Global Warming Caused Extinctions/Extirpations of Bird Species in Florida throughout the Pliocene and Pleistocene

Environmentalists fear the current climatic trend of global warming will cause numerous extinctions.  They have good reason to be concerned.  Dr. Steve Emslie surveyed most of the bird fossils ever found on the Florida penninsula and determined within which time span each species existed.  He discovered that waves of avian extinctions and extirpations occurred on the penninsula during times of global warming when sea level rose and flooded critical habitat.  (Note: Extinction means the permanent loss of a species; extirpation means a local extinction of a species that continues to exist elsewhere.)

This is a satellite map showing the potential encroachment of the ocean with a rise in sea level due to global warming.  With just a 5 meter rise in sea level (as occurred as recently as 130,000 years ago) the Everglades would be inundated.  Just think of the species extinctions, if that happened today.  The Everglades kite and dozens of land snails would become extinct.  Both of these images are  from the below referenced journal article.

Conversely, land area expands on the continental shelf when sea level falls during glacial expansion.  Glacial phases increase the amount and diversity of land habitat and thus the number of terrestrial species that can colonize the region.  An exposed land corridor allowed animals to reach the penninsula from Central America and Western North America.

The Avian Extinction Wave of ~2 Million Years BP

During the Pliocene from 3.5-3.0 million years ago and again from 2.5-2.0 million years ago, global cooling allowed a land bridge to emerge between North and South America.  This altered ocean currents and caused a major extinction of marine molluscs, especially salt water snails.  Scientists arbitrarily mark this extinction event as the boundary between the Pliocene and Pleistocene.  Much of what is now the Florida penninsula not only stayed above sea level but expanded in size due to the regression of the ocean from 2.5-2.0 million years ago.  Freshwater marshes commonly existed all over the penninsula.  Macasphalt Shell Pit and Richardsons Shell Pit are the fossil sites where bird skeletons of this age have been recovered.  About 2 million years ago, global warming melted the polar ice caps, and the ocean transgressed over much of what’s now Florida.  This sea level rise submerged freshwater habitat along what’s now the Georgia coast too, creating Trail Ridge (see my previous blog entry “The Geological and Ecological History of the Okefenokee Swamp part 1).  The ocean destroyed freshwater marshes and eventually left a layer of sea shells over the strata with the vertebrate fossils.  This destruction of habitat contributed to the extinction of at least 22 bird species (39% of the fossilized species known from these sites).  Almost all were small aquatic species such as gulls, ducks, and cormorants.

The Avian Extinction Wave of 1.6 Million Years BP

Global cooling once again caused sea levels to fall from 2.0-1.6 million years ago, creating a broad land corridor stretching along the Gulf Coast from Central America to Florida.  Glacial phases meant more dry land habitat and a variety of environments, including oak and pine forests, lightly wooded savannah, thorny brush, and freshwater marshes and lakes.  Numerous temperate, tropical, and western species of birds flocked to the region and left fossils at such sites as Inglis 1A and 1C, Haile 7C, DeSoto, and Pelican Shell Pit.  But global warming and sea level rise ~1.6 million years ago again caused the extinction/extirpation of bird species (at least 30 kinds–30% of regional avian species known), including the giant predator Titanis walleri, pygmy owls, and extinct species of eagles, vultures, condors, and cormorants.  The ring-billed kingfisher became extinct too, but a species of crow and two thrushes that used to live here may be the same as living tropical species still extant.

The Avian Extinction Wave of ~1 million Years Ago

Many bird species again colonized the region between 1.6-1 million years ago, leaving fossils at such sites as Leisey Shell Pit, and Haile 16A.  However, global warming and sea level transgression meant doomsday for 22% of the avian colonists, including extinct species of a loon, a pygmy goose, a rail, a bittern, a goose, a flamingo, a spoonbill, a heron, a woodcock, an ibis, a stork, and an auk.  Auks are arctic fish-eating birds, and this species must have ranged south to Florida during the height of Ice Ages.  Interestingly, J.J. Audubon reports puffins (as well as snowy owls) straggling as far south as Georgia during fierce 19th century winters.  The species of goose, pygmy goose, and flamingo that became extinct in Florida at this time survived in western North America until 11,000 years ago.

The Avian Extinction Wave of 11,000 BP

There aren’t many bird fossil sites in Florida dating between ~1 million-300,000 years ago, so a good survey wasn’t really possible for this time period, but sites dating from 300,000-11,000 are more abundant than from any other era.  The list of Rancholabrean Age bird and mammal fossil sites in Florida includes Reddick 1A, Arredondo 2A, Ichetucknee River, Rock Spring, Haile 11B, and Cutler Hammock. During this time period tropical, western, and northern temperate species colonized the state once more.  When the present day era began 11,000 years ago, global warming caused the submergence of the outer coasts of what’s now Florida.  However, this extinction/extirpation rate was low (14%), and most of the species were commensal scavenging birds.  Most scientists believe the reason for this wave of avian extinction was its correlation with megafauna extinction.  Scavenging birds had less food to eat, and as I thoroughly discuss in my book, overhunting by man is the most probable cause of megafauna extinction.  Woodward’s eagle, Grinnell’s crested eagle,  hawk-eagles, yellow headed caracaras, terratorns, California condors, and magpies disappeared from the state.  The latter two species still live in western North America, but were widespread in the east during the Pleistocene.  A few wetland birds became extinct or extirpated as well–a species of anhinga, a stork, and trumpeter swans; the latter species still also survies in small areas of the west.  Mexican grackles are another still surviving western species that used to live in Pleistocene Florida.  The southern lapwing, an extant tropical species lived in Pleistocene Florida.  Lapwings resemble plovers but are not closely related.  They prefer open habitat as did magpies.  Lapwings are on the increase in South America today as tropical forests are converted to pasturage.  Lapwings likely disappeared across southeastern North America when forests replaced Pleistocene prairies and savannahs.

References:

http://elibrary.unm.edu/sora/om/om050.pdf

For a species by species description of every extinct bird species in the Florida fossil record, be sure to click on the link above.

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