[MUD-Dev] RE: Realistic Ecological Models

Sasha Hart Sasha.Hart at directory.reed.edu
Sat Apr 27 00:27:47 New Zealand Standard Time 2002


[Dave Held]

> But I certainly welcome any insights gained from actual attempts
> to introduce ecology.

Here: a dump of thoughts on how to model population sizes .
Conspicuously absent are comments on how they are/can be/should be
used.

  -- No matter what your level of detail, you are going to reinvent
  the wheel.  Even if you think you are doing something radically
  simplified, or unrealistic, you can probably learn something from
  what people wrote.  (Learned the hard way).

  -- Growth is easy and absolutely essential. For typical
  assumptions it is exponential. This means that even if you change
  parameters (birth rate, initial population), you are going to end
  up with an absolutely vast number of critters quickly. Even if you
  are not instantiating each critter you need some assurance that
  your values aren't going to get bigger than the space you have to
  hold them. (Also, infinitely increasing populations are not too
  interesting or useful.)

  -- Population control I: carrying capacity. This is useful for
  putting an upper bound on population growth and for simulating
  starvation due to food shortages. A simple example: as population
  increases above the available supplies for being alive
  (e.g. food), you kill an amount of critters proportional to the
  difference.  (As it turns out, this essentially duplicates the
  logistic equation. The caps on population can be as hard as you
  want 'em). This is the way to go if you are worried about
  population sizes getting out of control. It is also the way to go
  if you want decent control for population sizes with a somewhat
  natural rationale ("an algal bloom has caused the jupiter muskrats
  to breed out of control!")

  -- Population control II: predation. Don't use these to control
  population.  I know, I know, this is what the natural world uses a
  lot of the time. I don't care. God has the deck stacked on his
  side; save yourself the heartache and use carrying capacity or
  other population cap strategies if what you need is a guarantee
  that population won't get above a certain level (max or
  avg). (Learned the hard way).

  Since predation can regulate population, it can give you the
  impression that carrying capacity doesn't matter. It does matter
  if someone kills your predators. If no one can kill your predators
  why are they there?  So don't forget carrying capacity even if it
  seems redundant.

  -- Dynamics need to be played out at the right rate. Not hard to
  engineer this, but it's important - if you want players to
  interact with it rather than just be affected by it, if you want
  them to be able to see signs, predict events and control them to
  any extent, they have to be able to see it, and they have to be
  able to keep up. Birth rate and more concrete instantiations
  (gestation time) are perfect.

Things I have a hunch about.

-- Food webs need diversity to be stable.
-- Diversity can be pretty tough to get and maintain.
-- Dealing with this problem may entail speciation or other
abstractions on genetics or at least variation in phenotype

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