Dr. Sheryl Shanholtzer
When populations of organisms are allowed to
grow under constant conditions, the increase in numbers follows a predictable
pattern. A type of graph called a population growth curve is used
to describe these patterns by showing the number of organisms on the y-axis
(vertical) and the time ,or number of generations, on the x-axis (horizontal).
These curves are usually S-shaped like Figure 1 because the numbers of
organisms start low, increase ever faster for a while and then level off
as they approach an upper limit beyond which they do not increase.
Figure 1: The logistic growth curve of a simple population.
The shape of an S-shaped population growth curve is the result of two processes. When the population is small relative to its resources, each individual organism is very successful at producing offspring, and the population grows each generation. So if each individual produces 3 offspring, the population will triple in each generation. When this occurs, population growth is said to be exponential, that is, it increases by a certain factor (by a factor of 3 in this example) in each generation; in three generations 33. If population growth continued in this exponential mode, the growth curve would resemble the upper line in Figure 2, and theoretically, would eventually reach infinity.
In real life, however, this can never happen. As population size increases, a variety of negative influences will begin to act either on the reproducing individuals or their offspring that decrease their reproductive success. Each adult will produce fewer offspring. Finally, when individuals produce, on average, only a single offspring, the population increase will stop. Many factors have a negative effect on reproductive success, and they may act on either survival or reproduction. Examples of such factors include availability of space, nest sites, food, and shelter, or disease, predation, and migration. These negative influences on reproduction and survival gradually put the brakes on population growth until it finally ceases and the population size reaches an upper limit (Figure 2).
Figure 2: Geometric and actual growth of a population
The upper limit of population size is called the carrying capacity (K) for that population. The carrying capacity is a characteristic determined by both the organism and the environment. Carrying capacity may be determined by the same factors that slowed the rate of growth -- availability of food, shelter, nesting sites, or space, disease, etc. As these environmental conditions change, the carrying capacity of an environment may also change.
Another way of looking at populations and population growth is from the point of view of the processes that result in changes of population size. There are four such processes: 1) birth, 2) death, 3) emigration and 4) immigration. Births and immigration represent gains in the population, while deaths and emigration represent losses. When:
Birth Rate + Immigration Rate = Death Rate + Emigration Rate
the population size will be stable, neither increasing nor decreasing. Again, many other factors influence these four rates.
The nature of the factors that result in population growth and stabilization are central questions in ecology, and have been the subject of a great deal of research. In real life, populations of species generally interact not only with members of their population but with other species as well as their physical environment. This makes understanding and predicting population growth in natural populations a very complex and difficult undertaking.
We are going to demonstrate the principles
of population growth by studying it in a simplified system - one with only
one species and a non-renewing environment. To do this we will
use flour beetles. The flour beetles of the genus Tribolium
are ideal subjects for this study. They are small, have short life
cycles, require little care because they spend their entire life-cycle
in dry flour, can be easily counted at intervals and can take the kind
of abuse that you can dish out. The vial of flour is their universe
-- it both supports them and limits them.
We will study the growth of flour beetle populations under the simplest possible conditions. Each pair of you will start with a population of flour beetles by adding ten beetles to a vial of fresh flour. We are starting with ten so we won't have to sex them. Out of ten individuals the chances are about 99.9% that at least one will be female. Because neither immigration nor emigration is possible under these conditions, we will see only the effects of births and deaths on population growth. Our general procedure is simple: periodically, the flour will be sorted through and the adult beetles counted, their numbers entered on the data sheet and plotted on graph paper.
For our study we will only look at the adult stage (unless you are ambitious and wish to count other stages as well). Adult females reproduce by laying eggs, and the number of eggs she lays per week can be viewed as her initial reproductive success. Her final reproductive success is best measured by how many new adults she produces.
1. Place 1 level scoop or tablespoon of flour into a vial.
2. Place 10 adult flour beetles into the vial and plug.
3. Write your name, instructor's name, and the date these were started on the vial label.
4. Return the vials to your instructor so that they can be placed in a 28o C incubator.
5. At intervals determined by the instructor, dump a portion of the contents of the vial onto a sheet of paper. Using a brush, sort through the flour and count the adult beetles. Put them into a holding vial using a curled sheet of paper, and repeat until all the flour has been sorted. Be sure to check that no adults or larvae remain attached to the paper.
6. Record the number of living and dead adults and note the larval and pupal stages.
7. Put all the insects back in the original vial. Mix by tilting and rolling the vial.
8. Fill in Table 1 each time you count.
9. Begin to produce their population growth curve by plotting the data
point each count.
Instructions for writing Beetle paper
A scientific paper is written in four sections, "Introduction," "Methods & Materials," "Results," and "Discussion." At the end an alphabetical (by author) list of "Literature Cited" should be included. For more information on writing a scientific paper, see "Instructions for Laboratory Papers."
Table 1. Data form.
|Date||Number of Adults||Comments, recorder(s) initials|