model for food chain

[ccmhsa...@gmail.com]

Hi, we are looking for an existing simulation modeling changes in food chains with coyote, mice and grass.  Please contact if you already created a model for these variables.  Thank you.

[Dale Frakes]

Have you looked at the Wolf-Sheep model in the Model Library?  You have Wolves eating sheep who eat grass, which is probably similar (at least a good starting point) for coyotes who eat rabbits who eat grass.

On 03/29/2019 09:52 AM, ccmhsa...@gmail.com wrote:

Hi, we are looking for an existing simulation modeling changes in food chains with coyote, mice and grass.  Please contact if you already created a model for these variables.  Thank you.

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Dale Frakes
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PSU Systems Science
dfr...@pdx.edu - http://web.pdx.edu/~dfrakes/

[Ken Kahn]

Other than the shapes the wolves can be any carnivore (coyote, shark, praying mantis, ...), the sheep can be any herbivore that the predator preys upon (e.g. mice, rabbits, some fish, ...), and grass is any vegetation that the prey can eat. Some mice are herbivores. 

It is only for pedagogic reasons that the wolf-sheep-grass model isn't simply the predator-prey-vegetation model.

UNLESS one uses empirically derived numbers for odds of reproduction and energy acquired from eating. But I don't think this makes sense for this model since it is unclear how long a tick is (maybe a year given the rate of reproduction) and the size of a patch (very large since it takes a year to cross one). This is a conceptual model and it doesn't make sense to turn it into an empirical one without major changes.

On Saturday, March 30, 2019 at 12:56:46 AM UTC+8, Dale Frakes wrote:

Have you looked at the Wolf-Sheep model in the Model Library?  You have Wolves eating sheep who eat grass, which is probably similar (at least a good starting point) for coyotes who eat rabbits who eat grass.

On 03/29/2019 09:52 AM, ccmhsa...@gmail.com wrote:

Hi, we are looking for an existing simulation modeling changes in food chains with coyote, mice and grass.  Please contact if you already created a model for these variables.  Thank you.

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[Steve Railsback]

And, in addition to the concerns Ken raises: "Wolf sheep predation" (with grass turned off) is really only a discretization of the classic "Lottka-Volterra" predator-prey model, originally published in 1920. Much of ecology in the past 3 decades has been about showing why the fundamental assumptions of this model -- that prey mortality is proportional to predator abundance and predator reproduction is proportional to prey abundance -- are wrong. The reason they are wrong is because animals and plants have behavior: the more wolves around, the more careful sheep are. Being careful *can* mean eating less and reproducing less, so sheep abundance goes down as an indirect effect of wolves, but the wolves don't get any food as a consequence.

Including behavior and avoiding such simplistic assumptions is exactly what we use individual-based models and NetLogo... Wolf sheep predation is a very nice demonstration model of many things but it is *not* a good example of an individual-based food chain model (or ecological model in general). If you want to use that model, just use the equations instead of NetLogo: https://en.wikipedia.org/wiki/Lotka%E2%80%93Volterra_equations

(Just by coincidence, we have a book in press about one way to to model adaptive tradeoff decisions, such as how to balance the need to eat against the risk of predation, in IBMs.)

(In a short course we once asked students to add predator avoidance behavior to the sheep in this model. They very quickly found a paper showing that real sheep respond to the presence of predators by grouping together in flocks. So the students quickly added the flocking behavior from the NetLogo library's Flocking model to the sheep.)

Steve R.