The non-renewable Resource (a Stock with no inflow) becomes a renewable
fishery with a true point of no return — an Allee threshold. Spawning
scales with density (~Fish^2) while natural deaths are linear (~Fish), so
below a critical density the deaths win and the stock slides to an
extinction it never recovers from; crowding deaths (~Fish^3) cap a healthy
stock at carrying capacity. A reinvesting fleet (Boats, Reinforcing)
overshoots the renewal rate and drags the fish under the threshold, then
starves and scraps itself.
The negative-positive-negative regrowth curve is the one shape the
proportional rule can't draw alone, so two relay Converters build it
(density to lift spawning to ~Fish^2, crowding for the ~Fish^3 ceiling) —
the Limits-to-growth crowding trick, doubled. At 16 nodes this is the
gallery's largest model and the only one with a Converter feeding a
Converter.
Tuned against the engine: Fish hold near 1000, cross the threshold (~200)
at t~40 as the catch overshoots, then go extinct and stay there; Boats
overshoot to ~450 and collapse back near their start by t=150. No
divergence; loops classify as expected (R: fleet reinvestment, birth
engine; B: natural/crowding/catch drains, scrapping).
The thirteenth sample points the language at a live debate: a classic trap,
Shifting the burden to the intervenor (Thinking in Systems, ch. 5), with AI as
the intervenor. Technical debt drives AI reliance, AI churns debt and lets
skills atrophy, and a thinner-skilled team refactors less — so the loop
Technical debt -> AI reliance -> atrophy -> Expertise -> refactoring ->
Technical debt is Reinforcing: addiction, not a fix. A learning Balancing brake
(practice toward a skill ceiling) is tuned to lose.
Maps the brief onto valid roles: Expertise and Technical debt are the stocks;
code quality and lead time read off them (inverse of debt / inverse of
expertise); model price lives in the AI-reliance factor. Buildable on the
existing rule vocabulary. Tuned against the simulator: over t=0..50 Expertise
slides 70 -> ~6 and Technical debt spirals 20 -> ~150, stopped (like
"Escalation") before the loop runs off-chart. The loop detector classifies the
four loops as expected (2 R, 2 B).
The twelfth sample fills the one classic dynamic the gallery was missing —
overshoot and collapse, the dynamic Thinking in Systems is built around. A
Reinforcing engine (Capital reinvesting its extraction revenue) runs on a
non-renewable Resource, the first Stock in the gallery with no inflow: it
overshoots the limit instead of settling at it, the dark twin of "Limits to
growth".
Buildable with the existing rule vocabulary (proportional only); the
non-negative-stock floor tames the bilinear extraction term as the Resource
runs out. Tuned against the simulator: Capital 5 → ~250 (t≈39) → ~5 by t=150,
Resource 1000 → ~6, no divergence. The loop detector classifies the three
loops as expected (R: investment→Capital→extraction; B: depreciation→Capital;
B: extraction→Resource).
A Stock can carry a display unit (°C, people, $, …): a new optional field, an
inspector text input, validation + round-trip in io, and a readout beside the
live value on the canvas. Equip the demonstrative samples (Water L, Balance $,
Coffee °C, Population/Epidemic people, Yeast cells). Also bumps the fill gauge
opacity for a more vivid level.
Recast the S-curve as a Reinforcing inflow plus a crowding-driven die-off that
grows with Yeast², using only the existing proportional rule. Yeast now climbs
20 → ~1000 as a true sigmoid, and the detector still classifies it R + B — the
balancing loop stays visible, which a single "logistic" rule would have hidden.
Drops the carrying-capacity converter (a faithful one needs a divide rule).
Give 10 of 11 samples initial values, rules, and a run window so they simulate
on load: Bathtub (linear), Savings/Escalation/Fixes-that-fail (Reinforcing),
Coffee (Balancing), Population, Epidemic (SIR), Tragedy (overshoot/collapse),
Drift (eroding goal), Predator-prey (damped). Limits to growth is left
diagram-only — its S-curve needs a saturating rule the vocabulary lacks.
Rename the coffee outflow "heat loss" to "cooling" so the balancing loop
reads more naturally, and stack carrying capacity above the source in
limits to growth so its link to crowding is a clean horizontal hop.
Snap all sample coordinates to multiples of 20 so they sit on the
editor grid and read cleanly. Computed midpoint() valves had to land
on grid points too, so a few endpoints were nudged (e.g. escalation
arsenals 300->280, limits-to-growth source -300->-280). Population is
re-laid as a grid cascade so its links no longer overlap the pipes.
Two more Meadows system traps: Fixes that fail (a balancing
road-building fix whose induced-driving side effect reinforces the
congestion it targets) and Drift to low performance (an eroding goal
that spirals actual performance and the standard downward).
Also re-lays-out the coupled-stock samples for readability: predator
and prey and escalation now use aligned two-row layouts, and fixes
that fail stacks its flow valves in one column so the backfire link
reads as a clean vertical.
Add a second tier past the four-model primer: Limits to growth, Predator
and prey, and Epidemic each introduce one structure the primer never
shows, plus two of Donella Meadows' system traps — Tragedy of the
commons and Escalation — to contrast healthy dynamics.
