Posts Tagged ‘American Midland Naturalist’

Pleistocene Bison Wallows

September 20, 2011

Prairie Violet (Viola pedatifida).  Laura Ingalls’s little toddler sister wandered off and was found in an old “buffalo” wallow overgrown with violets–probably this species.

I admire the clear and concise prose of Laura Ingalls Wilder, author of the famous Little House series for children.  Her stories offer a glimpse of life on the frontier before most of the conveniences we think we can’t live without were even invented.  In one of her books (I believe By the Shores of Silver Lake) she recounts an incident that probably was based on a true event.  (Her stories were a blend of fiction and nonfiction.)  This traumatic adventure occurred when Laura was in her early teens.  They were living in a house in the middle of the short grass prairie in South Dakota.  They were miles from any neighbors.  Laura was helping her mother and father plant small trees to form a windbreak for the house.  While toiling, they didn’t notice Laura’s baby sister, Carrie, had wandered out of sight.  After congratulating themselves for doing such a good job on the hedgerow, they experienced the bad shock of realizing the toddler was missing.  The whole family scattered in different directions searching in panic.  I endured that awful feeling once when my daughter got on the wrong schoolbus on the first day of middle school.  That was the second worst day of my life even though her whereabouts were unknown for only half an hour.  Laura eventually found her sister some distance away sitting in an old bison wallow overgrown with violets.  Carrie was amongst the flowers, pulling them, and saying “sweet, sweet.”

It occurred to me that bison wallowing must have had a significant impact on the environment, altering the habitat so that it favored some species of plants and inhibited others.  However, I didn’t think anybody had ever studied such an esoteric subject.  Happily, I was wrong and found a recent study about bison wallowing published in The American Midland Naturalist.

Photo of a bison wallowing from google images.

Bison create wallows when they repeatedly roll and kick their legs in the same area to rid themselves of parasites, to scratch insect bites, and to give themselves a dust bath which may help stymie ticks, fleas, and flies.  The wallowing forms circular depressions with compacted soil where water is retained longer than on adjacent prairie land.  Some wallows are larger than 2 acres.  There were millions of wallows across North America before the extirpation of the bison.  Most have been plowed under but some still exist and are known as relic wallows.  Cattle do not wallow, and accordingly, habitats created by wallows are limited to the few area where bison still exist.

Scientists studied plant diversity at the Konza Prairie Biological Station in Kansas which is owned by the Nature Conservancy and managed by Kansas State.  They compared the kinds of plant species found in active and inactive wallows and adjacent prairie.  They counted a total of 153 species of plants of which 10 were found only in adjacent prairie and 25 were found only in wallows.  Overall though, they found lower species diversity in active and relic wallows than in adjacent prairie.  Bison rolling, compacted soil, and excess moisture is evidentally intolerable to many species of plants.  Here’s a list of the 5 most common species of plants found in each type of habitat.

Inside Wallows……………Adjacent Prairie

1. Western Ragweed………..Big Bluestem

2. Sedges……………………….Heath Aster

3. Canada Bluegrass………..Sedges

4. Ridgeseed Spurge………..Western Ragweed

5. Hoary verbena……………Scribner’s rosette grass

Plant species diversity is also greater in grazed areas than in ungrazed areas.

During the Pleistocene there must have been tens of thousands of bison wallows, some abandoned, some active across the southeastern coastal plain and into the piedmont.  As I’ve noted in earlier blog entries (See https://markgelbart.wordpress.com/2010/06/11/were-there-three-species-of-bovine-roaming-southeastern-north-america-during-the-late-pleistocene/ and https://markgelbart.wordpress.com/2011/08/01/a-probable-pre-clovis-bison-butcher-site-in-washington/), I suspect there were 2 species of bison living in the region then.  The two species may have hybridized in some areas, and the long-horned bison is probably ancestral to the shorter-horned variety.  Long-horned bison (Bison latifrons) and northern bison (Bison antiquus) fossils have been found throughout the south, and they overlap geographically and temporally.  Long-horned bison may have been more of an open forest species, while its cousin may have preferred more open grassland.  Their wallowing behavior created habitats favorable for some species of plants and detrimental to others.  The common plants found inside and outside of Pleistocene wallows in the southeast undoubtedly differed from the list of plants found in the modern tall grass praire.  The composition of plants in Pleistocene bison wallows will likely remain a mystery.

Reference:

McMillan, Brock; Kent Pfeiffer, and Donald Kaufman

“Vegetation Response to Animal Generated Disturbance (Bison Wallows) in Tall Grass Prairie”

The American Midland Naturalist Jan 2011

How Unusually Cool Ice Age Summers Probably Shaped Periodical Cicada (Magicicada) Evolution

May 3, 2011

It sounds like everybody’s burglar alarm is blaring in Evans, Georgia.  But the noise doesn’t originate from annoying, malfunctioning security systems.  Instead, the 13 year periodical cicadas have emerged.  For over a decade these insects have lived a foot underground, well below the frost line, but now they’re ready to mate–a frantic affair that takes place within a timespan of 3-4 weeks.  Almost their entire lives, they’ve lived as nymphs, surviving on the xylem fluids of deciduous trees.  The urge to mate causes them to dig tunnels to the surface which they crawl through.  Sometimes they continue to crawl, making it halfway up a wall or a tree trunk before the winged adult bursts through the back of the shell of its thorax.  The holes from which they’ve emerged are visible, their molted shells scattered under foot like discarded shrimp exoskeletons at a Cajun seafood boil.

Photo from google images of a periodical cicada.

The unusually long period between emergences among the 7 species of periodical cicadas in the genus Magicicada puzzles scientists who hypothesize about its evolution.  Magicicada emergences in large numbers are obviously a defense mechanism known as predator satiation.  Like passenger pigeons, they occur in such a high population that they overwhelm the ability of predators to consume them.  Unlike other types of cicadas which are strong, fast fliers, periodical cicadas are clumsy and slow, but so many appear at once that predators are unable to consume most of them before they have a chance to mate and lay eggs.  Some scientists think their emergences every 13 or 17 years (depending on the species) is a way to keep predators from increasing their own populations after exploiting them as a food source, thus avoiding a cycle when predator numbers eventually escalate enough to decimate cicada numbers.  This seems an unlikely explanation to me for 2 reasons: cicadas are only available for a few weeks a year which is not enough time to have a significant long term impact on predator populations.  More bird nestlings may survive at first due to the abundance of cicadas, but then for the rest of the year, they must adapt to the normal supply of food.  Moreover, the average wild bird only lives 2 years–far shorter than 13 or 17 years.  Cicadas could avoid upswings in predator populations with much shorter periods between emergences.  Other scientists believe the odd high prime numbers emergences can be explained by a combination of predator cycle avoidance and interspecific competition among nymphs.  Although several entymologists have derived statistical models supporting this theory, I think the paleoclimate explanation proposed by R.T. Cox, C.E. Carlton, and independently by Yoshimura is more plausible.

Periodical cicadas depend specifically upon deciduous forests.  During the coldest stages of Ice Ages, deciduous forests north of the southern Appalachians were rare relics outnumbered by other environments such as spruce forests and prairies.  The bulk of deciduous forests then occurred south of the Appalachian mountains.  Even here, summer temperatures occasionally were too cold for cicadas in the Magicicada family.  They require temperatures above 68 degrees F for a period of 3-4 weeks for flight and mating.  Drs. Cox and Carlton assumed that during the coldest stadials (which lasted on average 1500 years) 1 in 50 summers failed to reach this temperature, and cicada reproduction failed.  Using a statistical formula, they estimated that over a 1500 year stadial, cicadas emerging every 6 years had a 4% chance of avoiding unusually cool summers; cicadas emerging every 11 years had a 51% chance of avoiding unusually cool summers; but cicadas emerging every 17 years had a 96% chance of avoiding unusually cool summers.  Cicadas emerging after shorter periods were eventually eliminated from the gene pool, while those with genes for longer cycles became dominant.

Map of Magicicada ranges from the below referenced paper.  The distribution of 13 and 17 year periodical cicadas supports the paleoclimatic explanation for their high prime number emergences.

17 year cicada species tend to live north of 13 year cicada species, even though the shorter cycle is a dominant gene.  Summers too cool for breeding would’ve occurred more frequently in the norther parts of their range, so those with 17 year cycles would’ve had a greater chance of avoiding them than those with the 13 year cycle.

Reference:

Cox, R.T. and C.E. Carlton

“Paleoclimatic influence in the evolution of periodical cicadas (Insects: Homiptra:Cicidae: Magicicada spp.)”

The American Midland Naturalist 120: 183-193 1988

Notes on my observations of periodical cicadas

–Periodical cicadas are slow.  I was able to catch one by simply picking it off the ground after if fell in flight and landed on its back.

–Birds are feasting on them.  Crows are catching the nymphs as soon as they crawl to the top of their tunnels.  I also saw a Canadian goose nab one that fell in a small lake.

–Wow! They are loud.

–They must be mole food during their nymph stage.

–Evans has become heavily developed.  Cicada habitat has been greatly reduced and covered with blacktop parking lots from which they can never emerge and escape.  I wonder if this is another creature that survived for millions of years, until the actions of Homo sapiens eventually renders them extinct.