Lotka Volterra model not working as intended

[Maxpark]

Good morning,
I am working on a Lotka Volterra model that Kumar Venkat has made(Link: https://www.cleanmetrics.com/pages/Wild ... ment-2.pdf). However, the model doesn't return an oscillating graphs as done in the pdf linked above. Not really sure what I have done wrong and would appreciate some feedback. I am currently using vensim PLE 10.2 on my MacBook.

I have also attached my Vensim model at the bottom.

Thanks, Max

[tomfid]

Nice to see a model with balanced units.

Generally to debug a problem like this, there are two things that are very helpful:
- Use synthesim overrides to freeze one or more of the stocks at a constant value; then you can more easily see how other things respond
- Use the table tool to walk through the numerical values at the start

In this case, where there seems to be explosive growth, the second approach worked for me. I think the problem may be primarily parameterization.

At time zero, the abalone birth rate is 50000 ab/mo, on a population of 500. If I make the abalone food conversion efficiency 100x smaller, the birth rate is more reasonable. However, the population then collapses, because too much kelp is eaten. So it might be better to modify the prey search parameter. Probably a few more parameter tweaks are needed.

[tomfid]

Part of the challenge here is that the standard description of the LV model is nicely physical, but challenging to parameterize. For example:

pred births = params*predator*prey

in typical SD practice might be written as

pred births = predator*fractional birth rate
fractional birth rate = reference birth rate*(prey/reference prey)

Normalizing by reference populations generally makes it easier to initialize the model, though it may also make the physics of mass/energy conversion more remote.

Another issue is with the LV model itself: with infinite prey (infinite food), predator births are infinite, but obviously that wouldn't really be the case, because food is not the only limiting resource (there's food processing time, gestation delays, etc.). That's the inspiration for the Holling Type 2 formulation of pred/prey interaction.