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Headwaters (stream orders 1-2) traditionally have been considered depauperate compared to mid-order streams (orders 3-4)-a conclusion that arises from a perception of streams as linear systems and emphasizes change in average a (local) diversity along streams. We hypothesized an opposite pattern for b (among-site) diversity and suggest that headwaters might account for a large degree of basin-scale  biodiversity if considered within the more realistic framework of streams as branching networks. We assembled pre-existing biodiversity data from across the globe to test this hypothesis broadly at the population-genetic (mitochondrial haplotype diversity within species) and community (species/taxonomic diversity) levels, with a focus on macroinvertebrates. We standardized 18 (9 headwater and 9 mid-order)  population-genetic and 16 (10 headwater and 6 mid-order) community-level ecoregional data sets from 5 global ecozones for robust comparisons of b-diversity estimates between the 2 stream-size categories. At the population-genetic level, we applied measures of among-site variation commonly used at both population-genetic (FST and WST) and community (S貥nsen's dissimilarity with both presence/absence and abundance data) levels and developed a novel strategy to compare expected rates of loss of c (regional) diversity as individual sites are eliminated sequentially from regions. At the community level, we limited analyses to S貥nsen's presence/absence measures. We found that S貥nsen's dissimilarity was significantly greater among headwaters than among mid-order streams at both population-genetic and  community levels. We also showed that individual headwater reaches accounted for greater proportions of genetic c diversity than did mid-order reaches. However, neither FST nor WST was significantly different between stream-size categories. These measures, which have been used traditionally for comparisons of population-genetic variation, measure proportions of total variation rather than solely among-site variation  (i.e., they also are influenced by within-site variation). In contrast, S貥nsen's dissimilarity measures onlyamong-site variation and, therefore, is presumably more useful for reflecting general b diversity. Overall results suggest that, on average, headwaters probably contribute disproportionately to biodiversity at the  network scale. This finding demands a shift in thinking about the biodiversity contributions of small headwaters and has strong conservation implications for imperiled headwater streams around the world.Griffith Sciences, Griffith School of EnvironmentFull Tex

Small but mighty: headwaters are vital to stream network biodiversity at two levels of organization