The Pleistocene Floral and Faunal Invasion of Southeastern North America

The Pleistocene fossil record of southeastern North America includes many species of plants and animals that had their origins in the neotropics of South America and the western grasslands of North America.  Disjunct populations of some of these species still exist in the southeast, especially in Florida.  Dr. David Webb may have been the first scientist to propose the former existence of a broad corridor on the coastal plain along the Gulf of Mexico that connected the southeast with these other regions.  This corridor has intermittently existed since the late Miocene, and most recently during the Last Glacial Maximum when sea level fell drastically as much of earth’s water became locked in glacial ice.  Dry land along the Gulf of Mexico emerged above sea level, greatly expanding this corridor as the map below shows.

Note how far dry land extended into the present day Gulf of Mexico during the LGM (~28,000 BP-~15,000 BP).  The state of Florida was twice its modern size.  A mixture of subtropical savannah, prairie, and scrub habitat probably covered most of this land.

The coastal prairies of Louisiana and Texas are a small remnant of this once more vast environment.

The coastal regions that emerged above sea level and most of south Florida supported a kind of lost world consisting of subtropical to warm temperate savannahs.  Paradoxically, climate in this corridor was warmer during the Last Glacial Maximum than today’s climate is there, and it was probably frost free.  During interglacials (such as the present one) and interstadials of the past, the gulf stream carries warm tropical water north where it cools, sinks, and returns south.  But during stadials, icebergs and meltwater break free from glaciers, and they cool and shut the gulf stream down, and the warm tropical water stays off the coast of the south Atlantic and in the Gulf of Mexico.  This created a thermal enclave in south Florida and along the Gulf of Mexico, but the palynological fossil record is incomplete, and it’s unknown exactly how far inland this thermal enclave extended.  It probably didn’t extend far because boreal taxa such as jack pine, red pine, white pine, spruce, and paper birch occurred as far south as the ridge and valley region of north Georgia.  There must have been an abrupt transition zone in the piedmont between the warm savannahs of the coastal plain and the cooler pine and spruce forests of the Appalachian Mountains.  Only 1 Pleistocene floral fossil site is known from the Piedmont region of Georgia–Nodoroc, a mud volcano that captured pollen and some framentary plant macrofossils.  But this site dates to just before the stadial that led to the LGM.  The pollen and macrofossils from this site indicate a mixed forest of northern and southern pines and oaks interspersed with meadows.

Ironically, during interstadials the Gulf Stream became re-established and warmed most of North America, but south Florida and the Gulf Corridor became cooler, though not subfreezing in south Florida.  Sea level rose and the corridor shrunk during interstadials.

Frequent tropical storms, lightning-ignited fires, and megafauna foraging gave the landscape its open appearance.  Longleaf pine savannahs and grassy prairies likely dominated the Gulf Coast Corridor.  Small areas avoiding the trio of landscape engineers became oak scrub.  Prairie acacia (Acacia angustissima) and hairy grama grass are among the notable plants that invaded the south through the corridor.  Prairie acacia is a close relative of the acacia tree that occurs so commonly on African savannahs.  Like African acacias, it was probably abundant, thanks to megafauna foraging.  Acacia trees produce edible pods eaten by elephants and formerly, mammoths and mastodons, and they spread the seed in their dung.  But the bark is poisonous and megafauna avoid eating it.  This explains the prevalence of acacia trees on landscapes where megafauna still live.  Unlike African acacias, the prairie acacia is merely a bush.  The modern range of prairie acacia includes Texas and Mexico with a disjunct population in Florida.

A thicket of praire acacia.  It was probably common on the Gulf Coast Corridor.

Hairy grama grass is among many species of western prairie grasses that have disjunct populations in the east.  A small disjunct population of hairy grama grass occurs on an island off the west coast of Florida.

Hairy grama grass.  A western species with a disjunct population on an island off the west coast of Florida.

Neotropical species of animals that invaded the south through the Gulf Coast Corridor included opposum, giant ground sloths, armadilloes, glyptodonts, tapirs, peccaries, capybaras, gompotheres, jaguars, ocelots, margays, vampire bats, Brazilian free-tailed bats, Mexican long-nosed bats, ghost-faced bats, terror birds, crested caracaras, great-tailed grackles, giant tortoises, indigo snakes, and whip snakes.  Western species of animals that invaded the south through the Gulf Coast Corridor included bison, lions, cheetahs, 13-lined ground squirrels, jack rabbits, hog-nosed skunks, badgers, pocket gophers, burrowing owls, ravens, magpies, prairie chickens, white-tailed kites, diamondback rattlesnakes, hog-nosed snakes, harvester ants, digger bees, some iceunomid wasps, and sand roaches.

The crested caracara is a tropical bird that colonized the Gulf Coast Corridor during the LGM.  In Florida caracaras and burrowing owls almost exclusively nest in heavily grazed pasture, suggesting their former close affinity with Pleistocene megaherbivores.  They prefer shortgrass environments over ungrazed tall grass areas.

Today, a few relic habitats remain from this once continous corridor.  Coastal prairies in Lousiana and Texas are perhaps the best example.  Florida dry prairies are another.  The formerly widespread longleaf pine savannahs on the coastal plain were a close analogue to the Gulf Coast Corridor but not an exact match because frosts do occur there.

Louisiana coastal prairie.  This is probably what much of the Gulf Coast Corridor looked like.

Just to the north of the warm subtropical grasslands, an environment consisting of an extinct temperate species of spruce (Critchfield’s) grew with modern hardwood species such as oak, maple, elm, hickory, and walnut.  The farther north and west from the Gulf Coast Corridor, the colder the climate was.  The spruce and deciduous forest likely predominated in this abrupt transition zone where the climate suddenly changed from tropical to temperate.  Tropical fronts often hit cold air and probably caused snowy winters here.  Summers were cool but winters moderate in the transition zone.  Birdwatching would have been interesting in the transitional zone between the warm Gulf Coast Corridor and the boreal forests of the mountains.  Harsh winters would have chased boreal avifauna farther south, and during summer tropical stragglers would have expanded their range north.

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2 Responses to “The Pleistocene Floral and Faunal Invasion of Southeastern North America”

  1. James Robert Smith Says:

    I was in a National Forest protected area a few years ago that had a patch of the dry prairie. I assume it’s the same type you mention in your (once again) excellent essay.

    Whenever I’m in Florida I always keep an eye out for the caracara but I’ve never seen one. We’re heading down there again this Spring so I’m going to try again. Of course I’ll have to be in one of the areas where they still exist. We’ll take trips to some of the counties where they’ve been recorded, so maybe I’ll get lucky this time.

  2. markgelbart Says:

    You might have better luck looking for caracaras in cow pastures instead of natural areas.

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