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. Altered systems are impaired in their ability to maintain intact biological functions.
This impairment can reduce 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 a lost service (fertile soil).
Ecosystem services also include life-support functions, such as cleansing, recycling, and renewal. When these functions are impaired, it can have direct impacts on the quality of life for all watershed inhabitants. Building a water treatment facility is another example of needing outside materials and energy to replace the water cleansing services previously provided by an intact ecosystem.
Even alterations from decades ago leave a legacy of change to the system. This change requires continued adjustment by the system components in an attempt to deal with the disturbance and find a new equilibrium.