Biodiversity is a measure that combines richness and evenness across species. It is often measured because high biodiversity is perceived a synonymous with ecosystem health. Diverse communities are believed to have increased stability, increased productivity, and resistance to invasion and other disturbances. That is, biodiversity can increase overall ecosystem stability when biodiversity is low, and decrease it when biodiversity is high, or the opposite with a U-shaped relationship. In order to measure the stability of an object, two factors need to be determined that are the width of the object's base and the height of the object's centre of mass. The position of the centre helps one to know whether the object will remain standing or tip over. The two key components of ecosystem stability are resilience and resistance. Resistance is an ecosystem's ability to remain stable when confronted with a disturbance. Resilience is the speed at which an ecosystem recovers from a disturbance.