Forest Dynamic Model - Dewdney (2000)

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WHAT IS IT?

This is the simulation model of Dewdney (2000). In his model, he used an arbitrary number of species, each with an arbitrary population size, preyed on one another in the following manner.

Within each iteration, two individuals (not species) are chosen at random.
One individual ingests the other, reproducing in consequence.

He called this model the multispecies logistic (MSL) system. The adjective "logistic" was chosen because the total biomass (number of individuals) remained fixed as a simple consequence of the basic trophic act (Dewdney, 2000). This is equivalent to assumed that community is saturated.

HOW IT WORKS

After each tick, the program displays a histogram of species versus abundances. Model code allows the user to select any number of species. The model has an "extinction" switch. With the 'extinction' switch "on" any species having abundance 1 will, when eaten, disappear from the community. With the extinction switch "off" species with abundance 1 will not be eaten, although they can (and do) end up preying on other species.This allows singleton species to increase their number (see Dewdney 2000).

HOW TO USE IT

Sliders

w1: Used to change the local community size. J_L = (w1+1)²
S: Number of species in the meta-community.

THINGS TO NOTICE

Stochastic drift determines the shape of the species distribution. A peak forms at the low end, and a long tail appears on the right. Distribution curve reaches "J-shape" in the long run. The higher the initial average abundance, "very abundant species were less likely to ingest other species as they approached the logistic limit (Dewdney, 2000)".

THINGS TO TRY

The model has an "extinction" switch. With the 'extinction' switch "on" any species having abundance 1 (singletons) will, when eaten, extinct from the community. This cause a mono-dominance (one species) community in long run and unstable community. With the extinction switch "off" species with singletons will not be eaten. This allows them to reproduce and increase their numbers. Rare species have advantage over the dominant species.

EXTENDING THE MODEL

It is possible to set the initial numbers of each species as Dewney (2000) suggested. However, this needs an additional parameter for each species.

This model does not assumed natural deaths due to aging of an individual. It is also possible to incorperates 'age' as a 'property' of an individual (turtle).

NETLOGO FEATURES

RELATED MODELS

Dewdney, A. (2000). A dynamical model of communities and a new species-abundance distribution. The Biological Bulletin, 198(1), 152–165.

CREDITS AND REFERENCES

For the model itself:

Punchi-Manage, R. (2023d). NetLogo Dewdney's (2002) Model. http://netlogo/models/Dewdney-2002-Model.

Please cite the NetLogo software as:

Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Comments and Questions

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Click to Run Model

extensions [table]
globals [i1 k1 x-cor y-cor local-richness-counts local-richness hist-frq-local rel-hist-frq-local sp n n2 time-spent specs time-spent-list abundance-list mean-time-spent data mean-life-comm k2 i2 h num j2 j s1]
breed [trees a-tree]
trees-own [species time-system]

to setup
  clear-all
  resize-world 0 (w2 - 1) 0 (w1 - 1)
  set JL (w1 + 1) * (w2 + 1)
  set k1 -1
  set n 2
  set sp 0
  set j 0
  set S w2
  set mean-time-spent []
  set abundance-list []
  set data []
  set mean-life-comm []
    while [k1 < w1 - 1 ] [
      set k1 k1 + 1
      set i1 -1
      while [i1 < w2 - 1] [
        set i1 i1 + 1
        create-trees 1 [
          setxy i1 k1
          set time-system 0
          set species random S
          set shape "circle"
        ]
      ]
    ]

  set local-richness-counts map count table:values table:group-agents trees [species]
  set local-richness length local-richness-counts
  set hist-frq-local sort-by > map count table:values table:group-agents trees [species]
  set rel-hist-frq-local map [i -> i / JL] hist-frq-local
  reset-ticks
end 

to go
  ingest-process
  reproduce-process
  set local-richness-counts map count table:values table:group-agents trees [species]
  set local-richness length local-richness-counts
  set hist-frq-local sort-by > map count table:values table:group-agents trees [species]
  set rel-hist-frq-local map [i -> i / JL] hist-frq-local
  tick
end 

to ingest-process
  if-else (extinction? = true)[
  ask one-of trees [
    set x-cor xcor
    set y-cor ycor
    die]
  ]
  [rare-species-protect]
end 

to reproduce-process
  ask one-of trees [
    hatch 1 [
      set xcor x-cor
      set ycor y-cor
      set time-system 0
    ]
  ]
end 

to rare-species-protect
  ask one-of trees [
    set sp species
    set n count trees with [species = sp]
  ]

  if-else (n > 1) [
    ask one-of trees with [species = sp] [
      set x-cor xcor
      set y-cor ycor
      die
    ]
  ]
  [rare-species-protect]
end

There is only one version of this model, created almost 2 years ago by Ruwan Punchi-Manage.

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