The Contribution of Headwater Streams to Biodiversity in River Networks¹

TLDR

Headwater streams, ranging from intermittent to perennial, harbor diverse taxa and provide unique habitats that link upstream and downstream ecosystems through migration, food export, and refuge from environmental extremes. This review examines the diversity of taxa that depend on headwater streams. The review highlights three unmapped headwaters that collectively support over 290 taxa, illustrating the breadth of headwater biodiversity. Loss and degradation of headwaters and their connectivity threaten the biological integrity of entire river networks.

Abstract

Abstract: The diversity of life in headwater streams (intermittent, first and second order) contributes to the biodiversity of a river system and its riparian network. Small streams differ widely in physical, chemical, and biotic attributes, thus providing habitats for a range of unique species. Headwater species include permanent residents as well as migrants that travel to headwaters at particular seasons or life stages. Movement by migrants links headwaters with downstream and terrestrial ecosystems, as do exports such as emerging and drifting insects. We review the diversity of taxa dependent on headwaters. Exemplifying this diversity are three unmapped headwaters that support over 290 taxa. Even intermittent streams may support rich and distinctive biological communities, in part because of the predictability of dry periods. The influence of headwaters on downstream systems emerges from their attributes that meet unique habitat requirements of residents and migrants by: offering a refuge from temperature and flow extremes, competitors, predators, and introduced species; serving as a source of colonists; providing spawning sites and rearing areas; being a rich source of food; and creating migration corridors throughout the landscape. Degradation and loss of headwaters and their connectivity to ecosystems downstream threaten the biological integrity of entire river networks.