River Turtle Population Suffers Decades After Harvesting
September 26, 2012

River Turtle Population Still Suffering Decades After Harvesting

Alan McStravick for redOrbit.com - Your Universe Online

A recently published study by researchers from the University of Florida has shown how the river-dwelling northern map turtles have struggled to rebuild their populations since an unfortunate era of harvesting in the 1970´s.

A previous study of the northern map turtle, which can be found in river systems from Arkansas to Quebec, was compiled by Max Nickerson of the Florida Museum of Natural History. As curator of herpetology, he collected data in 1969 and 1980, the results of which were used as a baseline for this most recent study. In 2004 it was determined that the northern map turtle (Graptemys geographica) had still not recovered from its previously recorded 50 percent loss in population that was caused by harvesting.

This most recent study, published in the September 14 issue of Copeia, shows how this species has been affected by both an increase in human population as well as the effects of global warming. The northern map turtle, a protected species in some states, is important in the maintenance of river ecosystems since they play critical roles in nutrient cycling nutrients and in maintaining the food-web dynamic.

“The importance of river turtles is really underplayed,” said lead author Amber Pitt, a Clemson University postdoctoral research fellow who conducted research for the study as a UF graduate student. “River turtles are long-lived, rely on the same water resources that we do and can serve as indicators of water quality. People should be concerned if turtles are impacted by poor water quality because we are likely being affected, too.”

Until 2000, this species was known as the common map turtle due to its wide geographic distribution. The name was changed to northern map turtle in order to emphasize the species´ increasing rarity, explained Pitt. The species is already listed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and may benefit from this research if it is used as a guideline for overall conservation and protection.

The researchers determined that harvesting has been the most probable culprit for the 50 percent population decrease between the years 1969 and 1980. This probability was based on an analysis of data originally published by Nickerson and Pitt in the Florida Museum of Natural History Bulletin in August.

That data also indicated that there were fewer adult females in the population than males. The adult female is larger than the male of the species and is generally preferred in the food trade. “This shows that harvesting, even if it´s a one-time event, can cause a turtle population to significantly decline and remain impacted for decades, because this species doesn´t reproduce quickly,” Pitt said. “It was really discouraging to see that even without the pressure of further harvesting, they couldn´t recover over that long time period, which is partially due to their biology but may also be associated with habitat degradation and disturbance.”

The research conducted in 2004 also focused on examining the general habitat of the northern map turtle over a 3-mile stretch of the North Fork of the White River in Ozark County, Missouri. The researchers used snorkels to tag and record information on the turtles. Their examination of the river also showed habitat degradation as a direct result of increased siltation, sedimentation and algal blooms.

Habitat degradation of a river system is defined as the general lowering of the streambed by erosive processes, such as scouring by flowing water and the removal of channel bed materials and downcutting of natural stream channels.

“What´s happening in these big spring-fed rivers is very important,” Nickerson said. “When you clear the banks of a river, you increase siltation, which affects the food sources, reproduction, plant growth, species composition and basic ecology of that section of the stream, and perhaps the entire river.”

River degradation has been partially caused by human recreation, which drastically increased by 2004, Nickerson explained. People swimming and boating also frighten turtles so they may not bask in the sun as much as needed to maintain their health and maximize egg production.

As mentioned by Nickerson, siltation can be devastating to an ecosystem. Silt is a granular material that is derived from soil or rock that has a grain size between sand and clay. Once introduced to a river system, it can reside on the river bed itself or occur as a suspension within the water. An increase in siltation occurs as a direct result of forestry, grazing pressures and small crop farming. As fine sediment loads increase, siltation works to smother a river bed and to kill off invertebrates and fish eggs, which results in a reduction in spawning or the abandonment of the habitat by fish altogether. Additionally, siltation acts as a vehicle for the transport of certain pesticides and phosphates into river systems.

Although scientists generally agree that many turtle populations are declining worldwide, little has been published on river turtle communities, said Don Moll, professor emeritus at Missouri State University and co-author of a textbook on freshwater turtles.

“This is a very important study because it follows the dynamics of this turtle community over a more than 30-year time period, and really it´s the only published river turtle study I can think of that does that,” Moll said. “It´s a real contribution in that sense – it´s so unique.”

One concern with attracting conservation efforts to river turtles may have to do with their small size because they do not garner as much public attention as larger aquatic species, Pitt said. Adult female northern map turtles are only about 11 inches long.

“Often times with conservation, you have the charismatic mega fauna that people care about, such as sea turtles – everybody cares about sea turtles, including me,” Pitt said. “But river turtles are facing just as many threats as sea turtles. People are also harvesting river turtles and there are very few laws in place to stop this harvest – it´s a global epidemic that is causing turtle populations to be wiped out.”