Healthy watersheds sustain dynamic, resilient ecosystems teeming with aquatic and terrestrial life. Ecosystems in a watershed, whether aquatic, riparian, or terrestrial, are strengthened by habitat and biological diversity. More diverse ecosystems have been found to be more resilient to disturbance (e.g. flood, fire, drought, outbreaks, overgrazing) and are more productive.
Ecosystem biodiversity is crucial to human welfare because ecosystems bestow:
"Human-induced changes to components of the Earth’s biodiversity have the potential to compromise the performance of ecosystems, both immediately and by impeding their ability to respond to altered conditions... Ecosystem goods and services, and the ecosystem properties from which they are derived, depend on biodiversity, broadly defined." (Hooper, et.al, 2006)
Biodiversity is threatened by human activities that alter and/or simplify the landscape, such as agricultural practices, urbanization, and channelization.
There are many examples of disturbances such as:
In response to these disturbances, plant and animal diversity declines because of habitat loss or degradation.
When a stream is straightened, the riparian habitats and in-stream biological communities become simplified. The loss of meanders changes relationships between all the components of watershed health.
Measurements of biodiversity often concentrate on the diversity of species. Diversity can be measured by the total number of species (species richness) and the evenness of abundances of those species (species evenness).
In addition to measuring species, biodiversity measurements consider the diversity of ecosystem types (e.g. grasslands, wetlands, rivers, lakes, forests, agricultural). Diversity increases as the number of species, species evenness, or the types of ecosystems increases.
For more detail about the diversity of changing stream habitats and the River Continuum Concept, read RCC.