The true global impact of species-loss caused by humans is far greater than expected, study reveals

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The extinction of hundreds of bird species caused by humans over the last 130,000 years has led to substantial reductions in avian functional diversity—a measure of the range of different roles and functions that birds undertake within the environment—and resulted in the loss of approximately 3 billion years of unique evolutionary history, according to a new study published in Science.

While humans have been driving a global erosion of species richness for millennia, the consequences of past extinctions for other dimensions of biodiversity are poorly known. Research led by the University of Birmingham highlights the severe consequences of the ongoing biodiversity crisis and the urgent need to identify the ecological functions being lost through extinction.

From the well-documented Dodo to the recent Kauaʻi ʻōʻō songbird declared extinct in 2023, scientists currently have evidence of at least 600 bird species having become extinct as a result of humans since the Late Pleistocene, when modern humans started to spread throughout the world.

Using the most comprehensive dataset to date of all known bird extinctions during the Late Pleistocene and Holocene, the paper "The global loss of avian functional and phylogenetic diversity from anthropogenic extinctions" looks beyond the number of extinctions to the wider implications on the planet.

Lead author Dr. Tom Matthews from the University of Birmingham explained, "The sheer number of bird species that have become extinct is of course, a big part of the extinction crisis, but what we also need to focus on is that every species has a job or function within the environment and therefore plays a really important role in its ecosystem.

"Some birds control pests by eating insects, scavenger birds recycle dead matter, others eat fruit and disperse the seeds, enabling more plants and trees to grow, and some, like hummingbirds, are very important pollinators. When those species die out, the important role that they play (the functional diversity) dies with them.

"In addition to functional diversity, each species also carries a certain amount of evolutionary history, therefore when that species becomes extinct, it's basically like chopping off a branch of the tree of life and all of that associated phylogenetic diversity is also lost."

The research found that the scale of anthropogenic bird extinctions to date has resulted in a loss of approximately 3 billion years of unique evolutionary history, and 7% of global avian functional diversity—a significantly larger amount than expected based on the number of extinctions.

Given the wide range of important ecological roles performed by birds, the loss of avian functional diversity in particular will likely have had far-reaching implications. These post-extinction aftershocks include reduced flower pollination, reduced seed dispersal, the breakdown of top-down control of insect populations—including many pests and disease vectors—as well as increased disease outbreaks due to reduced consumption of carrion.

In addition, the downsizing of the global avifauna documented in the research will likely affect the ability of many plant species to track present and future climate change.

Dr. Matthews concludes, "These results are a timely reminder that the current extinction crisis is not just about species numbers.

"By identifying declines in avian functional and phylogenetic diversity driven by human actions, our findings highlight the urgent need to understand and predict the impacts of past anthropogenic extinctions on ecosystem function in order to prepare for the magnitude of expected future loss from the projected 1,000 bird species that are expected to die out completely over the next two centuries.

"This information is vital for setting effective targets for global conservation strategies, as well as ecosystem restoration and rewilding efforts."

More information: Thomas J. Matthews, The global loss of avian functional and phylogenetic diversity from anthropogenic extinctions, Science (2024). DOI: 10.1126/science.adk7898. www.science.org/doi/10.1126/science.adk7898

Journal information: Science

Provided by University of Birmingham