Eastern Range Extensions of Western Fauna on Xeric Limestone Prairies

I wrote an article a few years ago about roadrunners (Geococcyx californiannus). (See: https://markgelbart.wordpress.com/2016/08/05/pleistocene-roadrunners-geococcyx-californianus/ ) I noticed roadrunners ranged into Arkansas–a curious eastern range extension–and I wondered why.  A few weeks ago, a scientist sent me a box of science books, and I found the answer to my question in 1 of them.  Xeric limestone prairies in Arkansas and Missouri provide excellent habitat for 3 species of western fauna including roadrunners, collared lizards (Crotophytus collaris), and Texas brown tarantulas (Aphonopelm hentzi).  Xeric limestone prairies are openings in woodlands that are created naturally but may be maintained with or without human influence.  Dry shallow soils, not more than 3 feet deep, on a bed of limestone or dolomite, favor the growth of grass over trees. Little bluestem grass dominates xeric limestone prairies, but Indian grass, side oats gramma, and big bluestem also grow on them with the summer annual grass, poverty dropseed, on areas with even shallower soils.  Grazing and fire help maintain these openings, but the dry shallow soils high in calcium can remain open without these influences.  Nevertheless, in the absence of fire or grazing woody encroachment can occur.  Juniper, blackjack oak, and black hickory may invade some xeric limestone prairies.

 Xeric Limestone Prairie in West Virginia.

Roadrunners primarily are a western species, but they have an eastern range extension into Arkansas because they like limestone prairies.

Texas brown tarantulas also range into limestone prairies in Arkansas.

Dry limestone prairies provide habitat for an eastern range extension of the collared lizard.

Reptiles like to sun themselves on the limestone rocks scattered throughout these prairies, and this attracts roadrunners that prefer open areas with lots of the insects, reptiles, and rodents they prey upon.  Collared lizards are 1 of the reptiles that like to sun themselves on rocks, and they may become prey for roadrunners,  but they are also predators that hunt insects and other lizards in this habitat..  Collared lizards are cannibalistic.  Texas brown tarantulas, yet another western species extending their range east on limestone prairies, are large spiders reaching 6 inches in length with a 4 inch leg span.  They can weigh as much as a McDonald’s quarter-pounder.  Their venom is not harmful to humans unless the person is allergic.  But their fangs are large and can cause a painful bite that may get infected.

Other species of animals common on xeric limestone prairies in Arkansas include 6-lined race runners, southern coal skinks, fence lizards, slimy salamanders, leopard frogs, box turtles, Bachman’s sparrows, field sparrows, prairie warblers, cerulean warblers, Kentucky warblers, painted buntings, brown thrashers, hawk wasps, and numerous species of grasshoppers.

Xeric limestone prairies are not confined to Arkansas and Missouri but are also found in parts of West Virginia, Alabama, Tennessee, Georgia, Indiana, Illinois, Ohio, and Pennsylvania.  However, limestone prairies in those states don’t host as many species of western fauna as those in Arkansas and Missouri.  The Mississippi River must be too big an hurdle for them.

Reference:

Cartwright, Jennifer and William Wolfe

“Insular Ecosystems of the Southeastern United States: A Regional Synthesis of Support Biodiversity Conservation in a Changing Climate”

U.S. Geological Survey Professional Paper 1828 2016

New Study of the Seda-DNA in Hall’s Cave, Texas

A new study of seda-DNA and bone DNA from Hall’s Cave documents the changes over time in the plant and animal communities on the Edward’s Plateau in Texas.  Previously, scientists had collected and identified thousands of bones in Hall’s Cave from 56 species of mammals, 30 species of birds, 9 species of amphibians, 3 species of reptiles, and 2 species of fish.  The bones date from the Last Glacial Maximum (~20,000 years BP) to the early Holocene (~9,000 years BP).  The new study extracted DNA from the bones but in addition took samples of DNA from the sediment. (Scientists call DNA from sediment samples, “seda-DNA.”)  Sampling DNA from the sediment has the added advantage of detecting the presence of plant remains that were otherwise unidentifiable, and the presence of animals that perchance left no skeletal remains at all in the cave.  For example 2/36 bone samples were from cat but 7/10 sediment samples detected cat.  Jaguars and bobcats urinated, defecated, and shed hair in the cave but left no skeletal remains.  The seda-DNA samples detected 36 of the 56 species of mammals known to have  occurred in the cave from fossil evidence but they found an additional 7 species of mammals as well as additional species of birds not collected as fossils here, including ducks and geese.  They also determined which species of woodrat lived in the cave, an identification not really possible by just looking at the bones.

Deer mouse, cottontail rabbit, and eastern woodrat were the most common species of small mammals found in the cave since the Last Glacial Maximum, and these species occurred throughout all climate phases.  White-tailed deer and bison were the most common large mammals found in the cave and they too were found throughout all climate phases, though they became less abundant over time.  Hackberry and oak were the most common plant species found in the cave, and they were found throughout all climate phases.  Hackberry still grows near the entrance of the cave.  According to local pollen studies, pine was the most common tree growing on the Edward’s Plateau during the Ice Age, but it is absent from the cave.  Pine simply didn’t grow near the cave.

Hall’s Cave.

Location of Edward’s Plateau.

Dendrogram of species found via DNA sampling in Hall’s Cave.  From the below referenced study.

The study sheds light on the changes that occurred on the Edward’s Plateau since the Last Glacial Maximum.  During the height of the last Ice Age weather patterns differed from those of today–more precipitation fell on southwestern North America whereas southeastern North America was more arid.  As a result, the Edward’s Plateau hosted a prairie environment with trees found at scattered locations.  Soils were much thicker because dense grass regularly decayed.  Deeper soils were good environments for prairie dogs, 13-lined ground squirrels, pocket gophers, and marmots.  Common large mammals included camel, pronghorn, and flat-headed peccary that were preyed upon by saber-tooths, dire wolves, and giant short-faced bears.  Birds that preferred treeless plains–prairie chickens, upland sandpipers, horned larks–abounded here then.

The environment changed here about 15,000 years ago during the Boling/Alerod Interstadial when temperatures and precipitation increased.  The prairie converted to open woodland and forest with widely spaced oak, ash, juniper, walnut, mulberry, and hackberry trees.  Plenty of grass still grew between the trees…enough to support a population of horses.  Many of the open plains animals disappeared from the record here including the pronghorn, camel, and flat-headed peccary.  Black-tailed jackrabbits, northern grasshopper mice, and prairie chickens all left the area as well.  However, turkey, bobwhite quail, and barking tree frogs moved onto the Plateau because they liked the newly expanded tree and thicket habitats.

12,900 years ago, during the Younger Dryas cold phase, the climate suddenly became much colder and dryer.  Vegetation decreased and the region became desert-like.  Small and large mammal and plant diversity decreased.  Following the end of this cold phase, temperatures and precipitation increased, though rainfall didn’t increase to the levels of the LGM and Boling/Alerod Interstadial.  Soils of the Edward’s Plateau were still thinner than they were during the LGM and today the region is dominated by a plant community of live oak, juniper, and hackberry.  Plant and small animal diversity rebounded but large mammal diversity did not.  The authors of this paper suggest man is likely responsible.  Plant and small mammal ranges adjusted to climate change, and they disappear and re-appear in the seda-DNA samples over time.  If not for overhunting by man, the same should hold true for large mammals.  14 species of large mammals that lived on the Edward’s Plateau during the late Pleistocene are either extinct or extirpated from the region.

Plant and animal composition does not stay constant, and the study found some non-analogue components living side by side.  Today, white-tailed jack rabbits and barking tree frogs have ranges that do not come close to overlapping, but both species lived on the Edward’s Plateau during the Boling/Alerod Interstadial.    Bog lemmings and least weasels ranged much farther south then and co-occurred with species of more southerly affinities.  Animal and plant communities are dynamic and always changing.

Range of barking tree frog.

Range of white-tailed jack rabbit.  White tailed jack rabbits and barking tree frogs both lived on the Edward’s Plateau during a warm interstadial of the last Ice Age, indicating the existence of non-analogue environments dissimilar to any that occur today.

Reference:

Seersholm, F.; et al

“Rapid Range Shifts and Megafaunal Extinctions Associated with Late Pleistocene Climate Changes”

Nature Communications 2020

https://www.nature.com/articles/s41467-020-16502-3#:~:text=Large%2Dscale%20changes%20in%20global,impacted%20ecosystems%20across%20North%20America.&text=Instead%2C%20five%20extant%20and%20nine,the%20end%20of%20the%20Pleistocene.

Squirrel Monkeys (Saimiri oerstedii) and their Co-Horts

Several species of birds follow foraging squirrel monkeys through the jungle tree tops of Central and South America.  Troops of squirrel monkeys look for insects and fruit to eat, often overturning leaves and thrashing branches.  Their actions dislodge hiding insects and lizards, and this exposes them to predators.  Double-toothed kites (Harpagus bidentatus) perch on nearby branches, looking for insects or lizards forced to leave their hidden refuges.  Normally, their prey blends in with their surroundings, but the kites are attracted to the exposed prey forced to move.  Gray-headed tanagers (Eucometis pencillata) and tawny-winged woodcreepers (Dendrocineeb anabatina) also find insects this way.  These birds also follow white-throated capuchin monkeys, and they are among the many species that follow army ants. This behavior has occurred for tens of millions of years, ever since monkeys accidentally rafted over from Africa.

Squirrel Monkey

Gray-Headed Tanager

Tawny winged woodcreeper

Double-toothed kite

Army ant columns attract even more birds than troops of monkeys do.   Fleeing insects and small vertebrates become prey to a family of 18 species of antbirds known as the Thamnophilidae, as well as cuckoos, thrushes, and chats.  The bird-ant relationship is parasitic.  Studies determined army ants obtain 30% less food when they are followed by antbirds.  It’s not always a beneficial relationship for the birds either.  4 ant stings is all it takes to kill the small birds.

Reference:

Boinski, S.; Peter Scott

“Association of Birds with Monkeys in Costa Rica”

Biotropics 20 (2) 1988

The Co-Existence of Feral Hogs and Peccaries in the Americas

An old episode of Anthony Bourdain’s No Reservations sparked my interest in Brazil’s Pantanal region.  Bourdain searched eastern Brazil in vain for a rare species of fish he wanted to eat.  He was told it was still abundant in a remote part of western Brazil, so he purchased a charter flight to take him there and satisfy his culinary curiosity. The pilot flew the small aircraft through storms, and Bourdain and his crew weren’t sure they were going to make it.  Then, after arrival, Bourdain suffered the worst back pain he’d ever felt, and his producer fell ill with a tropical fever.  Nevertheless, they continued filming and I was impressed with the quantity and quality of wildlife.  It reminded me of 18th century descriptions of Kentucky and early 19th century accounts of Oklahoma.  On a boardwalk through a jungle Bourdain saw a red monkey that no one could identify.  Though there are riverine forests, most of the Pantanal consists of vast treeless plains, variously flooded here and there.  The indigenous people who sparsely populate the Pantanal use it as pastureland, and large herds of feral hogs and peccaries intermingle with cattle.  Caimans and capybaras abound in the flooded parts, and huge flocks of wading birds crowd the water holes.  Incidentally, Bourdain did get to sample the fish.

Map of Brazil’s pantanal.

The Brazilian Panatanal is 1 of the richest wildernesses left on earth.  It encompasses 75,000 square miles and includes at least 12 different types of ecosystems.  463 species of birds, 209 species of fish, and 236 species of mammals live in the region.  It has the healthiest population of jaguars in the world.  Peccaries and wild pigs are the most common ungulates.

Feral hog and vultures feeding upon a dead cow in Brazil’s pantanal region.

Collared peccary.

White lipped peccaries.

Peccaries and pigs are superficially similar in appearance, but they are separated by over 25 million years of evolution.  There are 2 species of peccaries native to the Pantanal–the aggressive white-lipped (Tayassu pecari) and the collared (Pecari tajacu).  Feral hogs (Sus scrofa) are not native to the Pantanal and were introduced hundreds of years ago.  Scientists who study the interrelationships between peccaries and pigs expect the latter to be detrimental to the former, but their studies find this is not true.  A recent study examined the consumption of 37 plant foods among the 3 species, and they found minimal dietary overlap.  Feral hogs favored grugru palm nuts (Acrocomia aculeata), collared peccaries preferred bay cedar (Guazuma sp.), and white-lipped peccaries liked the fruit of a plant in the coffee family (Alibertia sessilis).  All 3 species did feed upon the fruit of Astralea phaleratata, a type of palm.  Palm nuts taste coconut-like.  There was more overlap in diet between white-lipped peccaries and wild hogs than between collared peccaries and wild hogs.  Collared peccaries foraged at times when they could avoid pigs and white-lipped peccaries.

Chart showing diet overlap between the 3 species.  From the below referenced paper.

Palm nuts from the grugru palm are the favorite food of feral hogs in the Pantanal.

Alibertia sessilis fruit ( relative of coffee)  is the favorite food of the white-lipped peccary in this region.

The study determined bay cedar was the favorite food of the collared peccary.

During the Pleistocene new species of mammals periodically crossed the Bering Land Bridge and invaded the Americas, and vice versa.  Like pigs and peccaries, many of the co-existed.  Deer, bear, and big cats came from Eurasia.  Horses and camels went from the Americas to Eurasia.  Co-existence was not always permanent.  Felids from Eurasia outcompeted many species of canids, a group of carnivores originating in North America.  Deer from Eurasia outlasted 3-toed American horses.  The composition of mammals on both continents changed over time, and co-existence between species can be temporary or long lasting.

Reference:

Galetti, M. ; et al

“Diet Overlap and Foraging Activity Between Feral Pigs and Native Peccaries in the Pantanal”

PLOS ONE 2015

 

Hippos in Colombia

I like to watch the Neflix series, Narcos, but I’m always disappointed in the ending of the story arcs.  The series chronicles the real story of the Colombian drug cartel.  The drug kingpins ingeniously avoid capture for most of the season until DEA agents eventually capture or kill them in the season finale.  This disappoints me because I root for the cartel.  These supposed bad guys are providing a product that people want, and that makes them heroes in my opinion. It’s the law enforcement agents who aim to ruin everyone’s fun.  The first 2 seasons featured the infamous Pablo Escobar, a man elected to Colombia’s parliament but was not allowed to take his seat due to his massive illegal drug operation.  (Decades of Civil War could have been avoided, if he had been allowed to serve in the legislature.) Pablo gave tens of millions of pesos to poor peasants–more than the Colombian government did.  Unfortunately, during 1993, American DEA agents murdered him.

Pablo Escobar.  Why do U.S. taxpayers pay the government to murder citizens in 3rd world countries?

Pablo Escobar loved animals and had his own private zoo.  Following his death, hippos from his zoo escaped into the wild and they are flourishing.  From a founding population of 4 the hippos in Colombia have increased to between 80-100, and biologists predict it could reach between 400-500 or possibly even 5,000 by the year 2050.  Hippos begin bearing young at 3 years of age and can keep giving birth every 2-3 years until they die between the ages of 40-50.  Adult hippos, even in Africa, have almost no natural predators other than man.  (Lions, with difficulty, can kill juvenile hippos.)

Hippos in Colombia.

Some researchers fear hippos, as an invasive species not native to South America, may be detrimental to the Colombian environment.  Concentrations of hippos can foul non-flowing water holes with excessive manure that turns them anoxic.  The concentrated nitrogen leads to algal overgrowth, resulting in water with no dissolved oxygen in it–killing all the fish.  However, other scientists think hippos may be filling an ecological niche formerly held by extinct Pleistocene megafauna and maybe  in some ways beneficial to the environment.  Hippos in Colombia inhabit floodplain lakes, cattle tanks, and streams.  75% of their range has been man-modified into farmland including cow pastures and palm oil plantations while the balance consists of the original tropical forest. In some areas, especially where the water flows, hippo manure fertilizes aquatic plants and increases fish and insect populations.  This means more food for birds.  Hippo wallowing and movement breaks up thick vegetation, and their aquatic trails connect ponds, allowing fish to migrate and colonize new areas.  Their close cropping of streamside vegetation creates lawn-like habitats that attract some species of birds.

Hippos may be a modern day substitute for the Pleistocene haplomastodons that used to occur in this part of South America.  Haplomastodons, a species of gompothere, were probably semi-aquatic. North American mastodons definitely were semi-aquatic–they ate aquatic plants and had fur similar to that of otters and beavers.  Some notoungulates endemic to South America may have also been semi-aquatic, but not enough is known about them to determine this for sure.

Illustration of a haplomastodon.  Fossils of this species have been found in Colombia.

Hippos are in Colombia to stay.  Years ago, some government jerks with a stick up their ass decided to start eliminating the hippos, and they killed a male popular with the locals and tourists.  The outrage among most Colombians put a stop to this.  Animal rights groups sued, and there are now no plans to wipe them out.

References:

Subalusky, A; et al

“Potential Ecological and Socio-Economic Efforts of a Novel Megaherbivore Introduction: The Hippopotamus in Colombia.”

Oryx December 2019

Svenning, J., and Soren Faulby

“Pre-historic and Historic Baselines for Trophic Rewilding in the Neotropics”

Perspectives in Ecology and Conservation 15 (4) 2017

The Red River Raft

I came across this remarkable phenomenon while re-reading America as Seen by Its First Explorers by John Bakeless. About 900 years ago, a flood washed a bend of land and all the trees on it into the Red River.  (The red clay substrate makes the water red, hence the name.) The trees and sediment caused a logjam and subsequent floods kept washing more and more debris into it so that by 1830 the logjam, known as the Red River Raft, was an incredible 165 miles long.  The sediment was so deep trees, bushes, bamboo, and grass sprouted on the logjam.  Forests of cypress, cedar, cottonwood, willow, sycamore, oak, and persimmon grew over the river, and many pioneers didn’t even realize they were passing over a river when they crossed it. Some of the trees were 10 inches in diameter. The logjam forced the river to shift positions, often leaving behind fertile soil where the Caddo Indians planted crops.  The impenetrable thickets and unnavigable river protected the Caddo Indians from European settlers and kept them isolated from other hostile tribes.  The logjam created 5 major lakes as well, and these attracted huge flocks of waterfowl.  Natural channels wove their way through the logjam, but it was impossible to travel through it by boat.

Map of the Red River and some of its tributaries.

Photo of part of the Red River Raft after it reformed during the 1870s.

Henry Shreve (the city of Shreveport was named after him) began dismantling the Red River Raft during the 1830s.  He used a giant winch on a steam boat to remove logs from the bottom up and he dug channels through the raft itself.  He successfully cleared the Red River Raft but warned that it could reform, and a few decades later it did.  Eugene Woodruff dismantled the reformed raft during 1873, but this increased water flow through the Mississippi River and flooded parts of Louisiana.  The Army Corps of Engineers was forced to build the Old River Control Structure to prevent disastrous flooding downstream.

Boat with winches used to clear trees from the Red River Raft.

Log jams over 100 miles long must have occurred sporadically during the Pleistocene, providing unique habitat for land and aquatic flora and fauna.

Proxy Evidence for an Increase in Human Populations at a site in South Carolina Circa 12,838 Years BP

The Younger Dryas Impact Hypothesis, or YDIH for short, is a crank theory that has been thoroughly debunked.  The YDIH proposes a comet impact on a glacier in the Northern Hemisphere 12,900 years ago sparked continent wide fires, caused a sudden drastic climate reversal, and resulted in the extinction of Pleistocene megafauna.  The Younger Dryas was a climate stage that lasted for about 1500 years; and it was a sudden return to cold, arid Ice Age conditions, following thousands of years of warmer, wetter climate.  It’s named for a flower that flourished in Europe during this climate phase.

Dryas octopetalus likes open, cold, landscapes.

The YDIH is fatally flawed.  There is no known impact crater, dating to the proposed time of impact.  The Hiawatha Crater recently discovered in Greenland has been proposed as the possible impact crater, but the lack of a young ejecta blanket suggests it is millions rather than thousands of years old.  Its ejecta blanket has eroded away–a process that would take a very long time.  There is no evidence of continent wide fires.  Instead, the sites first noted by YDIH proponents to have evidence of the fires dated to many different ages both before and after 12,900 years BP, and the fires were most likely caused by lightning or humans.  Many of the other claimed impact indicators date to different ages.  Megafauna extinctions and extirpations also occurred at various ages–not all at 12,900 years BP.  Some of the so-called impact indicators are not necessarily diagnostic of an impact but can have terrestrial origins.  Other scientists looking for impact markers have not been able to replicate the results of the original YDIH studies.

The Younger Dryas was caused by well understood cyclical climate variations known as Heinrich Events.  12,900 years ago, an ice dam in Canada broke, leading to an enormous influx of cold fresh water into the North Atlantic.  This shut down thermohaline circulation, causing temperatures in the Northern Hemisphere to plunge.  This is the explanation for the Younger Dryas I accept.  Nevertheless, the ridiculous YDIH just will not die.  A paper advocating this bizarre almost pseudo-scientific belief was published as recently as October of this year.  Though I reject the YDIH, this paper had interesting data that I interpret differently from the authors.

Scientists who published this paper took a 1 meter core of sediment from White Pond in South Carolina. White Pond is a ~32,000 year old Carolina Bay.  Carolina Bays are bodies of water formed by wind and water erosion during dry climate phases of Ice Ages, and they are found throughout the Carolinas and Georgia.  The scientists dated the sediment layer by layer and looked for charcoal and sporomiella.  The abundance of charcoal indicates fire in the environment, and sporormiella is a fungus that grows on megafauna feces and is used as proxy evidence for the abundance of large mammal populations.  The scientists found charcoal amounts peaked at 12,838 years ago, indicating lots of fire on the landscape.  Megafauna populations declined to a low point 12,752 years ago.  The authors of this study think the fire was caused by the comet impact, and the resulting changes in the environment led to the local extirpation of the megafauna.

Location of White Pond where this study took place.  Image from the below reference.

My interpretation is different.  I think the fires were set by humans to improve habitat for megafauna. (Indians set fire to southeastern landscapes until Europeans removed them from the region.)  Then it took 86 years for humans to wipe out megafauna at this locality.  Nomadic hunters possibly moved away after eliminating most of the big game, allowing megafauna populations to rebound until 10,399 years BP when they disappear again from the region, this time permanently.

Graph from the below referenced paper.  Note how megafauna populations rebounded until 10,399 years ago–2500 years after the proposed comet impact.  This suggests the proposed comet impact could not have been a factor in their extinctions.

The authors of this study note there is an unconformity in the core dating to the Younger Dryas.  Normally, during wet climate cycles sediment builds up as vegetation dies and turns into soil.  But during dry phases when vegetation is sparse, exposed soil erodes and is blown away by wind, and there is little to no sedimentation.  If this is the case, there should be a gap rather than a continuous line in the graph.  I’m not sure how this impacts the conveyed data.  However, the data is interesting to me because I think it shows when humans arrived at this locality in significant numbers, and how long it took them to extirpate the megafauna.

Reference:

Moore. C; et. al.

“Sediment Core from White Pond, South Carolina contains a Platinum Anomaly, Pyrogenic Carbon Peak, and Coprophilous Decline at 12.8 Ka”

Scientific Reports October 2019

https://www.nature.com/articles/s41598-019-51552-8

 

Why the Western Hebrides are Mostly Treeless

During fall of 1773 Samuel Johnson and James Boswell traveled together on the western islands of Scotland, also known as the Hebrides.  They wrote separate accounts about this journey, and both are included in  a volume I’ve been reading for the past couple of weeks.  Samuel Johnson frequently noted and joked about the scarcity of trees in the region.  I wondered why a temperate locality with plenty of precipitation was mostly treeless, so I researched the natural history of western Scotland and learned there is a fascinating and complex ecological explanation.

Map of the Western Hebrides.  Johnson and Boswell traveled on the inner islands.

Scottish peat bog and heather.  These are also known as moors.

The first explanation I found on google seemed implausible and I was right.  Some claimed the Vikings stripped the island of trees to prevent rivals from using the wood to build boats.  This is an unlikely explanation because a people who lived by pillaging would not be keen on all the labor required in felling and removing all that lumber.  Moreover, trees would grow back within a generation.  I dug deeper and found a better, more scientifically sound explanation.

During the Ice Age glaciers completely covered Scotland, and there were no trees.  11,400 years ago, the glaciers retreated and a scrub forest of birch, willow, hazel, and rowan advanced over the tundra.  These scrub forests co-existed with peat bogs.  By 8500 years ago, oak, elm, and Scotch pine began to grow as well but were uncommon and local in distribution on the Hebrides.  From 9300 years BP-7900 years BP open woodlands of birch, willow, aspen, and hazel with plenty of grassy meadows between the trees prevailed.  But then, peat bogs began to expand while areas consisting of woodlands contracted so that by 2500 years ago peat bogs were the dominant environment on the Western Hebrides.  This predates the Viking invasion by 1500 years.  Instead, natural disturbance and soil chemistry shaped the landscape of the region.

Severe storm events about 8000 years ago felled the trees.  Without trees drinking up the water, the water table rose.  Tree deaths caused a positive feedback loop for peat bog expansion at the expense of woodlands.  The cool moist climate slows down evaporation, and the water just sits on impervious bed rock.  Water dissolves acids in the rock, further helping the growth of acid-loving sphagnum peat and heath but reducing the fertility in the soil required by trees. Peat bogs are dominated by sphagnum peat, sedges, and carnivorous plants, while heath grows on the better drained sites.  Trees just can’t grow in these conditions.

About 400 years ago humans cleared the remaining woodlands on the islands and converted them to agricultural use.  Now, an organization known as the Hebridean Ark hopes to re-establish at least some forests on the islands.  They’ve planted 100,000 trees including rowan, birch, willow, hazel, juniper, and aspen.  According to Johnson and Boswell, attempts to plant trees on the islands during the 18th century mostly failed.  Modern scientists may have a better idea of what they are doing, however.

References:

Fossitt

“Late Quaternary Vegetation History of the Western Isles of Scotland”

New Phytologist 132 1996

Johnson, S. and James Boswell

A Journey to the Western Islands of Scotland and The Journal of a Tour to the Hebrides

Penguin Classics 1984

Did Pleistocene Tapirs Shit in the Woods?

The answer is not as obvious as it might seem.  A new study found extant lowland tapirs (Tapirus terrestris) defecate more often in degraded woodlands than in deep forests.  They spend more time in disturbed forest openings that have been logged or burned because they feed upon young plants sprouting in the increased sunlight after canopy tree removal.  The study suggests tapirs facilitate forest regeneration by defecating viable seeds in their dung.  Scientists estimated the average tapir shits about 10,000 viable seeds per year in disturbed forests–3X more than in undisturbed forests.

Lowland tapir standing near a forest edge.  They actually shit more next to the woods than in it.

Mountain tapir (T. pinchaque).  This is the only species of extant tapir adapted to cooler climates.  The extinct species of tapir that formerly lived in southeastern North America was likely adapted to temperate climates, like this species.

The extinct Vero tapir (Tapirus veroensis) roamed across southeastern North America during the Pleistocene, and this species likely played an important role in forest regeneration then as well.  Herds of mammoths and mastodons stripped bark from trees, often killing them.  This was especially true during droughts when mammoths, normally grass-eaters, were forced to dine on the edible parts of trees.  Flocks of passenger pigeons also wiped out whole sections of forest.  Tornadoes and ice storms left large gaps in the forest canopy.  Tapirs attracted to these disturbed areas helped them regenerate.

Studies of extinct tapir bone chemistry indicate tapirs preferred to eat plants that occurred in deep forests.  However, they likely ate the young saplings that sprouted in gaps within forests.  Some of the plants tapirs may have consumed included pokeberry, persimmon, pawpaw, Osage orange, honey locust, wild squash, blueberry, composites, maple, and oak. These are plants that quickly colonize forest gaps.  And tapirs didn’t often shit in the woods.  Instead, they crapped on the edge of the woods or in open gaps within the woods.

Reference:

Paolucci, L.; C. Rattis, R. Pereira, and D. Silverio

“Lowland Tapirs Facilitate Seed Dispersal in Degraded Amazonian Forests”

Biotropica Feb. 2019

The Last Glacial Maximum in the Georgia Piedmont–My Abundant Oases Hypothesis

Scientists estimate average annual precipitation in Georgia was just 15 inches during the Last Glacial Maximum (~24,000-~19,000 calendar years BP).  I’ve kept a rain gauge in my backyard here in Augusta, Georgia for 17 years, and I’ve carefully recorded precipitation.  Average annual precipitation in my backyard was 47.8 inches over this time period.  The driest year in my records was 2010 when just 29.5 inches of precipitation fell, and the wettest year according to my records was 2017 when 69.8 inches of precipitation fell.  The difference between present day precipitation totals and LGM precipitation suggests the floral composition must’ve been considerably different then, and many modern day species of plants must’ve retreated to small refugia.  However, there is no evidence of this, and in fact a couple lines of evidence indicate species abundant today were just as widespread during the LGM.

Genetic evidence shows that species common in deciduous forests occurred all the way to the glacial boundary during the Ice Age, despite pollen records indicating spruce forests dominated the landscape from north Georgia to the Ice Sheet.  DNA studies of eastern chipmunks, red and sugar maple, shagbark hickory, beech, and yellow birch suggest they all ranged right up to the edge of the glacier which expanded all the way to southern Ohio.  The ranges of at least 17 species of trees including persimmon, sweet gum, and river birch still reach their northern limits at the ghost boundary of where the massive glacier advanced.  (See: https://markgelbart.wordpress.com/2017/03/30/the-ghost-boundary-of-the-last-glacial-maximum-ice-margin/ ) Although genetic evidence reveals discontinuities between populations of species, these appear to be caused by geographical barriers such as major rivers and the Appalachian Mountains.  The genetic evidence suggests multiple diffuse refuges during the LGM for species that are common today.  It seems like a paradox, but I have an hypothesis that can explain this for the Georgia piedmont.  I call it my Abundant Oases Hypothesis, and it can probably be applied north of Georgia as well.

First, temperatures were much cooler during the LGM, so an average annual precipitation total of 15 inches would have gone much further then.  Evapotranspiration rates were much lower, especially during summer.  15 inches of rain and snow may have been the equivalent of 25 inches in today’s climate–similar to an average drought year today.  Second, Georgia’s piedmont soils are mostly clay, and they don’t drain as well as sandy soils.  Water was held longer near the surface when it did receive precipitation.  Third, the hilly terrain of the piedmont was a factor in contributing to oases where flora and fauna could flourish.  Rainwater flowed down hills (this is known as colluvial flow), so the bottom areas between them hosted more plant life.  Although most creeks dried up, there were plenty of areas at the bottom of hills where the water table came close enough to the surface to form intermittent springs.  Beavers dammed springs, making them deeper and helping them hold water longer.  Fourth, major rivers didn’t dry out completely and provided plenty of mesic refuge for species that could expand into oases during phases when annual precipitation increased.

Topographical map of the Piedmont National Wildlife Refuge in Jones County, Georgia.  Note the hilly terrain and abundant creeks.  During the LGM colluvial slope flow, intermittent springs, and lower temperatures helped reduce the negative effect of droughts.

My Abundant Oases Hypothesis is speculative.  There are no pollen studies dating to the LGM from piedmont Georgia.  There is 1 site in Winder, Georgia known as Nodoroc where pollen was collected that dates to just before the LGM when temperatures were warmer and precipitation was higher.  Oak and pine were the dominant species, and they co-occurred with hickory, fir, and spruce.  Beech, chestnut, birch, and maple were present.  The shrub layer consisted of hazelnut and blueberry/rhododendron.  There is no sediment dating to between 28,000 years BP-5000 years BP, suggesting land was eroding rather than accumulating sediment during this time period.

This is how I envision the Georgia piedmont landscape during the LGM.  The tops of hills were covered with widely spaced shortleaf pine and post oak.  These were slow growing and ancient because reduced CO2 levels in the atmosphere caused slow plant growth.  Grass, herbs, thorny patches with prickly pear cactus, exposed boulders and rocks, and bare earth occurred between the trees.  Following rare rain events, the ground burst into flower, but most of the year it looked dull and brown.  Deep gullies, red from exposed clay, were common on the hillsides–a result of erosion that commonly occurred due to storms and a lack of topsoil.  The bottoms of hills stayed green longer, and in some low areas hosted springs surrounded by marshy vegetation and deciduous woods of oak, maple, and beech.  These are the oases of my hypothesis.  Megafauna game trails connected these oases with each other, and the river systems where even more deciduous woodlands existed.  This system of oases is what supported the continued existence of species that were able to expand when climatic conditions improved.

Reference:

Soltis, et al.

“Comparative Phylogeography of Unclaciated North America”

Molecular Ecology 2006