Archive for the ‘Ecology’ Category

A 9 Mile Long Dogwood and Magnolia Grove in Alabama (circa 1775)

July 12, 2018

When William Bartram traveled through the south from 1773-1776 he observed many environments that today are either extinct or very rare.  In southern Alabama just east of Mobile he journeyed through a grove of dogwoods and magnolias that was 9 miles long.  This is how he described it.

We now enter a very remarkable grove of Dog wood trees (Cornus florida) which continuing nine or ten miles unalterable, except here and there a towering Magnolia grandifloria; the land on which they stand is an exact level; the surface a shallow, loose, black mould, on a stratum of stiff, yellowish clay; these trees were about twelve feet high, spreading horizontally; their limbs meeting and interlocking with each other, formed one vast, shady, cool grove, so dense and humid as to exclude the sun beams at noon-day.  This admirable grove by the way of eminence has acquired the name of the Dog woods.

The existence of an almost pure stand of dogwoods this large has long puzzled me.  Dogwood is a common understory tree throughout the south but I’m unaware of any natural location where it largely dominates as a canopy species.  Recently, I reread the passage, and the next morning I had a eureka moment–I believe passenger pigeon flocks created this unusually large stand of dominant dogwood trees.  The dogwood grove Bartram observed was likely the site of a massive passenger pigeon roost 50-100 years before he traveled through it.  Flocks of migrating passenger pigeons (Ectopistes migratorius) formerly caused eclipses of the sun lasting for 6 hours, and when they roosted their colonies would so damage the forest it would appear as if a tornado had struck.  The weight of the roosting birds would bust limbs and even crack enormous tree trunks in half.  The dung overfertilized the trees, often killing all of them.  These enormous colonies covered many square miles.  This explains the extent of Bartram’s dogwood grove.

Dogwood trees were already common in the understory of the forest, and the fruit ripens in the fall…exactly when passenger pigeons migrated to the south after nesting in the midwestern states.  It seems likely passenger pigeons fed on the dogwood and magnolia berries in the surrounding forest, and deposited the still viable seeds under their roosts in their dung.  Dogwood trees sprouted in the nutrient rich soil and thrived in the open sunlight created when the overstory trees were destroyed by the passenger pigeons.

Map of Alabama highlighting Conecuh County

Bartram’s dogwood grove was probably located in Conecuh County, Alabama.

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Passenger pigeon migrations eclipsed the sun.

Bartram describes adjacent open plains that also resemble a landscape recovering from a passenger pigeon invasion.  Most of the 70 mile forest surrounding the dogwood grove consisted of oak, hickory, black walnut, elm, sourwood, sweetgum, beech, scarlet maple, buckeye, and black locust with an understory of dogwood, crabapple, and plum.  (Chestnut and pine grew on rocky hills.)  But some pockets of treeless plains within the forest and alongside the dogwood grove were composed of shrubs covered in grape vines.  The shrubs included silver bud, buckeye, bignonia, azalea, and honeysuckle.

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Dogwood berries.  Passenger pigeons ate them.  They taste bittersweet to me.

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Flowering dogwood.  Bartram’s dogwood grove must have been beautiful during early March when the tree blooms.

I’ve always wondered how forests recovered following an invasion of passenger pigeons.  It didn’t occur to me until just recently that Bartram had described just such a site, though he was unaware of how the landscape he described originated.  In summary I shall list the lines of evidence for my hypothesis that Bartram’s dogwood grove was the result of a massive passenger pigeon roost 50-100 years earlier.

  1. The size of the site (9 miles in extent) is the same size as many passenger pigeon roosts described by colonists.
  2. Heavily fertilized soils support monocultures.  The site, fertilized by pigeon dung, supports just 1 dominant species with 1 minor component.
  3. From Bartram’s description all of the dogwood trees appear to be the same age, suggesting they all germinated during the same year.
  4. Passenger pigeons arrived in the region when dogwood trees bear fruit.  This makes my hypothesis plausible because passenger pigeons are the only species that could have planted dogwood seeds on such a large scale.
  5. Adjacent areas also appear to be recovering from a passenger pigeon invasion.  Bartram describes pockets of plains where there are no overstory trees, just shade intolerant shrubs covered in grape vines.
  6. The complete absence of overstory trees indicates a sudden traumatic tree-killing event in the recent past
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The Pliocene Marine Extinction Event

July 5, 2018

A major marine extinction event rubbed out at least 36% of the ocean’s vertebrate genera about 2.5 million years ago.  Scientists believe the extinctions were caused by a sea level fluctuation, resulting from glacial expansion.  Ice Ages increased in intensity during the late Pliocene and as more atmospheric moisture became locked in glaciers, sea level fell.  Habitat for many coastal species simply disappeared because their near shore environments rose above sea level.  A new study determined 55% of marine mammals, 43% of sea turtles, 35% of sea birds, and 9% of sharks and rays went extinct. I believe this estimate may undercount the actual loss because there are likely some extinct species yet to be discovered by paleontologists.  Many species of invertebrates became extinct as well.

Most of the genera lost were impressive and interesting.  Metaxytherium were a widespread genera of dugongs that grazed sea grass off coasts all across the world.  Thalassocrus were a group of aquatic sloths that evolved from giant ground sloths.  Giant predatory sperm whales (Livyatan) preyed on whales.  Psephopherus, giant sea turtles, laid their eggs on beaches.  The islands off the coast of South Africa, where several species of extinct penguins nested, became connected to land when sea level fell, and predators were able to invade and destroy their colonies. And of course the famous giant white shark, Megalodon, hunted the many species of now extinct whales that lived during the Pliocene.  Most species of baleen whales were smaller and more agile then because they had to avoid these large predators.

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Metaxytherium floridanum swam near and over what today is Florida.

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Thalassocrus, an aquatic genera of sloths.

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2 of the largest predators that lived during the Pliocene–Livyatan melvillei and Carcharocles megalodon.  Both grew to 60 feet long.  Baleen whales were smaller and more agile then, enabling them to escape predation.  The extinction of these predators allowed baleen whales to evolve to a greater size, so they can gorge on food, then fast when they migrate to warmer calving grounds where killer whales, their only modern marine predator, are uncommon.

During the Pleistocene new marine species evolved that were better adapted to the fluctuating sea levels of alternating glacials and interglacials.  New genera increased by 21%.  However, this means there is still a deficit of -15% fewer marine vertebrates than there were during the Pliocene.  Sea life may reclaim the land though, if sea levels keep rising.

An octopus recently found its way into a Miami parking garage.  If sea levels keep rising, marine life may reclaim territory it lost during Ice Ages.

Reference:

Pimiento, C. et. al.

“The Pliocene Marine Megafauna Extinction and its Impact on Functional Diversity”

Nature Ecology and Evolution 1 June 2017

A Pleistocene Cloud Forest

April 22, 2018

Cloud forests are lush environments unique to high elevations located within tropical latitudes.  Vines cover evergreen trees and ferns carpet the ground.  Cloud forests occur along the Andes Mountains from Central America to Argentina at elevations between 3600-10,800 feet, and most are frost free due to the tropical latitude, though they are cooler than lowland forests.  The low seasonality of cloud forests allows for a diverse assemblage of flora and fauna.  Some common plant species found growing in South American cloud forests are elephant ear, strangler fig, and walking palm.  Over 400 species of birds reside in cloud forests including an astonishing 30 species of hummingbirds.  Mammals such as tapir, peccary, brocket deer, jaguar, cougar, ocelot, and spectacled bear roam cloud forests.  Even more species of reptiles and amphibians abound in the thick vegetation of the understory.  Huge beetles and a butterfly with see-through wings are just some of the countless insects that thrive in cloud forests.

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Location of cloud forests around the world.

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A cloud forest in Ecuador.

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Walking palm trees.

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Invisible wings make this butterfly hard for predatory birds to see.

A site with evidence of a Pleistocene-aged cloud forest was unearthed during the construction of an highway in Ecuador.  Scientists examined pollen, geochemistry, and charcoal excavated from strata here dated to between 45,000 years BP-42,000 years BP. During those 3000 years the site went through 3 successional stages.  Pollen evidence suggests during the initial stage that it was a valley floor swamp dominated by grass, aster flowers, and plants in the nightshade family.  This environment was replaced by a forest of holly and plants in the Melstomataceeae and Weinmannia families.  Melastomataceae is a family of tropical flowering plants, and the Weinmannia family includes 65 tropical plants.  This stage succeeded to an environment dominated by alder, myrtle, and plants in the hedyosum genus which includes 65 tropical species.  The latter 2 stages consisted of plant compositions that don’t occur in present day cloud forests.

The authors of this study also measured the amount of sporormiella in the sediment.  Sporormiella is a dung fungus and is used as a proxy for megafauna abundance when fossil evidence is not available.  The amount of sporormiella suggests megafauna were present but not abundant.  Ground sloths, giant armadillos, and gompotheres (a type of mastodon) compose part of the regional fossil record here.  These species were likely the source of the sporormiella in the 42,000 year old sediment.

Fire is rare in montane cloud forests, but there are plenty of other agents of change that cause the environment to go through successional stages.  Landslides on steep slopes after heavy rains can demolish a forest, opening an opportunity for pioneer plants.  Wind throws and forest dieback from old age, disease, or insect infestation also opens up space for pioneer species.  Megafauna probably had just a minor impact on Pleistocene cloud forests because they were not abundant here and plant growth is rapid.  The authors of this study did find volcanic ash in the sediment.  Volcanic-sparked fires do burn some cloud forests, forcing the environment to regenerate through several successive stages.

Reference:

Loughline, N.; et. al.

“Landscape Scale Drivers of Ecosystem Change in the Montane Forest of the Eastern Andean Flank, Ecuador”

Paleogeography, Paleoclimatology, and Paleoecology 2017

The Oak Colonization of North America

October 19, 2017

Oaks are such an important part of the temperate forest ecosystem that it’s hard to imagine they originally evolved near the arctic circle.  During the Eocene about 45 million years ago the earth was mostly tropical and sea levels were much higher than they are today.  There were no ice caps, and climate at the poles was warm and temperate.  Nevertheless, for almost half the year the sun didn’t rise near the arctic circle, just as today night is nearly 6 months long in places like Alaska.  Seasonal darkness led to the evolution of deciduous trees that saved energy by dropping their leaves during winter when the sun didn’t rise.  This adaptation became a great advantage when worldwide climate cooled.  Deciduous trees pushed south because they were able to survive dormant cool seasons that began to occur during the start of the Oligocene ~33 million years ago.  Deciduous trees, especially oaks, replaced tropical species incapable of coping with winter frosts.  Deciduous trees didn’t waste energy with unnecessary growth during winter.

Evidence of the ancient forests where oaks originated exists near the arctic circle at a site known as Axel Heiberg Forest.  Today, this site is a polar desert, but wind erosion is gradually uncovering the forests that existed here 46 million years ago.  A series of floods, perhaps 1 every 10,000 years, covered these forests in sediment, so there are layers of tree stumps, roots, and fallen logs continuously being revealed, as winds strip the sediment away.  Sediment covered the forests rapidly during these catastrophic floods.  It is not a petrified forest because the geological conditions did not favor fossilization.  So once exposed to air, the ancient wood begins to decay, though the process is slow in cold arid conditions.  Scientists think the environment was a warm seasonal rain forest.  Tree composition consisted of dawn redwood, Chinese cypress, hemlock, pine, spruce, larch, gingko, and extinct species of birch, alder, sycamore, walnut, hickory, and oak.

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Location of Axel Heiberg forest–site of the oldest subfossil remains of oaks. Today, it is a polar desert, but during the Eocene it was a temperate seasonal rain forest.

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Subfossil wood from Axel Heiberg forest.

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Comparison between white oak leaves (top) and red oak leaves (bottom).  White oaks and red oaks ecologically complement each other and colonized North America at the same time.

Oaks are classified into 2 groups–red oaks and white oaks.  Genetic evidence suggests red oaks diverged from white oaks about 33 million years ago when they both began to colonize latitudes south of the arctic circle.  Red oaks produce crops of bitter acorns every other year, while white oaks produce more palatable acorns annually.  The strategic difference in acorn production is an ancient ecological balance, attracting squirrels and other seed distributors equally.  Genetic evidence also shows eastern red and white oaks are sister species to western red and white oaks.  Mexican oaks are sister species to eastern oaks, having diverged between 10-20 million years ago.  Oaks colonized eastern and western North America at the same time, then later eastern oaks invaded Mexico.

Mexico has more species of  oaks than any other region in the world (154 species).  If a region has more species of a genus, it usually is thought to be the region where that genus originated.  Instead, scientists believe Mexico has a greater number of oaks species because of differences in elevation in mountains closer to the equator.  Mexican mountains host many different ecological niches causing frequent speciation among oaks.  This explains why Mexico is home to more species of oaks than any other region in the world, though it is not where they originated.

Reference:

Hipp, Andrew; et. al.

“Sympatric Parallel Diversification of Major Oak Clades in the Americas and the Origin of Mexican Species Diversity”

New Phytologist September 2017

Inner Coastal Plain Deserts of the Ice Ages

October 4, 2017

A new study reinforces evidence, indicating some regions of southeastern North America were harsh environments during climatic phases when the ice sheets that covered Canada were expanding.  The scientists who wrote this paper took cores of sediment from 2 Carolina Bays (Jones and Singletary Lakes) located in Bladen County, North Carolina. Carolina Bays are elliptical depressions found on the Atlantic Coastal Plain that were formed during Ice Ages.  They were created by a combination of peat fires, and wind and water erosion.  The peat fires lowered the elevation, wind blew out the dried unconsolidated sediment, and wind-driven water shaped them into elliptical formations.  Jones and Singletary Lakes were also studied in the early 1950s in 1 of the first paleoecological studies of late Pleistocene environments of the south.  The new study analyzed pollen composition, charcoal abundance, and biomass; and the authors compared their results to the earlier study.  The data was dated using radio-carbon dating.

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Location of Bladen County, North Carolina.  This is the site of the study areas discussed in this blog entry.

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Photo of Singletary Lake, a Carolina Bay.  Scientists took a sediment core at the bottom of this lake and analyzed pollen, charcoal, and biomass abundance over the past 50,000 years.

Between ~60,000 years BP-~30,000 years BP climate fluctuated drastically between warm wet interstadials and cold arid stadials.  The glaciers covering Canada advanced then retreated then advanced again in fits and starts.  During glacial expansion more of earth’s atmospheric moisture became locked in glacial ice, causing prolonged droughts, but this moisture was released when glaciers were in a meltwater phase.  Oak and grass pollen increased during meltwater phases, and so did charcoal abundance.  An increase in vegetation meant there was more biomass to ignite and burn during electrical storms.  Oak and grass were fairly abundant from ~43,000 years BP-~32,000 years BP.  The environment mostly consisted of woodland and grassland during interstadials,  but about 30,000 years BP the situation deteriorated.

Ice sheets maintained a steady expansion from ~30,000 years BP-~21,000 years BP.  The initial drought that struck the region during this phase killed vegetation and caused a temporary spike of charcoal because the dead biomass was so flammable.  But after this initial spike, fire was rare to non-existent here.  Sand dunes rolled across the landscape because much of the region was sparsely vegetated.  I believe scrub oak thickets with thorny plants adapted to arid climates covered much of the landscape, but this type of environment doesn’t produce much pollen.  Thus, the amount of vegetation on the landscape then is understated in the pollen record.  For this reason I don’t believe the landscape was as bare as the authors of this study concluded when they wrote it was a “windswept sandy desert with riparian communities of pine and oak.”  Nevertheless, it was an harsh environment of thorny thickets interspersed with areas of bare soil and long distances between water and wetland environments where some trees and grass still grew.  Some tough species of mammals that could survive in this type of environment included horse, flat-headed peccary, helmeted musk-ox, and hog-nosed skunk.  Bison evolved into a smaller species more capable of living in a drier natural community. Overall, wildlife populations probably declined during this climatic phase.

About 21,000 years ago, the ice sheets began retreating and precipitation increased.  Oak and grass gradually increased in abundance, and eventually mesic species such as cypress, basswood, hemlock, and beech invaded the resulting wetter habitats.  ~12,000 years ago, man colonized the region and overhunted megafauna into extinction.  Human-set fires combined with an increase in biomass not being consumed by megaherbivores caused a great increase in fire frequency.

I’m skeptical of 1 claim made by this paper.  The authors estimated the average annual temperature and precipitation levels based on plant composition assumed from the pollen record.  During the Last Glacial Maximum they estimated the average January temperature at these sites was 20 degrees F, while the average July temperature was 68 degrees F.  However, they use 2 dubious assumptions.  They believe the pollen grains from northern species of pine can be distinguished from those of shortleaf pine, a southern species.  This is a doubtful assumption that I will examine more thoroughly in my next blog entry.  Moreover, the spruce pollen probably originated from an extinct species of temperate tree known as Critchfield’s spruce.  I don’t think they can estimate average annual temperatures based on pollen composition, unless the exact species are known with more certainty.

The outer coastal plain and the continental shelf, which was above sea level from ~80,000 years BP-~7,000 years BP, likely hosted richer environments than the inner coastal plain during stadials.  Sea breezes and weather fronts spawned in the Atlantic Ocean brought more moisture to the coast, allowing this region to maintain a mosaic of woodland, grassland, and wetland; while the inner coastal plain suffered greater aridity.  These fronts usually dissipated before they reached the inner coastal plain.  The coastal region likely served as a refuge for plants and animals that later re-colonized the inner coastal plain when climatic conditions improved.

Reference:

Spencer, Jessica; et. al.

“Late Quaternary Records of Vegetation and Fire in Southeastern North Carolina from Jones Lake and Singletary Lake”

Quaternary Science Review 174 October 2017

Arizona Sky Islands–Another Ecological Analogue for Pleistocene Georgia

September 18, 2017

Rapid climate oscillations, megafauna foraging, fire, and wind throws shaped the landscapes of southeastern North America during the Pleistocene.  The resulting environment in the piedmont region consisted of open oak and pine woodlands but with significant patches of closed canopy forests, savannah, prairie, scrub, and wetland.  This variety of habitats in close proximity supported a great diversity of wildlife.  The Pleistocene ecosystem in this region was unlike any extant environment.  Nevertheless, I’ve previously considered some regions as relatively close ecological analogues, resembling the Pleistocene piedmont.  Russian’s Far East was until recently a vast untracked wilderness of mixed forests with abundant game and apex predators.  (See: https://markgelbart.wordpress.com/2011/06/06/russias-far-east-the-modern-worlds-closest-ecological-match-to-pleistocene-georgia/ )  The Cross Timbers region of Texas and Oklahoma where the eastern deciduous forest gradually gives way to prairie may also be a vaguely similar analogue.  (See: https://markgelbart.wordpress.com/2012/06/13/the-cross-timbers-ecoregion-an-analogue-for-georgia-environments-during-some-stages-of-the-pleistocene/ ) I’ve come across a 3rd region that in some ways may resemble Pleistocene piedmont Georgia–the Sky Islands of Arizona, New Mexico, and northern Mexico.

Sky Islands are mountains that stand in the middle of the desert.  They host a variety of environments that change according to elevation.  A change of a few thousand feet in elevation equals the climatic difference of hundreds of miles in latitude.  In a day a man can ascend from an hot desert to temperate oak/pine woodland to boreal spruce/fir forests.  During Ice Ages the lowlands surrounding Sky Islands hosted continuous temperate forests, but now these forested environments are isolated on the mountains, surrounded by desert, hence the name Sky Island.

Mountains rise from the desert floor in Arizona, New Mexico, and northern Mexico.  They host diverse flora and fauna because the change in elevation supports a variety of environments adjacent to each other.

Sky Islands are rich in floral and faunal diversity because so many different natural communities are in such close proximity.  Sky Islands are home to 500 species of birds (over half of the species found in North America), 104 species of mammals, and 120 species of reptiles and amphibians.  Tree squirrels including Mexican fox squirrels, Arizona gray squirrels, and Mt. Graham red squirrels co-exist with rock squirrels (Spermophilus variegatus).  Rock squirrels live and nest in the ground, not trees.  13-lined ground squirrels, another species in the Spermophilus genus, also co-existed with tree squirrels in southeastern North America during the Pleistocene.  13-lined ground squirrels no longer occur in the region because they prefer open environments.  Their presence along with tree squirrels at some fossil sites suggest a more varied environment existed here during the Pleistocene.

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Rock squirrel (Spermophilus variegatus).  Sky islands are home to 7 species of squirrels.  During the Pleistocene a squirrel in the spermophilus genus also co-existed with tree squirrels in southeastern North America, suggesting a more diverse variety of habitats within the region.

Arizona Sky Islands are also famous for a small subspecies of white tailed deer known as the Coues.  For some reason the Coues deer is a popular trophy among deer hunters.  Jaguars and coati-mundi roam the Sky Islands as well.

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Coues deer–A small subspecies of white tail that lives on the Sky Islands of Arizona.

The oak savannahs and oak/pine woodlands of Sky Islands likely resemble natural communities that occurred in the piedmont region of Georgia during the Pleistocene, though they are composed of different species of trees.  Emory oak, Arizona white oak, Gambel’s oak, Canyon live oak, and blue oak grow with Arizona juniper, pinyon pine, yucca, bull grass, and bear grass.  Higher in elevation, silverleaf oak grows with ponderosa pine and Arizona pine.  Higher still, the forest may consist of ponderosa pine, Englemann Spruce, and Douglas Fir–trees of the northern Rocky Mountains.

Acorn Woodpecker Photo

Acorn woodpeckers are a communal species that hoards acorns.  They are a common species on Sky Islands.

The different types of forest attract many different species of birds.  Birds that prefer coniferous forests can be found with those that like oak forests. Tropical species including trogons, thick-billed parrots, buff-colored nightjars, and Arizona woodpeckers inhabit Sky Islands.  These species are found at few other sites north of the Rio Grande River.

 

The Chimney Top Fire

June 24, 2017

The Chimney Top is a series of dry rocky ridges located in the Great Smoky Mountains National Park where slate, schist, and phylite overlay erosion-resistant sandstone.  In some places precipitation has eroded away the top rocks, exposing the sandstone, and the formations resemble chimney tops, hence the name.  Last November, 2 unnamed juveniles set the surrounding forest on fire.  Drought conditions fed the fire, and it was fanned by 80 mph mountain wave winds.  Hot air from the fire rose up the mountain and when it met stable air, it ricocheted and accelerated downward in waves.  The fire burned over 15 square miles and spread into neighboring Gatlinburg, Tennessee, killing 16 people, 2 black bears, and uncounted small animals.  Yet, this forest will recover because many of the plant species growing on the ridge are well adapted to fire and in some cases even dependent upon it.

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Needles and cone of the table mountain pine.  This species depends on fire to open its cones.

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Fireweed also depends on fire.

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The Chimney Tops.  Erosion resistant rock explains the chimney-like formations.

Photo of a burned ridge on Chimney Top.

The Chimney Top environment consists of rock chestnut oak (Quercus montana), table mountain pine (Pinus pungens), and heath balds.  Rock chestnut oak is fire resistant, and it thrives in the rocky shallow soils on the ridge.  Table mountain pine also grows well in the shallow soils, and it depends upon fire to open its seed cones.  Although long exposure to hot sun opens table mountain pine cones, the process is best facilitated by fire.  Park service employees noted a rain of pine seeds in the air a few days after the fire.  In 5 years the burned over ridges will be covered with pine saplings and fireweed.  Some heath balds completely burned to the ground–an unusual occurrence here because this region is the rainiest spot east of the Mississippi.  Heath balds are evergreen shrub communities consisting of mountain laurel (Kalmia latifolia), Catawba rhododendron (Rhododendron catawbiense), various species of blueberries (Vaccinium sp.) and huckleberries (Gayluccia sp.), and 1 deciduous tree–mountain ash (Sorbus aucuparia).  Heath balds are often adjacent to grassy balds and surrounded by forests of red spruce and hemlock.  Heath shrubs thrive on shallow acid soils located on mountain slopes.  Both heath and grassy balds are of ancient origin.  (See: https://markgelbart.wordpress.com/2016/05/16/the-extinct-helmeted-musk-ox-bootherium-bombifrons-and-appalachian-grassy-balds-during-the-pleistocene/ )  Scientists studied heath balds and discovered they grow on a layer of peat underlain by charcoal.  This suggests heath balds occasionally do burn completely, yet regrow in the same location.  This fire gives scientists the first chance to ever witness the rebirth of a heath bald.

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Heath bald.

Forests are resilient.  The area in the photo below was clear cut during 1910.  The original forest consisted of chestnut, oak, and hemlock; many with trunks 5 feet in diameter.  The destruction of this locality spurred the creation of the Great Smoky Mountains National Park in 1926.  The 2nd growth forest that replaced the original tract is not as impressive but at least it is green.

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This area was clear cut in 1910.  It has nicely recovered but is not as impressive as it was originally.

Bodark Swamps

April 28, 2017

Botanists believe the Osage orange (Maclura pomifera) was restricted to bottomlands along the Red River drainage when Europeans discovered North America.  Here, it grew in pure stands known as Bodark Swamps.  (A disjunct relic population lived in the Big Bend region.) This relative of the mulberry and fig is a shade-intolerant, early successional species capable of surviving flood events that kill competing trees, perhaps explaining why they grow in pure stands. Early settlers cultivated the trees as hedgerows used to confine livestock, and farmers spread this species all over North America.  Osage orange hedgerows were much cheaper than fencing, and they were widely planted until the introduction of barbed wire in 1875.

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Range map of Osage orange.  There were probably additional disjunct relic populations located elsewhere on the continent that were never recorded by botanists.  This species was much more widespread during the Pleistocene.

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Illustration of an Osage orange tree and fruit.

There is some indirect evidence the pre-Columbian distribution of Osage orange was wider than range maps indicate.  Hagen’s sphinx moth (Ceratomia hageni) feeds on Osage orange leaves and nothing else.  This species of moth is locally abundant in the Black Belt prairie region of Mississippi–evidence Osage orange grew on the margin of this natural community before European conquest.  Compact clay soils in the Black Belt Prairie favor grass over trees, and shade-intolerant Osage orange grows well in this environment where they have less competition from other trees. Hagen’s sphinx moth has an erratic distribution.  When agriculturalists were spreading Osage orange seeds it doesn’t seem likely they brought the moths with them.  Relic populations of Osage orange probably occurred wherever this moth is common.

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Hagen’s sphinx moth, aka Osage orange sphinx moth.  Its only host plant is Osage orange.

Osage orange was even more widespread during the Pleistocene.  Mastodon dung excavated from the Aucilla River in north Florida contained Osage orange.  Fossil evidence of Osage orange reportedly was found in Ontario, Canada where it grew during warm interglacial times.  (The oft-repeated source of this information (Peattie 1953) mentions this but doesn’t cite his source.  I consider it a dodgy fact.  Who identified this fossil wood and from what site was it excavated?)  Osage orange became a relic species following the extinction of the mastodon.  A recent experiment determined Osage orange seeds can survive transit through an elephant’s gut but not an horse’s.  (See:https://markgelbart.wordpress.com/2015/04/10/asian-elephants-elephas-maximus-and-horses-equus-ferus-caballus-refused-to-eat-pawpaws-in-a-controlled-experiment/)  Horses and probably tapirs, a relative of the horse, consumed Osage orange, but this large fruit depended on mastodons and maybe mammoths for distribution across the landscape.  Elephants are capable of carrying viable seeds in their guts up to 40 miles before depositing them in great piles of fertilizer.  Without mastodons Osage orange range became more restricted.  Perhaps the Red River drainage and the Black Belt Prairie were where mastodons made their last stand.

Several characteristics of Osage orange show it co-evolved with megafauna.  The large fruits attract big mammals able to efficiently hold and transport the seeds in their guts.  Although horses, deer, squirrels and birds eat the fruit, they either destroy the seeds during consumption or pick at the fruit without distributing the seed.  Osage orange evolved thorns to deter megafauna from chewing on the tree itself.  And if the plant does get eaten, it is able to re-sprout from sucker roots.

Osage orange, along with yew, is considered some of the best wood for making bows.  Some archaeologists believe certain Indian tribes monopolized trade in Osage orange wood.

Osage orange fruit is not toxic, but it is considered inedible for human consumption.  Connie Barlow, author of The Ghosts of Evolution, reports it tastes like air freshener.  Some people think Osage orange fruit can be used as an insect repellant.  However, 1 scientist found 20 insect species on Osage orange fruit littering the campus at Louisiana State University.  The fruit is more likely to attract critters than repel them.  .

References:

Burton, James

“Osage orange: An American Wood”

U.S. Department of Agriculture Bulletin 1973

Ferro, Michael

“The Cultural and Entomological Review of the Osage orange (Maclura pomifera) and the Origin and Early Spread of “Hedge Apple” Folklore”

Southeastern Naturalist (13) Monograph 7 2014

Peacock, Evan; and Timothy Schauwetum

Blackland Prairies of the Gulf Coastal Plain

University of Alabama Press 2003

 

The Sourwood-Lettered Sphinx Moth-Black Bear Food Web

March 26, 2017

There are many intricate relationships between different species of plants and animals yet to be discovered.  The interrelationship of sourwood (Oxydendrum arboreum), lettered sphinx moth (Deidami inscripton), and black bear (Ursus americanus) was first noted in the scientific literature just last year.  Sourwood is a small tree, seldom growing to over 6o feet in height, that lives in oak forests and woodlands with acidic soils.  It is the sole species in its genus and a member of the blueberry and azalea family.  The leaves have a sour taste and can be chewed but shouldn’t be swallowed because they are mildly toxic with a high amount of oxalates.  Scientists were studying the occurrence of a major defoliation event of sourwood trees near Unicoi, Tennessee a few years ago.  Here, sourwood trees along with dogwood, summer grape, Virginia creeper, and greenbrier form the understory of a forest composed of red maple, black gum, northern red oak, pitch pine, Virginia pine, chestnut oak, scarlet oak, and striped maple.  They found the sourwood trees were being defoliated by larva of the lettered sphinx moth.

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A sourwood tree in fall foliage.

Lettered Sphinx - Deidamia inscriptum

The lettered sphinx moth.

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The larva of the lettered sphinx moth feeds upon grape, Virginia creeper, and peppervine; but just recently was discovered to have a preference for sourwood over all those plants in the Vitis family.

The lettered sphinx moth is the only species in its genus that lives north of Mexico.  Lettered sphinx moth larva were known to feed upon the leaves of plants in the grape family which also includes Virginia creeper and peppervine.   Lepidopterists refer to these plants as “host species.”  However, when scientists discovered sphinx moth larva defoliating sourwood they conducted an experiment–they put sphinx moth larva in terrariums and offered them grape leaves and sourwood leaves.  The sphinx moth larva preferred the sourwood leaves.  This suggests sphinx moth larva will choose sourwood leaves wherever the ranges of sourwood and species in the grape family overlap.

Scientists hypothesize the oxalates ingested from the sourwood accumulates in the caterpillar, and the toxicity discourages avian predators.  Nevertheless, bears are able to eat the caterpillars.  The authors of the below referenced study found evidence bears were consuming large quantities of sphinx moth caterpillars during the defoliation outbreak.  They saw stem breakage, claw marks on limbs, and bear scat filled with caterpillar remains all around the sourwood trees.  Moth larva provides lots of protein and fat, and the partially digested plant material in their guts likely contains beneficial vitamins for the bears.  The bear scat in turn helps fertilize the soil around the sourwood trees.

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Black bear feeding on forest tent caterpillars.  Caterpillars are nice fatty snacks for the bruins.

The interrelationship between sourwood, sphinx moths, and bears probably began during the Pleistocene or perhaps earlier; but it wasn’t noticed or recorded by people until last year.  There are countless other examples like this, yet to be discovered.

Reference:

Levy, Foster; David Wagner and Elaine Walker

“Deidamia inscripton (Lettered Sphinx Moth) Caterpillars feeding on Oxydendrum arboretum (Sourwood) and their Predation by Black Bears in Northeastern Tennessee”

Southeastern Naturalist 15 (3) 2016

Pleistocene Oysters (Crassostrea virginica)

March 14, 2017

Before humans harvested them, oysters lived longer, grew larger, and produced denser quantities of offspring.  Scientists compared oyster shells from Pleistocene-age oyster reefs with those from Native-American archaeological sites and modern harvests.  Pre-human contact oysters lived as long as 30 years, while oysters since human colonization never live longer than 6 years.  Pleistocene oysters grew up to 10.2 inches, pre-historic oysters from Native-American middens grew to 7.4 inches, and modern oysters reach 6.1 inches.  Native-Americans harvested oysters in a sustainable way, but populations of oysters since European colonization have been reduced by over 99%, despite restoration efforts.  Pollution and overharvesting have destroyed oyster numbers.  This is unfortunate because oyster reefs are a productive natural community, providing habitat for at least 303 species that have co-evolved with oysters over the past 135 million years, ever since these bivalves first evolved. Scientists estimate the original oyster population of Chesapeake Bay was capable of filtering the entire contents of this estuary in just a few days, so they help clean the water as well.  Modern day estuaries are suffering without more abundant populations of oysters.

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Ancient oyster midden.

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Pelican in front of a Georgia oyster reef at low tide.

A representative of every species living in oyster reefs could fill a big city aquarium.  Barnacles, mussels, clams, and bryozoans attach themselves to the reefs and live out their lives filter feeding just like their hosts.  Mud crabs (Eurypanopeus depressus) graze on the algae and detritus that accumulates on the reefs and sometimes feed upon the smaller oysters.  Oyster pea crabs (Pinnotheres) depend upon reefs for their very survival. The seashore springtail (Anurida maratima), unusual salt water insects, prey on microorganisms living on the reefs.  Amphipods, worms (Polydora and Polychaetas), anemones, mites, and hydroids are commensal animals dependent upon the existence of oyster reefs.  Boring sponges (Cliona) and starfish directly prey on the oysters.

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The seashore springtail is a true insect that lives on oyster reefs.

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The depressed mud crab grazes on algae, detritus, and small oysters on oyster reefs.

Many small species of fish swim in and around oyster reefs during low tide because the structure affords protection from predators.  Species of fish commonly found in Georgia oyster reefs include in order of abundance naked goby (Gobiosoma bosci), feather blenny (Hypoblennius hentzi), skilletfish (Gobiosox strumosus), seaboard goby (Gobiosoma ginsbingi), striped blenny (Chasmodes bosguianus), oyster toad fish (Opsanus sp.), and the crested blenny (Hypleurochilus geminatus).  During high tide larger fish such as sheepshead, black drum, and croakers move in and feed upon the shellfish and smaller fish living on the reef.

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The naked goby is the most common fish living in Georgia oyster reefs.  They feed upon worms, crustaceans, and dead open oysters.

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The skillet fish clings to oysters with its sucking mouth.

Land vertebrates forage oyster reefs during low tide.  Raccoons and wading birds find many a meal on the reefs.  Oyster catchers (Haematopus palliatus) specialize on feeding upon the oysters and other bivalves growing here.  Even boat-tailed grackles exploit oyster reefs–they eat the amphipods and pea crabs crawling over the reef.

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The American oystercatcher thrives on oyster reefs.

Oysters have a complex life cycle.  They expel sperm and eggs into the ocean water, and when these sex cells meet by chance they form larva.  (Oysters change sexes, so that males become females and vice-versa.  Some individuals are hermaphroditic  and expel sperm and egg at the same time.)  The larva lives in the zooplankton until they develop a foot.  The oyster senses pheromones from other oysters on a reef and will attach its foot to the structure where it will remain for the rest of its life, filter feeding upon diatoms, dinoflagellates, inorganic particles, bacteria, and marsh plant detritus.

Oyster reefs also have life cycles.  When oysters begin colonizing an area it is known as the clustering phase.  Oysters attach to each other and on old dead oyster shells during the accretionary stage, building reefs.  Eventually, oysters reach a vertical limit and start building the reef horizontally during the senescent phase.  Large reefs block sediment and shell debris carried by tidal currents and this action can create islands.  Little Egg Island in the middle of the Altamaha River mouth is an example of an island created by an oyster reef.

References:

Bahr, Leonard, William Larsen

“The Ecology of Intertidal Oyster Reefs of the S. Atlantic Coast: A Community Profile”

U.S. Geological Survey 1981

Lockwood, R.; K. Kusperck, S. Bonanani, and Gratt, A.

“Reconstructing Population Demographics and Paleoenvironment of Pleistocene Oyster Assemblages: Establishing a Baseline for Chesapeake Bay Restoration”

North American Paleontological Convention 2014

Rick, Turbin; et. al.

“Millenial-scale Sustainablity of the Chesapeake Bay Native American Oyster Fishery”

PNAS 2016

Wharton, Charles

The Natural Environments of Georgia

Georgia Department of Natural Resources 1978