Thursday, February 19, 2009

My Research Part II: Elemental Imbalances

I discussed previously the rather important Law of the Minimum. Essentially, that law states that growth is controlled not by the total of resources available, but by the scarcest resource (limiting factor). As with most biological principles, this law was derived from agriculture. Those raising crops all over the world were in the habit of shifting huge amounts of manure from feedlots to crop fields with the hope that whatever they were adding would cause the crops to grow more. In fact, in most cases, the crops would grow quite a bit more if just a single nutrient were added. And that's why the history of industrial fertilizers is really a history of how people can add nitrogen and phosphorus to soils.

At the same time that farmers and other agricultural experts were discovering that adding a few pounds of ammonia had the same effect as adding a hundred pounds of manure, ecologists were going around and trying to figure out what the limiting factor was for a whole bunch of plants and animals living in the wild. Over time a number of trends began to emerge and embed themselves into the background of ecological thinking: Autotrophs tended to be limited by nitrogen in terrestrial systems and phosphorus in aquatic systems, large mammals tended to be limited by food energy more than nutrients, etc. And so people tended to focus on particular elements because of their 'obvious' importance to particular processes.

In fact, some quite prominent research looking at how ecosystems function was done by looking pretty much entirely at a single element. One of the most famous and fascinating researchers of the early 1900s was Ray Lindeman, who spent his entire, short career studying how just 1 element (Carbon) was cycled in a bog. And this was and is somewhat interesting, but a little bit of common sense should cut in at some point and begin to wonder: Why just carbon? Is carbon being cycled in a form that is only carbon? The answer, of course, is that materials that contain carbon (like leaves and bugs and soils) also contain lots of other elements. All those elements are being cycled at once, in far more complex and interacting dynamics than you would ever even understand existed if you considered only one element.

(Side Note: Why was Ray Lindeman so interesting? Well, he was the first person to do anything like this, his papers essentialy began the field of ecosystem ecology, and he died tragically young before he even completed a postdoc. I'm sure I'm not the only ecologist to have read other papers from his lifetime, read his papers, and wondered in a somewhat dumbstruck amazement as to whether the entire science of ecology was set back a solid 20-25 years by his premature death.)

Let's imagine the biological parts of the carbon cycle for a moment. Atmospheric CO2 is taken up by plants, which are eaten by herbivores, who are eaten by predators. Along the way some plant material falls off and turns into soil, as does the feces and bodies of the animals. Within the soil, bacteria burn all that carbon for energy, releasing CO2 into the atmosphere, which goes back to the plants again. But what determines the amount of CO2 a plant takes in? If CO2 is the limiting factor, then the amount of CO2 taken in will be related to the maximum growth rate of the plant. But what if the amount of nitrogen in the soil is the limiting factor? Does the plant keep taking in more CO2 than it can use? Or does it taken in just as much CO2 as it can use with the amount of available nitrogen? Do different plants do different things?

The underlying problem here is that sometimes (probably most times) the resources available to an organism are not the ones that are optimum for its growth. To go back to the Liebig's barrel analogy from two weeks ago (here), organisms have no reason to build a long stave out of CO2, when they can only build a short one out of nitrogen. When an organism is faced with resources that do not match its elemental demands, we refer to it as an elemental imbalance. A big chunk of ecological stoichiometry is determining how elemental imbalances come to be, how organisms react to them, and how they influence the cycling of elements.

Let's explain some of the possibilities a little here. Let's go back to our hypothetical plant, and let's give it a name. Let's call it a mustard plant. Our hypothetical mustard plant is growth-limited by nitrogen in the soil, but has enough of everything else, and has a huge abundance of CO2 (because the atmosphere is essentially a limitless source of the stuff).

The first question is: Why? Why did the mustard plant evolve to need more nitrogen than was available here? There are lots of possible explanations for this, but one of the key issues is that chemical processes that make life alive are fundamentally constrained. It simply isn't possible to create a protein molecule without using some nitrogen. In many cases, the nutritional needs of an organism are determined not by evolution or biology, per se, but by the fundamental nature of the universe. These fundmental constraints make living organisms relatively homeostatic (that is, their body elemental composition doesn't change), at least compared to non-living materials.

The next question is: So what is this little mustard plant going to do about it? The answers to this depend a lot on the context of how they are asked. For instance, a continued shortage of nitrogen over evolutionary time scales might result in mustard plants evolving to minimize their nitrogen demands or possibly mustard plants will develop ways to extract more nitrogen from the soil. Alternatively, mustard plants might use that extra CO2 in other ways. For instance, many plants create secondary defensive compounds that consist of carbon compounds with little or no other nutrients. These defensive compounds may make the plant poisonious or unpalatable.

Finally, we might wonder: What is controlling the amount of carbon moving up the food change from this plant? If, for instance, nitrogen is limiting growth, then it may be directly linking to the amount of carbon being pulled out of the atmosphere. However, if the plants are producing the secondary defensive compounds with excess carbon, there may be a non-linear response. Plants begin to get starved for nitrogen and stop their basic growth process, but continue to produce new mass in the form of secondary defensive compounds, which deter herbivores from consuming, and therefore further reduce the amount of carbon moving up the food chain.

And all this is going on in the context of multiple plant and animal species competing for the same resources as well! Needless to say, this is an incredibly complicated process that will probably continue to keep ecologists occupied for, oh, say, the next ten thousand or so years.

And this is only the tip of the iceberg!

Thursday, February 12, 2009

Species Profile: Alligator Snapping Turtle

There are few more intimidating and mysterious animals in North America than the Alligator Snapping Turtle, Macrochelys temminckii. And few are more deserving of this aura than the mighty Alligator Snapping Turtle: Adults reach a whopping 175 pounds, are characterized by a row of nasty looking spikes down their back, and have a bite that is fully capable of breaking a broomhandle in half. Did we mention that one of their prey items is alligators (admittedly, small ones)?

Like all turtles, Alligator Snapping Turtles are long-lived. They do not achieve sexual maturity until approximately 11-13 years old (Dobie 1971), and they've been known to live at least 70 years (Gibbons 1987). I've always heard stories about a snapping turtle caught in the 1980s with a Civil War Era bullet lodged in its shell, but I'm essentailly unable to confirm that story (google has failed me).

Snapping turtles tend to attract amazing stories. According to unverified reports, the largest alligator snapping turtle ever was caught right here in Kansas, on the Neosho River, and weighed in at over 400 pounds. The largest verified specimen was 236 pounds, and I can't even imagine what I would do if I was dragging a seine net up and realized that a 200 pound snapping turtle was slowly crawling along the net towards me. I'm always a little freaked out by pulling up small common snapping turtles.

Oh, what was that? You didn't realize there were two kinds of snapping turtle? The far more numerous Common Snapping Turtle, Chelydra serpentina, is a totally different species. There are a lot of fairly obvious morphological differences (no spines on an adult common), you can see them side-by-side here. If you live in Kansas, the odds are you've never seen an Alligator Snapping Turtle in the wild. The common snapping turtle gets big (25-30 pounds in the wild), but not nearly as big, and frankly, that's part of what makes the alligator snapping turtle so much more awesome.

This is a 1-year old! (source)

So if you're a hundred pound turtle with no natural predators, what do you spend your time doing? Finding enough to eat! A study in Arkansas and Louisiana found a huge list of food items in the guts of 109 sampled individuals. By far the most abundant item was fish, but there were also crawfish, molluscs, other turtles, insects, Myocastor coypus (nutria), and even armadillos, possums, squirrels, hogs, snakes, eggs, and vegetation (adults are somewhat omnivorious; Elsey 2006). In all likelihood, several of these species were consumed post-mortem. Other studies have found acorns to be a significant source of food (see citations in Elsey 2006). In captivity, alligator snapping turtles have eaten just about anything put in front of them.

I wouldn't recommend trying this at home. (source)

Alligator snapping turtles are a uniquely North American species, and are restricted to river systems draining into the Gulf of Mexico. Which isn't to say these aren't extremely mobile guys. Unlike common snapping turtles, alligator snappers stick almost exclusively to large river systems. Instead of swimming they seem to mostly get around by just walking on the bottom of the river, which isn't the same as saying they don't swim at all. Individuals have been shown to move 27-30 km in a 3-year span and 16 km in a 2-month span (discussion in Shipman and Riedle 2008). That may not seem like a lot until you consider that individuals prefer to spend most of their time hiding out in cover, and get around by walking along river bottoms.

Despite all that movement, individual river systems seem to have genetically distinct populations of snapping turtles. Roman et al. (1999) found that 12 drainages had unique genetic characteristics, thereby justifying their management as separate units. This is probably explained by the fact that these turtles have never been found to move through terrestrial habitats (again, unlike common snapping turtles). In fact, common snapping turtles seem to be one large, genetically similar population across virtually all of North America.

Alligator snapping turtles are protected at the state level to some degree everywhere they occur, but have repeatedly failed to achieve federal status. This is borderline astonishing, since a wide range of data suggests that the species has been in rapid decline since the 1950s, continues to be threatened by the development of dams, and is considered a culinary delicacy in many parts of the United States. Overharvesting has been shown to dramatically affect the population structure of the species, even decades later. As with many other long-lived species, it is generally believed that the loss of reproductive adults will take a long time to recover (as I mentioned above, it takes over a decade for an individual to reach maturity). Within Kansas the Alligator Snapping Turtle is a Species in Need of Conservation.

Lit Cited:

Dobie, J.L. 1971. Reproduction and Growth in the Alligator Snapping Turtle Macroclemys temmincki (Troost). Copeia 4:645-658.

Elsey, R.M. 2006. Food habits of Macrochelys temminckii (Alligator Snapping Turtle) from Arkansas and Louisiana. Southeastern Naturalist 5:443-452.

Gibbons, J.W. 1987. Why do turtles live so long? BioScience 37:262-269.

Shipman, P.A. and J.D. Riedle. 2008. Status and distribution of the Alligator Snapping turtle (Macrochelys temminckii) in Southeastern Missouri. Southeastern Naturalist 7:331-338.

Thursday, February 5, 2009

My Research Part 1: The Law of the Minimum

Although I love talking about random wildlife and neat Kansas habitats, I am interested in updating my online explanations for my own research as well. Therefore I'm going to take us down a path towards understanding the fairly complex research that I do. So that I don't lose anyone, I'm going to try to start from some basic ecological concepts and work my way towards the more complex ones.

There are few more fundamental aspects of life than eating. Life does not spontaneously generate energy or material, and therefore all living things must pull nutrients from their surroundings in order to grow, survive and reproduce. On a personal, visceral level, every human understands this concept.

The early biologists (i.e., farmers) understood it too, and they understood that plants, as well as animals, needed to pull in nutrients to survive. Generally speaking, the first crops that would be planted on a plot of land would yield better than subsequent plantings. At least from the times of the early Egyptians, but probably long before that, the use of animal and plant waste products as fertilizers was common. There was very little understanding from those early farmers about what was actually being replaced with the additions, but whatever it was seemed to work. Adding manure or other fertilizers (dead plants, ashes, crushed seashells, etc.) prior to planting would cause greater yields.

Figuring out exactly what was happening had to wait until adequate understanding of chemistry was developed, and that happened during the lifetime of Justus von Liebig (mid 1800s). At the time, a lot of focus was given to the organic character of the soil. Much stock was put in the amount and quality of humus within the soil (Humus is degraded plant or animal matter that gives soil a dark brown or black color). Liebig, on the other hand, thought that humus was an essentially meaningless product for increasing plant yields, and argued instead that a single, inorganic component (ammonium) was far more important.

What Liebig understood is that a plant is made up of a few dozen elements. Agricultural plants needed all these elements to survive, but the repeatedly used soil was only deficient in one of them. When farmers of his day added manure or humic soil to their fields, they were adding a mix that contained all the elements, but all that mattered is that they were adding the 1 element that was missing. Liebig explained this by using a visual metaphor of a bucket constructed with staves of unequal length. Each stave represents a single nutrient that the plant needed, and the amount of water that the bucket can hold represents the plant's yield.

What Liebig explained is that the bucket would only hold as much water as the shortest stave. That is, a plant will only grow until it runs out of the nutrient that is least available. Liebig argued that for the agricultural systems in his neck of the woods (Europe) the nutrient in least supply was nitrogen, and that in order to increase yields, all one needed to do was add nitrogen (in the form of ammonia). Although there was some hiccups in the implementation of this concept, it has certainly proven to be correct. So much nitrogen fertilizer is used in the Mississippi basin, for instance, that it is causing the infamous Gulf of Mexico Hypoxic Zone.

However, what's true for plants is actually true for all living organisms. Liebig's Law states : growth is controlled not by the total of resources available, but by the scarcest resource (limiting factor). Notice the word resource and not element or nutrient here. That's because sometimes the limiting resource is not chemical. For instance, the growth of a population of hermit crabs may be limited by the availability of shells rather than any particular nutrient.

Liebig wasn't the first one to propose this idea, but he certainly made it popular. The effect on agriculture was slow but astounding. Fertilizers that targeted the missing nutrients in the soil (usually nitrogen) became extremely important to the boom in agriculture in the 1900s.

This idea of a limiting factor has really fascinated ecologists. Experiments have been on plant and animal species from aquatic and terrestrial habitats all over the world trying to get an idea of what the most likely limiting nutrients are. In general, important limiting factors for plants have included light, nitrogen, phosophorus, and the availability of pollinators, while larger animals (like humans) generally seem to be limited by food energy (the # of calories in the available food).

Limiting factors are an important component of evolutionary theory as well. This is, essentially, where evolution occurs. If a particular resource is limiting, and everyone in a population is competing for that resource, then individuals who can exploit that resource more efficiently will be more successful and more likely to pass on their genetic material.

Ok, I think that's enough for this week.