ANN ARBOR, Mich.—Loss of biodiversity appears to impact ecosystems as much as climate change, pollution and other major forms of environmental stress, according to a new study from an international research team.
The study is the first comprehensive effort to directly compare the impacts of biological diversity loss to the anticipated effects of a host of other human-caused environmental changes.
The results highlight the need for stronger local, national and international efforts to protect biodiversity and the benefits it provides, according to the researchers, who are based at nine institutions in the United States, Canada and Sweden.
"Loss of biological diversity due to species extinctions is going to have major impacts on our planet, and we better prepare ourselves to deal with them," said University of Michigan ecologist Bradley Cardinale, one of the authors. The study is scheduled for online publication in the journal Nature on May 2.
"These extinctions may well rank as one of the top five drivers of global change," said Cardinale, an assistant professor at the U-M School of Natural Resources and Environment and an assistant professor in the Department of Ecology and Evolutionary Biology.
Studies over the last two decades have demonstrated that more biologically diverse ecosystems are more productive. As a result, there has been growing concern that the very high rates of modern extinctions – due to habitat loss, overharvesting and other human-caused environmental changes – could reduce nature's ability to provide goods and services like food, clean water and a stable climate.
But until now, it's been unclear how biodiversity losses stack up against other human-caused environmental changes that affect ecosystem health and productivity.
"Some people have assumed that biodiversity effects are relatively minor compared to other environmental stressors," said biologist David Hooper of Western Washington University, the lead author of the Nature paper. "Our new results show that future loss of species has the potential to reduce plant production just as much as global warming and pollution."
In their study, Hooper and his colleagues used combined data from a large number of published studies to compare how various global environmental stressors affect two processes important in all ecosystems: plant growth and the decomposition of dead plants by bacteria and fungi. The new study involved the construction of a data base drawn from 192 peer-reviewed publications about experiments that manipulated species richness and examined the impact on ecosystem processes.
The global synthesis by Hooper and his colleagues found that in areas where local species loss this century falls within the lower range of projections (loss of 1 to 20 percent of plant species), negligible impacts on ecosystem plant growth will result, and changes in species richness will rank low relative to the impacts projected for other environmental changes.
In ecosystems where species losses fall within intermediate projections (21 to 40 percent of species), however, species loss is expected to reduce plant growth by 5 to 10 percent, an effect that is comparable in magnitude to the expected impacts of climate warming and increased ultraviolet radiation due to stratospheric ozone loss.
At higher levels of extinction (41 to 60 percent of species), the impacts of species loss ranked with those of many other major drivers of environmental change, such as ozone pollution, acid deposition on forests, and nutrient pollution.
"Within the range of expected species losses, we saw average declines in plant growth that were as large as changes seen in experiments simulating several other major environmental changes caused by humans," Hooper said. "I think several of us working on this study were surprised by the comparative strength of those effects."
The strength of the observed biodiversity effects suggests that policymakers searching for solutions to other pressing environmental problems should be aware of potential adverse effects on biodiversity, as well, the researchers said.
Still to be determined is how diversity loss and other large-scale environmental changes will interact to alter ecosystems. "The biggest challenge looking forward is to predict the combined impacts of these environmental challenges to natural ecosystems and to society," said J. Emmett Duffy of the Virginia Institute of Marine Science, a co-author of the paper.
Authors of the Nature paper, in addition to Hooper, Cardinale and Duffy, are: E. Carol Adair of the University of Vermont and the National Center for Ecological Analysis and Synthesis; Jarrett E.K. Byrnes of the National Center for Ecological Analysis and Synthesis; Bruce Hungate of Northern Arizona University; Kristen Matulich of University of California Irvine; Andrew Gonzalez of McGill University; Lars Gamfeldt of the University of Gothenburg; and Mary O'Connor of the University of British Columbia and the National Center for Ecological Analysis and Synthesis.
Funding for the study included grants from the National Science Foundation and the National Center for Ecological Analysis and Synthesis.
"This analysis establishes that reduced biodiversity affects ecosystems at levels comparable to those of global warming or air pollution," said Henry Gholz, program director in the National Science Foundation's Division of Environmental Biology, which funded the research.
U-M Sustainability fosters a more sustainable world through collaborations across campus and beyond aimed at educating students, generating new knowledge, and minimizing our environmental footprint. Learn more at sustainability.umich.edu.
In 2008, I and my colleagues published a study which appeared to confirm that current global warming could cause large losses of species and a loss of biodiversity. Four years later and a new approach to the data has changed our ideas on the effect of global warming on species. Our new study suggests that biodiversity actually increases in times of warmth - though this doesn’t provide much comfort in terms of current climate change.
The new research
Some of our previous findings are upheld in the new study. Origination and extinction are still more frequent in greenhouse periods. But biodiversity now increases rather than reduces with warmer temperatures. Furthermore, the effect of temperature still holds even after accounting for numerous other environmental factors which are thought to influence biodiversity.
A subtle difference is in the timing of events. The new study shows that extinction happens after biodiversity rises. This suggests that extinction may be caused by competition rather than being caused directly by temperature.
A warmer Earth may promote biodiversity in the very long term, but this probably depends on the origination of new forms of life, which we know generally takes thousands or millions of years. Hence, the new study doesn’t provide much comfort in the face of current climate change. In contrast, the majority of work on current ecosystems suggests that climate change will cause global loss of diversity, though it may increase locally in some places.
These new results do conform much better to the geographic pattern of life on Earth, which suggests that warm climates are good for biodiversity in the long term.
Learning from the past
In 2008, we used the most comprehensive fossil data then available, going back 540 million years. We compared diversity in these fossils as Earth’s climate fluctuated slowly from icehouse states to warmer greenhouse states. We found that warmer periods experienced greater extinction (including four of the big five mass extinctions) and reduced total diversity. After these extinctions there was a peak in the origination of new species.
Although our first study involved very long timescales, extinction can happen rapidly. And concerned citizens of the planet probably don’t want to risk environmental destruction on the scale of the end-Permian extinction. Many scientists readily accepted our findings, but the response from those studying the fossil record was mixed.
Three general objections were raised to the study. The findings seemed to contradict the spatial pattern of life on Earth, where there is greater biodiversity in warmer regions, rather than less. Second, we focused on the effects of temperature to the exclusion of other possible causes. Third, we used fossil data that contained potential biases.
After addressing these concerns, our new results will probably please those who were more sceptical of the 2008 work.
The use of new data was very important. Up until very recently, people examined broad trends in the fossil record by looking for the first and last fossil of each group they were interested in and assumed that the group was present at all times in between. There is some sense to this approach; it analyses what you know was really there; and the method definitely correctly identifies some important biological signals, such as the times of mass extinction.
However, the method is prone to bias. Some geological periods are better studied than others, so first and last fossils of different groups tend to concentrate in those regardless of the true underlying diversity pattern. In the last decade, a large effort by the scientific community has led to techniques to scrub away that bias. The key is to measure the fossil record much like an ecologist would sample a rainforest; by using a standardised sample, in this case one that is equally fair to the different time periods.
Preparing to change direction
As a scientist, these studies have taken me down a twisting intellectual path. The duty of a scientist, and I believe any responsible citizen, is to let your conclusions be guided by the evidence. But science develops by getting new and better data, hence we always need to be prepared to switch direction when the data tell us to. Although I have always known this in theory, I have never experienced it quite so prominently in my own work. It’s been a big test of how to conduct myself as a scientist. And I believe there is more to come.
There still isn’t much of a scientific consensus over the environmental drivers of diversity through time, and I’m sure this current study won’t be the last word on the subject. Still, it is very humbling to use data that contain the cumulative message of hundreds of fossil studies. Scientists often talk about “standing on the shoulders of giants”. Scientists also stand on the shoulders of thousands of dedicated workers who never achieve fame, but who accumulate the majority of what we know.