Pleistocene Caracaras

The pasturelands interspersed with woodlots that cover much of rural south Florida today likely resemble the coastal oak and pine savannahs of this same region during the Pleistocene. South Florida and the southern Gulf Coast were climatically out of sync with the rest of North America during Ice Ages. When climate phases of dry cold conditions struck the rest of North America, south Florida and the southern Gulf Coast experienced warmer wetter subtropical climates. The Gulf Stream of the present day carries tropically heated water north, keeping climates in the northern latitudes of North America relatively moderate, but during cold climate phases of the Ice Age, it shut down. Instead, this warm water stayed at lower latitudes ironically making climate along the southern Gulf Coast even warmer than present day conditions. This warm climate spurred frequent thunderstorms and hurricanes. Lightning-ignited fires and windstorms destroyed trees and created open savannahs where mammoths, bison, and horses further suppressed the growth of unbroken forests. Trees that survived fire and wind were spaced far apart, and woodlots were restricted to the vicinity of waterholes where the trees were protected by watery fire breaks. This warm savannah habitat occurred from south Florida, Alabama, Mississippi, Louisiana, and Texas all the way to eastern Mexico. Much of this land has been inundated by sea level rise since the end of the last Ice Age. Warm coastal savannahs were ideal habitat for many species of plants and animals including caracaras.

Crested caracara. Photo by Wisniewski.

Crested caracara range map. There is a disjunct population in south Florida. During the Pleistocene this range was continuous with its population in Central and South America. Warm savannah occurred along the Gulf Coast, much of which is now inundated by rising sea levels.

Two species of caracaras inhabited Gulf Coast savannahs during the last Ice age–the crested caracara (Polyborus plancus) and the yellow-headed caracara (Milvago reidei). The former still occurs as a relict population in south Florida. This species lived on dry prairies throughout much of Florida, but that type of habitat has largely been transformed to rural, suburban, and urban landscapes. A recent scientific study in Florida found 103 crested caracara nests, and most of those were on improved pastureland. They seem to prefer pastureland over what remains of their original dry prairie habitat. I think this is a clue they benefit from the presence of megafauna. During the present day this means herds of cattle, but formerly they accompanied now extinct and extirpated megafauna. Caracaras forage on the ground for carrion. There was an abundance of carrion during the Pleistocene. They hunt for insects, reptiles, and small mammals stirred up by grazing herds of megafauna. And along with swallow-tailed kites and other opportunistic birds, they hunt down small animals fleeing wildfires. The widely spaced trees and small woodlots located on the pastures or savannahs are used for nesting.

Study of crested caracara nests in south Florida. Most of their nests are located on cow pastures that resemble Pleistocene habitat. Image from the below reference.

Yellow-headed caracara.

Yellow-headed caracara range map. During the Pleistocene they also occurred in Florida.

Fossil remains of both species were found at the Cutler Hammock site located in Miami, Florida. Yellow-headed caracaras no longer occur in North America, but habitat during some phases of Ice Age climate was so favorable in this region that it attracted both species. The Cutler Hammock site is notable for having yielded many remains of large carnivores including dire wolf, saber-tooth, giant lion, jaguar, and cougar. Their kills helped feed a diverse population of avian scavengers.

Reference:

Morrison Joan

“The Crested Caracara in the Changing Grasslands of Florida”

Click to access 3-17145_p.21115_Mor_FDPC_d.pdf

See also: https://markgelbart.wordpress.com/2013/10/18/two-pleistocene-carnivore-dens-near-miami-florida-part-2/

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Some Plants and Birds that Favor Overgrazed Landscapes

Herds of Pleistocene megafauna were mobile and could never stay in one location for long or they would outstrip their food supply.  They seasonally traveled along the same corridors for decades or even centuries, until varying climatic changes altered their migration patterns.  During interstadials when forested conditions prevailed, grazers were forced to travel far and wide to satisfy their nutritional requirements, and during stadials when grassy environments were more common, the browsers had to migrate greater distances.  Many megafauna game trails led to valuable resources such as salt licks and water holes.  The megafauna greatly influenced the composition of plants, insects, and birds along these game trails, creating unique ecological associations and natural environments.  They trampled the ground, overgrazed and overbrowsed vegetation, debarked and killed trees, and unleashed large quantities of manure.

Most of the unique ecological associations formerly found on megafauna game trails no longer exist in North America, but the buffalo traces that early settlers encountered were a final relic.    Many buffalo traces led to the Blue Licks in Kentucky–a freshwater spring and salt lick.  The “immense” herds of buffalo had eroded the land everywhere here from up to 4-5 miles away.  Tree roots were visible due to this erosion.  Two species of now rare plants are thought to have been more abundant then because they depended upon habitat created by buffalo herds traveling along these traces.

Short’s goldenrod–a rare species known from only 3 sites.  These sites were formerly connected by a buffalo trace.

Goldenrod soldier beetle.  This is an important pollinator of Short’s goldenrod.

Short’s goldenrod (Solidago shortii) was discovered on Rock Island–a limestone outcrop located in the middle of the Ohio River Falls.  This was a former crossing spot for bison herds.  The Macalpine Lock and Dam later flooded this location, extirpating this population.  This species was later found growing on Blue Licks Battlefield Park in northern Kentucky and Harrison-Crawford State Park in Indiana.  All 3 known localities have 1 thing in common–they were formerly connected by the same buffalo trace.  Apparently, this species of goldenrod requires overgrazed trails, and they spread along this corridor, thanks to the bison.  It’s possible the goldenrod seeds were transported in bison dung.  Without trampling, grazing, and erosion, other species of plants outcompete and exclude this species of goldenrod.

Running Buffalo Clover–another rare plant that used to commonly grow on the buffalo traces.

Running buffalo clover (Trifolium stoloniferum) is another species now more restricted in range than in the past.  This is the only species of clover that does not have a rhizobial association.  All other species of clover and legumes have colonies of bacteria growing in their root nodules that help them absorb nitrogen.  However, running buffalo clover grew in patches frequently fertilized with nitrogen-rich bison dung, precluding their need for a rhizobial association.

Megafaunal trails likely had groves of wild fruit trees originating from seed-filled dung.  Persimmon, paw paw, plum, crabapple, wild squash, and passion fruit (maypop) probably were abundant on the sunny trails where large shade trees suffered high rates of mortality due to damage suffered from megafauna activity.  Dung beetles, scavenging birds, and predators all followed the mobile herds.  Thickets of shrubs and brier patches colonized abandoned megafauna trails and pastures, providing ideal habitat for rabbits.

 

 

 

 

 

 

Burrowing owls prefer overgrazed cow pastures or frequently mowed golf courses and airports.

The burrowing owl (Athene cunicularia) has an odd present day range.  It mostly occurs in western North America, but a relic population lives in Florida.  Recent studies determined the relic Floridian population prefers overgrazed pasture land.  They like short grass with few trees, so they can have a good view of approaching predators.  They’ve actually expanded their range in the past century to include land cleared by humans such as golf courses, cemeteries, and airports.  However, they are considered threatened by increasing development that converts cow pasture to suburbs.  Burrowing owls must have occurred throughout much of eastern North America during the Pleistocene when game trails provided a network of suitable habitat.

Audubon’s Crested Caracara also requires overgrazed grasslands.

The crested caracara (Caracara cheriway) also occurs as a relic in south central Florida.  The rest of its range includes South America north to Texas.  Formerly, a coastal grassland habitat connected Florida with Texas, but during the Holocene forested habitat, unsuitable for this species, replaced this coastal grassland environment.  This is a scavenging bird that eats carrion, bird nestlings, disabled small animals, and insects.  They inhabit open ground where they search for food under woody debris and even cow dung.  Caracaras were also more widespread during the Pleistocene, but are more limited than burrowing owls because they are less cold tolerant.

Refererences:

Morrison, Joan

“The Crested Caracara in the Changing Grasslands of Florida”

Proceedings of the Florida Dry Prairie Conference 2004

Noss, Reed

Forgotten Grasslands of the South

Island Press 2013

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.