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The model represents two fields in 19th’s century England, before the industrial revolution. Three sheep farmers [A, B, C] pastures their animals [blue, gray, red]. Each of them wants to put the most of sheep as possible. They (in the model) start placing a fixed number of animals, sharing them out between the two fields. Sheep can reproduce, eat grass and die (if they don’t find enough grass).

When a farmer become “important” enough (i.e. he has much more sheep than any other farmer) he can enclose the field and chase away sheep that he does not own.

SPLITTING: You can share sheep of a farmer between the two fields.

REPRODUCTION SHEEP: It is the probability of a sheep reproducing at each time step (only in INTERACTION_OFF - read on)

GROWTH GRASS FIELD: How long it takes for grass to regrow once it is eaten.

INTERACTION —

Off: Sheep reproduce independently of other animals;

On: We suppose the sheep be very, very, prolific. The farmer can place in the field only few animals, according to the percentage of sheep he has and his ARROGANCE.

ACTIVE SIMULATION —

Off: Model go on as long as you want.

On: When the percentage of sheep of a farmer reach the QUOTA FOR APPROPRIATION, he encloses the field. The sheep of another color go in the adjoining field or die.

The model starts with a fixed number of sheep, shared between two fields. At each tick sheep wander randomly, using their energy. If a sheep arrives on a “patch of grass” it eats and increases its energy. If the energy is 0, the sheep dies. If INTERACTION is OFF, sheep can reproduce according to the growth-tax REPRODUCTION SHEEP. If INTERACTION in ON, sheep “born” when they have a place in the field.

The formula used in INTERACTION_ON is:

Probability of maintaining a sheep = ARROGANCE * no. of sheep / total amount of sheep

NOTE: When the probability is >1 (considering that it does not exist) in the model it is actually 1.

Julia Schindler (2012)

Rethinking the Tragedy of the Commons: The Integration of Socio-Psychological Dispositions

Journal of Artificial Societies and Social Simulation 15 (1) 4

http://jasss.soc.surrey.ac.uk/15/1/4.html

breed [a1] breed [a2] breed [b1] breed [b2] breed [c1] breed [c2] globals [ po1 po2 ] ;; -_-_-_-_-_-_-_-_-_-_-_-_-_- SETUP -_-_-_-_-_-_-_-_-_-_-_-_-_- to Setup clear-all initial setup-patches setup-turtles reset-ticks ask patches with [ pxcor = 0 ][ set pcolor yellow] end to setup-patches ask patches [ set pcolor green ] end to initial set po1 1 set po2 1 end to setup-turtles create-a1 Splitting_A [ set shape "sheep" set color blue ] ask a1 [ setxy random-xcor random-ycor if xcor > 0 [ set xcor xcor - max-pxcor ] if int(xcor) = 0 [ set xcor -1 ] ] create-a2 (Ewes_A - Splitting_A) [ set shape "sheep" set color blue ] ask a2 [ setxy random-xcor random-ycor if xcor < 0 [ set xcor xcor + max-pxcor ] if int(xcor) = 0 [ set xcor 1 ] ] create-b1 Splitting_B [ set shape "sheep" set color grey ] ask b1 [ setxy random-xcor random-ycor if xcor > 0 [ set xcor xcor - max-pxcor ] if int(xcor) = 0 [ set xcor -1 ] ] create-b2 (Ewes_B - Splitting_B) [ set shape "sheep" set color grey ] ask b2 [ setxy random-xcor random-ycor if xcor < 0 [ set xcor xcor + max-pxcor ] if int(xcor) = 0 [ set xcor 1 ] ] create-c1 Splitting_C [ set shape "sheep" set color red ] ask c1 [ setxy random-xcor random-ycor if xcor > 0 [ set xcor xcor - max-pxcor ] if int(xcor) = 0 [ set xcor -1 ] ] create-c2 (Ewes_C - Splitting_C) [ set shape "sheep" set color red ] ask c2 [ setxy random-xcor random-ycor if xcor < 0 [ set xcor xcor + max-pxcor ] if int(xcor) = 0 [ set xcor 1 ] ] end ;; _-_-_-_-_-_-_-_-_-_-_-_- END SETUP -_-_-_-_-_-_-_-_-_-_-_-_ turtles-own[energy] ;; _-_-_-_-_-_-_-_-_-_-_-_-_-_- GO -_-_-_-_-_-_-_-_-_-_-_-_-_-_ to go move-turtles eat-grass reproduce check-death regrow-grass tick end to move-turtles ask turtles [ right random 360 forward 1 set energy energy - 1 ] ask a1 [ if xcor >= 0 [ set xcor (xcor - max-pxcor)]] ask a2 [ if xcor <= 0 [ set xcor (xcor + max-pxcor)]] ask b1 [ if xcor >= 0 [ set xcor (xcor - max-pxcor)]] ask b2 [ if xcor <= 0 [ set xcor (xcor + max-pxcor)]] ask c1 [ if xcor >= 0 [ set xcor (xcor - max-pxcor)]] ask c2 [ if xcor <= 0 [ set xcor (xcor + max-pxcor)]] end to eat-grass ask turtles [ if pcolor = green [ set pcolor lime - 3 set energy energy + 10 ] ] if Active_simulation [ if po1 = 1 [ if ( count a1 / (count a1 + count b1 + count c1 ) ) * 100 >= Quota_for_appropriation [ if po2 = 1 [ create-b2 (count b1) [ set shape "sheep" set color grey ] create-c2 (count c1) [ set shape "sheep" set color red ] ] ask b1 [die] ask c1 [die] output-print "A encloses field 1!" set po1 0 ] if ( count b1 / (count a1 + count b1 + count c1 ) ) * 100 >= Quota_for_appropriation [ if po2 = 1 [ create-a2 (count a1) [ set shape "sheep" set color blue ] create-c2 (count c1) [ set shape "sheep" set color red ] ] ask a1 [die] ask c1 [die] output-print "B encloses field 1!" set po1 0 ] if ( count c1 / (count a1 + count b1 + count c1 ) ) * 100 >= Quota_for_appropriation [ if po2 = 1 [ create-b2 (count b1) [ set shape "sheep" set color grey ] create-a2 (count a1) [ set shape "sheep" set color blue ] ] ask b1 [die] ask a1 [die] output-print "C encloses field 1!" set po1 0 ] ] if po2 = 1 [ if ( count a2 / (count a2 + count b2 + count c2 ) ) * 100 >= Quota_for_appropriation [ if po1 = 1 [ create-b1 (count b2) [ set shape "sheep" set color grey ] create-c1 (count c2) [ set shape "sheep" set color red ] ] ask b2 [die] ask c2 [die] output-print "A encloses field 2!" set po2 0 ] if ( count b2 / (count a2 + count b2 + count c2 ) ) * 100 >= Quota_for_appropriation [ if po1 = 1 [ create-a1 (count a2) [ set shape "sheep" set color blue ] create-c1 (count c2) [ set shape "sheep" set color red ] ] ask a2 [die] ask c2 [die] output-print "B encloses field 2!" set po2 0 ] if ( count c2 / (count a2 + count b2 + count c2 ) ) * 100 >= Quota_for_appropriation [ if po1 = 1 [ create-b1 (count b2) [ set shape "sheep" set color grey ] create-a1 (count a2) [ set shape "sheep" set color red ] ] ask b2 [die] ask a2 [die] output-print "C encloses field 2!" set po2 0 ] ] ] end to reproduce ifelse Interaction [ ask a1 [ if energy > 50 [ if random-float 1 < ( Arrogance_A * ( count a1 / (count a1 + count b1 + count c1 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ask a2 [ if energy > 50 [ if random-float 1 < ( Arrogance_A * (count a2 / (count a2 + count b2 + count c2 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ask b1 [ if energy > 50 [ if random-float 1 < ( Arrogance_B * ( count b1 / (count a1 + count b1 + count c1 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ask b2 [ if energy > 50 [ if random-float 1 < ( Arrogance_B * (count b2 / (count a2 + count b2 + count c2 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ask c1 [ if energy > 50 [ if random-float 1 < ( Arrogance_C * ( count c1 / (count a1 + count b1 + count c1 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ask c2 [ if energy > 50 [ if random-float 1 < ( Arrogance_C * (count c2 / (count a2 + count b2 + count c2 ) ) ) [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] ] [ ask turtles [ if random 100 < Reproduction_sheep [ hatch 1 [ set energy 50 ] set energy energy - 50 ] ] ] end to check-death ask turtles [ if energy <= 0 [ die ] ] end to regrow-grass ask patches [ if pxcor < 0 [ if random 100 < Growth_grass_field_1 [ set pcolor green ] ] if pxcor > 0 [ if random 100 < Growth_grass_field_2 [set pcolor green ] ] ] end