Moving Headwater Streams to the Head of the Class

the establishment of the chemical signature for water quality in the landscape. High levels of habitat diversity among and within these small streams create niches for diverse organisms, including headwater-specialist species of aquatic invertebrates, amphibians, and fish. Headwaters also act as refugia for riverine species during specific life-history stages and critical periods of the year, such as warm summer months. Like the alveoli (the final branches of the respiratory tree that serve as the primary gas exchange units of the lungs), headwater streams are characterized by strong and vital interactions with the systems that surround them. Terrestrial inputs—dissolved nutrients, toxins, and particulate matter, for example—play a central role in determining the physical and chemical conditions of headwater streams (Likens and Bormann 1974) and in regulating the composition and productivity of biotic communities in these streams (Wallace et al. 1997). Because of this close terrestrial‐aquatic linkage, the ecosystem services provided by headwaters and the species they support tend to be very sensitive to natural and anthropogenic disturbance of surrounding lands. Along with other distinctive qualities, this close connection creates a unique set of challenges and opportunities related to the protection of headwaters, and to research in these systems. Conservation challenges and opportunities