Source of non-linearity
The nonlinear behavior of complex systems is caused by certain phenomena, there are several sources of non-linearity according to the studied system, we will give a few sources that can be fairly general:
Threshold: It is common in environmental systems, it is defined as the point of changes in the system's behavior. For instance, in fluid dynamics, when the shear stress is lower than that necessary to entrain a fluid particle, the particle remains motionless. When the shear stress threshold is exceeded, the particle is entrained.
Saturation: In biological systems, it is one of the effect that are responsible of non-linearity. This is linked to the existence of an optimum level of availability of a resource. When the availability increases (up to this optimum), each unit supplied has an increasing effect. Beyond the optimum, the effect of a unit increase declines, and may even be retarding
Competitive relationships: As for the previous sources of non-linearity, this source can also be found in biological systems where several organisms compete for resources. For instance, in geomorphic systems, soil erosion may limit vegetation cover, while vegetation inhibits soil erosion.
Forces and Boundary conditions: Applied forces depend on changes in the shape or size of the boundaries of the system. For instance, the wind loads or wave loads on an environmental system.
