Interventions

An intervention changes the ecosystem: it destroys habitat, moves individuals, converts land, or alters what a layer will do next. That is a different thing from a layer change, which changes one layer's values as a pure function of elapsed time - and the two are deliberately separate mechanisms.

	changes	is a function of	applied
`AbstractLayerChange`	one layer's values	elapsed time only	redundantly on every MPI rank
`Intervention`	the ecosystem	a schedule	once, identically everywhere

An intervention answers three questions, and each is a type rather than a callback - so what one does is visible without running it:

eco = makeeco()
simulate!(eco, 5year, 1month_mean_duration,
          intervention = Intervention(AtTime(2year), RandomCells(20), Deactivate()))
count(parent(eco.habitat.active))          # 20 cells destroyed

When - the schedule

EveryStep, AtTime, AtTimes, BetweenTimes and NeverScheduled (for disabling one without removing it).

A one-off schedule fires on the step that reaches its instant, not on one that equals it: elapsed time accumulates as a float and a run's steps need not land on the instant exactly, so an equality test would silently never fire.

Where - the region

AllCells, ActiveCells, CellMask (your own boolean grid), RandomCells and SpreadingCells (a contiguous cluster).

A count may be exact or a rate. A rate means each candidate is taken independently over the step - a binomial draw - which is how habitat is actually lost:

eco = makeeco()
simulate!(eco, 10year, 1month_mean_duration,
          intervention = Intervention(EveryStep(), RandomCells(0.05 / year), Deactivate()))
100 - count(parent(eco.habitat.active))    # cells lost, drawn not dictated

What - the operations

A closed set: Deactivate, Reactivate, SetLandCover, SetChange, AddAbundance and RemoveAbundance. Closed on purpose - a callback could do anything, including the things that break reproducibility and MPI, whereas six named operations can each be checked once and then trusted.

Deactivate kills what lives there. Destroying a cell is not a pause: a deactivated cell is skipped by the simulation entirely, so anything left in it would neither breed nor die - a frozen population with no ecological meaning. Reactivate does not bring them back either; it makes the cell habitable again so that dispersal can recolonise it, as vegetation returns to a slag heap once it stops being used. To restock deliberately, add AddAbundance alongside it.

Several operations, one region

Operations after the first share the same resolved region, applied in order - clear ground and plant a crop on exactly the ground you cleared:

eco = makeeco()
simulate!(eco, 5year, 1month_mean_duration,
          intervention = Intervention(EveryStep(), RandomCells(0.02 / year),
                                      Deactivate(), AddAbundance(5, 500)))
converted = findall(.!vec(parent(eco.habitat.active)))
all(eco.abundances.matrix[5, converted] .>= 500)

true

Two interventions could not do this: each resolves its own region, and a random one draws its own cells, so the crop would be planted somewhere other than the cleared ground.

Several interventions

InterventionSet applies them in the order written, every step.

Reproducibility

Selections come from a counter-based stream - hash((seed, :intervention, k, step)) - which generalises the per-species scheme. So a run replays exactly, every MPI rank and thread computes the same selection without communicating, and species streams stay reserved for birth, death and dispersal, meaning adding an intervention cannot re-phase the demography.

Ordering

Interventions run inside update!: after the population dynamics, after the clock advances, and before the layer update. So a SetChange installed on a step takes effect that same step rather than one step late.

Worked examples

examples/interventions/ recreates a published set of scenarios - steady warming and drying, a seasonal cycle, random and clustered habitat loss, land conversion, and generalist and specialist invasions - and examples/interventions.jl runs them as part of the test suite. examples/models.jl reuses the same scenarios verbatim, following them through time with a full suite of diversity measures.

Contrast with a layer change, which is the other mechanism: Time in EcoSISTEM and examples/VaryingClimate.jl cover it. The distinction is not stylistic. A layer change is a pure function of elapsed time, so it can be applied redundantly on every MPI rank and still agree; an intervention mutates the ecosystem, so it must be applied once and identically everywhere. That is also why the operation set here is closed rather than a callback - a callback could draw from the global RNG or write a layer's matrix, and either would silently desynchronise ranks.