Friday, June 4, 2010

Of Mice and Megafauna

The travails associated with opening my summer cottage each spring help explain why I enjoy some of the recent research on the extinction of the North and South American megafauna. This fauna of very large mammals which went extinct at the end of the Pleistocene Epoch (about 11,500 years ago), included such creatures as 15 foot tall woolly mammoths (image on left below), “smaller” mastodons at roughly 8 feet high (image on right), giant ground sloths the size of elephants, bear-sized giant beavers, and saber-toothed cats on the order of today’s Siberian tiger.

The link between the megafauna extinction research and my summer cottage is the lowly house mouse (Mus musculus) which checks in with a body of between 3 to 4 inches in length, complemented by a tail of perhaps equal length.

In the slender volume entitled The Ecology of a Summer House (1984), the late biologist Vincent Dethier painted a loving portrait of nature within the confines of a summer bungalow in Maine over the course of summer and into winter, ending with the deep snows of December. A renowned expert on flies and on insect behavior, he brought a scientist’s precise perspective to the subject, and coupled it with an artist’s sensitivity to life and death. As was only natural, mice and summer homes were joined in the book.

For as long as I can remember there had been wood mice in the bungalow. They were year-round residents, true natives. Each summer when we opened the house there would be numerous signs of their winter occupancy despite all efforts to discourage it. (p. 22)

Wood mice (Apodemus sylvaticus), despite their destructiveness and, perhaps because of their “air of delicate charm,” were treated by Dethier with a gentle hand. Still, he was enough of scientist to experiment with one mouse mother and her pups to see if she really knew how many she had (she didn’t).

Every spring, when I first approach the front door of my summer cottage, it is with a sense of anticipation tinged with definite dread, the latter a feeling I suspect Dethier never experienced with his bungalow. Though the cottage has been without its human occupants for nearly all of the fall and winter seasons, it has not been unoccupied. The wintering-over residents are most likely to have included, among others, house mice, not Dethier’s cuter wood mice.

A first order of business in opening the cottage for the season is searching for evidence of mice amid the dust and cobwebs. Sure, this involves some scanning for destruction, but that’s usually well hidden, waiting to be discovered late one night when, in desperate need of sleep, I unfold the sofa bed or reach into the bottom of the chest with the blankets.

The best evidence, the telltale sign, that few, some, or hordes of mice partied here in my absence is mouse scat. Though it’s hard to be precise in using this evidence to measure the extent to which the mice wintered over within these walls, with experience I have developed an instinctive internal gauge about these things.

Upon reflection, I have had to conclude that my annual spring “analysis” of mouse scat and its implications for the state of the cottage predisposes me to appreciate recent research on the extinction of the American megafauna.

This research grapples with what appear to me to be among the core questions of paleobiology: When did some set of events occur? What were the causes? What were the consequences? In this case, the key event is the extinction of the megafauna. Scientists know these large animals were still around at roughly 15,000 years ago and by the end of the Pleistocene were gone. Timing is everything. It is particularly critical for weighing the various alternative explanations offered up for this extinction. These extinction theories include (1) climate change dooming the megafauna, (2) newly arrived Paleo-Indians hunting the animals to extinction, (3) those same Paleo-Indians bringing some virulent disease that decimated the megafauna, or (4) the impact of a comet setting off a catastrophic chain of events that led to the extinction. (For an overview of these theories, see End of the Big Beasts by Peter Tyson, on NOVA Beta Evolution page, March 1, 2009.)

I think that one avenue of research on the megafaunal extinction is particularly brilliant and therein lies the summer cottage link. This thinking begins with the understanding that the herbivores among the megafauna consumed a huge amount of plant biomass and, as a result, must have generated copious amounts of dung, as in, say, mastodon scat. It also stands to reason that this waste would have become home to dung-living fungi, particularly Sporormiella, which produce spores on dung. Further, as a result of ingesting so much cellulose, it makes sense that the herbivore megafauna emitted vast amounts of methane.

A couple of recent analyses are very clever in using this scenario to fashion answers to questions surrounding the megafaunal extinction. Jacquelyn Gill of the University of Wisconsin and her colleagues analyze cores of sediment taken from the bottom of a lake in Indiana (supplemented with data from New York lakes), and measure changes in the presence in the cores of Sporormiella spores. (Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America, Science, November 20, 2009). The spores are washed from the dung into the lakes by “slopewash” and their relative abundance at different levels in the cores are taken to reflect the waxing and waning of the megafauna. I love this example of that “indirect” scientific approach – if something cannot be measured or witnessed directly, find some associated effect that can be.

What do Gill et al. find? They conclude that the inception of a significant decline in the Sporormiella spores pegs the beginning of the megafaunal decline at 14,800 years ago, with full collapse at about 13,700 years, and apparently extinction at about 11,500 years. So, the ultimate extirpation of this fauna took awhile. With these dates in hand, they offer various conclusions, among them, that the “rapid-extinction hypotheses” are wrong, so no comet impact (even the most likely candidate occurred at 12,900 years ago which is well after the onset of the decline) and no “Paleo-Indian blitzkrieg” bringing the megafauna down. They acknowledge that humans may well have contributed to the decline. Of interest, among other findings, they posit that significant changes in vegetation followed the megafaunal decline, and that the megafauna decline began during a warm period and well before the sudden cooling associated with the Younger Dryas period.

The second piece of research is one I just came across. It is a tightly reasoned, mathematical analysis that explores the potential impact of the megafauna extinction on the amount of methane in the atmosphere and the consequences for the climate. The authors, led by biologist Felisa A. Smith of the University of New Mexico, marshal data on 114 megafauna herbivores that died out at the end of the Pleistocene. (Felisa A. Smith, et al. Methane emissions from extinct megafauna, Nature Geoscience, published online May 23, 2010.) Using estimated data on the body masses of these herbivores, their per square kilometer density, and their ranges, the authors calculate that these animals’ combined annual methane production was 9.6 teragrams (9,600,000 metric tons). When those animals went extinct, that methane contribution to the atmosphere ended. Since methane is a greenhouse gas and its loss could have had substantial consequences for the climate, Smith et al. relate their findings to data on atmospheric methane concentrations derived from ice-core records. Their ultimate conclusion?

We find that the loss of megafauna could explain 12.5 to 100% of the atmospheric decrease in methane observed at the onset of the Younger Dryas. . . . [O]ur calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.

Though each of these studies addresses a different, though related, set of questions, there is one critical area in which they don’t agree – the time period over which the extinction occurred. As noted, the megafaunal march to extinction revealed in Gill’s data begins about 14,800 years ago, reaches collapse about 13,700 years, and extinction by 11,500 years. Smith, in contrast, places the timing of the extinction across a time period from 13,400 years ago to perhaps 12,500 years, supported in this by the decline in methane concentrations shown in the ice-core data. As a consequence of these differences, Gill would have the decline and collapse of the megafauna take place well before the beginning of the Younger Dryas cooling, a climatic change Smith suggests could itself be attributed in part to the extinction. Significantly (I think), the ice-core data in Smith’s piece seem to show rising methane concentrations for several hundred years after Gill would have the megafauna decline begin in earnest. More research and thinking may be in order.

Regardless, both of these analyses show intellectual virtuosity in their efforts to extract meaning from their data. I enjoyed them both. Smith’s analysis was especially hard to resist because in her acknowledgements she thanks the NPR news quiz show Wait Wait . . . Don’t Tell Me for “providing critical stimulus and motivation to pursue the project.”

As for the role of humans in the demise of the megafauna, we don’t have an answer, but many of us certainly are inclined to believe it wasn’t inconsequential. I turn back to Dethier and the wood mice in his summer bungalow.

The mice, like all other creatures in the house, lived in the territory of nature’s greatest predator, ignorant of the power of life and death that he held over them. Nothing is too large to be slain or too small to be obliterated. In the world at large we can kill them all, from the sperm whale to the virus. What was I to do with the mice? (p. 24)

Source of Images

Both images are from the Smithsonian Institution and were taken by Dane A. Penland in 1977. The mammoth picture can be found here and that of the mastodon here.

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