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Community Structure v_4

by David W. Rudge and William Merrow (Submitted: 08/07/2008 )

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WHAT IS IT?

NetLogo-Coummunity Structure invites students to explore the effects of competition and predation on the stability of an ecosystem. Students will investigate whether and why population size of a given species changes over time in terms of the direct and indirect effects of the presence of other species.

To investigate these issues, students will study a simulated meadow ecosystem as it changes over time. Nine species are represented, six plant species that compete with one another for space and three predator species, two of which compete with one another for a single species of plant. After studyiing the system with all 9 species present, students will systematically explore this ecosystem by first studying competition amongst the three grass species and competition amongst the three vine species. After studying each predator in isolation, students will consider the role of each predator in the ecosystem as a whole in terms of its direct and indirect effects on other species.

USING THE SIMULATION

1. To get a feel for the simulation, click on the light blue SETUP button to populate the field with all nine species. Click the light blue GO button to start the simulation. Clicking the GO button a second time stops the simulation. (The speed of the simulation can be increased by turning off the Meadow view button, the button to the immeadiate right of the red arrow.)

2. View the RELATIVE BIOMASS bar graph to keep track of the current total amount of biomass of each species relative to one another. The PRODUCERS and CONSUMERS line graphs chart fluctuations in populations over time relative to one another. (Note that to faciliate study a smaller y-scale is used to measure predator biomass because biomasses associated with producers (plants) are always much larger than that associated with consumers in any stable ecosystem.)

For each species, does the relative biomass number flucutate at random when all 9 species are present? Or is it the case that over time the ossilations appear to be converging on a single range of values (the species' carrying capacity (k))?

COMPETITION AMONGST GRASS SPECIES

Smooth crabgrass (light green), Red Clover (pink) and English Plantain (dark green) represent grass species that compete with one another for space in this meadow. We need to figure out among these three species which is the most dominant and which is the least dominant competitor.

1. Stop the simulation if you haven't already done so by clicking on the GO button.
2. Make sure the three grass SPECIES? buttons are ON and set all of the other SPECIES? buttons (e.g. ivy?) to OFF and click the SETUP button to populate the meadow.
3. Slide the "ADJUST SPEED" parameter at the top of the meadow view box to the left to slow the simulation.
4. Click the GO button to start the simulation. Watch a given square (pink, light green or dark green) to see what replaces it (black = blank space). Do this for all three species. This will provide you with a basis for predicting which of the three is the best and second best competitor.
5. Click the GO button once more to stop the simulation. Slide the "ADJUST SPEED" parameter back to the right to speed up the simulation.
6. Now predict what you should see in the PRODUCERS line graph if indeed one species of grass is a better competitor than the other two, and one of the remaining two is a better competitor than the last.
7. Test your prediction by running the simulation (click the GO button). What do you suppose will happen over a long period of time in a meadow composed of only these three species if the trends you see continue?

COMPETITION AMONGST VINE SPECIES

Poison Ivy (purple), Smooth Sumac (red) and Virginia Creeper (yellow) represent vine species that also compete with one another for space in this meadow. We need to figure out among these three species which is the most dominant and which is the least dominant competitor.

1. Stop the simulation if you haven't already done so by clicking on the GO button.
2. Make sure the three vine SPECIES? buttons are ON and set all of the other SPECIES? buttons (e.g. grass?) to OFF and click the SETUP button to populate the meadow.
3. Slide the "ADJUST SPEED" parameter at the top of the meadow view box to the left to slow the simulation.
4. Click the GO button to start the simulation. Watch a given square (purple, red or yellow) to see what replaces it (black = blank space). Do this for all three species. This will provide you with a basis for predicting which of the three is the best and second best competitor.
5. Click the GO button once more to stop the simulation. Slide the "ADJUST SPEED" parameter back to the right to speed up the simulation.
6. Now predict what you should see in the PRODUCERS line graph if indeed one species of vine is a better competitor than the other two, and one of the remaining two is a better competitor than the last.
7. Test your prediction by running the simulation (click the GO button). What do you suppose will happen over a long period of time in a meadow composed of only these three vine species if the trends you see continue?

COMPETITION AMONGST PLANT SPECIES

Based upon your experiences so far, think through how you could test whether the grass species are better or worse competitors than the vine species.

PREDATION

The effects of predation on this system can be studied by thinking through what would happen if that predator were introduced into an ecosystem containing only prey species. This will provide insight into the direct effect of the predator on those species.

Consider a very simple ecosystem composed of the three grasses. You've already determined what would happen if this system were allowed to run its course with no interference by another species. Knowing that Voles eat all three species of grass, what do you suppose will happen if you introduce Voles into such an ecosystem?

1. Stop the simulation if you haven't already done so by clicking on the GO button.
2. Make sure the three grass and Vole SPECIES? buttons are ON and set all of the other SPECIES? buttons (e.g. ivy?) to OFF and click the SETUP button to populate the meadow.
3. Now predict what you should see in the PRODUCERS line graph after the Vole is introduced.
4. Run the simulation by clicking the GO button to test your prediction. What do you suppose will happen over a long period of time in a meadow composed of only these four species if the trends you see continue? Will the introducton of the vole species cause the extinction of one or more of the grass species if you ran this simulation long enough? Why or why not?

Deer eat the Smooth Sumac and Poison Ivy vine species. Raccoons eat the Poison Ivy and Virginia Creeper vine species (and also Voles). Predict what will happen over time if deer are introduced into a meadow with only Smooth Sumac and Poison Ivy and use the simulation to test your prediction. Predict what will happen over time if the raccoons are introduced into a meadow with only Poison Ivy and Virginia Creeper, and again test your prediction using the simulation.

What do you suppose would happen if Virginia Creeper was introduced into a meadow containing Smooth Sumac, Poison Ivy and Deer?

What do you suppose would happen if Smooth Sumac was introduced into a meadow containing Poison Ivy, Virginia Creeper and Raccoons?

Can Deer and Raccoons coexist in a meadow that contains only Poison Ivy (a vine they both prey on)?

Can Voles and Raccoons coexist together in a meadow containing only the three grass species? Compare the carrying capacity of the Vole population before and after the Raccoons are introduced. Compare the carrying capacity of the grass species populations before and after the Raccoons are introduced. Are you surprised by the effect of Raccoons on the grass populations? Consider also what would happen if you introduced Poison Ivy and/or the Virginia Creeper to this system.

PUTTING IT ALL TOGETHER

Ultimately what we want to do is figure out the effect of each of the predator species on the remaining eight species in this meadow ecosystem. To do this, we will first develop a set of predictions for what we think will happen when one of the three predators is removed. These predictions will include both consideration of what will happen to the species it preys upon (direct effects) and also consideration of what will happen to the other species (indirect effects). We will then test each of these predictions by running the simulation.

First remind yourself of the dynamics of the community by examining it once more with all nine species present.

1. Stop the simulation if you haven't already done so by clicking on the GO button.
2. Make sure all of the SPECIES? buttons are ON and click the SETUP button to recolonize the meadow with all 9 species.
3. Run the simulation by clicking the GO button. Try to estimate what the carrying capacities of each population are when all 9 species are present.

Next consider what would happen to the ecosystem as a whole if you were to remove the Voles.

4. Stop the simulation if you haven't already done so by clicking on the GO button.
5. Make sure all of the SPECIES? buttons are ON except for the Voles and click the SETUP button to populate the meadow.
6. Now predict what you should see in the PRODUCERS and CONSUMERS line graphs after the Voles are removed. Think about this with respect to the effect of its absence on the prey species (direct effects) and then think through what effect (if any) it's absence will have on the other species (indirect effects).
7. Run the simulation by clicking the GO button to test your predictions. Is the resulting system a stable one? or does it appear the system might lose some of its species diversity.

Repeat these last 7 steps for the other two predator species (Deer, Raccoons).

FINAL THOUGHTS

The last activity illustrates that even though the three predator species all prey on plants their respective effect on the ecosystem as a whole is not the same. A predator whose presence appears to maintain an ecosystem's diversity is referred to in ecology as a keystone predator.

CREDITS AND REFERENCES

This is based on elements from the several WolfSheep Predation NetLogo Programs by Uri Wilensky and was inspired by an earlier simulated ecosystem program called "Ecobeaker", originally developed as part of the BioQuest Library by Eli Weir.

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