Friday, February 17, 2012

The Chemistry of Turtles

The following article is a guest post by Sean Sterrett. I would like to feature more guest posts here in the future; please contact me if you're interested in contributing. Want to see more posts from Sean or other potential future guest bloggers? Encourage them by letting them know what you think in the Comments. Don't forget to subscribe to this blog here.

Sean Sterrett currently lives in Athens, Georgia and is a Ph.D. student at the University of Georgia researching the ecological roles that reptiles play in aquatic ecosystems.  Sean hails originally from Indiana, where he received his B.Sc. degree in Biology from Butler University.  Sean first travelled to Athens in 2004 to pursue a M.Sc. degree; he studied the impacts of land use on riverine turtle communities (I wrote about helping him with his fieldwork here).  Sean is an avid freshwater kayaker (a great way to see turtles), vegetable gardener and homebrewer.  

The field of conservation biology is often described as having arisen in response to urgent and global biodiversity losses in the latter part of the 20th century.  Since then, scientists have dedicated their careers (and lots of money) into figuring out how to improve the ability of animal populations to persist while conserving the habitats that they rely on.  Most scientists would agree that there are two major reasons for learning about how humans have impacted wildlife and natural ecosystems: intrinsic and ecological values.

Intrinsic value, or the ethical value placed upon something for its own sake, is one reason that people are inspired to learn about and study animals and ecosystems.  This has even led to a new sub-discipline of ecological philosophy known as "deep ecology."  For me, intrinsic value is rather simple to describe. I do not want to live in a world without its amazing animals, like tigers, elephants, pythons and sea turtles. Recognizing that I valued these animals inspired me to become a biologist.

The ecological value of animals, on the other hand, is more challenging to pinpoint.  Numerous studies have described what happens when certain species are reduced in numbers or even removed from their natural habitats. One striking example involves Gray Wolves, Canis lupus, in Yellowstone National Park. After being persecuted for many years, all of the wolves in Yellowstone National Park were killed off (they have since been reintroduced). Surprisingly, the disappearance of the wolf had trickling effects all the way down to evergreen trees.  Here’s how it happened: when wolves were removed, the pressure was taken off of populations of prey, such as Elk, Cervus elaphus. Elk are herbivorous and feed heavily on plant material.  Therefore, when the wolves were removed from the equation, elk populations grew larger and browsed plants at an increased rate.  So, wolves indirectly influenced forest structure and vegetation communities. These types of indirect effects are known in the ecological literature as trophic cascades.

Historically, top predators kept many of these cascades in check. Because I study reptilian ecology, it is worth mentioning that there are likely similar impacts occurring due to the loss of other top predators, such as Eastern Diamondback Rattlesnakes, Crotalus adamanteus. However, many of these impacts have yet to be studied or measured.

Predator and prey relationships are not the only ways wildlife species influence their surroundings. Here, I want to highlight the chemical roles of vertebrate animals.  Many studies have described how fish recycle and transport nutrients within aquatic ecosystems.  For example, gizzard shad (a small fish) feed in the bottom of lakes and can liberate stored nutrients, which then become incorporated into food webs. I would hope this might impress you?  If not, I will add that this type of nutrient "addition" to a lake can meet or even exceed external sources of nutrients, such as streams or atmospheric inputs.  There is a growing body of research describing how nutrients flow through ecosystems. After reading many papers on the subject, I now consider the world as one big chemical reaction. 

Trapping turtles may sometimes offer big surprises
When I was an undergraduate at Butler University in Indianapolis, Indiana, I was more interested in helping out on the Urban Turtle Ecology Research Project (UTERP) than studying for chemistry classes. Ironically, now as a PhD. student at the University of Georgia, those same laws of chemistry that I did not want to learn are now major components of my current research on turtle ecology.  So, how is it possible to integrate freshwater turtles and chemistry?

Well, just like the gizzard shad, turtles eat a variety of plants and animals.  Part of what they eat goes toward growing and maintenance of their body.  Other parts are used as energy for reproduction.  When turtles excrete ("poo" and "wee"), they are getting rid of waste materials that they don't need.  But one creature’s waste is another’s treasure. That "waste" material is made of elemental nutrients (remember the periodical table of elements?). These nutrients are used by many other life forms, especially aquatic plants and algae, which make up the base of most food webs.

There are many different species of freshwater turtle and some can be found in just about every type of wetland, including those uninhabitable by other animals. Turtles can make up a significant amount of animal biomass in aquatic ecosystems (the total weight of turtles in a lake can be close to the weight of all the fish). They also have some interesting characteristics that make them different from fishes and other aquatic organisms in their habitats.  First, turtles live very long lives. Each animal can influence its aquatic habitat for many years.  Second, turtles have one of the most formidable evolutionary defense structures of any animal: a shell. Their shell is composed largely of bone, which is packed full of calcium and phosphorus. When a turtle dies and decomposes, these nutrients are then released into the environment.

Incubation trials of riverine turtles
 in the Lower Flint River Basin
One objective of my doctoral research is to figure out how turtles impact aquatic ecosystems via excretion and retention of nutrients; nutrients that are necessary for functioning aquatic ecosystems.  I'm studying these questions using very common species in the southeastern U.S., a global hotspot for turtle diversity.  The photo below shows how I'm approaching the problem of determining the rates of excretion from turtles.  This process involves first catching turtles, then placing them in filtered water and taking a series of water samples for nutrient analysis.  Although it sounds simple, the process is labor intensive and logistically challenging, especially considering the hefty amount of materials I need to pack into my remote field sites, some of which are most accessible by boat.

So...turtles are certainly important members of aquatic ecosystems as consumers. I hope my research allows us to determine how they are chemically influencing their habitats and other animals. What if there weren’t any turtles around, would their chemical roles be missed?

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RIPPLE, W., & BESCHTA, R. (2004). Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems? BioScience, 54 (8) DOI: 10.1641/0006-3568(2004)054[0755:WATEOF]2.0.CO;2

SCHAUS, M., VANNI, M., WISSING, T., BREMIGAN, M., GARVEY, J., & STEIN, R. (1997). Nitrogen and phosphorus excretion by detritivorous gizzard shad in a reservoir ecosystem Limnology and Oceanography, 42 (6), 1386-1397 DOI: 10.4319/lo.1997.42.6.1386

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