An important feature of the MSY model is how harvested populations respond to environmental fluctuations or illegal offtake. Consider a population at formula_21 harvested at a constant harvest level formula_19. If the population falls (due to a bad winter or illegal harvest) this will ease density-dependent population regulation and increase yield, moving the population back to formula_21, a stable equilibrium. In this case, a negative feedback loop creates stability. The lower equilibrium point for the constant harvest level formula_19 is not stable however; a population crash or illegal harvesting will decrease population yield farther below the current harvest level, creating a positive feedback loop leading to extinction. Harvesting at formula_33 is also potentially unstable. A small decrease in the population can lead to a positive feedback loop and extinction if the harvesting regime (formula_22) is not reduced. Thus, some consider harvesting at MSY to be unsafe on ecological and economic grounds. The MSY model itself can be modified to harvest a certain percentage of the population or with constant effort constraints rather than an actual number, thereby avoiding some of its instabilities.