The Great Barrier Reef is the largest living structure on Earth. It might also represent the most prolific cradle for new types of animals on the planet, according to new research published in the January 8 edition of Science.
"In the oceans, new species and genera tend to originate in the tropics and in the shallows near shore," says paleobiologist Carl Simpson of Humboldt University in Berlin, one of the researchers on the new paper. By using a massive collection of data on fossils from mollusks to South American mammals, which records where a fossil was found, how often it is found and what accompanied it, Simpson and his colleagues find "that a majority of genera first evolve in reefs and then later expand to other habitats."
In fact, of the 6,615 seabed invertebrates surveyed in the so-called Paleobiology Database, 1,426 evolved in a reef ecosystem. And the result is not just an artifact of reef and shallow-water fossils being relatively more studied. "Reefs are actually rare compared to other habitats," Simpson notes. "If anything, there is a bias against finding that reefs are cradles."
The phemonenon of reefs acting as cradles of biodiversity seems to have peaked in the Paleozoic—from 542 million to 251 million years ago—for reasons that are unclear. One possibility is that by seeding other environments with new species, reefs undercut their own preeminence as cradles. "The consequence would be that reefs become one of many important cradles, rather than the most common cradle," Simpson says.
And reefs do not seem to have helped the oceans rebound from mass extinction events, like the one at the end of the Paleozoic era that wiped out as much as 90 percent of marine life. That's likely because "reef-building as a process had to recover from mass extinction events, because the ecosystem engineers that built reefs were severely affected," says marine scientist Richard Aronson of the Florida Institute of Technology, who was not involved with the study. "Reefs in general were not available to enhance biodiversity rebound because they first had to be reconstituted as viable ecosystems."
But coral reefs do offer a variety of ecological niches and "bumpiness," as Aronson puts it, or a "great variety of physical spaces, [water] flow regimes and other ecological opportunities." That has made them cradles not just for sponges and the like but also snails, shrimps, urchins, fishes and even extinct animals like trilobites.
Plus, vicious competition for space and food on a reef leads to a lot of "turnover," Simpson notes, or extinctions that allow for new species to develop as new habitats form or ecological niches open. "If turnover is high, then the brief window of opportunity for new species is a common occurrence."
According to some mathematical estimates, 99.9 percent of all species that have ever existed are now extinct. As it stands, estimates of the number of species on Earth at present range from five million to as many as 100 million, with science having identified only two million members of the biodiversity extant today. That means literally billions of species have come and gone in the 4.5 billion years Earth has existed.
New research will be needed to determine exactly why reefs are such efficient cradles for new life-forms. But the threats faced by coral today—from rising ocean acidity to agricultural runoff and rampant disease—do not bode well for marine biodiversity in the near future. After all, it is possible that during mass extinction events the destruction of reefs might have played a role in the decline of diversity in other marine environments, as well. As Aronson asks: "If modern reefs continue to degrade, will that have cascading evolutionary consequences for other ecosystems by cutting off the supply of new genera?"