Cellular automata models : Null Models for Ecology Jane Molofsky Department of Plant Biology University of Vermont Burlington, Vermont 05405
Mar 27, 2015
Cellular automata models :Null Models for Ecology
Jane MolofskyDepartment of Plant Biology
University of VermontBurlington, Vermont 05405
Cellular automata models and Ecology
• Ecological systems are inherently complex.
• Ecologists have used this complexity to argue that models must also be correspondingly complex
Cellular automata models in Ecology
• Search of ISI web of science ~ 64 papers
• Two main types– Empirically derived rules of specific systems– Abstract models– Many more empirical models of specific
systems than abstract models
1-dimensional totalistic rule of population dynamics
• Individuals interact primarily locally
• Each site is occupied by only one individual
• Rules to describe transition from either occupied or empty
• 16 possible totalistic rules to consider
Molofsky 1994 Ecology
Ecological Scenarios
• Two types of competition– Scramble– Contest
• Two scales of dispersal– Local– Long
Possible neighborhood configurations
0 10
0 010 00
1 10
1 01
0 11 1 11
1 00
0 1 32Sum
Transition Rules
0 1 1 0Local Dispersal
Long distance Dispersal 1
0 1 32Sum
1 1 0
Long distance dispersal
Local Dispersal
Totalistic Rule Set
2 states, nearest neighbors 28 or 256 possible rules
Scr
am
ble
Conte
st
Totalistic Rule Set
• How often we expect complex dynamics to occur?– Ignore the 2 trivial cases– 6/14 result in “chaos”, 6/14 periodic, 2/14
fixation
• How robust are dynamics to changes in rule structure?
Totalistic Rule Set
2 states, nearest neighbors 28 or 256 possible rules
Scr
am
ble
Conte
st
Do plant populations follow simple rules?
Cardamine pensylvanica
Fast generation timeNo seed dormancySelf-fertileExplosively dispersed seeds
1-dimensional experimental design
Grown at 2 different spacings (densities)
Molofsky 1999. Oikos
Do plant populations follow simple rules?
• In general, only first and second neighbors influenced plant growth.
• However, at high density, long range interactions influenced final growth
Experimental Plant Populations
Fast generation timeNo seed dormancySelf-fertileExplosively dispersed seeds
Replicated 1-dimensional plant populationsFollowed for 8 generations
Two dimensional totalistic rule
• Two species 0, 1• von Neumann neighborhood• Dynamics develop based on neighborhood sum • 64 possible rules• Rules reduced to 16 by assuming that when only
1 species is present ( i.e sum of 0 or 5), it maintains the site the next generation
• 16 reduced to 4 by assuming symmetry
Rule system
Species 0
Species 1
1
4
2
3
3
2
4
1
Positive 0 0 1 1
Negative 1 1 0 0
Allee effect 0 1 0 1
Modified Allee 1 0 1 0
Biological Scenarios
Positive Frequency Dependence
Negative Frequency Dependence
Allee effect
System Behavior
1,0
0,1
1,1
0,0
P1
P2
P2= probability that the target cell becomes a 1 given that the neighborhood sum equals 2
P1= probability that the target cell becomes a 1 given that the neighborhood sum equals 1(0.2,0.4)”Voter rule”
clustering
Ergodicperiodic
Phase separation
Molofsky et al 1999. Theoretical Pop. Biology
00
00
10
01
0.98,0.98
0.35, 0
0.31,0
0.27,0
D. Griffeath, Lagniappe U. Wisc.Pea Soup web site
Probability that a migrant of species 1 arrives on the site
Probability that the migrant establishes:H1=0.5 + a (F1-0.5)
Probability that a site is colonized by species 1
2211
111 DHDH
DHP
Neighborhood shape
Frequency dependenceDispersal
Moore neighborhoood
Positive frequency dependence
Molofsky et al 2001. Proceedings of the Royal Society
Spatial Model
Determine the probability that a
species colonizes a cell
One individual per grid cell
Update all cells synchronously
Stochastic Cellular Automata
Probability that a migrant of species 1 arrives on the site
Probability that the migrant establishesh1=0.5 + a (f1-0.5)
Probability that a site is colonized by species 1
Transition Rule
P1 = h1f1/(h1f1 + h2f2+ h3f3+ h4f4+ h5f5+ h6f6+ h7f7+ h8f8+ h9f9+ h10f10 )
DispersalFrequency dependence
1. The neutral case
(a=0)Ecological Drift sensu Hubbell 2001
2. Positive Frequency(a=1)
Generation 0
Generation 100,000
Molofsky et al 2001. Proc. Roy. Soc. B.268:273-277.
3. Positive Frequency20 % unsuitable habitat
Generation 0
Generation 100,000
4. Positive Frequency Dependence40% unsuitable habitat
Generation 0
Generation 100,000
The Burren
Interaction of the strength of frequency dependence and the unsuitable habitat
Number of species after 100 000 generations
Molofsky and Bever 2002. Proceedings of the Royal Society of London
Invasive species
Local interactions: Yes, reproduces clonally
Exhibits positive frequency dependence: Yes
High levels of diversity: Yes
No obvious explanation: Yes
Lavergne, S. and J. Molofsky 2004. Critical Reviews in Plant Sciences
Consideration of spatial processes requires that we
explicitly consider spatial scaleEach process may occur at
its own unique scale
Competition may occur over short distances but dispersal may occur over longer distances
Grazing by animals in grasslands may occur over long distances while seed dispersal occurs over short distances
Negative frequency dependenceTwo species, two processes
dispersalfrequency
dependenceProbability that a migrant of species 1 arrives on the site
Probability that the migrant establishes:H1=0.5 + a (F1-0.5)
Probability that a site is colonized by species 1
2211
111 DHDH
DHP
D1
F1
Interaction neighborhoodDispersal neighborhood
Each process can occur at a unique scale
Focal site
Molofsky et al 2002 Ecology
Local Frequency DependenceLocal Dispersal
Strong Frequency Dependence
a = -1
Intermediate Frequency Dependencea = -0. 1
Weak Frequency Dependence
a = - 0.01
For local interactions when frequency dependence is strong (a
= -1)random patterns develop because
H1 = 1 - F1,
D1 = F1
P1 = (1- F1 ) , F1 / (1- F1 ) , F1 + F1 (1- F1 )= (1- F1 ) , F1 / 2( (1- F1 ) , F1 )
= 0.5
Weak Frequency Dependencea = -0.01
Local DispersalLocal Frequency Dependence
Long DispersalLong Frequency Dependence
Dispersal and Frequency Dependence at same scale
Local Frequency Dependence Long Distance Dispersal
Strong Frequency Dependencea = - 1
Weak Frequency Dependencea = - 0.01
Why bands are stable?
Local, strong, frequency dependence(over the large dispersal scale, the two species have the same frequency: D1=D2)
Because the focal, blue, cell is mostly surrounded by yellow, is stays blue
Local DispersalLong Distance Frequency
Dependence
Strong Frequency Dependencea = - 1
Weak Frequency Dependencea = - 0.01
Why bands are stable?
Local dispersal
(over the large interaction scale, the two species have the same frequency: H1=H2)
Because the focal, blue, cell is mostly surrounded by yellow, is becomes yellow
How robust are these results?
Boundary ConditionsTorus, Reflective or Absorbing
Interaction NeighborhoodsSquare or Circular
UpdatingSynchronous or Asynchronous
DisturbanceHabitat Suitability
Effect of Disturbance
Local Frequency DependenceLong Distance Dispersal
Local Dispersal Long Distance Frequency Dependence
Strong Frequency Dependence a = -1Disturbance = 25 % of cells
Habitat Suitability
Local Frequency DependenceLong Distance Dispersal
Local Dispersal Long Distance Frequency Dependence
Strong Frequency Dependence a = -125 % of cells are unsuitable
Processes that give rise to patterns…
Strong Negative Frequency Dependence only if equal scalesWeak Negative Frequency Dependence only if long distance dispersal
Strong Negative Frequency Dependence only if unequal scales
Weak Negative Frequency Dependence only if dispersal is local
Weak Positive Frequency Dependence only if dispersal is local
Strong Positive Frequency Dependence most likely if local scales only
Negative frequency dependence
If dispersal and frequency dependence operate over different scales, strong
patterning results
Striped patterns may explain sharp boundaries between vegetation types
Need to measure both the magnitude and scale of each process
Next step
• Non symmetrical interactions
• For non-symmetrical interactions, what is necessary for multiple species to coexist? Most multiple species interactions fail but we can search the computational universe and ask, which constellations are successful and why?