Trail #97 in the Cohutta Wildlife Management Area

I think the name of Trail #97 is the Etterle Creek Trail, but I didn’t write it down and now I can’t remember for sure.  

Our trip 2 weeks ago to Land Between the Lakes was an 8 hour drive.  I decided to break 1 of the travel days in half and stay in Chatsworth, Georgia, so we could hike the Birdsong Trail on Grassy Mountain.  When we got to the mountain the paved road became a gravel road.  The gravel road was in good condition, but  I’m never too thrilled with driving on unpaved roads.  I would have kept going because the trail sounded like a great bird-watching destination, but my daughter suggested we stop and walk on any of the perfectly lovely trails that we kept passing by.  On the route to Grassy Mountain, CCC Road turns into Lake Conasauga Road which leads to the Birdsong Trail.  I didn’t know if we had reached the latter road yet and had no idea how long it would take to get there.  Winding mountain roads are slow-going, so because we had a 4 hour drive to Clarkesville ahead of us anyway, I agreed to stop at Trail #97 instead.

The trail is little more than a wide ledge between a steep mountain rise on one side and a creek gorge on the other.  The trail is about 400 yards long and dead ends at a gorgeous shoal on the creek where enormous Paleozoic-age boulders rest.  Dominant trees in the adjacent forest are white oak, sweetgum, and hemlock.  The white oaks include 2 different leaf variations.  Some of the white oak leaves had such fat leaves, I thought I was looking at a different species, however, upon studying a tree field guide, I learned that some white oaks do grow much fatter leaves than others of the same species.  Sweetgum prefers warm moist conditions; hemlock prefers cool moist conditions, so both species reach a happy medium in this locality which is southern but at a high elevation.  I only saw 1 dead hemlock tree here.  I walked about a half mile up the road where the trail begins and could see in the distance a whole hillside of healthy hemlock trees.  Evidentally, the trees here are still unaffected by the scourge that’s wiping them out elsewhere.  Also growing in the nearby woods were mountain laurel, beech, white pine, river birch, loblolly pine, shortleaf pine, and post oak.

Big boulders across the trail make for a bit of a rugged hike. 

View of the gorge.

A healthy hemlock tree.  Almost all the ones I saw at this locality were healthy still.

Another view of the gorge from the trail.

The trail is a wide ledge with a gorge on one side and steep rock like this on the other.

I thought prohibition ended. 

Boulders at a shoal at the end of the trail.

A ten inch tall waterfall!

The creek is eroding through to bedrock.

The hillside in the background is an almost pure healthy stand of hemlocks.

I saw 2 species of birds–a belted kingfisher, and a common crow, but it was the latter that had successfully captured a fat minnow.  I’d never seen a crow catch a fish before.  Kingfisher’s are interesting birds that burrow and nest in muddy creek banks.  Two human fishers were fly-casting for trout at a bridge down the road.  The water here was cool and tasted good.

How Unusually Cool Ice Age Summers Probably Shaped Periodical Cicada (Magicicada) Evolution

It sounds like everybody’s burglar alarm is blaring in Evans, Georgia.  But the noise doesn’t originate from annoying, malfunctioning security systems.  Instead, the 13 year periodical cicadas have emerged.  For over a decade these insects have lived a foot underground, well below the frost line, but now they’re ready to mate–a frantic affair that takes place within a timespan of 3-4 weeks.  Almost their entire lives, they’ve lived as nymphs, surviving on the xylem fluids of deciduous trees.  The urge to mate causes them to dig tunnels to the surface which they crawl through.  Sometimes they continue to crawl, making it halfway up a wall or a tree trunk before the winged adult bursts through the back of the shell of its thorax.  The holes from which they’ve emerged are visible, their molted shells scattered under foot like discarded shrimp exoskeletons at a Cajun seafood boil.

Photo from google images of a periodical cicada.

The unusually long period between emergences among the 7 species of periodical cicadas in the genus Magicicada puzzles scientists who hypothesize about its evolution.  Magicicada emergences in large numbers are obviously a defense mechanism known as predator satiation.  Like passenger pigeons, they occur in such a high population that they overwhelm the ability of predators to consume them.  Unlike other types of cicadas which are strong, fast fliers, periodical cicadas are clumsy and slow, but so many appear at once that predators are unable to consume most of them before they have a chance to mate and lay eggs.  Some scientists think their emergences every 13 or 17 years (depending on the species) is a way to keep predators from increasing their own populations after exploiting them as a food source, thus avoiding a cycle when predator numbers eventually escalate enough to decimate cicada numbers.  This seems an unlikely explanation to me for 2 reasons: cicadas are only available for a few weeks a year which is not enough time to have a significant long term impact on predator populations.  More bird nestlings may survive at first due to the abundance of cicadas, but then for the rest of the year, they must adapt to the normal supply of food.  Moreover, the average wild bird only lives 2 years–far shorter than 13 or 17 years.  Cicadas could avoid upswings in predator populations with much shorter periods between emergences.  Other scientists believe the odd high prime numbers emergences can be explained by a combination of predator cycle avoidance and interspecific competition among nymphs.  Although several entymologists have derived statistical models supporting this theory, I think the paleoclimate explanation proposed by R.T. Cox, C.E. Carlton, and independently by Yoshimura is more plausible.

Periodical cicadas depend specifically upon deciduous forests.  During the coldest stages of Ice Ages, deciduous forests north of the southern Appalachians were rare relics outnumbered by other environments such as spruce forests and prairies.  The bulk of deciduous forests then occurred south of the Appalachian mountains.  Even here, summer temperatures occasionally were too cold for cicadas in the Magicicada family.  They require temperatures above 68 degrees F for a period of 3-4 weeks for flight and mating.  Drs. Cox and Carlton assumed that during the coldest stadials (which lasted on average 1500 years) 1 in 50 summers failed to reach this temperature, and cicada reproduction failed.  Using a statistical formula, they estimated that over a 1500 year stadial, cicadas emerging every 6 years had a 4% chance of avoiding unusually cool summers; cicadas emerging every 11 years had a 51% chance of avoiding unusually cool summers; but cicadas emerging every 17 years had a 96% chance of avoiding unusually cool summers.  Cicadas emerging after shorter periods were eventually eliminated from the gene pool, while those with genes for longer cycles became dominant.

Map of Magicicada ranges from the below referenced paper.  The distribution of 13 and 17 year periodical cicadas supports the paleoclimatic explanation for their high prime number emergences.

17 year cicada species tend to live north of 13 year cicada species, even though the shorter cycle is a dominant gene.  Summers too cool for breeding would’ve occurred more frequently in the norther parts of their range, so those with 17 year cycles would’ve had a greater chance of avoiding them than those with the 13 year cycle.

Reference:

Cox, R.T. and C.E. Carlton

“Paleoclimatic influence in the evolution of periodical cicadas (Insects: Homiptra:Cicidae: Magicicada spp.)”

The American Midland Naturalist 120: 183-193 1988

Notes on my observations of periodical cicadas

–Periodical cicadas are slow.  I was able to catch one by simply picking it off the ground after if fell in flight and landed on its back.

–Birds are feasting on them.  Crows are catching the nymphs as soon as they crawl to the top of their tunnels.  I also saw a Canadian goose nab one that fell in a small lake.

–Wow! They are loud.

–They must be mole food during their nymph stage.

–Evans has become heavily developed.  Cicada habitat has been greatly reduced and covered with blacktop parking lots from which they can never emerge and escape.  I wonder if this is another creature that survived for millions of years, until the actions of Homo sapiens eventually renders them extinct.

The Invasion of the Little Lobsters

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