In modelling biological communities with ODEs, given that individuals are somewhat finite entities, I am trying to avoid infinitesimal densities which are mathematically correct but make no sense. More specifically, I want to set an extinction threshold at a certain small value (eg 1e-12), to ensure that extinct species are actually extinct and do not come back from the dead later. Currently doing this with a callback (see below). It works but I was wondering if there is a more elegant or efficient way of setting to zero values of the solution under a certain threshold?

My current approach (simplified, this is of course the drug concentration decay example with a large threshold)

```
function f(du,u,p,t)
du[1] = -u[1]
du[2] = -u[2]
end
threshold = 0.2
condition(u,t,integrator) = any( (u .< threshold) .& (u.>0))
function affect!(integrator)
integrator.u[(integrator.u .< threshold) .& (integrator.u .> 0)] .= 0
end
cb = DiscreteCallback(condition,affect!)
u0 = [1, 0.5]
prob = ODEProblem(f,u0,(0.0,10.0))
sol = solve(prob,Tsit5(),callback=cb,saveat = 0.1)
plot(sol)
```