Refuges in Time: Huffaker's Mites

Susan Harrell and Sunitha Vege

Huffaker studied the role of dispersion in the predator-prey association between the predatory mite, Typhlodromus occidentalis, and the phytophagous mite, Eotetranychus sexmaculatus. The phytophagous mite fed on oranges interspersed among rubber balls in a tray. Huffaker wanted to know if an adequately large and complex laboratory environment could be set up in which the predator and prey could coexist with sustained oscillations. He tested a wide variety of setups where he manipulated dispersal abilities of the mites and heterogeneity of the habitat. The surface and the distribution of oranges utilized were varied. Increasing patchiness resulted in coexistence of the predator and prey by allowing refuges in time. We will attempt similar experiments and show which factors promote coexistence.

Outline of This Lab

In this lab, we'll examine factors that allow coexistence of a predator and its prey. We are going to watch interactions of a predatory mite population and its prey, the six-spotted mite, on a setup of oranges interspersed among rubber balls in a tray. Oranges will be replaced as they are used up, so that the six-spotted mite will have a continuous food supply.

Initially we will examine persistance of the six-spotted mite in the absence of its predator. This mite maintains a persistant population on a variety of experimental setups. Next, we will examine persistance of the populations when the predatory mite is added to the system a few days later. You will be able to change the experimental setup by changing the number of oranges available, the area of orange available for eating, and the patchiness of the setup. You will also be able to change the dispersal ability of each of the mite species.

In many cases, the predatory mite will rapidly increase its own population size, consume all of its prey, and then become extinct itself. A few prey may escape and attain large populations in the absence of predation. Your job will be to determine what factors allow the predator and prey to coexist for longer periods of time.

The Lab

1. Run Ecobeaker (double-click on its icon).

2. Open the situation Huffaker.sit (use the OPEN command in the File menu).

After the situation loads you should see windows laid out in the following way:

The big window in the upper left is the experimental setup. It will be a view of the tray with oranges on it and will show the actions of the mites. To the right is a graph titled "Population Sizes" which will monitor the growth of each mite population over time. In the upper right is a window labelled Species which shows the species used in the model. Below this is a window labelled Habitats, which show the habitats used in the model.

3. Run the simulation (click GO in the control panel).

In the experimental setup, you will see 9 oranges. You will first run it without the predator. A few individuals of the six-spotted mite will be placed on a single orange and allowed to eat, disperse, and multiply. The food resource of the six-spotted mite in the simulation is the sugar. Every few time steps, sugar will be replenished. In the graph you will see the population grow rapidly and then persist at some level. To stop the model, click the 'STOP' button in the control panel.

4. Now try changing the experimental setup. Add more oranges by clicking on "orange" in the Habitat window. Change the distance between circles to 5. Since you will have more circles, we wish to have a similar area as the previous experiment. Therefore change the radius of the circles to 5 as well (click on 'OK') To make sure the six spotted mites start on a single orange, click on "orange1" in the Habitat window. Change the radius of this circle to 5, and the distance to 35 (click on OK).

Run the simulation (click on RESET before running any simulation). Click on GO in the control panel.

You should now see 16 smaller oranges with approximately the same area as the 9 oranges before. Again watch the graph window. Stop the simulation. Is the population able to persist? At what population level does the six-spotted mite persist? Record this number.

5. Now try changing the patchiness of the oranges. Click on the "six-spotted mite" in the Species window. Then, press Action Parameter. Change the Top depth to 35 and Bottom depth to 70. Click on OK. This will create 16 circles: the top two rows are rubber balls. The mites can only feed on the bottom 8 oranges.

Run the simulation (click GO in the control panel).

The six-spotted mite should eat only on the bottom two rows of oranges. Again watch the graph window. Stop the simulation. Is the population able to persist? At what population level does the six-spotted mite persist? Record this number.

6. We will compare this to an environment in which the oranges are more widely dispersed. First, click on six-spotted mite in the species window. Then, click on action parameter and change the Top depth from 35 to 0. Again click on "orange" in the habitat window. Change the distance between the circles to 10. Click OK. Here we will keep the area of the circles about the same as the previous experiment. Also, you will need to click on "orange1" in the Habitat window and change the distance between circles to 30. Click OK.

Run the simulation (click GO in the control panel).

You should have 9 oranges of the same size as before but more widely dispersed. Imagine there are rubber balls placed between these oranges on which the mites can move but have no food source. Again watch the graph window. Is the population able to persist? At what population level does the six-spotted mite persist? Record this number.

7. Now we will add a predator to the system and monitor the populations of the two mite species.

Click on predatory mite. Then, click on settlement parameter and change Num Immigrants from 0 to 200. This will allow approximately 5-10 predatory mites to be introduced on the single starting orange.

Run the simulation (click GO in the control panel). Do the predator and mite populations coexist? How long do the two species persist? Think of a hypothesis to explain the pattern. (STOP)

8. Now, we are going to cluster the oranges. To do this, click on 'orange' and change the distance between the circles to 5. Click on OK. Then, click on "Orange1" and change the distance between the circles from 30 to 35. Click on Ok. Then, click on "six-spotted mite" in the species window. Then, click on action parameter and change the Top depth from 0 to 35. Click on Ok. Run the simulation several times (click GO in the control panel). Do the predator and mite populations coexist? On average, how long do the two species persist? Compare these results to those from step 7.

9. Click on six-spotted mite in the species window. Then, click on action parameter and change the Top depth from 35 to 0. Click on Ok. Run the simulation several times (click GO in the control panel). Do the predator and mite populations coexist? On average, how long do the two species persist? Compare the results to the those from the prior simulations.

10. Now, we are going to examine how dispersal ability affects coexistence of the two species. Click on "six-spotted mite" in the species window. Click on action parameter and change Distance to look for 'table' from 15 to 2. Click on OK. Run the simulation several times (click GO in the control panel). What general pattern do you note. How and why is this different from the previous simulations?

Other things to try: You can also try changing how quickly the predator comes into the system by changing the predatory mite's settlement parameters. For instance, change time to settle to an earlier or later time and see what happens.

11.In the beginning of the lab, you changed the number of oranges while keeping the total area the same. You will repeat it but this time with the predator present in the system. First, click on six-spotted mite in the species window. Click on action parameter and change Distance to look for 'table' back to 15. Click on OK. Click on 'orange' and change the Radius to 7 and the distance between the circles to 6. Click on OK. Click on 'Orange1' and change the distance between circles to 28. Click on Ok. Run the simulation several times (click GO in the control panel). If extinction occurs, record the time at which it occurs for the predator and/or prey numbers.

12. Develop a hypothesis that would allow for the coexistence of the predator and prey. In this, include how and why dispersal ability, number of oranges, and distance/patchiness of the oranges effect the probability of survival of the predator and prey.

Notes and Comments

As you may have seen in this lab, the outcomes were variable within a single simulation step. Stochastic events can play a strong role in the outcome of predator/prey relationships, at times even causing extinction of the two species when you might expect coexistence. However, there are still general patterns of coexistence with increasing patchiness. Habitat patches allow for refuges in time so that even if prey populations on a single orange are driven to extinction by the predator, another empty habitat is available for them to colonize. This game of 'hide and seek' allows for prolonged coexistence of the predator and prey provided the predator is not able to outrun the prey.