The Ups and Downs of Hydrologic Connectivity

So yesterday I linked to an article about the benefits of dams. Then I read through my emails, and find someone has sent me this article, entitled:

"Ecological benefits of reduced hydrologic connectivity in intensely developed landscapes."

Really?

Ok, so the fundamental point of this article is actually pretty obvious. Take a look at the following figure from that paper:

If you think about the caption here, I think you'll find that it doesn't make any sense unless you replace 'hydrologic connectivity ' in the first sentence with 'aquatic ecosystems' or something similar. The entire paper treats hydrologic connectivity as an essentially 'physical' process that affects the entire ecosystem, so I think that is what is being implied here.

What are we seeing in this figure? What we are seeing is a simplistic representation of multi-dimensional environmental space. Instead of an n-dimensional space (where each dimension is a different environmental variable), this figure has been condensed down to just three dimensions: Biological variables, environmental variables, and physical variables. I think this is a good way to think about the effects of different management practices.

Now the first example the authors describe involves changing just the physical characteristics of a particular location. So, for instance, you restore natural stream flows to an area otherwise regulated by a dam. Assuming you are successful at doing this, you've moved the ecosystem properties towards the 'desired' portion for physical characteristics, but you haven't actually gotten into that area because the biological and chemical characteristics are still way outside that 'idea' space.

This is the only idea the authors treat explicitly with this figure, but almost everything else they talk about can be related back to this figure. That example shown in the figure itself (moving from X to X') implies that it is possible to change the physical environment without changing the biological and chemical environment. That's probably not the case very often. As the article I linked to yesterday shows (and numerous examples are also in this paper), removing a physical barrier can open up a previously isolated reach to invasion by non-indigenous species. When you move closer to the 'ideal' space by moving along the physical axis on this figure, you're actually moving further away on the biological side.

That's just one example of how things can go wrong. There are lots more in this paper. The authors break them into 7 categories, but I really think you could group them into 3:

1. Attracting native species to bad habitats
2. Allowing non-indigenous species to invade native refuges
3. Radically altering watershed biogeochemical dynamics (This one is tenuous to me, they talk about removing farm ponds and consequently losing all the sediment and nutrient removal those ponds do. I'm skeptical that this is really occurring, but I can get into that later. They equate these farm ponds to fulfilling the same functions beaver dams used to do...again...I'm skeptical.)

Unlike the paper I referred to the other day, this paper seems to fit a need in the literature. There's been a general sense in the stream ecology community that if you fix the physical habitat then the ecological community will follow along. This has led to work by people like David Rosgen to restore streams so that they function morphologically like natural streams. This view has also been supported by ecological theory (e.g., Poff and others suggesting discharge is a master variable controlling stream ecosystems). With the examples in this paper, Jackson and Pringle suggest it isn't going to be that simple:

Restoration of hydrologic connectivity in a disturbed landscape moves an aquatic system towards a new ecological state, with which we often have little experience and which may have undesirable ecological attributes. (pg 44)

They also quote Saunders and Tyus (1998):

The potential for success of flow management strategies will depend on the extent to which target species or communities are limited by other factors, such as contaminants or the presence of nonnative species, that may not be responsive to changes in the flow regime (pg. 427)

This kind of thinking can lead very quickly to paralysis. The reality is that almost all our ecosystems are under assault from some anthropogenic stressor. Figuring out the right way to alleviate these stressors almost always involves 1) great cost, 2) great societal commitment, and/or 3) great desire. There's never much difficulty finding people who care greatly about fixing these problems, but the money and commitment tend to be more scarce. So the people who care tend to focus on the easiest problems to fix. Removing a dam is easy. All you need is a back-hoe and an operator. Removing an invasive species is hard. In the U.S. we spend billions of dollars just to keep invasives at bay, I'm not aware of any that have actually been eliminated.

So as a result, people tend to plunge ahead fixing what they can (without thinking about the other impacts) or they tend to get bogged down by the myriad of other possible negative outcomes and don't do anything.

I'm not really sure there's a great solution here.

Jackson, R.C. and C.M. Pringle. 2010. Ecological benefits of reduced hydrologic connectivity in intensively developed landscapes. BioScience 60:37-46. doi:10.1525/bio.2010.60.1.8

PS. Let's think about those axes a little differently. How easy do you think it is to move an ecosystem along those axes? My feeling is that moving something along the biological aspect is impossible for some biological variables: Once a disease or invasive species becomes established, it's just there forever. On the chemical side, we probably can change the chemistry of an aquatic ecosystem pretty readily, but it isn't going to be cheap. The physical axis, on the other hand? Well, we move dirt all the time...it's practically the default state of being for humans. That's why we see the emphasis in restoration on physical restoration.

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!

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.

Toxistoichiometry: Integrating Ecological Stoichiometry and Ecotoxicology

In my current job I haven’t been doing a lot of original research, but that hardly means I am abandoning my wide range of research interests. I wanted to explain today a little bit about the “big idea” that I’ve been chipping away at for years. What follows are pieces from a talk I’m going to give to the Society of Environmental Toxicology and Chemistry in Milwaukee on November 12^th.

Evaluation of toxicity is most often done by the use of standard bioassays on commonly used test organisms. For example, you can pull up an EPA-approved method for determining the acute or chronic toxicity of any chemical to Daphnia spp. Theoretically, anyone in the world can follow the same protocol and get the same result. In reality, considerable differences occur between labs and even between Daphnia strains (not to mention different species), but this overall approach has been accepted for years as a means of estimating toxicity. The problem is that this method only estimates relative toxicity within a specific context. Relating the results of these short-term, laboratory tests to real-world scenarios is not only difficult, it may be impossible. For example, frequently used protocols often provide organisms with an abundance of high-quality resources, while in nature organisms are often faced with either low quantity or low-quality resources. A number of studies have tried to understand how the effect of a toxin varies with food quantity, but few have adequately addressed how food quality alters the effect of a toxin. Imbalances between the nutritional quality of a food source and an organism’s dietary needs are common in nature and appear to play a role in individual physiology, population dynamics, community interactions, and ecosystem processes. The study of how nutrient imbalances alter ecological relationships is often referred to as ecological stoichiometry. Organisms respond to changes in the stoichiometric ratio of elements in their food with corresponding variations in growth, reproduction, assimilation, and excretion. Essentially, nutritional imbalances have a large effect on ecosystem function.

My interest is in linking ecological stoichiometry with ecotoxicology. Initially, I have focused on how food quality affects acute toxicity in aquatic organisms. My collaborators at the University of Notre Dame and Trent University and I have shown that some toxins (iodine, cobalt, fluoxetine in particular) are stoichiometrically explicit, meaning the effects on organisms vary based on the quality of food the organism is getting. Other toxins do not appear to be stoichiometrically explicit (bendiocarb, triclosan, methanol) although it is obviously harder to prove.

The implications of stoichiometrically explicit toxins are presently unknown, but one can imagine a situation where the concentration of a toxin allowed by law is harmless when organisms are fed high-quality food, but detrimental when they are given poor quality food. Obviously, in nature species are often faced with food shortages or poor food quality, thus the actual effect of toxins may be much greater than the estimated effects. Evaluating the risk a chemical poses to an ecosystem therefore requires a more context specific approach. Are the receiving ecosystems frequently nutrient stressed? Are times of pollutant release going to coincide with occasions when ecosystems are nutrient stressed? I’m not sure whether regulatory agencies are equipped to permit in this way.

This kind of work is obviously just a first crack at the idea, and in many ways a very basic approach. Without this kind of base data, however, addressing more interesting questions becomes difficult. I believe that toxistoichiometry has the potential to provide a new axis of understanding for ecotoxicology.