Posts Tagged ‘prairie dogs’

The Unraveling of Pleistocene Ecosystems in Southeastern North America

July 23, 2017

The entrance of humans into southeastern North America and the subsequent extinction of megafauna species had a profound effect on the region’s ecosystems.  Evidence from other regions suggests megafauna populations began to collapse during the Boling/Alerod interstadial, a warm phase of climate following the Glacial Maximum that lasted from ~15,000 years BP-~12,900 years BP.  Megafauna populations should have been increasing during this climate phase because the increase in precipitation and warmer temperatures fostered greater plant growth and species composition diversity.  However, the improved climatic conditions benefitted people and in turn the presence of humans is almost always disastrous for wildlife.  When the men failed on a day’s hunting trip, they still had plenty of food, thanks to the women who gathered wild plant foods and trapped fish, turtles, and birds in their nets all day.  The increased precipitation also caused an expansion of wetland habitat and aquatic resources. Under these environmental conditions human populations rose rapidly, much to the detriment of large animals.  Within a few thousand years humans wiped out almost all of the large mammal species in the south, so for the first time since a few million years after the KT impact that knocked off the dinosaurs, the region was nearly devoid of large megafauna populations.

Few scientists have studied the details of megafauna extinctions in the south and how they impacted the ecosystem.  One study (Smith, F.A. 2015 referenced below)  looked at all the Pleistocene sub-fossils found in Hall’s Cave, Texas–a site with well dated chronology and a continuous record of mammals from 22,000 years BP to the present.  A statistical analysis suggests many Pleistocene mammals were positively associated with other species.  This means complex interrelationships between species probably existed, though we have no way of knowing what they were.  This blog entry is my attempt, as an educated layman, to imagine what occurred as Pleistocene ecosystems unraveled.

I propose the first animals to be overhunted to extinction in the southeast were ground sloths, pampatheres, glyptodonts, and giant tortoises.  Ground sloths and pampatheres were important keystone species because they constructed extensive deep burrow systems.  (I also believe giant tortoises, like their extant relative the gopher tortoises, dug burrows.  Unfortunately, a paleontologist labeled giant tortoises as “non-burrowing” in a 1950’s era paper, and no qualified vertebrate zoologist has ever challenged his assumption.  I am unaware of any study of Hesperotestudo   anatomy that determined whether they could burrow or not.  If they couldn’t dig their own burrows, they must have been dependent upon ground sloth and pampathere burrows.) These burrow systems provided refuge, not only for ground sloths and pampatheres (a kind of giant armadillo) but many other species as well.  They served as shelter for hundreds of species of small vertebrates and invertebrates, much like the tunnels of prairie dogs and gopher tortoises do today.  I hypothesize ground sloths were immune to snake bite venom because their burrows likely served as winter dens for rattlensakes.  Large predators, bears, and peccaries probably used abandoned ground sloth burrows for shelter.  Ground sloths excavated large quantities of subsoil too and when mixed with topsoil these mounds supported unique plant communities.  It seems likely the plants that grew around ground sloth burrows were edible for ground sloths and many other herbivores.  I recently watched an episode of Expeditions with Patrick McMillan that showed a prairie dog town surrounded by mallow flowers.  Fossil coprolites indicate globe mallow was a favorite food of 1 species of ground sloth.

I doubt ground sloths lived in dense colonies like prairie dogs.  I guess there was normally 1 active ground sloth burrow every 3-5 square miles.  Intraspecific competition, if it existed in ground sloths, may have limited population density.  The burrows helped ground sloths survive climatic extremes, and the large powerful animals were able to hold their own against predators, but they were defenseless against men with projectile weapons.  They were the easiest of all the megafauna for men to kill–the most meat from the least effort–and therefore were the first to be hunted into extinction.  The increased frequency of fires set by humans may have also contributed to their extinction.  Instinct told them to take refuge in their burrows during lightning storms when the flashes of electricity ignited natural fires, but fires set by humans could overcome them at any time of year during sunny conditions.  Ground sloths could not outrun fires.  All the animals and plants that benefitted or completely depended upon ground sloths, pampatheres, and giant tortoises had to re-adapt to their absence.

Image result for prairie dog colony

Photo of a prairie dog colony.  Though giant ground sloths and giant tortoises probably didn’t live in dense colonies, they also dug burrows throughout their range and were important keystone species that played as an important role in ecosystems as prairie dogs do today.  They were likely the first organisms to be wiped out by humans in North America.

The largest most dangerous predators were the next group of animals to be eliminated from the landscape.  Giant short-faced bears, giant lions, and fanged cats had no fear of people, yet they were no match for groups of men with projectile weapons.  Many of these animals were killed while contesting carcasses with people.  The decline of large carnivores likely occurred simultaneously with the extermination of mammoths and mastodons.  During the Boling/Alerod interstadial mastodon populations should have been increasing because this semi-aquatic species benefitted from the expansion of wetlands.  But humans drove mammoths and mastodons away from their favorite foraging grounds and watering holes, and they disrupted their migration routes.  The reproductive rates were too slow to keep up with human hunting pressure.

African elephants influence their environment today.  In Kruger National Park elephants uproot 1500 mature trees annually.  They convert forest into open savannah.  Mammoths and mastodons likely kept environments in southeastern North America in a constant state of flux.  The environment was patchy with various stages of forest succession located adjacent to other stages–meadow next to shrubby thickets alongside 2nd growth and mature woodland.  There were groves of large seeded fruits such as Osage orange, pawpaw, honey locust, and persimmon that had been planted in the excrement of the proboscideans.  After mammoths  and mastodons were eliminated the patchy woodland and grassland transformed into a monolithic mature forest that supported few large mammals.  The loss of patchy habitat hurt populations of llama, peccary, and tapir.  Even some small animals disappeared from the region, as their favored micro-environment converted to deep forest.

Next came the slaughter of horses and bison.  With mammoths and mastodons gone, the final populations of horses in southeastern North America were hemmed into smaller grasslands because forests expanded now that trees weren’t being uprooted with the same frequency.  This made them more vulnerable to human hunters.  Bison benefitted from their co-existence with horses.  Bison feed on the nutritious new growth spurred by horses grazing tall grass.  But the elimination of horses also meant bison, those that avoided human hunters, had a hard time surviving in the region.

Image result for mustangs and bison

Pleistocene horses may have improved the quality of grazing for bison.

The loss of megafauna spelled the end of the line for a long list of commensal species including condors, and extinct species of vultures, eagles,  storks, and cowbirds.  There wasn’t enough prey for dire wolves and even extinct subspecies of jaguars and cougars.  Genetic evidence suggests all North American cougars descend from a population originating in South America 10,000 years ago.  Eventually, cougars recolonized the region, probably from a population that evolved a tendency to avoid man and prey on small game as well as deer.  And after European diseases decimated Indian populations, bison, and horses introduced by the Spanish recolonized the region as well.

References:

Malhi, Y. et. al.

“Megafauna and Ecosystem Function from the Pleistocene to the Anthropocene”

PNAS 113 (4) 2015

Smith, F.A. et. al.

“Unraveling the Consequences of the Terminal Pleistocene Megafauna Extinction on Mammal Community Assembly”

Ecography 39 2015