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Areas of Application 77
☞ The Predator_Prey model is located in the folder \Examples\Continuous\Standard Block Mod- els. This model uses blocks from the Value and Plotter libraries.
Model details
This model illustrates some interesting continuous modeling concepts:
• Becausethemodelhasfeedback,theHoldingTankblocksaresettoInputsare:integrated (no delay). For more information, see “Integration vs. summation in the Holding Tank block” on page 709.
• The Holding Tank block has a want input connector that is used to tell the block how much of the contents you want removed. The amount wanted and the amount that is actually removed (indicated at the get connector) may differ, as discussed below.
• Bydefault,theHoldingTankblockissettonotallowitscontentstobecomenegative.This means that the population of hare cannot be reduced below the available amount, no matter what is requested through the want connector. The amount that is actually removed is reported at the get output connector. If the tank were allowed to become negative, the amount at the want connector and the amount at the get connector would be the same. Since it can’t go negative in this model, the amount at want could be higher than the amount actu- ally removed and reported at get.
• Dividing the hare population by the area of the ecosystem (100 hectares) determines how many hares there are per hectare. The lynx can eat every hare in each hectare, but if they eat too many and the hare population decreases, the lynx mortality rate increases. This contrib- utes to the cycles seen in the model.
• Noticethat,ateachpointintime,theresultintheHaresKilledblockishigherthanthehare population shown on the plotter. This occurs because hares are constantly being born and the lynx kill not just the previous step’s population but also some of the new births.
• TheLookupTableblockissettoOutputis:interpolated,whichmeansthatintermediate values can be used. For example, an input value of 65 (which is halfway between the Hares Eaten values of 60 and 70) will cause the block to output 0.175 (which is halfway between the Lynx Death Rate of 0.2 and 0.15).
Further exploration
If you review the initial assumptions for this model, you can probably see several enhance- ments that could be made.
• Thebirthratesforboththehareandthelynxcouldvarybasedonmodelconditionsoron outside factors. For instance, the birth rate might be dependent on the health of the parents, the level of crowding, or the amount of pollution in the ecosystem.
• You might add a predator or an additional food source for the lynx.
• Themodelassumesanunlimitedfoodsupplyforthehare.Toafoodsource,theharewould
be considered the predator, so modeling a food source would follow the same logic as adding
the lynx predators.
• Hares could have predators other than lynx.
• Youcouldfactorinoutsideconditions,suchasthetimeofyearandexpectedweathercondi-
tions. Then examine the effects of those conditions on mortality rates.

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