As we near 2011, I know I must try to prepare myself for 2012, and the (next) wave of apocalyptic doom-saying that, according to the ‘fin-de-cyclists’, was predicted by the ancient Mayan mathematicians over 3 thousand years ago. Of course, they never predicted an End, per se, it’s just that they only extended their 260 day, cyclical calendar* to December 22, 2012 (near the winter solstice, a fitting day to be sure); they probably thought that was just plenty.
It’s easy to dismiss such cataclysmic claims as being the product of mis-educated, or under-informed, mystical types, but I remind myself that science too has fanned its share of doom and gloom scenarios…Consider the Y2K hysteria of but a decade ago. The predictions surrounding Y2K (the “millennium bug”) ranged from peoples’ bank accounts being frozen to failed air traffic control to mass power outages and the collapse of human civility, if not civilization (how would we ever get along without computers?).
So OK, both science and pseudo-science have had their end-of-life-as-we-know-it predictions (science veils its doom saying in purely “what if?” terms; pseudo-science selectively edits out what doesn’t fit the predictions), but, as a student of geology as well as biology, I still have to wonder: could we be entering (or already in) a new Mass Extinction phase? What evidence supports, or refutes, this possibility?
Our methods for counting our planet’s biodiversity — both in the past and in the present — are getting better. But in truth, the only way we can make accurate guesses about the future is to look for evident trends over vast stretches of geologic time, and make the best estimates that we can. Looking at the fossil record, we see that the Earth (its biosphere) has experienced quite a few “mass” extinctions in its history– some more massive than others — such as the Permian Extinction (about 260 million years ago [mya], which ended existence for 90% of the world’s lifeforms), and, the famous KT extinction that brought about the end of the dinosaurs’ dominance (about 65 mya).
But how does the present measure up against the past?
An IUCN report from 2007 estimated that each year, some 400 species of plants and animals go extinct (in the wild). That’s a little over one species per day. But to understand the significance of that number, one has to compare it to the “normal” rate of extinction, what’s known as the background extinction rate. Analyses of the fossil record have shown that this background rate has remained relatively consistent over the eons.
Different estimates of this rate have been attempted, with the most recent ones utilizing data from the Paleobiology Database (PaleoDB). Typically, a researcher will take a random sample from each time period and extrapolate from these counts. Critics of these estimates say that less populous species get left out of the count, so the arrived-at extinction rate is an under-estimation. A new approach to data sampling called “shareholder sampling” was utilized by paleobiologist John Alroy (Sept 3, Science). This method tracks how frequently certain groups appear in each geologic period and calculates a target number of counting samples that are representative of that proportion.
These older and newer estimates give us a range for our current epoch’s extinction rate of 10 to 100 times the background extinction rate. Much of this can be attributed to human activities such as over-hunting/harvesting, development, and pollution.
In each case of mass extinction, in each long era or eon of evolutionary change, the extinction was never total; some species adapted and survived, just barely, and some new species (or clades of species) arose, flourished and re-diversified. At some points in the fossil record, such as during periods of dramatic climate change, biodiversity in some taxonomic groups clearly declines, while in others, biodiversity actually increases.
Some groups can have sudden diversity spurts lasting tens or hundreds of millions of years, and then, suddenly, their diversity numbers just crash. An example of this is the ancient group of bivalves called brachiopods, which flourished from 540 to 250 mya, surviving through the Permian Extinction. But some time after 250 mya, their numbers took a steep dive. Despite this, though quite rare, some forms of the ancient brachiopod persist to this day.
It’s as if evolution wants to create, to preserve, life, despite it’s annihilating tendencies. Another example of this would be tiny sponge-like creatures called Ponifera, which survived the ancient ice age known as the Cryogenian, that ended about 635 mya. Descendants of these primitive sponges remain with us, continuously filtering plankton and providing in turn a food source for many other marine life forms.
However, in examining the fossil record, paleobiologists will note that the biodiversity of large taxonomic groups (such as marine plankton) has tended to arrive at an equilibrium point, in which total species’ biodiversity levels off, neither increasing or decreasing significantly. And the thing is, there’s no way to predict with certainty if any given group has attained, or will attain, its biodiversity “cap”, or upper/lower limit. Further, this diversity limit is not fixed; apparently, this limit changes, spontaneously, every 200 million years.
Despite these uncertainties and fluxes in diversity, paleobiologists will also point out that the background extinction rate for individual species has in fact remained consistent; the average species evolutionary time span lasts but a few million years. Well, that was before the rise of homo sapiens sapiens, anyway.
And we might never get the chance to describe many of these, given the accelerating rate of negative human impacts on biodiversity (overall biodiversity is declining), and the current rate of extinction that’s well above the background rate; one could readily make the claim that we are indeed experiencing a mass extinction.
For this reason primarily, some scientists are suggesting that the Holocene Epoch may be over, and that a new epoch — the Anthropocene — may be officially upon us.
Perhaps, in naming this epoch after our kind, we might grow more self-conscious about what we are doing in it. What we do clearly impacts diverse ecosystems. These ecosystems drive the biodiversity and evolutionary fitness of countless animals and plants. There are some points in geologic time in which failed ecological conditions prevented recovery of species richness. There are critical thresholds beyond which biodiversity cannot recover. Despite the resilience of some, many more will die out, never to be replaced. Our ocean’s coral reefs seem to be one type of ecosystem that is facing this evolutionary challenge.
But I believe, if we can take away biodiversity, then we also have the power to restore it. We can assist nature, perhaps, in finding a new equilibrium point for those critical species upon which our precious, life-giving ecosystems depend. We can become a positive force for planetary biodiversity. This will benefit our survival, in the long run. And, in so doing, we would truly bring honor to the title: the Anthropocene Epoch
* According to Wikipedia, the Mayan Calendar is actually a composite of two calendrical systems: the Tzolk’in and the Haab’ systems; 260 and 365 days, respectively. Neither system numbered years. Years were arrived at by adding the two systems together. A full cycle (“calendar round”) ends/begins (repeats) every 52 years. The end of any calendar round was generally viewed as a period of disorder and “bad luck”.
All Images: public domain
Top Image: Dutch school, 17th Century illustration, “Dronte” (Dodo)
