GENERAL LAB INTRODUCTION

Imagine an organism moving through its environment searching for food.  Every time it encouters a prey item, certain “decisions” have to be made:  pursuit and capture of prey takes energy, but passing up a prey item and continuing to search also takes energy.  Will the energy used in pursuit be less than the energy gained by consumption—if the prey is caught?  Is the prey too difficult to catch, making it a better choice to continue searching and not pursue?  Is the prey item a high-energy food source?  The relative densities of predators and prey great affect encounter rates also—as does the heterogeneity of the habitat, by providing places to hide (for prey) or ambush (for predators).  If prey are infrequently encountered, the predator should be less likely to be selective about attempts to capture than if prey are encountered frequently.  This type of cost-benefit analysis of predator-prey interactions (summarized by MacArthur and Pianka in 1966) is referred to as optimal foraging theory.

Our lab today will expand on last week’s predation lab, focusing on some possible optimal foraging dynamics at work in your mosquitofish-brine shrimp model.  We will become a bit more sophisticated in our experiments quantifying predator-prey interspecies interactions between Gambusia holbrooki and Artemia sp. (brine shrimp).  Agani, your fish were collected from three different [and very familiar!] aquatic settings on campus:  1) in the shallow wetland region of Hennington Pond; 2) in the deeper lake areas of Hennington Pond, and; 3) the FIU Gym Pond.  Your prey are laboratory raised brine shrimp.  Your experiments this week (which you will design) should include the following additional parameters, as you think they might shed insight into the kinds of “decisions” that Gambusia are making about preying on Artemia:  1) varieties of predator size in the same container; 2) varieties of prey size in the same container; 3) different kinds of habitat heterogeneity; 4) light versus dark predation and the importance of visual versus olfactory cues, and; 5) interactions among these to make things really interesting.   As with last week, your dependent variable, will be predation rate, as # prey consumed per predator per unit time, although be thinking about how you can make the units of this dependent variable reflect selection for larger prey items.  That is, a single fish showing a predation rate of 5 Artemia per 5 minutes on small Artemia may actually consume less energy than a fish the same size that eats only 2 large Artemia in 5 minutes.

LAB INSTRUCTIONS

1.  Generate several testable hypotheses as a class that you can test with today’s exercise.  Use the controls on predator-prey interactions listed above as helpful hints, and think about key differences between the 3 habitats where you will be catching your predators.

2.      Divide into groups and work as teams in the lab.  Work should be divided up so that all team members get to experience each aspect of the exercise.  In other words, don’t make one person record data for the entire lab exercise!

3.      From here, follow last week’s lab instructions as the experimental approaches are very similar.  Be sure to match your experimentation to your questions.

Answer/address all of the following questions on your own paper.  In some cases, this will require computer printouts of spreadsheets, graphics, or statistical output.

1.       Present a clear list of the hypotheses that you tested in your predation experiments using Gambusia and Artemia.  After each, briefly detail the experiment you designed to test this question.

2.      Briefly present and discuss your results for each experiment and question described above.  Relate these back to such factors as the source environment of the fish (e.g. did fish from wetland areas of Hennington Pond show different results from fish that were living in the deep water areas of this pond?  Why?).

3.      Present a clear list of the hypotheses that you tested in your expanded experiments designed to test some optimal foraging theories using Gambusia and Artemia.  After each, briefly detail the experiment you designed to test this question.

4.      Briefly present and discuss your results for each experiment and question described above.  What did you learn about the cost-benefit trade-offs of predation by mosquitofish?  How do your results differ with the source environment of the fish.  Why?