image: Fossil pollen records offer our best insights into past rates of vegetation change. A lake, or other suitable environment, is cored to retrieve the layered sediments which contain pollen grains that accumulated over thousands of years. By identifying and counting the different pollen grains researchers can then reconstruct the local vegetation composition. Finally, the rate of vegetation change is estimated from the changes in pollen abundances through time.
Wherever ecologists look, from tropical forests to tundra, ecosystems are being transformed by human land use and climate change. A hallmark of human impacts is that the rates of change in ecosystems are accelerating worldwide.
Surprisingly, a new study, published today in Science, found that these rates of ecological change began to speed up many thousands of years ago. "What we see today is just the tip of the iceberg" noted co-lead author Ondrej Mottl from the University of Bergen (UiB). "The accelerations we see during the industrial revolution and modern periods have a deep-rooted history stretching back in time."
Using a global network of over 1,000 fossil pollen records, the team found - and expected to find - a first peak of high ecosystem changes around 11,000 years ago, when the Earth was coming out of a global ice age. "We expected rates of ecological change to be globally high during this transition because the world was changing fast as glaciers retreated and the world warmed" explains co-lead author Suzette Flantua. However, the timing of peak changes varied among regions, suggesting a fairly complex suite of climate and ecosystem changes during the end of the last ice age.
More surprisingly, the team found a second period of accelerating change that began between 4.6 and 2.8 thousand years ago and has continued to present. Remarkably, these recent rates of change are now as fast or faster as the massive ecosystem transformations that accompanied the end of the last ice age.
To achieve this global analysis, the team developed new statistical tools that allowed them to calculate rates of change in stratigraphic sequences. "We found a way to compare the many records across different continents and smaller regions" explains Alistair Seddon at UiB and Bjerknes Centre for Climate Research. "That was a major breakthrough as it allowed us to demonstrate that different regions experienced peak rates of change at different times".
The study was made possible by scientists working together to create a global resource of fossil data, called the Neotoma Paleoecology Database (https://www.neotomadb.org), available freely to everyone. "Paleoecology is quickly transitioning from a local science to a global science, powered by the ongoing growth in open-access global databases and by the urgent scientific need to better understand how ecosystems respond to environmental change, at local to global scales." John (Jack) Williams said, co-senior author and a paleoecologist at the University of Wisconsin-Madison (USA).
More work is needed to understand and confirm the causes of these accelerations in rates of vegetation change. "Although some patterns seem more obvious than others, we are actually not sure which changes were caused by humans, climate, or both", clarifies Mottl. The next step forward is therefore to compare these global networks of fossil data to independent evidence of past climate change and archaeological findings. "Bringing multiple lines of evidence together should help us better understand how climate, humans, and ecosystems interacted." says Flantua who is also keen to increase the number of pollen records from the southern hemisphere where gaps in the analyses remain. An ongoing ERC project called HOPE (Humans on Planet Earth), led by Prof. J. Birks (UiB) will make strides forward and as more data is integrated, more decisive conclusions can be presented on the role of humans in shaping modern-day ecosystems.
Sadly, one of the co-founders of Neotoma and study co-author, Dr. Eric C. Grimm, passed away unexpectedly as this work was nearing completion. Eric was a much-loved palynologist who made enormous contributions to the analyses of pollen records from around the world. Thanks to his advocacy of open and shared data and software, he built the foundation for this study, and undoubtedly for many studies ahead. The other authors have dedicated this paper to his memory.