From Fresh Water to the Sea: The Turtle Bridge is Open
By Whit Gibbons
In the 1970s, 129 research papers on freshwater turtles were published
in the three major herpetology journals (Copeia, Herpetologica, Journal
of Herpetology) that publish scientific papers, compared to only 39 papers
on sea turtles during the same period. Prior to and during that decade,
the most notable contributor to research on both groups was Archie Carr.
Otherwise, the overlap among authors in the two categories of turtles was
low, virtually zero.
During the next decade and into this one, the situation has changed
in two ways. First, the number of research papers on sea turtles (106)
has increased relative to the number on freshwater turtles (135). Second,
many of the authors of freshwater turtle papers were formerly authors of
sea turtle papers. This is a healthy sign, for all of the species under
study are turtles and are more similar to each other than they are to any
other group of organisms.
The same basic biological principles apply. All turtles lay eggs and
begin life as vulnerable prey items for a vast array of predators. Most
develop self-protection through large size and body armor. Perhaps the
greatest similarity between marine and freshwater turtles is the documented
longevity of individuals, which exceeds that of almost all other animals.
Therefore, they should be studied as a group for many aspects of their
biology, although different questions may be asked relative to certain
physiological questions influenced by salinity levels. Or, certain ecological
questions may need to be approached differently, such as ones involving
the immense ocean distances traveled by some of the sea turtles compared
to overland travel by freshwater species. Also, most turtles face innumerable
human-caused perils, some that are similar and some that differ depending
on whether the turtle lives in the ocean, in fresh water, or on land.
My objective is threefold: (1) to discuss the connectedness between
research ecology conducted on sea turtles; (2) provide accounts of significant
research conducted on freshwater turtles that was carried out by former
sea turtle researchers; (3) recognize the conservation and protection problems
faced by all turtles that will be aided greatly by the combined efforts
of all turtle conservation ecologists.
Some might argue about whether advances in ecological knowledge of freshwater
turtles preceded that for sea turtles by a few years, or about whether
sea turtle researchers have benefited scientifically from the advances
made by freshwater turtle ecologists. But few are likely to disagree that
successful conservation and protection for sea turtles has greatly exceeded
that for freshwater turtles in virtually all categories, and has preceded
it by decades. Sea turtle conservationists have done a fine job of garnering
public support for protecting marine species. An increasing number of freshwater
species would profit greatly if such successful efforts could be directed
toward their plight as well.
Crossing the Bridge: Setting the Example
Five individuals associated with the University of Georgia’s Savannah
River Ecology Laboratory (SREL) are investigators who have published a
significant study on freshwater turtles as well as having conducted research
with sea turtles prior to moving inland. Figuratively, they have crossed
the bridge between salt water and fresh. Some have subsequently moved back
to the ocean and can’t seem to make up their minds.
Nat Frazer emerged early from the sea, having conducted his doctoral
work on nesting patterns of loggerhead sea turtles on Little Cumberland
Island, Ga., and is noted for his published model on estimated survivorship
patterns of the species. In later years he developed two of the most complete
actuarial life tables ever constructed on the demography of any turtle,
based on the foundation of research data on two freshwater species (eastern
mud turtle and yellow-bellied slider) already available at SREL, while
continuing to publish material based on his interest in sea turtles.
Stephen Morreale made his debut as a turtle biologist through his research
with Jim Spotila, Ed Standora, and G. J. Ruiz, which resulted in their
1982 publication in Science on the importance of temperature dependent
sex determination to conservation of the green sea turtle. Both Spotila
and Standora had conducted freshwater turtle research at SREL, thus developing
the bridge between freshwater and the sea from the landward side. When
Steve came to SREL, he used several years of data already collected during
field studies by others, augmented with data of his own, and published
a paper on terrestrial movement patterns of freshwater turtles. The paper
established the maxim that adult male freshwater turtles characteristically
move overland more frequently and for greater distances than do females.
Rebecca Yeomans came to SREL as a graduate student after assisting Anne
and Peter Meylan with their studies of sea turtles in Panama. For someone
whose only previous research experience had been with sea turtles, Rebecca
did an outstanding field experiment with freshwater turtles at SREL. The
essence of her Master’s thesis was published in Animal Behavior in 1995,
a study that convincingly demonstrated that under certain conditions adult
slider turtles can orient unerringly for long distances overland to locate
wetlands that contain standing water. The study was an extension of earlier
research indicating that some turtles had the capability of leaving a drying
wetland and finding their way to the nearest available water, at distances
of more than a half mile.
Vinny Burke received his initial research training studying ridley sea
turtles with Steve Morreale and Ed Standora in New York. Although Vinny
published several important scientific papers on the diet of ridleys in
the Atlantic waters off Long Island as a result of this earlier work, his
most significant contribution to turtle ecology was part of his doctoral
dissertation on freshwater turtles at SREL. His landmark publication in
the journal Conservation Biology in 1995 on the terrestrial hibernation
and nesting sites of four species of turtles provides the documentation
needed to demonstrate the inadequacy of federal wetlands delineation regulations
now in place. The evidence Vinny has provided underscores the need for
stringent rules governing the establishment of terrestrial buffer zones
around freshwater wetlands.
Tony Tucker conducted research on leatherbacks on St. Croix while a
student at the University of Georgia and SREL. His publication on reproductive
variation of the marine leviathan contributed greatly to our understanding
of sea turtle conservation issues. However, while at SREL he turned his
attention to another species, one neglected in much of the scientific literature
for many years, the diamondback terrapin of the salt marshes. Tony’s publications
on the diet and other aspects of the biology of terrapins on the South
Carolina coast completes the turtle bridge in a geomorphological and physical
sense as well as a professional one.
Such examples of turtle researchers who gained their initial experience
with sea turtles and then applied their knowledge to research efforts with
freshwater species were practically non-existent even two decades ago.
Sea turtles were one group of animals with their own suite of investigators;
freshwater turtles were another. The two seldom overlapped in regard to
biological findings or even conservation measures.
Today the connection between the two is needed more than ever. Not only
do we need the cross-communication from a research perspective, but the
two former groups, which are rapidly melding into one among turtle researchers,
need to function as one in conservation efforts. Turtle biologists, whether
they conduct their research in salt water or fresh, need to join forces
in efforts aimed at ameliorating the worldwide threats to this beleaguered
group of animals we know as turtles.
Whit Gibbons is a professor of ecology at
the University of Georgia Savannah River Ecology Laboratory. Patrica J.
West and Tracey D. Tuberville made constructive comments on the manuscript.
Manuscript preparation was aided by Financial Assistance Award Number DE-FC09-96SR18546
from the U.S. Department of Energy to the University of Georgia Research
Foundation.
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