All ecosystems are nested within a system of larger ecosystems. For management or study purposes, we arbitrarily assign boundaries to ecosystems. Instead of managing for individual parcels of land, ecosystem management uses elevation as the management boundary, water drainage being the element common to all lands. Within this boundary all water flows or drains to a common point. This is called a watershed scale and provides a manageable area with definite boundaries and common characteristics.
Ecosystems change constantly.
Changes in species composition , structure and function occur through succession , a slow, orderly sequence of development of structure and complexity over many years. Change also occurs due to random disturbances and recovery from fire, drought, flood and insect damage and due to human-induced impacts. Natural disturbances are common and extremely important to a healthy ecosystem . Disturbance and recovery from disturbance regulate energy flow and nutrient cycling and maintain age, species, genetic and structural diversity in an ecosystem. Efforts to reduce disturbance from fire, flood and insects have reduced biodiversity , ecosystem health and natural ability to recover from these outbreaks.