Cotton can self-pollinate or cross-pollinate with the help of bees that transfer pollen between the flowers of different plants. If conditions are favorable, the pollen grain will germinate after it is stuck to the stigma and form a pollen tube, which extends through the tissues of the pistil. Once the pollen tube reaches the ovary, fertilization can occur.

This image was taken with an Environmental Scanning Electron Microscope (ESEM).

All in all, the period since the ice age abated about 15,000 years ago has been pretty interesting. Melting ice raised the oceans, flooding the Bering Strait land bridge across which the Americas were populated. Temperatures rose around the globe, leading to the invention of cities, armies, writing and bacon.

Here’s an enduring question. Why were the giant mammals that made the Americas more zoologically diverse than Africa all exterminated within a few thousand years after the big melt-down? Bye-bye beavers as big as black bears, giant sloths, saber-toothed cats, and the elephant-like mastodon.

[svgallery name="mastadon"]

As Australian paleontologist Christopher Johnson wrote in Science this week, all 10 species of mammals weighing more than a ton had gone extinct in North America by 10,000 years ago.

Why?

Many theories are proposed for the sudden disappearance: An impact of a comet or asteroid around 12,900 years ago. Rapid ecological changes that accompanied the warming. Widespread wildfires. And hunting – the “overkill” hypothesis. Although similar disappearances roughly coincided with the arrival of people in Europe, Eurasia and Australia, and hunger is certainly the ultimate motivation, did people actually lay waste to entire groups of large mammals?

The debate may seem academic, and it has been one of the most brutal and tenacious debates in academia.

Reading the dung calendar

Now we get some solid evidence that the extinction of the mastodon and other large herbivores closely followed the arrival of humans in North America, and that it preceded a pervasive change in type and prevalence of trees.

The new evidence, contained in research by Jacquelyn Gill and Jack Williams of the University of Wisconsin-Madison, and colleagues, was published in Science this week, and although it does not prove the overkill hypothesis, it does usher a new type of evidence into the debate: spores of fungi that grow in herbivore dung.

Between 14,800 and approximately 13,700 years ago, fungal spores of the genus Sporormiella declined by up to 98 percent in sediments found in lakes in Indiana and New York State.

Mastodons eat black ash trees as the last ice age begins to abate. Image courtesy Barry Roal Carlsen, University of Wisconsin-Madison.

For decades, students of ancient ecology have been poking through pollen in sediments to see what plants were alive when the sediment was deposited. Pollen are durable structures, but it turns out that Sporormeilla spores are equally tough, and if you have the patience (Why Filers immediately excuse ourselves at this point!) counting spores provides a good gauge of the number of herbivores.

Because the same sample also contains pollen and charcoal, it’s also possible to document the co-existing plant community, and get an idea of the extent of wildfires.

Fungi are a new addition to the paleoecologist’s toolkit, says Gill, first author of the paper, and a graduate student in Williams’s lab. “Only recently have fungal spores been getting any attention; we used to basically ignore them if we counted them at all, but now we realize they are a good source of information about early conditions.”

Being skeptics, we asked whether the decline could simply represent a change in conditions that was less conducive to preservation, but Gill says not. “If so, you would expect other proxies to show similar transitions. Since the same sediment that contains the spores also contains pollen, we’d expect to see pollen disappear, but we don’t.”

The dating game

Having a firm date for the decline of mastodons and other large herbivores is mainly helpful for eliminating some possible explanations, says Gill. The decline started almost 2,000 years before the putative impact of a comet or asteroid. And a change in climate apparently did not cause a broad habitat loss, Gill adds. “The extinction started before the habitat changed; the vegetation is relatively stable until after the extinctions began. We do have evidence of warming taking place, but if climate change is causing the extinctions, it’s not through a loss of food.”

A major ecological change did follow the elimination of large mammals, however, as documented by pollen representing a new assembly of trees, including ash and ironwood, which had probably been held in check by hungry herbivores, growing along with less nutritious conifers like spruce and larch. Once the grazers left, these trees began to dominate the landscape — and then became fuel for wildfires that burdened younger sediment with charcoal.

Graduate student Jacquelyn Gill holds a sediment jar with a scrap of charcoal being prepared for carbon dating. Photo: The Why Files

Although the sexy “overhunting” hypothesis is sure to get a boost from the Science paper, Gill says one study hardly proves the case. And as Johnson notes in his commentary in Science, the Clovis people who spread across much of North America arrived more than 1,000 years after the decline began. Evidence for earlier North American populations is sketchy and scarce, but it is arising, Johnson added.

A second focus of the Gill paper may be equally important: the effect, rather than the cause, of the extinctions. “What happens when half of the species larger than a German shepherd go extinct in North America?” Gill asks. “Elephants eat 300 pounds of food a day, and when animals like the mastodon are rapidly taken out, you would think the landscape would notice, but that has been absent from the discussion. People were underestimating the power of these fungal spores to tell about the local presence of animals and vegetation.”

– David J. Tenenbaum

Pollination is the transfer of pollen (the plant equivalent of sperm) to eggs. Some plants rely on wind or gravity, but many require animals to do the transportation. The smell of the flower alerts pollinators that the plant is ready to be pollinated, and when the animals arrive to collect pollen and/or nectar, pollen gets transferred.

Plants and pollinators often display a long history of mutual evolution, Iltis adds. When Charles Darwin saw a flower with a foot-long tube during the 1800s, he predicted the existence of a moth with an equally long “tongue” that could reach the female parts at the bottom of the tube. This moth was discovered more than a century later!

The minty, oily or sharp smells produced when you crush a leaf or stem play a defensive role, Iltis says. These smells come from chemicals that are often toxic to animals, and thus serve as a one-two punch: they smell (and taste) terrible, and then they make you sick if you ignore your senses and take a bite.

During the long struggle for existence, Iltis says, evolution has shaped every part of plants – including their chemical composition. But pollination is a troublesome subject: many crops are under threat as honeybees succumb to “colony collapse disorder.” Although the cause is unknown, environmental disturbance likely plays a role, Iltis says.

Ragweed pollen as seen under a microscope

Tissue, please… In honor of the sneezin’ season, this CSI is common ragweed pollen as seen under a microscope. Ragweed pollen is the principal cause of hay fever and can also trigger asthma. But for all the itchy throats and watery eyes, this tough little plant is just trying to survive.

The common ragweed is an annual that grows to about 3.5 feet tall, has hairy stems, divided leaves, and simple greenish-yellow flowers. Arguably not much to look at, the ragweed does not rely on the help of flying creatures to transfer pollen from plant to plant. Thus it lacks the bright, smelly flowers that attract the birds and bees.

The ragweed relies on a process called wind pollination to procreate. The light, powdery pollen forms in the anther, or male part, and breezes through the air to the pistil, or female part, where fertilization occurs. Structurally, the pollen grain is multi-layered. The outer layer, or exine, not only protects the center nuclei responsible for fertilization, but also makes the grain virtually indestructible. A single ragweed plant can generate a million grains of pollen each day during its peak season, mainly late summer and early fall. Not all pollen reaches its intended destination in the pistil and instead lands in the human nose and many geologic sediments. In fact, grains of ragweed pollen have been found 400 miles out to sea and 2 miles high in the air. By studying geologic pollen sediments scientists have been able to make remarkable discoveries about the origin and evolution of plant life.

Pollen image courtesy of NIAID.

Triprojectus unicus

This CSI is a picture of a pollen grain from an extinct group known as triprojectates. This particular beast, Triprojectus unicus, was common about 65 million years ago in the Late Cretaceous, when the dinosaurs still ruled the roost. The nasty-looking recurved spines may have allowed the pollen grains to hitch rides on passing insects. Pollen survives in the fossil record because their external walls were, and are, composed of sporopollenin, a rugged organic polymer that can endure the rigors of the environment and the process of fossilization. It is from little gems like these that scientists learn about the flora — and the larger environment — of the age of the dinosaurs.

Special thanks to Andrew MacRae of the University of Calgary who has a nifty gallery of pollen from the past.