Gretchen C. Daily, a Stanford University scientist, defines ecosystem services as "the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life."
The health of a watershed is enhanced when the five components — hydrology, connectivity, geomorphology, biology and water quality — have a functional relationship within that ecosystem. Interactions occur at all scales of time and place, and the pieces come together into a synergistic whole.
Significantly altering any of the five basic components of a watershed sends reverberations throughout the system. An altered system is more limited in its ability to react to change and to deliver intact function over time. This includes the systems' capacity to deliver ecosystem goods and services to its human inhabitants. The natural production of ecosystem goods, such as food, forage, timber, fuels, fiber, pharmaceuticals and industrial products can falter and require outside inputs of energy and material. The addition of fertilizer made from fossil fuels and shipped to a farm is an example of an outside input needed to replace the lost service provided by fertile soil.
Ecosystem services also include life-support functions, such as cleansing, recycling, and renewal. When these functions are diminshed, it can have direct impacts on the quality of life for all watershed inhabitants. For example, if natural water cleansing services are lost, a water treatment facility must be built. This requires a long term commitment of material, energy and maintenance to replace a service previously provided by an intact ecosystem.
The impact of alteration leaves a legacy of change in the system. Changes that occurred decades ago may continue to require adjustment by the system components in an attempt to deal with the disturbance and find a new equilibrium.
Overgrazing on steep slopes was one practice that contributed to a legacy of eroded soil in the stream systems
Sediment filled stream channels contribute to the altered hydrology, higher flood flows, and eroding stream banks of today