Intrinsic and Extrinsic Limiting Factors Dropbox
Consider the meaning of the term “ecology.” How do the ideas of energy and chemical cycles, community structure, biodiversity and succession fit together to form the basis of the way the natural world works? Explain how each of these ideas works and find ways to link between them. Present examples from your experience to demonstrate an understanding of the principles involved.
Reading Materials To this assignments
Defining Ecology as a Science
Let’s open this lecture with a decent definition of the term “ecology.” This is a textbook definition:
“The scientific study of how organisms interact with their environments.” Source: Essential Biology with Physiology
Does this really tell us anything? What do they mean by “scientific study” or “environments”? A scientific study is simply a study that is repeatable because the scientific method is utilized. A series of documented steps are taken to either support, or refute, a given hypothesis. The term “environments” may be more problematic!
Now, consider this definition:
Ecology is the scientific study of interaction between or among organisms and their environment. Two distinct things are going on here: interactions between or among living entities, and interactions between living entities and the environment.
We often use the terms “biotic” and “abiotic.” Biotic refers to biology. Anytime you see an “a” placed in front of a term, it means “without.” Therefore, ecological studies are two-fold, including both the living and non-living factors in the environment. Examples of abiotic factors are sunlight, water, temperature, wind, rocks, soils, and periodic disturbances. You can easily see how each would affect organisms and that none of these are living entities!
Back to the science of ecology! Much of ecological studies are just that: studying some aspect of the environment. “Study” usually means to observe, monitor, and record what happens over a prescribed period of time. As we will see in this and next week’s lecture, much of ecology is trying to figure out how to “fix” things once we have messed them up.
A more modern and scientific approach has been dubbed “experimental ecology.” Experimental ecology is a process of asking “what if” in an ecological setting. It takes place in the environment, where a “test” area is set up.
One example is the Hubbard Brook experiment, in which a portion of timber was removed to see what the effects on water runoff would be. Part of the ecosystem was left untouched, the experiment allowing for comparison between the test area and the untouched area.
This approach does away with the old “it’s different times, so you can’t extrapolate those data” arguments, against the “observation only” ecological studies.
Levels of Interaction—From Organisms to the Biosphere
When discussing ecology, we must consider the level of interaction that is taking place. For example, if we want to look at the way a particular organism copes with its environment, we would be studying ecology at the organismal level. If we are more concerned with how a population (a population being a group of individuals of the same species) deals with the environment, we turn to the science of population ecology.
An example would be factors that affect the number of individuals within a population. What if we want to see how a group of populations (populations of different species) interacts? We turn to the science of community ecology. A community is a group of different populations within a given area. A particular area will have a specific community based on the abiotic factors, the critters that get there first, and how they deal with one another.
Considering the abiotic factors, along with the community they live in, we have the science of ecosystem ecology. An ecosystem consists of the community and the world around it. If we examine the ecosystem on a global scale, we are looking at the biosphere.
Now that we’ve looked at the various levels of interactions, let’s examine how organisms respond to their environment. These responses are called adaptations. Anything that helps an organism to better cope with its surroundings or adapt to the situation is an adaptation. Adaptations to the environment fall into one of three categories:
Adaptation: Physiological responses
Physiological responses refer to the function of the organism. You are already familiar with one example of a short-term response, the goose bump! This is a physiological response of a muscle constriction to raise the hair on your skin. (It just so happens that we don’t have a thick fur that rises to increase the insulation—we just get the bumps!)
Blood vessel constriction in animals to conserve heat is another, less familiar response—as we see in cats. Other physiological responses are more long-term. They are referred to as acclimation, since the organism becomes attuned to the climate.
If you live in the “lowlands,” take a trip to Denver to see the effects of acclimation, or lack of it! You will feel the effects of the lower oxygen levels immediately and it will take you some time to become accustomed to the change in the air. Your body will produce extra red blood cells to compensate for the lower oxygen levels. The increase in red blood cells is an example of acclimation.
Plants of the desert regions are another great example of acclimation. They have acquired physiological capabilities to store water, even to the point of developing different modes of photosynthesis to best conserve water. Life History Concept
The life history concept is an important part of the discussion on how organisms adapt to the environment. This concerns the life cycle of an organism and how it functions in the environment. The functional role of an organism in the environment is termed the niche.
The terms r- selection and K- selection are most often used by ecologists to define the life history of an organism and this is a misnomer at best! Bet you’d like an explanation, right? Here goes!
r – selection These are organisms that put their energy into r-eproduction. They tend to be small, short-lived, with most of their efforts going into reproducing the next generation. Mice are a good example of r-selection organisms. As you may be able to figure out, these guys are found in areas where periodic disturbance is an integral part of the environment.
K – selection These organisms are larger, longer living and put their efforts into higher quality offspring. K refers to “karrying” (sic), the concept of carrying capacity of a given area, which means the maximum number of individuals that the area is able to sustain. In this case, organisms strive to exploit a more stable environment to produce the best, largest individuals. The offspring are well developed and parental care is commonly involved. Elephants are a good example of K-selection organisms.
These are the extremes of life history traits, the “endpoints” of a continuum. Imagine a line with two endpoints: there are an infinite number of points on this line. In other words, any number of combinations of r- and K- selective characters can exist. An organism may lean more toward one extreme or the other, but it is not fully r- or K- in nature.
Energy Flow Within Ecosystems
We can’t discuss ecology without talking about the energy flow within biological systems. To do this, we must consider the Earth as a whole. The Earth is literally “floating” in space—in essence, a vacuum. What does this tell you about things like carbon, oxygen, and the other substances that are essential to life? They are all available in finite amounts. In fact, only one thing is replenished on this Earth—all others must be recycled within the closed system of our planet.
Have you guessed what can be replenished? It’s sunlight, or more correctly, radiant energy from the sun. Radiant energy is waveform energy of the magnetic spectrum. We are most familiar with a narrow part of this energy, the visible spectrum. We know from the laws of thermodynamics that energy cannot be created or destroyed—it just changes form. We also know that disorder in the universe is increasing—the scientific term for this is entropy.
What happens to this radiant energy as it enters the Earth’s system? Biologically speaking, the waveform energy is converted to chemical bond energy via the process of photosynthesis (known as primary production). We have a conversion of energy from one form to another, so what about the increase in entropy? Anytime an energy conversion takes place, a little energy is lost in the form of heat. (Remember that heat is random molecular movement. It doesn’t get any less ordered than this!)
Both energy conversion and an increase in entropy occur in biological systems. Even if no living organisms existed on the planet, energy conversions would still be taking place. With radiant energy, we see higher-level energy coming in to the Earth, and heat going out. This energy transfer is the only “flow” arriving to and leaving from this planet. All other materials of life depend upon the cycles going on or at the Earth’s surface, to make their way into the world of biology. We are living in a closed system!
Chemical Cycling within Ecosystems
Looking at the chemicals of life, we can see a dependence on cycling to keep them available for life. This means that there must be both abiotic and biotic components to the cycle. We often refer to the abiotic component as having a reservoir or sink. The chemical is more readily available in the source as compared with the sink, which is a place of long-term storage.
There must be some way of getting the chemical out of the abiotic sink and into the biological world—and a way to get the chemical back into the abiotic world!
Four “biggies” come to mind, when we talk about chemical cycling in the environment:
Water (which takes care of hydrogen and oxygen)
Let’s take a moment to examine these cycles in a greater detail.
The Carbon Cycle
Carbon is frequently called the “building block” of life, since many of life’s molecules contain carbon. Where do you suppose we find the most carbon in the world (besides in us)? You’ve likely heard of the “greenhouse effect.” This “effect” is due to excess CO2 (carbon dioxide) being released into the atmosphere from the burning of fossil fuels (oil, gas, coal, etc.).
As you can probably guess, the atmosphere is a major source of CO2 on this planet. A lot of carbon can be found in the fossil fuels. Where did they come from? Why, the Earth’s crust, of course! It was the primary production of the fossil plants that first sequestered energy from the sun and the elemental carbon into biomass, which much later became the crude oil of fossil fuels. (Biomass is the amount of organic material in an ecosystem.)
When looking at the abiotic component of this cycle, we see both an atmospheric component and a less available source, the ground. In this case, the ground is the sink. There must also be a process to release carbon back to the biological world. We refine and then burn it, converting the hydrocarbons to energy, CO2, and water.
We’ve removed massive amounts of carbon from the sink. We’ve also done away with much of the biomass in certain areas, thus upsetting the delicate balance of this cycle. Two examples of this are the loss of the rainforest and the changes to the environment due to row-cropping.
A more direct line of reincorporation of carbon in living systems is through detritus. The detritus is comprised of dead organisms that are in various states of decay (think: recycling). This stuff is being broken down by the detritivores, which include the bacteria and fungi, along with some invertebrates. As you will see in the following sections, these guys are usually involved in the recycling of inorganic molecules in the biological systems.
The Nitrogen Cycle
Without nitrogen, there would be no DNA, no RNA, no proteins—and thus, no life! Oddly enough, the major source of nitrogen is not usable in living systems!
You may remember from high school Earth science classes that the atmosphere is mostly nitrogen (about 78% by most estimates). Nitrogen exists as a diatomic molecule (N2) and we cannot incorporate this directly into the world of biology. As with most elements, nitrogen molecules first enter the biological realm from the primary production of plant biomass. Plants are able to incorporate biomass only when in the form of nitrates and ammonia. (You may have heard these chemicals mentioned in fertilizers.)
We know that a great deal of nitrogen can be found in the atmosphere and it is incorporated in the biomass of living organisms. But how does it get from point A to point B? Of course, once nitrogen is incorporated in biomass, the detritivores can easily recycle it back into the biological world. Just how nitrogen gets there in the first place is quite interesting!
A chain of conversions is carried out by very specific bacteria. Some of these guys are free-living in the soil, while others form close associations with the roots of many plants. One major group of plants is the legumes (peas and soybeans are prime examples). These plants have built-in nitrogen conversion factories. If you happen to live in an area of large row-crop farms, you may notice that the farmers will plant corn for a few years, and then change to soybeans for a year or two. Why? The soybeans have the capability to fix nitrogen in the soil and act to replenish what has been removed by the corn.
Another by-product of fossil fuel burning is the release of nitrogen in the form of nitric acid. This is the stuff of acid rain. It is interesting to note that the nitrogen cycle is the only major element cycle that depends on living organisms converting the molecules
to usable forms