Predator Prey Interactions and Population Change

Predator Prey Interactions and Population Change

Introduction

Background

Organisms interact in many different ways. Some of the interactions have to do with feeding patterns. These feeding relationships make up what are called food chains.

Predator organisms feed upon other organisms, called prey. The predators depend on the populations of these prey organisms. The number of predator organisms depends on the numbers of prey. Correspondingly, the number of prey organisms is limited by the number of predators that feed on them. In other words, the size of predator and prey populations aredependent on each other. This relationship depends upon the specific kinds of organisms and the conditions in which they live.

Goals

In this investigation you will model the interactions between a population of owls and their prey, a population of mice. You will measure the sizes of the populations as they change over several generations, and you will graph the data you obtain.

Lab Warmup

Concepts

Owls are an example of predator organisms. They feed on smaller organisms such as mice. The mice therefore serve as prey for the owls. As predators, owls occur high in a food chain of forest organisms. Mice occur lower on the food chain. In modeling predator/prey interactions, one needs to make simplifying assumptions. In this investigation, you will assume that the owls feed only on mice. You will also assume that all owls that can catch and eat a certain number of mice will survive and reproduce. These assumptions are like patterns that exist in nature, but do not mirror them exactly. The assumptions are useful however, in simplifying the model so that population patterns emerge.

Materials

Beans (mice)

Paper Owls

Procedure

1. You will simulate 25 generations of mice and owls that live within a habitat. Your habitat will be your table.

1. Spread 100 mice evenly within the habitat. Set the remaining mice aside for later use.

1. Assume that there are two owls in the first generation. Hold each owl approximately 18 inches above the habitat and drop it onto the habitat. Catch as many of the mice as you can with one drop. Remove and count the captured mice.

1. Each mouse not captured is assumed to produce one offspring. Add to the habitat one offspring for each mouse not caught.

1. Determine whether each owl survives and reproduces. An owl must capture at least three mice to survive and it produces one offspring for every three mice it catches.

1. In column D, record the number of mice caught.

1. In column E, record the number of owls starved.

1. Calculate the number of surviving mice and offspring and record the result in Column F.

1. Calculate the number of surviving owls and offspring and record the result in Column G.

1. Copy the results from Columns F and G, Generation 1 to Columns B and C, Generation 2. These numbers represent the population size of mice and owls present at the start of the second generation.

1. Repeat the above steps until you have simulated 25 generations. Use the following limitations:
2. At the beginning of each generation there must be at least three mice
3. At the beginning of each generation there must be at least one owl
4. The mouse population cannot exceed 200

Analysis and Conclusions

1. Compare the changes in mouse and owl populations during the first few generations.

1. Compare the changes in mouse and owl populations during the middle generations.

1. Compare the changes in mouse and owl populations during the later generations.

1. Compare your model of interactions between the owl and mouse populations with what might actually occur in a community that includes owls and mice. How do the assumptions you made in this activity account for differences between the model and real life.

1. What do you think would happen if a second animal species that feeds on mice was added to the simulation?

1. What do you think would happen if the prey of the mice (i.e. grain) was added to the simulation? Design an experiment to check your prediction.