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.
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