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Chapter 6: Principles of ecology

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Edited by Peter Moyle & Douglas Kelt
By Mary A. Orland, July 2004

So far we have discussed how the cause underlying biological phenomena can be rooted both in events occurring at the next lower scale of organization, as well as interactions with factors at the same or higher scales of organization. These two different forms of causality result from principle of emergent properties in biological organization, and represent the fundamental difference between reductionist and holistic philosophies of scientific inquiry. Another way that these two approaches are described is by the terms ultimate and proximate causality. The proximate cause is the mechanism that allows for something to happen, and the ultimate cause is the drive that makes it happen. These can be summarized as how (proximate cause) and why (ultimate cause). Within in the context of biology, the proximate cause is nearly always the reductionist explanation from the next lower scale. The ultimate cause is the conditions in the environment that place certain constraints on the organisms, and the selection based drive for the organisms to survive in the context of that environment and their interactions with other organisms. The following is an illustration of now ultimate and proximate causality work together to explain the biology of some desert rodents of North America kangaroo rats and pocket mice.

Why don't kangaroo rats drink water? This will be the question around which we demonstrate the concepts of ultimate and proximate causality in biology; note that this question occurs at the scale of an individual organism. Kangaroo rats are small rodents adapted to life in the desert. Like many desert creatures they live in burrows underground during the day to escape the desert heat (and predators), and they are active at night. They have both longer life spans and slower reproductive rates than typical rodents of their size, which is probably an adaptation to the unpredictable food supply and harsh conditions of the desert. These animals eat predominantly seeds, which they gather in fur-filled pouches on either side of their mouth. They store these seeds in caches, from which they can later feed for many months. This seed caching behavior is also an adaptation to the low productivity and unpredictability of desert ecosystems. These animals are predominantly found in the very arid areas of North America, including parts of the desert that get less than 5 inches of rain on average per year, with dry years that can be as low as 1 inch per year (or less!). For the most part, therefore, there is little standing water available to them because even most streams are dry in most of the year. When these animals are kept in captivity in research labs they commonly do not drink water that is given to them and have even been known to bury their water dishes with sand. It should be noted, however, that the dozen or so species of kangaroo rat do vary somewhat in their ability to live without water.

Figure 6.3 Merriam's kangaroo rat (right) is highly tolerant of dry desert conditions, and is found in the sandy creosote bush habitat of the Colorado Desert, shown here at the University of California Boyd Deep Canyon Desert Research Center. Kangaroo rat photo source: Dr. Lloyd Glenn Ingles © California Academy of Sciences

Explanations for why kangaroo rats do not have to drink water can be found at all the scales of biological organization, and all of them are correct. First we will look to lower scales of biological organization for explanation. Kangaroo rats have unusually large kidneys that allow them to remove nearly all of the water from their urine before excretion (Schmidt-Nielsen 1997). This is the result of an elongated loop of Henle within the kidneys, which in turn are the result of genes that code for proteins that form those structures. Kangaroo rats are also very efficient at utilizing water that they literally produce as a byproduct of carbohydrate metabolism from other cellular processes, and they preferentially eat seeds that have higher carbohydrate levels and lower protein levels. These explanations on the physiological, cellular, and genetic scales all qualify as proximate causes for the fact that kangaroo rats do not need to drink water. This is also a reductionist approach to explaining a biological phenomenon on the organismal scale.

What about explanations at and above the scale of the organism itself? These start with the classic natural selection argument that the reason that kangaroo rats do not need to drink water is that those animals that are less dependent on standing water have a higher probability of surviving and reproducing in desert conditions. Through time natural selection should favor those individuals that were more drought tolerant, and hence eventually led to the evolution of species that do not need to drink standing water at all. This process occurs because of interactions between individuals of the same species, so evolution by natural selection is a population scale explanation, one scale higher. We can also look for explanations on yet higher scales. Given that the desert is a stressful environment, why are kangaroo rats even living there at all? Why are they not living in a place with more rainfall, so that they would not have to have all these special adaptations that allow them to live without drinking water? The answer to this question is on the community scale of biological organization.

The reason kangaroo rats live in an environmentally extreme habitat is competition with other species of rodents. Kangaroo rats presumably could survive in many more productive habitats that have more food and water. However, there are other species in those habitats that are better competitors under those less stressful conditions, probably in large part because they are faster reproducers. Trade-offs exist between different species traits, so the very traits that allow a kangaroo rats to thrive in the desert may not make them good competitors under less arid conditions. The long life spans and slow reproductive rates that are adaptive in the harsh, unpredictable desert environment would result in lower reproduction and therefore lower competitive ability is less harsh environments. Furthermore, the energy invested in the physiological structures, such as enormous kidneys, that allow for kangaroo rats to live without standing water is energy that will not be invested in reproduction. Therefore, one reason why kangaroo rats do not need to drink water is lack of interspecific competition (competition between species), which is a community scale explanation.

It is also possible to find an explanation for why kangaroo rats do not need to drink water at the global biosphere scale. If there were no deserts in North America to begin with, then there would be no need for kangaroo rats to be so drought tolerant. There are deserts in North America because of the nature of the Hadley cell global circulation patterns, which create deserts around the world at approximately 30° latitudes. This is discussed in much more detail in Chapter 3, but suffice it to say that if these global circulation patterns did not exist, the deserts of North America would be greatly reduced (recall that the Great Basin is a rainshadow desert, so it likely would still be present in this imaginary Hadley cell-free world), and at the very least we would expect fewer species of kangaroo rats, or perhaps none at all.

The concepts of ultimate and proximate causality also are relevant to situations other than biology. For example, imagine that you have an internship at a government agency in Sacramento (which, by the way, is a great opportunity available to UC Davis students), and that you are on the Yolo Causeway driving your car to work. If we were to ask the question "what is causing you to drive to Sacramento?" we could give different explanations based on proximate versus ultimate causes. The proximate cause behind your driving to Sacramento is the workings of your car engine, because the fact that it is burning gasoline and turning chemical energy into mechanical and kinetic energy is what is causing you to move toward Sacramento. In other words, this proximate cause explains how you are driving to Sacramento. The ultimate cause considers why you are driving to Sacramento. It starts by looking at the benefits to you as an individual for going to your job; you are driving to Sacramento because you need to make money, and because you are gaining valuable job experience to help your future career. Of course, there are larger-scale ultimate causes in addition to this, such as that you are fulfilling a role in the government agency, and that agency is in turn fulfilling a role in the governing of California that was deemed important by policy makers. If any of these ultimate or proximate causes did not exist, such as if the car engine did not work, if you did not benefit as an individual from the job, or if the agency had no need for your labor, then you would not be driving to Sacramento. Notice that the proximate causes focus on smaller scale explanations, such as the mechanics of your car, while the ultimate cause are larger scale, such as your individual economic needs and the existence of the job to begin with. The parallels with the ultimate and proximate causes in the kangaroo rat example should be readily apparent.

In summary, some of the general principles of ecology presented in this essay are as follows:

The following chapters will give you more detailed information of principles in population, community, and ecosystem ecology that will be quite useful for studying wildlife, and for understanding the consequences of human alterations of natural systems.

Barbour, M. B. Pavlik, F. Drysdale, and S. Lindstrom. 1993. California's Changing Landscapes. California Native Plant Society, Sacramento, California.
Berger, J. P. Stacey. L. Bellis, and M. Johnson. 2001. A mammalian predator-prey imbalance: grizzly bear and wolf extinction affect avian neotropical migrants. Ecological Applications 11(4) 947-960.
Fryxell, J., J. Falls, E. Falls, R. Brooks, L. Dix, and M. Strickland. 1999. Density dependence, prey dependence, and population dynamics of martens in Ontario. Ecology 80(4): 1311-132.
Levin, S. 2002. Complex adaptive systems: exploring the known, the unknown, and the unknowable. Bulletin of the American Mathematical Society 40(1): 3-19.
Masters, Gilbert M. 1996. Introduction to Environmental Science and Engineering.
Schmidt-Nielsen, K. 1997. Animal Physiology: Adaptation and Environment, 5th Ed. Cambridge, UK: Cambridge University Press.
Williams, G. 1966. Adaptation and natural selection: a critique of some current evolutionary thought. Princeton University Press.
Wilson, D. S. 1980. The natural selection of populations and communities. Benjamin/Cummings Publishing.

Table of Contents

1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
4. Biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife

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