An Assessment of the Mechanistic Basis for the High Endemism and Landscape-Scale Biodiversity in Headwater Streams

Frequent observations suggest that headwater streams have lower species diversity within a site than larger streams, but higher beta diversity, and thus gamma diversity, across a catchment. This pattern of diversity includes taxonomic richness and genetic diversity. There are several mechanisms that potentially contribute to the overall high diversity of freshwater organisms in headwaters, although these mechanisms are interdependent. Here I review those mechanisms in an attempt to isolate each and review the available evidence. These include the high numbers of headwater streams, heterogeneity of habitats and resources, founder effects, colonization dynamics, isolation, and strong selection, all leading to diversification of forms. While mechanisms are not independent of each other, I review some of the evidence in support of each. There are a number of directions for future research, including more explicit sampling designs to better address taxonomic richness and for a broader range of taxa. It will be interesting to find ways to partition the relative importance of different mechanisms in contributing to the high variation in diversity among headwaters. The importance of headwater streams to global biodiversity conservation is clear, but will be more evident when better assessments of these small systems are available.