Streams and Their Valleys - Fundamentals of Ecosystem Science - page 263

remain, although considerably more scientific attention has been paid to urban streams in
recent years (e.g., Meyer et al. 2005; Walsh et al. 2005 ).
Working in urban streams has made me realize the extent to which scientific under-
standing of stream ecosystems has not been adequately incorporated into environmental
policy and enforcement. Nowhere is this more apparent than in society's current treatment
of headwater streams. Despite their importance in river networks, these small first- and
second-order streams ( Box 16.1 ) are not appreciated, catalogued, or adequately protected.
Although individually small in size, headwater (first- and second-order, Box 16.1 )
streams comprise over 70% of stream length in U.S. river systems. Their total length is not
known precisely because mapping has been done at a scale that does not recognize small
streams, particularly those that flow intermittently ( Meyer and Wallace 2001 ). Yet small
streams are hotspots of intense biogeochemical activity on the landscape because of tight
connections to terrestrial ecosystems and the frequent exchange between flowing water
and sediments ( Meyer et al. 2003 ). Small streams are also hotspots of high species richness
(e.g., Meyer et al. 2007 ). Their productivity supplies food resources to riparian communi-
ties of insects, birds, and bats as well as downstream ecosystems ( Figure 16.6 ). Ecosystem
services provided by small streams include maintenance of discharge regimes, regulation
of sediment export, processing and retention of nutrients and organic matter, and provid-
ing habitat and refuge for many highly valued species (e.g., salmon; Lowe and Likens

Drifting
insects

Unique
chemistry

Export

Riparian
fauna

Benthic
organisms

Refuge from
high flows

Emerging
insects

Migrants

Thermal
refuge
Refuge from
predators

Dissolved and
particulate
organic matter

Export

Riparian
vegetation

Refuge from
competitors
Refuge from
invasive species

Organic matter
storage

Migrants

Nutrient
transformations

Spawning sites

Export
downstream

Nursery habitats

Migrants
from
downstream

Rich feeding
areas

FIGURE 16.6 The role of headwater streams in river networks. Factors on the right result in habitats that sup-
port both unique headwater species and migrants from downstream; the contributions of headwater streams to
riparian and downstream ecosystems are indicated on the left. (From Meyer et al. 2007 .)