| Fundamentals of Populations and Population Growth |
In this section we will give a brief introduction to the fundamentals of populations, their growth, and the balance amongst different populations. Population ecologists study and analyze every aspect of populations and the factors that can impact their numbers in a given ecological system.
"A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community. It is wrong when it does otherwise." "Conservation is a state of harmony between men and land" Aldo Leopold Lets start by describing what an ecosystem, which is short for ecological system, is. Imagine yourself standing in the middle of a forest observing everything around you. You will see a group of plants, animals, soil, sunlight, air, water, minerals, and nutrients1. The plants and animals in that forest depend on each other for survival. What is more difficult to see are the interrelationships between the living organisms (some which can't be seen without a microscope) and the environment. The dependencies between all of these elements can be very complex. Complex systems like this, with different organisms and their interrelationships with the physical environment and contained in a specific unit of space (or area) are called ecosystems. "Contained in a specific unit of space" means that all the elements in the ecosystem might have intricate and complex interdependencies between each other but only within the confines of that space and independent from anything outside the area. Of course, things like the energy radiated by the sun or the climate and air are special cases. Ecologists are scientists that study complex ecosystems with the fine and interesting dependencies amongst all the different elements. Their objective is to understand what every element brings into the system and what every element needs from the system. They try to understand, from a scientific point of view, what keeps everything living and existing in a balanced and stable way for very long periods of time. By the time you finish this lesson you will have gained a good appreciation of this.
What is a population? Very simply, a population is a group of organisms of the same species that live in a particular area. The number of organisms in a population changes over time because of the following: births, deaths, immigration, and emigration. Of course, births and immigration increase the size of the population; whereas, deaths and emigration decrease the size. The increase in the number of organisms in a population is referred to as population growth. There are factors that can help populations grow and others than can slow down and even prevent populations from growing. Factors that limit population growth are called limiting factors. However, before we go into the limiting factors, let's talk about the biotic potential of a population.
If things were perfect for a population and all the individuals in the population survived and reproduced at the maximum rate, that growth rate is called the biotic potential. The biotic potential is used as a reference when looking at growth rates of populations. Are the growth rates close to the biotic potential or far from it and how far? That type of analysis helps population ecologists understand if the conditions for the population are adequate.
It is certainly not common for a population to grow at its biotic potential for a considerable period of time; however, there are situations where this can happen. For example, when fish are introduced into a lake where there is plenty of food and space and there are no predators, the fish can reproduce at their biotic potential, but not for a long time. Another example is when a scientist grows E. coli2 on a petri dish with ideal nutrients. The bacteria will reproduce and grow at its biotic potential, which for E. coli means that the population doubles in size every 20 minutes!. The graph of a population growing at its biotic potential, which is called exponential, can be very steep. The graph and the mathematical equations, which represent this kind of growth, are included in the Facts about Population Growth Mathematical Models page.
For the two previous examples as well as for all populations, there are factors that will limit their growth. Some of these factors depend on the population density. The most common density-dependent factors that limit population growth are:
- Food and water supplies - A given supply of food and water might be enough for a small population density. However, that same supply might not be enough for a high-density population, and competition among the individuals of the population would develop.
- Light - Light is a very common resource needed by plants. A plant density increases those plants that don't get enough light will not grow strong enough and might even die.
- Space - This is an obvious limiting factor, especially if you think of fish in a lake, bacteria in a petri dish, or plants in a forest.
- Predators - Higher densities of a prey population attract more predators and as the number of prey increases, so does the number of predators. On the other hand, if the number of prey decreases, so does the number of predators.
- Diseases - Diseases can certainly have an impact on birth rate and thus affect growth rate. Since many diseases are contagious, they are therefore dependent on density.
- Parasitism - Parasitism is a relationship where one organism (the parasite) feeds on the tissues or body fluids of another organism (the host). In this type of relationship the parasite benefits and the host is harmed, sometimes to the point of killing the host. Like diseases, since parasites spread easier in a high-density host, their impact depends on the density.
There are also limiting factors that don't depend on the population density. These density-independent factors are abiotic factors such as weather storms, fires, earthquakes, or floods. Any of these factors can have a severe impact on population sizes regardless of density.
To conclude this section, we will describe the carrying capacity of an ecosystem. The area occupied by a population does not have unlimited resources such as food, water, and supplies to build and keep a dwelling. These factors limit the population growth and many times bring about death rates that equal the birth rates. When this happens, the population size reaches a stable balance. So one could say that there is a certain number of individuals of the population that can be supported by the environmental resources in a given ecosystem. That is called the carrying capacity of that ecosystem. The graph of a population that grows until it reaches a stable size based on the carrying capacity of the ecosystem is called an S-shaped curve. The graph and the mathematical equations, which represent this kind of growth, are included in the Facts about Population Growth Mathematical Models page.
[Note 1] -- One can not see the air, although one sees it through the things (such as leaves or tree branches) that the air moves. Minerals and nutrients can also not be seen although one knows that they are there.
[Note 2] -- You can visit the What the Heck is an E. coli? site for lots of interesting information on the E. coli bacteria.
Last Updated: Monday, 13-Nov-2000 05:22:04 GMT
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