Human pressure has a hidden impact on plant diversity

An unprecedented survey of nearly 5,500 plant communities across 119 regions has uncovered a hidden ecological deficit: in landscapes with heavy human pressure, typical habitats contain barely one-fifth of the species they could support.

The international DarkDivNet consortium – whose members include the Biodiversity and Evolution Research Group at the University of the Basque Country (UPV/EHU) – reached this conclusion by analyzing “dark diversity,” the pool of plant species that should thrive in a site yet are absent.

Measuring human pressure on plants

Conventional field studies tally the species that are present, but the DarkDivNet team flipped that approach.

For every one-hectare plot, the researchers used climate, soil, and regional species pools to predict the full roster of plants likely to live there under natural conditions. They compared this “potential community” with the species that actually grew on the ground.

Conventional approaches to measuring biodiversity, which focus solely on counting the species currently present and ignore those that could potentially inhabit an area, often downplay the full extent of human influence on ecosystems.

Because the technique captures loss before it becomes obvious, it serves as an early-warning system and a yard-stick for restoration goals.

From pristine reserves to patchy farmlands

The DarkDivNet project was conceived by Meelis Pärtel of the University of Tartu and launched in 2018. Since then, research teams on every continent have followed a single protocol to ensure results are comparable.

One contribution came from lecturers Idoia Biurrun-Galarraga and Juan Antonio Campos-Prieto (UPV/EHU), who mapped 55 plots in Spain’s Gorbeia Nature Reserve, focusing on beech woods and upland heaths.

Data gathering spanned five field seasons and navigated hurdles such as COVID-19 travel bans and political unrest affecting partner institutions.

To rate human pressure, the scientists turned to the Human Footprint Index, a composite measure that integrates population density, land-use conversion, and infrastructure such as roads. Scores ranged from virtually undisturbed wilderness to intensively farmed or urbanized territory.

The resulting pattern was stark. Sites in low-footprint zones harbored about one-third of their potential species – already a significant shortfall caused mainly by natural dispersal limits. Where the footprint was high, realized diversity slumped to 20 percent.

Human pressure beyond plant life

The deficit is not confined to places where bulldozers roar or fields extend to the horizon. Modelling showed that roads, pollution, and land-use change depress diversity hundreds of kilometers away, even inside protected areas.

According to the authors, the results “are alarming because they show that human disturbance exerts a much greater impact than initially thought, even reaching protected areas far from the source of human impact.

Pollution, deforestation, overgrazing, and forest fires can exclude plant species from their natural habitats, preventing them from recolonizing.”

Yet the study did uncover a mitigating factor. Landscapes in which at least one-third of total area remains intact fare markedly better; species stranded in little-touched blocks can drift back into damaged patches.

“The negative influence of human activity was less pronounced when at least one third of a region’s area remained well preserved, which supports the global goal of protecting 30% of the planet’s surface,” the authors wrote.

The finding lends empirical weight to the “30 by 30” conservation target adopted in the 2022 Kunming–Montreal Global Biodiversity Framework.

Dark diversity and hidden issues

Identifying which species should be present but are missing allows land managers to diagnose underlying problems – be it altered fire regimes, livestock browsing, or lost pollinators – and to direct re-introductions where they stand a fighting chance.

In restoration projects, the metric can reveal whether ecosystem recovery from human disturbance is on track long before plant counts return to pre-disturbance numbers.

Because the same protocol works in grasslands, forests, and wetlands, DarkDivNet data provide a common currency for comparing impacts and gauging trade-offs among land-use policies worldwide.

The approach can also highlight priority corridors between intact refuges, where recolonization pressure is greatest.

Future research directions

The network plans to extend its reach to under-sampled biomes and to layer animal and fungal dark-diversity maps onto the botanical baseline, producing a multidimensional picture of ecological debt.

Remote-sensing teams are testing whether patterns of missing species leave spectral fingerprints detectable by drones or satellites.

For now, the project delivers a sobering message: even where forests still stand tall and fields appear green, a silent erosion is under way.

By documenting both visible and invisible losses, dark-diversity analysis reveals the full scope of human influence – and spotlights how much of the living world can return if true refuges are safeguarded across at least 30 percent of every region.

The global tally of species we fail to see, in other words, may be the most important conservation number we need to track.

The study is published in the journal Nature.

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