Exercise 1: Functional Response of Predators to Prey
Reference: Holling, CS. 1959. Some characteristics of simple types of predation and parasitism. Canadian Entomologist 91: 385-398.
The relation between prey population density and predator feeding rate (individuals eaten per hour) is called the functional response of a predator. At low prey density, the feeding rate is proportional to prey density. At higher prey density, the predator cannot eat all of the prey that are available, because of limitation of handling time. This means that no matter how many prey are available, the feeding rate is constant.
Holling's classic experiment on the shape of the functional response was done with humans, and we will repeat it today. Blindfolded human subjects were presented with different numbers of sandpaper discs in a 1m2 area. They were allowed 1 minute to collect as many discs as they could (1 at a time). The relation between number of discs collected and the density of discs (number per m2) is the functional response.
Form groups of 4 people. 1 will be the forager (blindfolded), 1 will measure handling time with a stopwatch, 1 will keep track of the 1 minute time limit for foraging, 1 will be the recorder of data.
Use the following numbers of sandpaper squares, and thumbtack them down to the board in random positions.
5, 10, 50, 100
- Have the subject search by tapping the tip of the index finger on the board (no sweeping allowed)
- Record the average time taken to pick up the sandpaper squares and place them in a jar at the side of the board. This is the handling time.
- Record the total number of squares collected after 1 minute.
- Make 3 replicate measurements at each density. Use a different person for each replicate, and make sure each one does all 4 densities.
Repeat the experiment using a pencil to tap the surface of the board instead of using the index finger.
Plot your data on graph paper or use Excel on the computers to do the plots.
- What is the relation between average handling time and the maximum number of sandpaper squares collected in 1 minute?
- How does foraging rate change when you switch from tactile (finger tapping) to auditory (pencil tapping) to detect prey?
- How confident are you in your results? Did your replicates show the same pattern? Were there differences among individuals in foraging ability? Is there an average pattern?
- How do your results compare to Holling's? Are the qualitatively similar (do the graphs have similar shape)? Are they quantitatively similar (do your data fall on the same lines as Holling's)?
- How might you decide whether your results differ from Holling's? What tests might you use? Try them and see what you get.
- Holling used randomly distributed prey. What do you think would happen if prey were clumped?
- What kinds of organisms do you think would show the kind of functional response you measured today?