Posts Tagged ‘Pleistocene survivors’

The Invasion of the Little Lobsters

November 12, 2010

Tens of millions of years ago, probably when dinosaurs still stalked the earth, armies of salt water crustaceans inhabited the shallow seas, salt marshes, and estuaries so abundant in that watery age.  During the Eocene when the Atlantic Ocean periodically receded from the North American continent, these marine crustaceans began to evolve the ability to survive in brackish water habitats that became more and more common, and eventually they evolved into freshwater organisms.  The ecological niche they colonized had few, if any, competitors, and many species evolved.  Southeastern North America has the most species on the continent–evidence that this is the region where crayfish first colonized fresh water.  68 species of 8 genera are known to occur in Georgia today.

Illustration of crayfish anatomy from Crayfishes of Georgia by Holcombe Hobbs.

Biologists like to study crayfish because they represent good case studies of evolution.  Barriers often form in river drainages, isolating different populations of crayfish, and accordingly this affords frequent opportunity for speciation to occur.  Scientists use cladistics (the comparison of anatomical characteristics) to determine the evolutionary relationships between species.  Of course, DNA testing is an even more accurate way to determine these relationships.

Holcombe Hobbs was the foremost crayfish expert in Georgia, and he wrote a very thorough book in 1981–The Crayfishes of Georgia which is available online   http://si-pddr.si.edu/dspace/handle/10088/5545  for free.  He knew of 65 species in state.  In the 30 years since his book was published, only 3 additional species have been discovered in Georgia surveys, showing  just how thorough he was.

I find most interesting the burrowing habits of crayfish which create little mud chimneys in wet fields and stream sides.  Crayfish also burrow at the bottom of streams, but these don’t result in visible chimneys.

Illustration of crayfish burrows from The Crayfishes of Georgia.  Note the chimney-like hills.

It’s likely the ancestors of crayfish lived in intertidal zones near the sea shore.  At low tide they survived dessication by digging burrows in the sand and mud, similar to many modern day organisms.  Later, this habit helped them survive drought when they colonized upland freshwater habitats.  Crayfish are yet another example of a Pleistocene survivor because they were able to survive the many dry climate phases of the Ice Ages.  Today, upland crayfish dig burrows down to the water table.  Other species dig burrows directly in stream beds.  The ones that dig on land usually tunnel around tree roots.  According to Dr. Hobbs, these can be difficult to excavate when collecting specimens.   Some species defend their burrows aggressively; others retreat deeper into side chambers.  During droughts and cold weather, crayfish plug the tops of their chimneys to stay warm and moist.  If the water table falls below the level of their burrow, they become dormant.

Much crayfish habitat has been destroyed by the creation of reservoirs which are like deserts for this animal.  They don’t like deep water.  However, man has created a lot of crayfish habitat.  Crayfish do like to live in roadside ditches.

Here’s a list of major rivers and drainage systems in Georgia and the number of known crayfish species each holds.

Altamaha–22 species

Chattahoochee–14 species

Chatooga–9 species

Coosa–15 species

Flint–14 species

Ogeechee–16 species

St. Mary’s–7 species

Satilla–10 species

Savannah–20 species

Suwanee–8 species

Tennessee–12 species

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When my family first moved to Athens, Georgia in 1976 (I was 13), I delighted in rambling around the woods that surrounded the partially developed residential neighborhood, not far from Cedar Shoals High School.  It was mostly second growth pine and oak interspersed with old fields and plum thickets.  There was even a pond site with a small waterfall that led to a chain of beaver ponds.  (Developers have since destroyed this last described landscape and built a shopping center over it.)  One day, a newfound friend and I took a dip net to one of the creeks we frequently followed.  Along the creeks, we always saw the handlike prints of raccoons, and occasionally the empty shell of a crayfish that had provided a meal for the former.  Simply by swinging the net across the bottom of a stony section of the creek, we collected two dozen crayfish in a short period of time.  We put the crustaceans in a bucket and brought them h0me, setting the container in a backyard lawn.  The next morning, the crayfish had vanished–a mystery for us then because we didn’t know crayfish are nocturnal and can travel over land at night.

Crayfish taste delicious–much like shrimp but without the iodine flavor.  Eat them fresh, not previously frozen.  Fresh, the tail meat is delectable, and the juice from the heads (actually the brain) is rich.  Previously frozen, they smell bad upon preparation.  The tail meat is still palatable, though nothing special, but the juice from the heads tastes like a mixture of mud and rotten fish.

References:

Hobbs, Holcombe

The Crayfishes of Georgia

Smithsonian Publications 1981

Pleistocene survivors–Oaks

March 8, 2010

The purpose of this blog is to promote my newly published book–Georgia Before People: Land of the saber-tooths, mastodons, vampire bats, and other strange creatures.

I just approved my book for distribution. Within the next 6-8 weeks it will be available from http://www.amazon.com, and will show up in book search databases. It’s already available for purchase at http://stores.lulu.com/GeorgiaBeforePeople

I will use this blog to highlight some points that I didn’t cover completely in my book, or to discuss new findings that didn’t make it into my book.

Pleistocene survivors

Think about this: Every animal and plant that you see today is a Pleistocene survivor. All extant species lived during the Ice Age, otherwise they wouldn’t be here today. Oak trees are among the most abundant living species found in temperate North America today. Pollen studies from Georgia and other southeastern sites show that oak trees were fairly common here during the Ice Age as well. Oak consistently makes up about 12% of pollen samples at various Pleistocene sites in Georgia. Though pollen studies have some drawbacks (which I discuss in my book) when estimating actual tree populations, this means that about 1 in every 8 trees in the Pleistocene forests of Georgia were oaks. Like today, they shared the landscape with southern pines and hickory, but unlike today, Pleistocene forests in Georgia also consisted of northern species of pines and spruce trees as well, at least in the north and central parts of the state. A more equable climate with cooler summers and mild winters allowed more species of trees (as well as small animals) to co-exist than do in today’s Georgia.

Oaks require a great deal of sunshine to germinate and do well. This means that in Pleistocene Georgia, there were plenty of sunny locations for oak trees to sprout up on. This implies an open woodland kind of forest rather than the closed canopy woods that began to dominate in the Holocene. Fire, drought, ice storms, wind, and megafauna browsing and trampling are the factors that kept Pleistocene landscapes more open.

Man is the reason oaks continue to thrive in today’s environment. Men cut down trees, clear the land for agriculture, than abandon it, thus allowing oaks to return.

I visited protected areas in the Appalachian mountains of North Carolina last summer where there has been little lumbering for the past 100 years. Surprisingly, the forests here have few oaks. The dominant trees are maple, tulip, beech, white pine, and hemlock. These trees shade out oaks. This makes me think that the Great Smoky Mountains National Park has less wildlife than it should because the habitats are undisturbed. Oak is an important mast tree–the acorns providing an important food source for all kinds of wildlife. After the blight wiped out the chestnuts, oaks became even more important, but without lumbering or fire to open the landscape, the supply of acorns is declining, and accordingly, the wildlife in this park is a pitiful remnant of what it should be. Park officials probably wouldn’t agree with me when I say some of it should be lumbered for the purposes of diversifying habitat.

One more thing to consider about oaks in Georgia. I’ve studied the range map of the bur oak–this is a tree that likes rocky calcareous soils and thrives in the cross timber region of midwestern North America where prairie meets woods. A very small scattered population exists in northern Alabama and Mississippi, but none have been found in Georgia. It’s possible that during the Pleistocene, bur oaks were more widespread and may have occurred in state. Unfortunately, plant macrofossil sites in Georgia are so rare we’ll never be able to confirm my suspicion.

Next up: A discussion of Pleistocene eagles.