A Downward Spiral

Courtesy Wikimedia Commons

These days, there’s a fair amount of talk, unfortunately, about species extinction. But what does that mean, really, for our ecosystems going forward, long after our lifetimes? We can look to rocks—to the fossilized past—for clues.

Recently, geologists at Brown University created a 50-million-year fossil record in western Canada for ammonoids—small, spiral-shelled mollusks related to today’s nautilus—to learn more about two of the five mass extinctions in Earth’s history: the first at the end of Permian period, 250 million years ago, when 90 percent of species disappeared; the second at the conclusion of the Triassic, 200 million years ago, when 72 percent vanished. It’s thought that global volcanic eruptions caused these catastrophic extinctions by spewing greenhouse gases into the atmosphere, drastically altering the world’s climate.

There were two main types of ammonoid: one that simply drifted, and another that actively swam after prey. Following these mass extinctions, however, the swimming version is conspicuously absent from the fossil record. This, the researcher’s theorize in the journal Geology this month, suggests a lack of “ecological redundancy” or, in essence, a radically simplified food chain—so simple, in fact, that it prolonged the ocean ecosystem’s recovery. Why? After these mass extinctions, the remaining ecosystem (composed of those species left standing ... or, drifting) was more vulnerable to minor environmental changes, as compared to an ocean at climax diversity. Smaller bumps in the road, so to speak, stalled and set back the ocean life’s long, winding drive for complexity.

In other words, redundancy is a measure of an ecosystem’s resilience. Evidence for the ancient ocean's minimal resilience, post-mass-extinction, can be found in “chaotic carbon episodes,” during which carbon isotope values—tied to changing burial rates of dead organisms and the availability of nutrients (i.e. overall biomass)—fluctuated. After the Permian and Triassic extinctions, isotope values finally stabilized, returning to "normal" levels, just when swimming ammonoids reappeared. What this may signal is that, after millions of years of species immigration and evolution, the ancient ocean’s ecosystems were resilient once more.

This theory has bearing, of course, on the world's sixth mass extinction: right now. Almost a third of marine species, including many gamefish, have declined drastically in the past forty years, and coral reefs, especially, are in trouble. In 2010, we saw coral bleaching—when stressed polyps expel the algae that lives symbiotically inside them—second only to 1998, most notably in Southeast Asia. Severe bleaching bouts can be permanent, leading to the death of coral colonies. The phenomena is exacerbate by periodic El Nino/La Nina events, and yes, by climate change. Worldwide, reefs have diminished by 19 percent since 1950, and another 35 percent could give up the ghost over the next 40 years as oceans warm. That’s bad news for ecological redundancy: Though corals cover just .2 percent of the world’s oceans, they account for 25 percent of marine species.

Reefs have been described as "the ocean’s canaries”; their loss heralds a downward spiral for ecological redundancy, for biodiversity—the same indicated by the disappearance of ammonoids 200-250 million years ago. "It's definitely a cautionary tale,” Jessica Whiteside, an assistant professor at Brown, said in a press release about her team’s study. "Because we know it's happened at least twice before. And you have long periods of time before you have reestablishment of ecological redundancy." Just how long? Well, swimming ammonoids reappeared in the fossil record after about 10 million years.

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