Fossils resembling miniaturised popcorn that date back millions of years provide the first statistical evidence that number of species on Earth depends on how the environment changes, according to a new study.
By analysing the fossil record of microscopic aquatic creatures called planktonic foraminifera, the research from University of Southampton in the UK shows that environmental changes put a cap on species richness.
"While the idea of infinite species on a finite Earth is clearly fanciful, the relevance of upper limits to diversity is still a fractious debate amongst evolutionary biologists, ecologists and palaeontologists," said Thomas Ezard, from the University of Southampton.
"We are the first to show statistically that this upper limit is environmentally dependent," Ezard said.
"It's intuitive that a changing environment alters how many species we see - the spatial gradient of more species in the tropics than at the poles is pervasive evidence for its large-scale impact," he said.
"However, analyses of how species numbers have changed over time have assumed that any limit has always been the same, even through periods of massive climate upheaval," Ezard said.
"Our data reject this idea of fixed rules for competition among species and instead show that the limit to the number of species that can co-exist on Earth is much more dynamic. Climate and geology are always changing, and the limit changes with them," he added.
While previous research typically focused individually on either biological, climate change or geological explanations, this new research examined the co-dependence of these factors on how species interact.
Looking at the fossil history of 210 evolutionary species of macroperforate planktonic foraminifera in the Cenozoic Era from 65 million years ago to the present, the study found that the number of species was almost certainly controlled by competition among themselves and probably kept within a finite upper limit.
"We used mathematical models to reveal how environmental changes influence both the rate of diversification among species and how many species can co-exist at once," Ezard said.
"Our results suggest that the world is full of species, but that the precise fullness varies through time as environmental changes alter the outcome of competition among species," he added.
"Scientists have long argued that environmental changes are likely to impact the number of species that can co-exist on Earth, but the fossil record is usually too incomplete for powerful statistical testing," said Andy Purvis from the Natural History Museum.
"Microfossils - especially planktonic foraminifera - give us a record with almost no gaps," Purvis said.
The study was published in the journal Ecology Letters.