BISC 111/113:Lab 2: Population Growth

Objectives

1. To gain experience in experimental design and to set up your Tribolium experiments.
2. To learn how to use the computer program Excel for graph construction and the computer program JMP for statistical tests.

Lab 2 Overview

Population Growth Background

It is accepted that environments on the Earth are finite and therefore have limited resources, so it follows that no population can grow indefinitely. Certainly no organism exhibits its full reproductive potential. Darwin, for example, calculated that it would take only 750 years for a single mating pair of elephants (a species with a relatively low reproductive potential) to produce a population of 19 million. This is vastly in excess of the current total population and elephants have existed for millions of years. Some species might exhibit population explosions for a short time (e. g., algal blooms), but their population inevitably crashes. Most populations, however, are relatively stable over time, once they have reached an equilibrium level.

Population ecology is the discipline that studies changes in population size and composition, and also tries to identify the causes of any observed fluctuations. A population is made up of interbreeding individuals of one species that simultaneously occupy the same general area. Fluctuations in population sizes could be caused by environmental conditions as well as by predation and interspecific competition, e.g.. It can be particularly challenging to follow and understand the population dynamics of a species in the "real" world. Therefore, scientists have often used controlled lab experiments to understand the basic concepts of population ecology. Many classical experiments have explored population dynamics of and inter- and intraspecific competition in the flour beetles, all members of the genus Tribolium.

While Tribolium can survive on a number of finely ground grains, these particular beetles are cultured in 95% whole wheat flour and 5% brewer's yeast. Tribolium thrive at a temperature range of 29-34 oC and a humidity of 50-70%. Under optimum conditions one would expect a new generation roughly every 4 weeks. The "confused" flour beetle (T. confusum) (Fig. 1, Table 1)was so named because it was often confused with its closely allied species T. castaneum.

Because a female flour beetle can live at least a year and lay 400-600 eggs in her lifetime, one can imagine the potential for overcrowding. High density can lead to several interesting phenomena, such as an increase in the incidence of cannibalism, where the adult beetles will eat the eggs; larvae will eat eggs, pupae, and other larvae. If conditions are crowded and stressful the beetles will often produce a gas containing certain quinones that can cause the appearance of aberrant forms of young or can even kill the entire colony. There have also been reports that overcrowding leads to an increase in the transmission of a protozoan parasite (Adelina tribolii).

Arthropods need to molt in order to grow. Tribolium beetles, like all other members of the insect order Coleoptera, undergo complete metamorphosis, passing through four distinct phases to complete their life cycle: egg, larva, pupa, and adult. An egg is laid from which hatches a larva. This larva molts into a second and then third larval stage (or instar) increasing in size in the process. The third instar turns into a pupa from which finally an adult is released. The pupa is a quiescent stage during which larval tissues and organs are reorganized into adult ones.

Setting up the Experiment

Your charge is to set up an experiment dealing with population ecology of the flour beetle. You should start your experiment with at least 20 beetles per container, since they are not sexed. This should ensure enough females in your starting population. We have found that 1 gram of food per beetle will keep them reasonably healthy until the end of the experiment.Discuss the appropriate number of replicates.

Some of the factors you could consider investigating are:

Size of starting populations (intraspecific competition)

Food supply (e.g., arious milled flours, prepared grain products, and/or brewer's yeast)

Environmental structure (effects of environmental patchiness on population growth,, e.g. habitat size, light availability, refuge use, or "dilution" of the food/habitat volume with inert materials).

Biological control of populations in grain storage facilities (e.g., application of plant volatiles.)

Briefly outline hypothesis you are testing in your lab notebook. Provide details of your experimental design (starting number of beetles, number of replicates, variables, etc.).

Prepare a PowerPoint slide describing the experimental design and post it to the lab conference on Sakai.

Descriptive Statistics

The t-test: Comparing Means of Two Samples

Assignments

1. Prepare a PowerPoint slide descriing the experimental design and post it to the lab conference on Sakai.
2. Write a preliminary Materials and Methods section text for your Tribolium experiment.
3. Prepare a column graph of Tribolium means ± SD from historical data.
4. In preparation for the Plant Biology series, visit the College Greenhouse, paying particular attention to the different adaptations of plants located in the Desert, Tropical, Subtropical and Water Plant rooms. Greenhouse Map.
5. With your bench mates, prepare a presentation of key characteristics of your plant.