigns, which survey the area of EU countries to measure the size, number of species, age, and volume of trees, among other characteristics.

Depending on the period, a decline or expansion has been observed for forest cover. Yet, alone, this is insufficient to account for the evolution of forest ecosystems.

The resilience of forests goes hand in hand with the diversity of tree species. This can be studied using data collected by NFIs which allow its evolution to be reconstructed [11].

This time, only recent data, from 2005 to 2015, are available for Europe. These allow illustrating a 1% decrease over this period in the proportion of forests formed by only a single tree species.

The proportion of forests with between 2 and 5 species has increased by more than 1%, while that of forests with more than 6 species has remained stable in comparison.

The introduction in past centuries of new species and the planting of coniferous species to the detriment of broadleaved forests have changed the composition of European forests.

More than half of the growing stock volume — total volume of living trees — in 2020 is composed of spruce or pine.

This large increase in the share of conifers in forests in Europe is mainly attributed to the interest in their rapid growth, and thus wood productivity in the short term.

Annual growth over the last 30 years of the growing stock volume of broadleaves of +1.6% against +1.2% for conifers has been observed, thus illustrating the better long-term yield of broadleaved or mixed forests.

On the other hand, the forest bird population indicator is stable since 1980, with a slight increase observed since 2010, reflecting the state of generalist forest bird populations but also the viability of the ecosystem.

While forest area is increasing in Europe, and certain forest biodiversity indicators are stable, the growth of growing stock volume is slowing down each year [11].

By increasing tree mortality, these factors reduce the duration of carbon storage in living biomass and in soils.

These disturbances thus counteract management efforts to increase the capacity of forests to fulfil ecosystem services such as CO2 absorption and storage — equivalent to 10% of EU emissions — which help mitigate climate change [14].

Other indicators testify to the fragility of European forests, such as defoliation — leaf loss — which is increasing for 4/5 of trees, and the age structure of trees, with 3/4 of forests considered even-aged (belonging to the same age class).

Climate change, by causing droughts and soil desiccation, contributes to further weakening trees. A robust understanding and monitoring of forest ecosystem disturbances would be useful in an effort for sustainable forest management.

The full extent of biodiversity remains unknown and expands as it is studied. Despite this, substantial knowledge for certain species groups, such as vertebrates or plants, allows for the monitoring of their evolution. Genetics has enabled the reconstruction of the Tree of Life where all species are classified. Nearly one-third of the described and assessed species are threatened with extinction. Indeed, a rapid decline in the species conservation index is observed, translating into a threat to species diversity in Europe and globally.

The example of European grassland ecosystems is used to substantiate this observation. Butterfly populations can be employed as a biological indicator due to their characteristics, which render them sensitive to environmental variations. Through the index, a halving of grassland butterfly populations is observed between 1990 and 2023. A study in the Netherlands provides an order of magnitude of the losses likely experienced by butterfly populations in Europe in the 20th century, bringing the total decline in European grassland butterflies to a level above 80%.

A positive trend is observed in the recent evolution of forest area; however, the perspective of its evolution since the start of the Holocene shifts the view on the influence of humans on forest ecosystems. Indeed, at their peak extent, before the effect of human pressures took hold, forests exceeded 60% of Europe’s area while they represent around one-third today.

The recent evolution of forests attests to a relatively stable diversity; however, over the last few centuries, a strong increase in the proportion of conifers has been observed, reaching more than half of the growing stock. This has often occurred through the planting of monocultures, which have permanently transformed European forest ecosystems. These transformations have affected the resilience and growth of forests, which are less diverse in tree species — one-third of forests are composed of a single species — and poorly diversified in age. The ecosystem services provided by forests — such as CO2 storage — have also been diminished. Weakened forests are facing more severe disturbances caused by winds, droughts, bark beetles, amongst others exacerbated by climate change.

The observed signals are consistent with a general decline in biodiversity in Europe. The factors causing this decline are often linked to habitat loss and production intensification practices, compounded by pressures related to climate change. More recently, positive signals have been observed in the evolution of European forest ecosystems, whose diversity and area are slowly increasing. Despite this, one-third of European forests are still monospecific.