Dung fungus

…and the extinction of megafaunal species

Animal dung has many uses, both to its producers and to other organisms. I’ve written about some of these before: Beavers and many other rodents recycle some of their dung to extract more nutrients; terns sometimes defend their nests by dive-bombing intruders with messy fecal bombs; otters establish latrines that notify all comers that the place is occupied, and so on. Bears eat salmon and distribute the nutrients over the landscape, feeding the vegetation and some insects that become prey to birds; whale dung nourishes whole communities of marine invertebrates. When flying squirrels defecate after eating truffles, they distribute truffle spores for the next generation of truffles.

In summer, hikers find blue splats on boardwalks, indicating that a thrush (probably) has eaten some blueberries and defecated the seeds. Birds distribute seeds all over the place, usually just a few in any one spot. In contrast, bears sometimes have whole gut-loads of berries, and deposit thousands of seeds in a single place. If a junco or a mouse doesn’t come to the bear scat and tear it apart to extract seeds, there will be what we called a ‘bear garden’ of seedlings, germinating in dense stands in which the intense competition permits only a few individuals to survive [photo]. But those few may get a head start from the nutritious medium they grow in.

Recently, however, I have read about another use for animal dung—this one by paleontologists that are interested in the well-known extinction of large mammals such as mammoths and mastodons when the Pleistocene glaciers retreated, beginning about 20,000 years ago. By about 10,000 years ago, most of the North American mammals that weighed more than about one hundred pounds, and all of the species that weighed more than a ton or so, had disappeared (34 genera in all). There has been intensive, sometimes vitriolic, debate about the reasons for this geologically rapid loss of all these spectacular creatures. Maybe it was driven by climate change (either directly or through vegetation and habitat changes), or the crash-landing of a meteorite that set off massive fires, or human predation by the early humans that populated North America.

Fossilized dung (called coprolites) of ancient herbivores such as mammoths and mastodons may offer a means of sorting out the major causes of these extinctions. There is a fungus known by the tongue-tangling moniker of Sporormiella that requires passage through the digestive tract of an herbivore to complete its life cycle, producing spores in the dung of mammals and some birds that eat vegetation. The spores are known to occur in the gut contents and coprolites of mammoths; when they were deposited on the landscape, they got washed down into lakes, where they accumulated in the sediments—and researchers can find them. Along with the spores, pollen accumulated, allowing researchers to track changes in the composition of the surrounding vegetation. The material in the sediments can be dated quite accurately, allowing researchers to compare times of vegetation change with the demise of the large mammals as indicated by a drastically lowered level of Sporormiella spores.

Sediments from a lake in Indiana revealed that the megafaunal (and fungal spore) decline began about 14,800 years ago and took about a thousand years before final extinction. The principal changes of vegetation and habitat, from savanna to various woodlands, happened after the fungal spores diminished. Following the decline of the mammals, along with the vegetation changes, came an increase of fire and the frequency of charcoal in the deposits, reflecting the more wooded habitats. Thus, it is not reasonable to argue that vegetation changes and fire caused the megafaunal extinction, but it remains possible that the absence of large herbivores contributed to the vegetation changes.

All of that happened well before the arrival of extraterrestrial objects that struck the earth about 12,900 years ago, so that suggested cause can be discarded. What about the human factor? Could prehistorical hunters have eliminated the herds of large mammals, as they are known to have done in many other areas, especially islands? Maybe, but it would have been some very early people; the demise of the big mammals occurred before the arrival of the famous Clovis culture that was previously thought to be responsible.

So by eliminating some of the disputed factors involved in the megafaunal extinction, the little spore-bearing fungus has helped to clarify the arguments and focus attention on the remaining possibilities. That’s how science progresses.

It has been useful in other locations as well. A precipitous decline in Sporormiella spores in cave deposits in western North America also marked the demise of ancient herbivores, and its resurgence marked the arrival of domestic livestock on the landscape. In the Great Plains, the abundance of these spores in sediments provides a record of grazing intensity by bison, allowing researchers to interpret the past effects of bison on grasslands, as registered by datable fossil pollen deposits.

In New Zealand, the fungal spores have been found in the dung of native herbivorous birds such as the takahe and kakapo parrots, and the frequency of the spores tracks the decline of these bird populations. The spores also occurred in the dung of the now-extinct moas, large herbivorous birds that were exterminated by human settlers. When red deer were introduced to New Zealand, the spores tracked the spread of the deer populations across the countryside.

In Madagascar, giant lemurs, elephant birds, pygmy hippos, and giant tortoises all were virtually wiped out by human settlement, hunting, and habitat destruction. Sporormiella spores in sediment cores have been used to reconstruct the pre-human distribution of their habitats and plot the time-course of the animals’ demise in various regions of the island. As in other regions, the eventual introduction of domestic livestock brought a rise in the occurrence of the spores.