Researchers from Yale and Princeton say the scientific community is in dire need of a new way to compare the cascading effects of ecosystem loss from human-induced environmental changes to major crises of the past.
For too long, scientists have relied on metrics that compare current rates of species loss with those characterizing mass extinctions in the distant past, according to Pincelli Hull, assistant professor of Earth and planetary sciences at Yale , and Christopher Spalding, astrophysicist at Princeton. .
The result has been projections of extinction rates in the coming decades that are on the order of a hundred times higher than anything observed in the last millions of years of the fossil record.
“The problem with using extinction rates in this way is that their assessment is fraught with uncertainty,” said Hull, who has conducted extensive research into the massive extinctions of marine life in the ancient world. “We need a better thermometer for biodiversity crises.”
Additionally, the researchers said predictions of mass extinction do not fully reflect the severity of damage to an ecosystem when species are depleted but not fully wiped out.
In a new study in the journal Proceedings of the Royal Society B, Spalding and Hull highlight deep gaps in the way mass extinctions are projected and propose a new model for assessing biodiversity loss.
Part of the problem, they said, has to do with comparing the extinctions found in the fossil record over millions of years with the human-influenced extinctions of the last century alone. Mass extinctions in the ancient world were generally characterized by “pulses” of extinctions, preceded and followed by quieter periods; the longer period reduces the historical average because it includes the surrounding quiet periods.
In addition, there are large gaps in the ancient fossil record. For example, it’s well documented that frog species are at high risk of extinction today – but frogs are only rarely found in the fossil record. In addition, some habitats with many extinctions today – such as islands – are also not represented in the ancient fossil record. Rather, the fossil record tends to be dominated by larger species and geographically larger habitats.
“It is difficult to deduce with certainty whether the current rates are objectively higher than those in the fossil record,” Spalding said. “Meanwhile, we know that ecosystems can be totally decimated, but suffer very few extinctions. In that sense, extinction rates can even underestimate our influence on the biosphere.”
Spalding and Hull were careful to describe the perilous state of the natural world today, beyond the number of species extinctions. According to a report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) in 2019, nearly 75% of all freshwater resources on Earth are used for agricultural and animal production; human activities have dramatically altered 75% of all ice-free land environments and 66% of marine environments.
The proposal of Spalding and Hull is to change the metric from the loss of species to the changes in the rocks under their feet.
“Humans change the rock record as soon as they enter an area, whether it’s agrarian societies, beaver trapping, or river dams,” Hull said. “We are completely changing the way the Earth is formed and it can be seen in the rocks left behind.”
The researchers said that a variety of measurable parameters – such as the chemical makeup of sediments and rock grains – are more easily compared to older timescales.
“Historical comparisons offer hope that we could begin to understand the relative scope and possible ramifications of our modification of the biosphere,” Spalding said. “If we think these comparisons are important, we have to do them correctly.”
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