Hello,
I am working on a model of unicellular organisms (plankton) that float around and consume particles of food (nuts). Nuts have size 0.2, plankton start from size 1. When they consume nuts, they add the size of the latter to their own. When they reach size 2, they devide into two plankton of size 1. If a plankton cell has gone without eating for 30 ticks, it dies and breaks down into n=size/0.2 nuts.
I have taken several steps to ensure mass conservation in the system.
If a plankton cell is e.g. size 1.8 and encounters two nuts, it will only consume one (generally, plankton can only eat as many nuts as will bring them up to size 2):
to eat
let my-radius size / 2
let nearby-nuts nuts with [distance myself < (my-radius + (size / 2))]
;; update starvation counter
ifelse not any? nearby-nuts
[set ticks-without-eating ticks-without-eating + 1]
[set ticks-without-eating 0]
;; eat while under size 2 and there are nuts to eat
while [size < 2 and any? nearby-nuts] [
let this-nut one-of nearby-nuts
let nut-size [size] of this-nut
set size size + nut-size
ask this-nut [ die ]
set nearby-nuts nearby-nuts with [self != this-nut]
]
end
When they starve, they break into n=size/0.2 nuts of size size/n
to starve?
;; if plankton hasn't eaten for 30 ticks, it dies and breaks into nuts
if ticks-without-eating >= 30 [
let num-to-hatch (size / nuts-size)
set breed nuts
set shape "circle"
set size (size / num-to-hatch)
set color brown
hatch num-to-hatch [fd 0.1]
die ;; Remove plankton after starvation
]
end
Finally, when they reproduce, they halve their size:
to reproduce
;; if plankton reaches size 2 or greater, split it into two new plankton
if size >= 2 [
set size (size / 2) ;; reset size of the current plankton
set ticks-without-eating 0
hatch 2 [fd 0.1]
die
]
end
However, total mass (sum [size] of plankton + sum [size] of nuts) in the system is not conserved; it slowlly decays at increments I cannot reconcile with the magnitudes involved. Plankton size also behaves unexpectedly; it should not be possible to take values other than 1, 1.2, 1.4, 1.6, 1.8, 2 but that is not the case. Nut size on the other hand stays at 0.2 as expected.
Weirdly, the problem goes away if I change the size of nuts from 0.2 to 0.1.
Can someone help me understand what is going on? I would welcome any advice on how to prevent this instability.
Here is the full model:
show code
breed [ plankton a-plankton ]
breed [ nuts nut ]
plankton-own [
ticks-without-eating
]
to setup
clear-all
create-plankton num-plankton
[
set shape "circle"
set color green
set size one-of [1]
set ticks-without-eating 0
setxy random-xcor random-ycor
]
create-nuts num-nuts
[
set shape "circle"
set color brown
set size nuts-size
setxy random-xcor random-ycor
]
reset-ticks
end
to go
ask plankton [
move
eat
starve?
bounce-plankton
reproduce
]
ask nuts [
float
]
tick
end
to move
right random whimsy
left random whimsy
forward speed
end
to float
right random 90
left random 90
forward 0.1
end
;to eat
; let my-radius size / 2
; ;; all nuts that are touching (within radius)
; let nearby-nuts nuts with [distance myself < (my-radius + (size / 2))]
;
; ;; if plankton hasn't eaten, increment the counter
; ifelse not any? nearby-nuts
; [set ticks-without-eating ticks-without-eating + 1]
; [set ticks-without-eating 0]
;
; if size < 2 [
; ;; eat each nearby nut
; ask nearby-nuts [
; let nut-size size
; ask myself [ set size size + nut-size ]
; die ;; remove the nut
; ]
;
; ]
;end
to eat
let my-radius size / 2
let nearby-nuts nuts with [distance myself < (my-radius + (size / 2))]
;; update starvation counter
ifelse not any? nearby-nuts
[set ticks-without-eating ticks-without-eating + 1]
[set ticks-without-eating 0]
;; eat while under size 2 and there are nuts to eat
while [size < 2 and any? nearby-nuts] [
let this-nut one-of nearby-nuts
let nut-size [size] of this-nut
set size size + nut-size
ask this-nut [ die ]
set nearby-nuts nearby-nuts with [self != this-nut]
]
end
to starve?
;; if plankton hasn't eaten for 30 ticks, it dies and breaks into nuts
if ticks-without-eating >= 30 [
let num-to-hatch (size / nuts-size)
set breed nuts
set shape "circle"
set size (size / num-to-hatch)
set color brown
hatch num-to-hatch [fd 0.1]
die ;; Remove plankton after starvation
]
end
to bounce-plankton
ask plankton [
let my-radius size / 2
ask other plankton [
let other-radius size / 2
if distance myself < (my-radius + other-radius) [
set heading random 360
forward 0.1
]
]
]
end
to reproduce
;; if plankton reaches size 2 or greater, split it into two new plankton
if size >= 2 [
set size (size / 2) ;; reset size of the current plankton
set ticks-without-eating 0
hatch 2 [fd 0.1]
die
]
end
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