1. Equilibrium
2. Second Predator - Overkill
3. Second Order Predation

Equilibrium Mode

To run the model in dynamic equilibrium mode click on the EquilibriumTest switch in the number 4. box.

This over-rides the settings of the variables controlled by the first three sliders and calls for a one time 0.2 pulse reduction of carnivores at -11000 years B.P.

Overkill Mode - H. sapiens enters the New World

Deactivate the Second-Order Predation mode by setting the slider AmtHsKillCrn to zero. Pull slider control in the number 2. box to zero.
Note:the 'U' button will reset the slider.

The first slider (1. AmtMigrateHs)controls the amount of migration. The default, 200 (0.2K) is based on the model done by Whittington and Dyke (1989).

Press 'Control R' to start the model. (The question mark button shows the documentation for the slider.)

Now look at what happens if 500 (0.5K) people enter the New World. Look at what happens if 1000 (1.0K) do.

(Note: It may be easier to place your cursor in the little window in the slider and type in the number. You have to press enter before the change will work.)

Reset the first slider to the default migration value of 200 (0.2K) by clicking on the 'U' button at the right side of the slider.

Herbivores Needed by H. sapiens per year

The third slider (3. FoodNeedHs) controls how many pounds of herbivore is necessary to support one pound of H. sapiens per year. The default - 10 lbs.. - is half what it takes to support a large carnivore. See what happens if you reduce the amount to 5 lbs. Now see what happens if you increase it to the same amount as a non-human carnivore - 20 lbs. - now increase it to more than a non-human carnivore - 30 lbs.

Whittington and Dyke (1984) derived a prey destruction value of 3.862 animal units (a.u.=1000 lbs.) per person per year. If we assume a 100 pound human then this means 38.62 pounds of herbivore per pound of human per year. This is the top setting for the slider.

Note: to change the range, double click on the body of the slider.

Second Order Overkill Mode - H. sapiens reduces Carnivore Populations

To run the model in Second Order Overkill Mode carnivore populations have to be reduced by H. sapiens. This is controlled by the second slider - (2. AmtHsKillCrn). The default value is 0.025. Or 1 unit of carnivore for every 40 units of humans.

Click on the 'U' button on the second slider to run the model in default Second Order Overkill Mode.

Press 'Control R' to start the model. Look at the other pages of the graph pad to see what is happening with Herbivores and Vegetation.

(Note: If the graph is flat you may have forgotten to reset the slider which controls the migration of H.sapiens (slider 1.). If there aren't any H.sapiens they cannot kill carnivores.)

Now see what happens if people reduce carnivore populations by 0.05

Note: It may be easier to place your cursor in the little window in the slider and type in the number. You have to press enter before the change will work.

Now see what happens when people only hunt carnivores at 0.01. As you see there is no extinction. Running the model this way and then running the model in simple Overkill mode gives a clue to why people would be motivated to reduce carnivore populations:

• Overkill mode - H. sapiens end value is 833.79 K
• Second Order Overkill mode, AmtHsKillCrn= 0.014 H. sapiens end value is 863.33 K
H. sapiens are more successful when they reduce carnivore populations slightly. There are more herbivores and fewer carnivores so people have more to eat. It is only if carnivore killing has exceeded the threshold value (0.014 in this model), that extinctions occur.

An intermediate case occurs when the AmtHsKillCrn=0.015 Mixed Feeders go extinct at -7159 years. To run this simulation you you will have to change the time setting. On the tool bar at the top of the screen click on the 'Run' menu and pull down to 'Time Specs'. This will activate a window where you can set beginning and ending times, dt, and the integration method. Change the ending time to -7000 and click 'OK'. Then set the AmtHsKillCrn to 0.015.

You have completed the first level of interacting with the model.

To interact with these variables:

• Click on the '?' button to see the documentation. You will notice that some of the graphs do not have a '?' button. These are variables related to grazers, ruminant and non-ruminant. I have supplied the documentation in the box surrounding the graph.

• Double click on the graph to change the slope of the curve. Most of the changes I have experimented with make the entire model unstable but do not make Overkill more likely.

• The easiest way to make the model unstable is to change the slope of the curve controlling how the environment is apportioned to trees and grass.

• The easiest way to prevent extinctions when running in Second Order Overkill mode is to match the H. sapiens hunting curves to the Carnivore hunting curves. The current configuration assumes that H. sapiens is more apt to switch to gathering when herbivores are scarce and to loose interest when they can obtain the maximum number of herbivores to support existing populations and reproduce at a maximum rate.

If you find an combination which is stable and which makes Overkill more likely than Second Order Overkill please e-mail me at elin@quaternary.net

References

1989. Simulating overkill: experiment with the Mossimann and Martin model. In, Martin, P.S. & Klein, R.G. (eds.) Quaternary extinctions: A prehistoric revolution. Univ. Arizona Press, Tucson.