Environmental changes have led to mass extinctions that slashed the number of species on Earth. A new study challenges the view that without such dramatic events species numbers would rise forever, instead concluding that environmental conditions probably set an upper limit on the number of species that can exist at a time.
Dr Thomas Ezard of the University of Southampton has studied the tiny marine animals known as foraminifera laid down in marine deposits dating back to the extinction of the dinosaurs. Foraminifera have long been used as indicators of environmental conditions, with species replacing each other as the ocean warms or cools. Ezard's findings have been published in Ecology Letters.
"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,” Ezard said in a statement. "We are the first to show statistically that this upper limit is environmentally dependent."
Globigerinella siphonifera is one of the species of foraminifera explored in the study. GLOW research cruise
“The foremost regulator of life on Earth is life itself,” the paper notes. As the number of species rises in an ecosystem extinction rates increase or diversification decreases. What has been debated is whether this reaches a firm maximum, or if “as standing diversity increases, biotic competition can slow – but not halt – diversification.”
Even the most casual observer can see that tropical rainforests contain far more species than polar ice caps. As the paper puts it: “Energy-rich environments support larger populations and provide more ways to construct a species-specific niche.”
“However,” Ezard added, “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 and his co-authors argue that environmental conditions such as available energy need to be taken into account when testing if there is a true upper bound on species numbers.
From observing 210 foraminifera species come and go over 65 million years of deep sea cores, the authors concluded it is highly probable that any set of environmental conditions do indeed create a hard limit. No matter how long the oceans went undisturbed in certain eras, after a certain point newly arrived species replaced others, rather than adding to total numbers. Numbers only increased with environmental changes such as warmer temperatures, not with time. Sophisticated statistical modeling of their observations supported this interpretation.
Co-author Professor Andy Purvis of the Natural History Museum noted that foraminifera are one of the few groupings of animals capable of properly testing the question, because we have almost continuous fossil records for them. Other environments almost always involve large gaps that prevent us reaching such firm conclusions.