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

 

 

Ants, Flies, and Trillium

Woodland herbs in the trillium family have evolved interesting traits that lure insects into helping them reproduce.  The trillium family belongs to the lily plant order, and it includes 45 species and 5 hybrids found in North America and Asia.  More species live in North America and the center of trillium evolutionary origin is probably the southern Appalachians.  Species of trillium were likely widespread in the tropical to temperate forests that occurred across both continents during the Miocene over 5 million years ago.   This is likely when trillium evolved a dependent relationship with bees, ants, and flies.  Some species of trillium, such as the great white (Trillium grandiflorum) produce flowers that attract bees and wasps with a sweet smell.  But others, such as the inaptly named little sweet betsy (T. cunacateum), have flowers that smell like rotting meat, and the red color looks like flesh, so they attract flies.  These flying insects help pollinate the trillium plants.

Little sweet betsy–more accurately known as bloody butcher because they smell like rotting meat. The flesh color and carrion smell of the flowers attract flies, and in turn the flies pollinate the flowers.

By contrast the flowers of the great white trillium smell sweet and attract bees and wasps for pollination help.

Ants disperse trillium seeds throughout the forest.  Aphaenogaster picca is an example of an ant species that mostly preys on termites but takes advantage of the passive trillium seeds when they have a chance.

Trillium produce a fruit with over a dozen fat covered seeds that attract ants.  This layer of fat is known as the elaiosome, and it surrounds a hard seed.  The ants carry the fat rich seeds back to their nest, but discard the hard seeds after consuming the nutritious coating.  This helps spread the species throughout the environment. Most of the species of ants that consume trillium seeds normally prey on termites, worms, and grubs–the trillium seeds are an easier meal that puts up no resistance.  Plants that depend upon ants to disperse their seeds are called myrmecochorous.

For years many scientists were mystified at how trillium and other plant species so rapidly recolonized land formerly covered by glaciers. This is known as Reid’s paradox defined as the unexplained mystery of how some plant species apparently recolonized post glacial habitats faster than their modern dispersal rates suggest.  It is mathematically impossible for ants to have dispersed trillium across New England and parts of Canada in less than 10,000 years. However, scientists recently discovered deer feces containing viable trillium seeds.  Apparently, white tail deer eat trillium fruit and spread the seeds into new territory.  This would answer Reid’s paradox.  However, high populations of white tail deer can be detrimental to trillium populations.  Trillium plants are perennials, capable of surviving from the same root for up to 25 years, but repeated deer consumption of the leaves can kill the plant.  Deer also choose to eat the largest trillium plants, thus selecting for smaller and less productive individual trillium plants.  I hypothesize extinct tapirs and possibly long-nosed peccaries also fed upon and dispersed trillium during the Pleistocene.

The relict trillium (T. reliquum) is known from just 50 sites along the fall line, mostly in Georgia. A genetic study determined this species survived in 2 different Pleistocene refuges during Ice Ages when much of the landscape became desert like.  There is actually greater genetic diversity on the eastern and western parts of its range rather than in the central part.

The relict trillium, also known as the Confederate wake robin.  It is an ancient species that became rare because of Ice Ages.

References:

Gonzalez, E.; and J.L. Hamrick

“Distribution of Genetic Diversity among Disjunct Populations of the Rare Forest Understory T. reliquum”

Heredity 95 2005

Velland, Mark; et. al.

“Dispersal of Trillium Seeds by Deer: Implications for Long Distance Migration of Forrest Herbs”

Ecology 84 (4) 2003

The Vice-Versa Climate Phases of South Florida during the Late Pleistocene

Drastic climatic fluctuations occurred during Ice Ages.  Sudden warm spikes in average annual temperatures followed by rapid onsets of much colder climate phases altered the quantitative composition of plant species in the environment.  In most of North America oaks and other broad-leafed trees increased in abundance during warm wet interstadials but waned during arid cold stadials when coniferous parkland forests consisting of pine and spruce expanded in response to the changing climate.  These fluctuations were part of a feedback loop.  The warmer temperature phases gradually caused giant ice dams to weaken and break.  Torrents of cold freshwater from glacial lakes with floating icebergs flooded the North Atlantic, shutting down the Gulf Stream which during present day conditions moderate temperatures.  These deluges of iceberg studded meltwater are known as Heinrich Events, and they caused average annual temperatures to drop by 15-20 degrees F within decades.  In response to the drop in temperatures the Ice Sheets expanded for thousands of years until the next warm phase.  Climate also became drier because moisture for potential precipitation became locked in glacial ice.  However, evidence from a lake in south Florida suggest climate there was out of sync with the rest of North America north of the Rio Grande.

Graph depicting Heinrich Events and the subsequent fall in temperature.  Pollen evidence from sediment in Lake Tulane, Florida reveals the floral response to these climatic shifts, including the most recent 6 Heinrich Events.

Lake Tulane is located near Avon Park, Florida.

Scientists have taken numerous cores of sediment from Lake Tulane, located in south central Florida.  Lake Tulane is a very old body of water and has probably existed for almost 5 million years since this region emerged above sea level.  The sedimentary record goes beyond the limits of radio-carbon dating (50,000 years). The pollen composition in these cores shows the local environment’s response to Heinrich Events, but curiously it is the opposite from that of the rest of North America.  When the rest of North America experienced a warm wet interstadial, south Florida became cooler and drier (though mostly not sub-freezing).  The landscape transformed into an environment dominated by scrub oak, Florida hickory, red cedar, ragweed, grass, asters, staggerbush (Lyonia sp.), and rosemary (Ceratiola).  Scrub oak thickets surrounded by open spaces prevailed.  These are all drought-tolerant but shade-intolerant species, and red cedar is fire-intolerant, indicating the rarity of lightning-induced fires.  Conversely, when the rest of North America suffered cold dry stadials south Florida was warmer and wetter and pine forests spread across the land because lightning-induced fire frequency increased, and pine is fire tolerant.  Scientists find macrofossils of aquatic plants in the sediment representing dry phases because the lake was shallow enough to support emergent marsh vegetation, but these are absent during the wetter phases.

A brand new study introduced a new line of evidence that supports assumptions based on the earlier pollen evidence.  The authors of this study looked at variations in carbon and oxygen isotopes from Lake Tulane plant leaf waxes.  Scientists can understand the historical precipitation characteristics by studying the isotopic composition of plant leaf waxes.  (Plants synthesize organic compounds by using hydrogen atoms they absorb from water molecules.  The isotopes vary depending upon their source.)  They discovered that during climatic phases when scrub oak and ragweed dominated, average annual precipitation dropped by 22%.  The source of precipitation was different too.  Oak phase precipitation mostly came from storm fronts, but pine phase precipitation originated from tropical oceanic storms.

The vice-versa climate of south Florida was tied to the shifting Gulf Stream.  Under non-Ice Age conditions the Gulf Stream carries tropically-heated water to the North Atlantic as far as the coast of Canada, moderating temperatures.  When it shut down following Heinrich Events, the warm water stayed near the coast of south Florida, keeping it warm and wet while the rest of North America suffered dry cold conditions.  The Gulf Stream eventually restarted, bringing warmer wetter temperatures north, but this caused cooling and aridity in south Florida.

I hypothesize the Gulf Stream periodically began to restart within stadials, then shut down with new influxes of meltwater.  These partial changes likely influenced temperatures near the coasts of Georgia and South Carolina.  The climate may have temporarily been warmer in this region even during colder climate stages, and the composition of species here may have varied as well, though it involved different species than the Florida endemics.  Eventually, when the Gulf Stream restarted for longer periods, coastal Georgia may have experienced warmer climates centuries before northern latitudes of North America did.

Common ragweed.  It was much more common in south Florida during the late Pleistocene than it is today.

There is no modern analogue for the abundance of ragweed (Ambrosia artemisifolia) in south Florida during the late Pleistocene.  Ragweed prefers cooler nights than occur in present day Florida for germination, and its abundant presence during the late Pleistocene indicates cooler nights prevailed, even during the out of phase warmer climate stages.  Ragweed is a tough plant that grows on disturbed sites such as abandoned agricultural fields and vacant lots.  It produces up to 32,000 seeds per plant, so it is able to survive heavy foraging by herbivores.  The seeds persist indefinitely and can wait for centuries before germinating when an environment transforms into a sunny one.  This means it can lay dormant through several stages of forest succession, and then colonize the habitat when conditions become favorable.  This may explain why it was so successful during the late Pleistocene when both sudden climatic fluctuations and megafauna foraging greatly disturbed and altered the landscape.  Many animals consume ragweed.  Rabbits and meadow voles eat the leaves while birds including juncos, cowbirds, quail, purple finch, mourning dove, goldfinch, and red-bellied woodpeckers eat the seeds.  Rabbits and meadow voles were common in south Florida during the Pleistocene, though the latter is presently restricted to 1 county in the state.  Farmers report livestock prefer giant ragweed (Ambrosia trifida) over clover.  It’s unclear if this species lived in south Florida during the late Pleistocene.  It is found in a couple counties, and this may represent relic populations, but they also may be invasive.  Livestock will also eat common ragweed, though they don’t like it as much.  Pleistocene megafauna such as horses, bison, and mammoth likely ate ragweed and grass, and they probably occurred in large herds during both climatic phases.  Other species probably common during the oak phase were Harlan’s ground sloth, northern pampathere, flat-headed peccary, and giant tortoise.  Flat-headed peccaries preferred thorny thickets, and the other 3 liked open environments.  Predators such as saber-tooths, lions, jaguars, and dire wolves fed upon the herbivores.  Ragweed survived megafauna foraging by producing large numbers of seeds, but 2 other plant species in this environment survived because their leaves were toxic–rosemary and staggerbush.  I think both climatic phases in Florida supported approximately similar populations of megafauna.

References:

Arnold, T. Elliott; et. al.

“Climate Response of the Florida Peninsula to Heinrich Events in the North Atlantic”

Quaternary Science Reviews 194 2018

Grimm, Eric; et. al.

“A 50,000 year old record of Climate Oscillation from Florida and its Temporal Correlation with Heinrich Events”

Science 9 July 1993

Grimm, Eric: et. al.

“Evidence for Warm Wet Heinrich Events in Florida”

Quaternary Science Review 25 Sept 2006

Gulf Fritillary and Passion Flower Vine

Butterfly migration is even more amazing than bird migration.  Bird migration includes the same generation, but butterflies that begin migrating north never live long enough to return south.  Instead, butterflies gradually expand their range north as the weather warms; breeding, laying eggs, and dying.  The next generation advances farther north.  Then, several generations later, they begin moving south, retreating before killing frosts.  The gulf fritillary (Augraulis valinae) is an example of a migratory butterfly.  They winter in Florida, south Texas, and Mexico, but generations of them migrate as far north as Pennsylvania.  Gulf fritillaries were named because they are some times seen fluttering over the Gulf of Mexico.  Their larva feed upon passion flower vine (Passiflora incarnata) foliage.  The adults obtain their energy from nectar in flowers , and as the below photo represents, they often find some nutrition in animal feces.  Gulf fritillaries are particularly fond of lantana, a non-native shrub that rapidly colonized Florida during early Spanish occupation.

Gulf fritillary snacking on dog feces.

Passion flower.  Spanish conquistadors thought it symbolized the passion of Christ.

The fruit of passion flower is edible.  The seeds are covered in a gelatinous substance with a sweet-sour flavor and a tropical aroma.  Brazil produces and consumes the most passion fruit.  Imported passion fruit is occasionally available in the grocery store.

There are between 520-700 species of passion flower vine–taxonomists disagree about the number of species.  96% of them occur in the Americas, indicating this is where they originated.  Other species live in southeast Asia, Australia, and Pacific islands.  They probably colonized these regions by rafting on clumps of debris ripped from the land by  tropical storms.  P. incarnata and the crinkled passion flower (P. gracilis) are the only species that evolved to live in temperate climates.  P. gracilis  is restricted to 1 county in South Carolina, while P. incarnata ranges throughout eastern North America.  During the Miocene when most of North America was sub-tropical there were probably many species of passion flower native to North America, but just 2 evolved the ability to survive frosty seasons.

Passion flower vines are shade intolerant but drought tolerant.  They prefer disturbed areas, and I’ve found them growing on vacant lots in my neighborhood.  This species was well adapted to live during the Pleistocene when rapid climate change and megafauna foraging often drastically altered local landscapes.  Mammoths and other large animals girdled and uprooted trees, opening up the canopy so shade intolerant passion flower vines could thrive.  Many vertebrates, perhaps peccaries, fed on the fruit and distributed the still viable seeds in their dung.  Long Ice Age droughts also killed trees and let passion flower vine spread in the available sunshine, climbing over grass and tree saplings and across bare sandy soils.

When the Spanish conquistadors conquered the Americas, they found passion flower vine growing everywhere.  The soldiers were super religious, though they ignored 1 of the 10 commandments when they were butchering the Indians.  They thought passion flowers symbolized the crucifixion of Christ, known as the passion by religious zealots.  Supposedly, the 5 petals and 5 sepals represent the 10 apostles.  The 72 filaments = the number of thorns in Jesus’s crown.  The 3 stigmas = the cross.  The 3 stamens = the wounds in Jesus’s hands.  The leaf lobes resemble the spear wounds.  The dark spots under the leaves represent the 33 pieces of silver given to Judas to betray Jesus.  The flowers die after just 1 day, just like Jesus died after a day on the cross.  And the petals reclose like the tomb enclosed Jesus.  Some superstitious priest sure had an overactive imagination.

Did Large Carnivores Influence Dune Formation in Ice Age Georgia?

Over 100 years ago Australians built a 3480 mile long fence to keep dingoes away from livestock. For ecologists this provides a grand experiment of how the exclusion of a large predator influences ecosystems. However, there exists a considerable amount of conflicting scientific literature about this. Many studies report overgrazed regions on the dingo-less side of the fence that have poor soils as a result. The fence bisects a national park. One study confined to part of this park counted 85 dingoes and 8 kangaroos on the side of the fence with the dingoes, and 1 dingo and 3200 kangaroos in a comparably sized lot on the side that is supposed to be without dingoes. Tame livestock, feral goats and hogs, and rabbits along with the kangaroos contribute to these overgrazed landscapes. Parma wallabies, the greater bilby, and small rodents thrive on the side of the fence with the dingoes because the large canines suppress populations of smaller predators. Another study that claims to be more comprehensive than any other found no differences between either side of the fence. The authors of this study suggest there are no differences because dingoes have never been completely eliminated on the supposedly dingo-less side of the fence. They say other studies concluding there is a difference are local and anecdotal.

Dingo on a sand dune.

I think the most interesting study is a recent paper that found the presence of dingoes influenced sand dune formation in arid regions. On the dingo-less side of the fence sand dunes were larger and stabilized with shrubby plants growing on top. On the side of the fence with dingoes sand dunes were more shallow, bald, and dispersed by wind because plant growth was sparse. This seems counterintuitive. But this difference in dune formation is caused by the suppression of small carnivore populations. Dingoes reduce populations of foxes and feral cats (neither of which are native to Australia). In turn dusky hopping mice and rabbit populations increase, and they eat the seeds of plants and shrub saplings that keep dunes stabilized.

This last study is most interesting to me because sand dunes rolled across parts of Georgia during the coldest driest stages of Ice Ages, and I wonder if large predators influenced their shape and pattern. The arid climate caused some small rivers in Georgia to run dry. Wind blew the riverine sand into big dunes that are still evident today, though scrubby vegetation has since stabilized them. (See: https://markgelbart.wordpress.com/2012/04/09/the-ohoopee-sand-dunes/ ) I’ve hypothesized overgrazing by megafauna alongside shrinking water holes located in the river bed may have contributed to the erosion leading to sand dune formation. But maybe the presence of large carnivores played a role as well. Dire wolves, jaguars, and cougars suppressed populations of bobcats and foxes; causing an increase in rodent and rabbit numbers. The small herbivores stripped the vegetation bare, allowing sand dunes to roll. On the other hand hawks, owls, and snakes probably always remained abundant, and they likely provided a check on rodent and rabbit populations. Nevertheless, the notion large carnivores may have influenced dune formation in Georgia is an intriguing idea.

References:

Glen, A.; and C. Dietman, M. Soule, and B. Mackey
“Evaluating the Role of the Dingo as a Trophic Regulator in Australian Ecosystems”
Australian Ecology August 2007

Harris, Emma
“Dingoes have Changed the Actual Shape of the Australian Desert”
The Atlantic July 6, 2018

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

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.

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

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.

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

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

The Pliocene Marine Extinction Event

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.

Metaxytherium floridanum swam near and over what today is Florida.

Thalassocrus, an aquatic genera of sloths.

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

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.

Location of cloud forests around the world.

A cloud forest in Ecuador.

Walking palm trees.

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