Human impact on Amazon forests is transforming its ecological functions and evolutionary history

A new study reveals that the impact humans are having on the Amazon rainforest is so profound it is even changing the evolutionary history and functionality of the forests.

As the world gathers at COP30 to discuss climate in the world’s largest rainforest, the focus is often on the carbon that these forests either store or sequester.

However, tropical forests are much more than just carbon: they are among the biggest reservoirs of biodiversity on the planet. For example, the Amazon holds up to 16,000 species of tree alone, with a single hectare frequently holding more than 300 species. In comparison, the UK has just 32 species of native tree and the whole of Europe has around 450.

A new study, “Multi-faceted assessment of Amazonian tree diversity reveals pervasive impacts of human modifications,” published in Global Change Biology, by an international team of researchers from Brazil and the UK and led by scientists at the universities of Lancaster and Oxford, shows that human disturbances—from logging and fires of primary forests, or the clear felling and forest recovery after land abandonment (secondary forests)—brings about a comprehensive change in these hyper diverse tree communities.

And the effects of human disturbance are so great they swamp the different ways scientists have at looking at changes in diversity.

Scientists have spent the last two decades advancing ways to measure changes in biodiversity, developing ‘functional’ approaches that use traits (such as bark thickness, wood density, leaf area and micronutrient content of leaves) to understand the link between tree species and what they do in an ecosystem, and ‘phylogenetic’ methods that assess how each species relate to one another through evolution.

“While these advances are important, our results show that when it comes to understanding human influences on rainforests, it does not matter much—human influences are so profound that all measures are changing, and it is the disturbance itself that determines the degree of change,” said Brazilian-born Dr. Erika Berenguer, of Lancaster University and the Environmental Change Institute at the University of Oxford, who co-led the study.

The researchers sampled more than 55,000 trees of different sizes across 215 plots in two regions of the Eastern Amazonia. The plots represented a range of human impacts—from undisturbed primary forests, to primary forests that had been selectively logged, primary forests that had been logged and burned, and secondary forests.

For each plot, the scientists measured the diversity of species of trees, the diversity of functional types of trees (linking the tree traits to the ecological functions those trees have in the forests), as well as the diversity of tree evolutionary groups within the forests. This helped to get a comprehensive measure of the composition and ecological diversity of the forests.

The expectation is that forests that are more diverse—composed of more species, more functional types and more lineages that have evolved along different paths—should be more resilient because of that different evolutionary history and functional contribution.

They found that all human disturbances—including logging, wildfires and clear felling followed by regrowth negatively impacted all measures of tree diversity.

The findings highlight that even activities that are considered as sustainable management, such as selective logging, have profound impacts on the diversity of trees, which constitute the structure of a forest and represent the primary energy source for a multitude of other species.

Secondary forests allowed to regrow after clearance were found to be the most ecologically and evolutionarily distinct from their undisturbed primary counterparts—which researchers say is unsurprising considering clear felling is the most severe form of human change.

“Disturbed primary forests and secondary forests had lower numbers of tree species, but also lower numbers of evolutionary lineages and functional types of trees,” said Dr. Cássio Alencar Nunes of Lancaster University and the Universidade Federal de Lavras in Brazil, and who co-led the study.

“However, it wasn’t just the numbers that were lower, but also the identity of the species, lineages and functional types changed after disturbance. Disturbance is not only resulting in impoverished tree diversity, it is changing the species composition of human-modified Amazonian forests.

“An example of this is we find disturbed forests see a greater prevalence of ‘pioneer’ tree species and much fewer of the larger, slower-growing species we find in undisturbed forests.”

The study’s findings highlight the damaging impacts of humans on the diversity of tropical forests and the importance of protecting the remaining undisturbed areas from selective logging and wildfires.

Professor of Conservation Science at Lancaster University, Jos Barlow, said, “Our findings show that human-modified forests are fundamentally different from their undisturbed counterparts.

“As the Amazon faces mounting human pressures, conserving remaining undisturbed forests is essential—not only for their carbon-storing potential but also to preserve the deep evolutionary heritage that has shaped one of our planet’s most diverse ecosystems.

“However, our results also demonstrate the value of disturbed forests when compared to forest regeneration after clear felling. This highlights the importance of novel protection mechanisms such as the Tropical Forest Forever Facility (TFFF) that provide a way of funding the conservation of all forests, and not just those that remain unimpacted by humans.

Dr. Berenguer said, “Although the focus of COP30 is mostly on carbon, linking climate discussion with biodiversity is essential if we want to overcome the climate and biodiversity crisis. Ultimately, biodiversity is what guarantees the provision of ecosystem services such as carbon sequestering and stocking.

“Given that disturbed forests presented impoverished and distinct tree compositions, it is expected that large areas of the Amazon are already unable to provide the full range of ecosystem services found in undisturbed primary forests.”